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R T GOWAN ECHNOLOGY ROUP

Courtesy U.S. Navy. Photo by Ensign John Gay

Use of Thermal Sprayas an Aerospace ChromePlating Alternative

Report to:William GreenGeo-Centers

Rowan Project #: 3105JSF3

Contract Number: N00173-98-D-2006, D.O. 0002Subcontract Number: GC-3363-99-004P.O. Number: 28578MK

Report Number: Final

Date: October 27, 2000

Authors: Keith Legg(Rowan Technology Group, Principal Investigator)

John Sauer(Sauer Engineering)

UNCLASSIFIED NON-PROPRIETARY - Distribution Statement A

(mailto:[email protected])

mailto:[email protected]

Rowan Technology Group Project #: 3105JSF3 ; Report #: Final

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EXECUTIVE SUMMARY Thermal spray coatings have been used for many years in aircraft turbine engines as wear and erosion resistant coatings, thermal barriers, and clearance control coatings. As increasing environmental and safety issues have driven a search for chrome plating alternatives, engineers have found that thermal spray coating, long used for gas turbine engines, can be a very cost-effective alternative to hard chrome plating. Although the initial driver for the substitution of thermal spray coatings for chrome was environmental, the alternatives are now being widely adopted because of their better performance, higher reliability, and lower life-cycle cost. This document summarizes the current state-of-the-art, property and performance data, and usage of thermal spray coatings as replacements for hard chrome plating on aerospace components. The information covers the use of hard chrome for both original equipment and for overhaul and repair. Its purpose is to provide in one place a summary of information on thermal spray coatings that will be useful for engineers engaged in the design and maintenance of aircraft components. This document is designed as an electronic book, with links to guide the user directly to information of interest. The document itself contains data summaries and examples, with a large number of underlying full-text references (available at the click of a mouse) to provide as much detail as possible. The information is current as of August 2000, but the document is intended to be readily revised and updated as more information is generated. After a brief introduction, the document is broken into four parts:

Part 1. Aerospace Usage of Chrome � An overview of the types of components and applications in which hard chrome is currently used in the aircraft industry, and the requirements for chrome replacement.

Part 2 Overview of Thermal Spray � Types and principles of thermal spray, especially High Velocity Oxy-Fuel (HVOF) and Plasma Spray � the two primary chrome replacement technologies. This Part includes thermal spray equipment and powders, thermal spray producibility and quality control, stripping, and finishing.

Part 3. Thermal Spray Data � Summary of current data on structure, properties, and performance of thermal spray coatings � hardness, adhesion, embrittlement, corrosion, fatigue, wear, hydraulic rig testing, landing gear rig testing, and flight testing. The text contains data summaries and graphs, with the underlying data accessible via full-text documents.

Part 4. Specifications and Qualified Components � Summary of thermal spray specifications, and of thermal spray-qualified applications and components.


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In summary, the data shows that in all critical respects HVOF coatings perform as well as (and in most cases better than) hard chrome. This is certainly true in critical areas, including hardness, wear, fatigue, corrosion, hydraulic testing, and extended flight testing. HVOF can be applied to almost any material without causing hydrogen embrittlement, and in many cases the fatigue debit can be completely eliminated. As a result HVOF coatings (primarily tungsten carbide cermets) are now specified on more than a hundred components on Boeing aircraft, and are used extensively for overhaul and repair of landing gear cylinders and axles, and flap and slat tracks. The new Boeing 767-400 is specified for HVOF-coated or chrome plated landing gear, whichever customers request. Parker-Hannifin is eliminating chrome plate, and using thermal spray coatings on all new aerospace hydraulic actuator designs. Airlines such as Delta, Lufthansa, and United are all qualifying HVOF for landing gear overhaul. There are several standard and widely used aerospace specifications for thermal spray processes and for the powder materials they employ. However, thermal spray is not a simple drop-in replacement for chrome plate. As a dry spraying process rather than an electroplate it fits differently into the OEM production and overhaul sequence. Although it can be done in-house, and is in fact available at most repair shops and DoD depots, OEMs frequently contract it out. Furthermore, HVOF coatings, the most common chrome alternative, cannot be used on internal diameters, although plasma spray can be used on diameters down to about 2�. Thermal spray cannot be used to replace thin dense or flash chrome, since it cannot be made thin enough. The process lends itself to a large number of different coating materials and a wide range of deposition conditions. This makes it highly flexible but more complex to use. Therefore the specifications for a thermal spray coating must be properly defined, and the process optimized to fit both the material being processed and the coating material being applied. For example:

• Since it utilizes a torch or plasma gun, it is possible to overheat heat-sensitive components, making proper temperature measurement and control an essential part of the process specification.

• The coating material must fit the substrate material. The most common coating material is tungsten carbide, but thermal sprayed hard alloys, such as Tribaloy, give better fatigue performance on aluminum alloys.

• The thermal spray coating must be optimized properly for the application. Some thermal spray coatings have performed poorly because they used the wrong coating material or used a deposition process that was optimized for the wrong application. For example, thermal spray coatings optimized for wear resistance may have as large a fatigue debit as chrome (or even larger). Re-optimizing the coating for fatigue has reduced, or even eliminated, the fatigue debit while still retaining superior resistance to wear.


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• The finishing specifications for thermal spray coatings are not necessarily the same as for chrome. Thermal spray coatings must in general be finish-ground or superfinished to a much finer surface than is typical for chrome plate. For example, a 16 µinch finish is typically specified for chrome plated hydraulics. Using HVOF coatings with this finish leads to very rapid seal failure. With a 4 µinch or better finish, however, both seal life and rod life are greatly extended.

In summary, the thermal spray process is highly recommended and growing as a replacement for hard chrome plate, but it must be used properly, with accurate specifications, a qualified sprayer, and proper account taken for the materials and applications in which it is used.


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ACKNOWLEDGEMENTS This compilation would not have been possible without the assistance of a great many people in the aerospace industry. We would therefore like to acknowledge the many people and organizations that have contributed to this document. Funding for the work was provided by the Joint Strike Fighter Program Office, NADEP Jacksonville, poc Jean Hawkins. HCAT information has been provided by the members of the HCAT team, courtesy of Bruce Sartwell, NRL, the team leader. Many documents and other information have been provided by companies such as Boeing, Messier-Dowty, Praxair, Sulzer Metco, Southwest Aeroservice, Metcut Research, and the National Research Council of Canada, among others. Many of the documents have been provided by courtesy of ASM International, Materials Park, OH 44073-0002, other documents by courtesy of Gorham Advanced Materials, Gorham, Maine, and by numerous individual authors as indicated in the text.


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HOW TO USE THIS REPORT This document is designed to be used electronically, so that it can be a living document that can be updated with the latest information as it becomes available. It is extensively hyperlinked to permit the reader to jump immediately to items of interest in the text. Many items in the text � figures, tables, references, section headings, etc. � are hyperlinked, and can be identified by their blue text. Clicking on the text takes you to the item. The report is designed to contain the most important information within the text. Details and backup information are provided in the form of attached documents, which can be recognized by the yellow boxes, like the one below. Clicking on the icon within the box brings up the document, making all the details readily available. Later cross-references to these documents are shown in blue, and the Document can be opened by clicking on the blue text. When you have finished with the Document, just close it to return to the Report. If you need to keep it available you can switch between Document and Report by clicking on the �Window� menu button and choosing which item to view.

These documents were created by Adobe Acrobat in .PDF format, and can be read with Acrobat Reader . Most computers already have this utility installed for browsing the web. If yours does not, it can be obtained free of charge by clicking on the link below, which will take you to the Adobe web site.

A note on the use of the Acrobat Reader - Make sure to open the menu item File/Preferences/General and uncheck the box �Open Cross-Doc Links in Same Window�. This will ensure that the Document opens in a separate window from the Report. Also click the button �Show navigation pane� to put the Bookmarks and Thumbnails for navigation at the left edge of the screen.

"HVOF Applications Listing SWA.PDF"

http://www.adobe.com/


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TABLE OF CONTENTS

Executive Summary........................................................i

Acknowledgements ......................................................iv

How to use this report ...................................................v

Table of Contents .........................................................vi

Index of Tables.............................................................xv

Index of Figures ..........................................................xix

Table of Documents..................................................xxiii

Table of Acronyms ...................................................xxvi

1. Introduction.............................................................................. 1

1.1. Documents ........................................................................... 2 1.2. Recent data on health effects of Cr6+.................................... 2 1.3. Progress in chrome replacement .......................................... 3

Part 1. Aerospace usage of chrome ............................6

2. Typical Chrome Plated Components ....................................... 6

2.1. New equipment usage.......................................................... 6 2.2. Overhaul and repair usage ................................................... 7 2.3. Landing gear components .................................................... 7 2.4. Hydraulic actuators............................................................... 9

3. Chrome replacement options and requirements .................... 10

3.1. Hard chrome replacement criteria....................................... 11 3.2. Thermal spray for hard chrome replacement ...................... 13


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Part 2. Overview of thermal spray .............................14

4. Principles of thermal spray..................................................... 14

4.1. Summary............................................................................ 14 4.2. Documents ......................................................................... 14 4.3. General .............................................................................. 15 4.4. Thermal spray processes ................................................... 16 4.5. Factors determining coating properties............................... 18 4.6. Applications of common thermal spray coatings ................. 19 4.7. Limitations of thermal spray................................................ 20

4.7.1. Line of sight issues.......................................................... 21 4.7.2. Heating issues................................................................. 21 4.7.3. Coating thickness............................................................ 21

5. Thermal spray coatings ......................................................... 23

5.1. Summary............................................................................ 23 5.2. Thermal spray materials ..................................................... 23

5.2.1. General ........................................................................... 23 5.2.2. Powders frequently used for chrome replacement 26

5.3. Typical structural properties of thermal spray coatings ....... 27 5.4. Typical applications of thermal spray coatings.................... 29

6. Types of thermal spray processes ......................................... 31

6.1. Flame spray........................................................................ 31 6.2. Arc spray ............................................................................ 31 6.3. Plasma spray...................................................................... 32 6.4. High velocity oxy-fuel (HVOF) spray and detonation gun.... 33

7. Thermal spray producibility .................................................... 34

7.1. Summary............................................................................ 34 7.2. Documents ......................................................................... 34 7.3. Quality Control Of the Thermal Spray Process ................... 35

7.3.1. Choice of powder ............................................................ 35 7.3.2. General ........................................................................... 35 7.3.3. Metallography.................................................................. 37


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7.3.4. Hardness......................................................................... 39 7.3.5. Tensile/Adhesion............................................................. 40 7.3.6. Temperature monitoring .................................................. 40 7.3.7. Monitoring residual stress................................................ 43

7.4. Process optimization and control ........................................ 44 7.4.1. General ........................................................................... 44 7.4.2. Example 1 � Optimization of WC-Co ............................... 46 7.4.3. Example 2 � Optimization of WC-CoCr ........................... 48

7.5. Stripping ............................................................................. 52 7.5.1. Documents...................................................................... 52 7.5.2. Stripping of WC-Co ......................................................... 53

7.5.2.1. Southwest Aeroservice.............................................. 53 7.5.2.2. Sulzer-Metco............................................................. 54 7.5.2.3. Lufthansa .................................................................. 55 7.5.2.4. Other specifications................................................... 55

7.5.3. Stripping of WC-CoCr...................................................... 56 7.5.4. Stripping of Tribaloy 400 ................................................. 56 7.5.5. Water-jet stripping ........................................................... 57

7.6. Finishing............................................................................. 57 7.6.1. Documents...................................................................... 57 7.6.2. General requirements...................................................... 57 7.6.3. Specifying the surface finish............................................ 58 7.6.4. Superfinishing ................................................................. 61 7.6.5. Rig test experience.......................................................... 63 7.6.6. Flight experience............................................................. 64

7.7. Inspection........................................................................... 65

8. Thermal spray equipment ...................................................... 66

9. Thermal spray services.......................................................... 67

Part 3. Thermal Spray Data ........................................69

10. Coating structure ................................................................ 70

10.1. Summary............................................................................ 70


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10.2. Documents ......................................................................... 70 10.3. General .............................................................................. 71 10.4. Microstructure..................................................................... 71

10.4.1. General Description and Test Methods ........................ 71 10.4.2. Microstructural Features .............................................. 73

10.4.2.1. Porosity/Voids ......................................................... 73 10.4.2.1.1. Comparison of Porosity vs. Particle Velocity ..... 73

10.4.2.2. Matrix/Carbide Phases/Composition........................ 74 10.4.2.3. Transverse Cracks .................................................. 76

10.4.3. General trend of microstructural features ..................... 76 10.5. Phase Determination and Effect ......................................... 76

10.5.1. General Description and Test Methods ........................ 76 10.5.2. Phase Determination and Effect Results ...................... 77

10.5.2.1. Carbide Phase Comparison vs. Process Type......... 77 10.5.2.2. Carbide Degradation Indexing ................................. 79

10.5.3. General Trend of Carbide Phase Distribution............... 79

11. Coating properties .............................................................. 81

11.1. Summary............................................................................ 81 11.2. General Background........................................................... 81 11.3. Hardness............................................................................ 82

11.3.1. Documents................................................................... 82 11.3.2. General Description and Test Methods ........................ 82 11.3.3. Hardness Results......................................................... 82 11.3.4. General Trend of Hardness Results ............................. 85

11.4. Adhesion ............................................................................ 85 11.4.1. Documents................................................................... 85 11.4.2. General Description and Test Methods ........................ 85 11.4.3. Tensile/Adhesion Results............................................. 86 11.4.4. General Trend of Tensile Results................................. 86

11.5. Residual Stress .................................................................. 86 11.5.1. Documents................................................................... 86 11.5.2. General Description and Test Methods ........................ 87 11.5.3. Residual Stress Results ............................................... 89

11.5.3.1. Almen strip .............................................................. 89


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11.5.3.2. Almen/Residual Stress Comparison ........................ 90 11.5.3.3. Modified Layer Removal Technique ........................ 93 11.5.3.4. Residual Stress by X-ray Diffraction ........................ 95 11.5.3.5. Residual Stress by Neutron Diffraction .................... 96

11.5.4. General Trend of Residual Stress Results ................... 98

12. Coating performance.......................................................... 99

12.1. Summary............................................................................ 99 12.2. General Background......................................................... 100 12.3. Documents ....................................................................... 100 12.4. Test Protocol Summaries ................................................. 101

12.4.1. Start-up test Protocol ................................................. 101 12.4.2. JTP for Landing Gear................................................. 102 12.4.3. Other Protocols .......................................................... 102

12.5. Corrosion.......................................................................... 103 12.5.1. Documents................................................................. 103 12.5.2. Corrosion Test Methods............................................. 104

12.5.2.1. Atmospheric Methodology ..................................... 104 12.5.2.2. Simulated Cabinet Testing..................................... 104

12.5.3. Corrosion Data........................................................... 108 12.5.3.1. Simulated Cabinet Results from Lufthansa............ 108 12.5.3.2. Cabinet and Atmospheric Testing - HCAT ............. 111

12.5.3.2.1. ASTM B117 Salt Fog Testing.......................... 111 12.5.3.2.2. GM 9540P/B Testing....................................... 114 12.5.3.2.3. Atmospheric Testing ....................................... 114 12.5.3.2.4. Interpretation of results.................................... 115

12.5.3.3. Electrochemical Testing of Carbide Coatings ........ 115 12.5.3.3.1. Interpretation of results.................................... 117

12.5.3.4. Corrosion Work Planned in JTP Landing Gear ...... 117 12.5.4. General Trend of Corrosion Results........................... 118

12.6. Fatigue ............................................................................. 118 12.6.1. Documents................................................................. 118 12.6.2. General Description and Test Method........................ 119 12.6.3. Fatigue Results .......................................................... 123

12.6.3.1. Comparison of Hard Chrome vs. HVOF WC-Co and


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T400...................................................................... 123 12.6.3.2. Comparison of Hard Chrome vs. HVOF WC-Co for

Landing Gear......................................................... 127 12.6.3.3. Comparison of Hard Chrome vs HVOF WC-CoCr . 132 12.6.3.4. Other ..................................................................... 134 12.6.3.5. Comparative Study of Compressive Stress Effects on

Fatigue for HVOF .................................................. 134 12.6.3.5.1. Interpretation of results.................................... 137

12.6.4. General Trend of Fatigue Results .............................. 137 12.7. Wear � Erosion, Abrasion, Sliding, Fretting ...................... 138

12.7.1. Documents................................................................. 138 12.7.2. General Description and Test Methods ...................... 139 12.7.3. Test Methods ............................................................. 141

12.7.3.1. Erosion Testing per ASTM G 76............................ 141 12.7.3.2. Abrasion Testing ................................................... 142 12.7.3.3. Sliding/Fretting Wear Methods .............................. 142

12.7.4. Wear Results ............................................................. 143 12.7.4.1. ASTM G 65 Erosion Testing .................................. 143

12.7.4.1.1. Interpretation of results.................................... 145 12.7.4.2. ASTM G 76 Abrasion Testing ................................ 145

12.7.4.2.1. Interpretation of results.................................... 147 12.7.4.3. Other Abrasion Tests............................................. 147 12.7.4.4. Sliding and Fretting Wear Results ......................... 148

12.7.4.4.1. DARPA program � GEAE/NU.......................... 148 12.7.4.4.2. JTP for Landing Gear...................................... 150

12.7.5. General Trend of Wear Results.................................. 151 12.8. Impact .............................................................................. 152

12.8.1. General Description and Test Methods ...................... 152 12.8.2. Impact Test Results ................................................... 152

12.9. Hydrogen Embrittlement................................................... 153 12.9.1. General Description and Test Methods ...................... 153

12.9.1.1. Embrittlement Testing:........................................... 153 12.9.2. Lufthansa embrittlement tests .................................... 153 12.9.3. Hydrogen Embrittlement Tests Planned - HCAT ........ 154 12.9.4. General Trend of Hydrogen Embrittlement Results ... 154


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12.10. Creep ............................................................................ 155 12.10.1. General Description and Test Methods...................... 155 12.10.2. Documents ................................................................ 155 12.10.3. Creep Testing Results ............................................... 156

12.10.3.1. Results for HVOF WC-Co and T400...................... 156 12.10.3.1.1. Test conditions.............................................. 156 12.10.3.1.2. Results.......................................................... 157 12.10.3.1.3. Interpretation of results.................................. 157

12.10.4. General Trend of Creep Results ................................ 157

13. System performance ........................................................ 158

13.1. Summary.......................................................................... 158 13.2. Rig tests ........................................................................... 159

13.2.1. Hydraulic Seals � Green, Tweed Phase 2 hydraulic rig test ........................................................................... 159

13.2.1.1. Documents ............................................................ 159 13.2.1.2. Test Description .................................................... 159 13.2.1.3. Test Conditions ..................................................... 159 13.2.1.4. Results .................................................................. 160 13.2.1.5. Interpretation of Results ........................................ 161 13.2.1.6. Comments............................................................. 162

13.2.2. Landing Gear Pins � Boeing landing gear rig test ...... 163 13.2.2.1. Documents ............................................................ 163 13.2.2.2. Test Conditions ..................................................... 163 13.2.2.3. Results .................................................................. 164 13.2.2.4. Interpretation of results.......................................... 164 13.2.2.5. Comments............................................................. 164

13.2.3. Rig tests under development � Messier-Dowty .......... 164 13.3. Flight tests........................................................................ 165

13.3.1. Lufthansa................................................................... 165 13.3.1.1. Documents ............................................................ 165 13.3.1.2. Test Conditions ..................................................... 165 13.3.1.3. Results .................................................................. 166 13.3.1.4. Interpretation of results.......................................... 166 13.3.1.5. Comments............................................................. 166


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13.3.2. Delta .......................................................................... 166 13.3.2.1. Test Conditions ..................................................... 167 13.3.2.2. Results .................................................................. 169 13.3.2.3. Interpretation of results.......................................... 170 13.3.2.4. Comments............................................................. 170

13.3.3. F-18 landing gear repair............................................. 170 13.3.4. Flight tests under way or under development............. 171

Part 4. Specifications and qualified components...172

14. Specifications and standards for thermal spray ................ 172

14.1. Documents ....................................................................... 172 14.2. Boeing thermal spray specs � method, powder, grinding.. 172

14.2.1. Boeing Thermal Spray Spec � BAC 5851 .................. 172 14.2.2. Boeing Powder Spec � BMS 10-67 ............................ 174 14.2.3. Boeing Grinding Spec � BAC 5855 ............................ 174

14.3. Hamilton-Sundstrand � HS 4412 ...................................... 174 14.4. Society of Automotive Engineers - AMS 2447 .................. 174 14.5. American Welding Society � AWS C.2-19-XX .................. 175 14.6. AMS standards under development.................................. 176

15. Qualified Thermal Sprayed Airframe Components............ 177

15.1. Documents ....................................................................... 177 15.2. Usage of thermal spray in Gas Turbine Engines............... 177 15.3. Summary of thermal spray coatings on non-engine

components ................................................................. 179 15.4. Boeing � qualified thermal sprayed components .............. 180 15.5. Landing gear .................................................................... 181

15.5.1. OEM Production - Boeing 767-400 landing gear ........ 181 15.5.2. Flight tested landing gear repair - Canadian F-18 MLG

axle........................................................................... 183 15.5.3. Other qualified landing gear applications.................... 184 15.5.4. Boeing overhaul manual revision ............................... 184 15.5.5. Delta Airlines qualified landing gear repair � Boeing 737,

757, 767 ................................................................... 185 15.5.6. Qualified landing gear repair ...................................... 186


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15.6. Hydraulics ........................................................................ 186 15.6.1. P&W F-119 engine convergent nozzle

actuator .................................................................... 187 15.6.2. Flight test � Sikorsky CH-53 blade damper ................ 187

15.7. Production - Flap and slat tracks ...................................... 188 15.7.1. OEM tracks - Boeing.................................................. 188 15.7.2. OEM tracks - Bombardier........................................... 188 15.7.3. Flap track repair � Bombardier Dash 8....................... 189 15.7.4. O&R of tracks � Boeing and other aircraft .................. 190

15.8. Other components............................................................ 191

References .................................................................193


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INDEX OF TABLES Table 1. Hard Chrome Alternatives Team members (full list available on

HCAT web site). ............................................................................... 4 Table 2. Some typical OEM chrome plated components......................... 6 Table 3. Some differences between OEM and O&R chrome

replacement.................................................................................... 10 Table 4. Hard chrome replacement criteria. .......................................... 12 Table 5. Typical characteristics of thermal spray coating processes. .... 17 Table 6. Some fundamental terms that define the quality of thermal spray

coatings. ......................................................................................... 19 Table 7. Some common thermal spray coatings, their structure,

performance, and applications........................................................ 20 Table 8. Some major thermal spray powder classifications................... 24 Table 9. Important parameters defining thermal spray powders and

electric arc wire............................................................................... 25 Table 10. Examples of thermal spray powder used in chrome

replacement operations. ................................................................. 26 Table 11 Comparison of thermal spray coating processes � general

properties. ...................................................................................... 28 Table 12. Comparison of thermal spray coating processes - permeability,

thickness. ....................................................................................... 29 Table 13. Some applications of thermal spray coatings.. ...................... 30 Table 14. Producibility summary and links. ........................................... 34 Table 15 Commonly Used Quality Control Tests................................... 36 Table 16 Common Characteristics Evaluated in Metallographic

Specimens...................................................................................... 38 Table 17. Thermal spray process parameters....................................... 45 Table 18. Design of Experiment analysis tool. ...................................... 47 Table 19 Response vs. Coating Property............................................. 48 Table 20. Comparison of hydrogen versus propylene DOE. ................. 48 Table 21. Electrolytic stripping method for HVOF WC-Co (Courtesy

Southwest Aeroservice).................................................................. 53 Table 22. Electrolytic stripping method for HVOF WC-Co (Courtesy

Sulzer Metco). ................................................................................ 54 Table 23. Electrolytic stripping method for �aged� HVOF WC-Co

(Courtesy Lufthansa). ..................................................................... 55 Table 24. Electrolytic stripping method for "new" HVOF WC-Co (Courtesy


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Lufthansa). ..................................................................................... 55 Table 25. Electrolytic stripping method for HVOF WC-Co (NTS/NADEP

Cherry Point). ................................................................................. 56 Table 26. Seal life in HVOF-WC-Co sprayed landing gear.18,................ 64 Table 27. Qualified providers for Boeing 5851 thermal spray coatings, as

of June 2000 (Source, Boeing Aircraft Corporation)........................ 67 Table 28. Common microstructural Characteristics Observed in Tungsten

Carbide Materials. .......................................................................... 72 Table 29. Features seen in Figure 35. .................................................. 75 Table 30. Physical properties of coatings produced by different guns. .. 78 Table 31. Effect of Gas Flows and Cooling Gases on Retained Carbon.

....................................................................................................... 78 Table 32. Retained C and XRD phases. ............................................... 78 Table 33. Microhardness for various HVOF coatings and equipment

(Courtesy Praxair Surface Technology). ......................................... 83 Table 34. Comparison of Microhardness Values and Resultant Variation

(Courtesy Sulzer Metco and SUNY Stony Brook). .......................... 84 Table 35. Qualitative techniques for measuring residual stress............. 88 Table 36. Common quantitative residual stress measurement techniques.

....................................................................................................... 89 Table 37. Zone analysis of thermal spray coatings. .............................. 92 Table 38. WC coating system designations for Document 24. .............. 93 Table 39. Comparison of Residual Stress by Varied Techniques.......... 96 Table 40. Experimental set-up for neutron diffraction............................ 96 Table 41. Summary of performance tests. ............................................ 99 Table 42. Chemistry of Tribaloys. ....................................................... 101 Table 43. Materials and heat treats for HCAT Landing Gear JTP. ...... 102 Table 44 Common Corrosion Testing Methods................................... 104 Table 45 GM9540 Protocol for Corrosion testing. .............................. 106 Table 46. Visual ranking criteria (ASTM B537-70). ............................. 107 Table 47. Coatings tested (Lufthansa). Note: 25µm+0.001�................ 109 Table 48. Summary of corrosion ratings for coatings tested by Lufthansa.

..................................................................................................... 110 Table 49. Coatings and substrates - HCAT corrosion testing. ............. 111 Table 50. Corrosion of 4340 steel with HVOF and Cr coatings -

appearance and protection rankings............................................. 113 Table 51. GM9540P/B corrosion of 4340 steel with HVOF and Cr

coatings - appearance and protection rankings............................. 113


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Table 52. HVOF coatings used for Comparison of Electrochemical Corrosion Potential. ...................................................................... 116

Table 53. HCAT/C-HCAT corrosion test matrix for landing gear steels and coatings. ................................................................................ 117

Table 54. Fatigue testing variables. .................................................... 122 Table 55. Materials and Substrates in Study....................................... 123 Table 56 Fatigue Test Parameters...................................................... 123 Table 57. Fatigue Matrix for Initial Validation ...................................... 124 Table 58. Substrate and coating materials - landing gear JTP. ........... 128 Table 59. Test conditions for landing gear JTP. .................................. 128 Table 60. Tungsten Carbide Coating System Designations (Volvo) .... 135 Table 61. Fatigue Test Parameters for Volvo Evaluation .................... 136 Table 62. Four Primary Wear Mechanisms......................................... 140 Table 63. Erosion Results as Conducted By Praxair........................... 144 Table 64. ASTM G76 data from Praxair. ............................................. 146 Table 65. ASTM G76 Data from NRC. ................................................ 146 Table 66. ASTM G76 Data from NRL and Sulzer Metco. .................... 147 Table 67. Average wear coefficients, K, expressed in units of 10-4 mm3/N-

m, for the various coating/substrate combinations. ....................... 148 Table 68. Fretting test parameters. ..................................................... 149 Table 69. Wear test variables for DOE factors. ................................... 151 Table 70. Creep test parameters ........................................................ 156 Table 71. Summary of rig and flight testing data. ................................ 158 Table 72. Hydraulic test conditions. .................................................... 159 Table 73. Stroke and frequency profile for hydraulic tests................... 160 Table 74. Military flight tests of HVOF-coated components................. 171 Table 75. Boeing thermal spray coating types. ................................... 173 Table 76. AMS 2447 HVOF Coating specifications. ............................ 175 Table 77. Summary of thermal spray-qualified non-engine components.

(Click on links to access data directly.) ......................................... 179 Table 78. Summary of Boeing components specified for thermal spray.

..................................................................................................... 180 Table 79. Other landing gear components qualified for OEM HVOF WC-

Co (Courtesy Southwest Aeroservice). ......................................... 184 Table 80. Landing gear components commonly repaired with HVOF WC-

Co (Courtesy Southwest Aeroservice). ......................................... 186 Table 81. Flap and slat tracks specified for thermal spray coating with


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Super D-gun WC-18Co (Courtesy Boeing). .................................. 189 Table 82. Bombardier Dash 8-100, -200, -300 flap tracks qualified for

HVOF repair (Courtesy Vac Aero). ............................................... 190 Table 83. Common flap/slat track repairs using HVOF WC-Co (Courtesy

Southwest Aeroservice)................................................................ 191 Table 84. Other OEM HVOF WC-Co applications (Courtesy Southwest

Aeroservice). ................................................................................ 192 Table 85. United Airlines O&R components qualified for HVOF in place of

chrome plate................................................................................. 193


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INDEX OF FIGURES Figure 1. Boeing 737 nose landing gear inner cylinder. Shiny areas are

chromed - piston and four axle journals (Courtesy Sulzer Metco). .... 8 Figure 2. Boeing 767 main landing gear (Courtesy Sulzer Metco). ......... 8 Figure 3. F-18 main landing gear Oleo Attach Pin (Courtesy Boeing). .... 9 Figure 4. F/A-18 E/F aileron servocylinder, manufactured by HR Textron

(Courtesy Boeing)............................................................................. 9 Figure 5. Thermal spray process schematic (left); close-up view of

surface (right). ................................................................................ 15 Figure 6. Types of thermal spray processes. Types covered in this report

shown in green.8 ............................................................................. 16 Figure 7. Structure of Thermal Spray Deposit at 100-500X.8................. 18 Figure 8. Example of powder definition for sintered irregularly shaped

88/12 Tungsten Carbide Cobalt powder.......................................... 27 Figure 9. Components of a typical plasma spray system. ..................... 32 Figure 10. Typical HVOF coating cross sections; Ni-Al left (200x), WC-Co

right (500x). (Courtesy Praxair-TAFA)............................................ 38 Figure 11 Tensile Assembly from ASTM C-633 .................................... 40 Figure 12. Typical non-contact temperature arrangement for HVOF..... 41 Figure 13 Typical Temperature Plot From a Spray Cycle (J. Schell,

GEAE, Courtesy HCAT). ................................................................ 42 Figure 14. Almen �N� Test Strip. ........................................................... 43 Figure 15. Almen holding fixtures (Electronics Inc.). ............................. 44 Figure 16. Almen measuring instrument (Electronics Inc). .................... 44 Figure 17. Kinetic versus thermal energy for the main thermal spray

technologies. .................................................................................. 46 Figure 18. Best propylene results. Degradation index = 4.25. .............. 49 Figure 19. Best hydrogen results. Degradation index = 3.46................ 49 Figure 20. Graph showing the temperature/velocity profile with varied fuel

types............................................................................................... 50 Figure 21. Microstructure/morphology of selected powders. ................. 51 Figure 22. Particle size distribution for the three best powders. ............ 51 Figure 23. Typical surface profile. ......................................................... 59 Figure 24. Three different surfaces with the same Ra........................... 60 Figure 25. Other surface roughness parameters................................... 60 Figure 26. Definition of bearing ratio. .................................................... 61


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Figure 27. Effect of various finishing methods on an HVOF coating at 175x (Courtesy Supfina). ................................................................ 62

Figure 28. Surface finishes obtained on Chrome and HVOF WC-CoCr by various finishing methods.18............................................................ 63

Figure 29. Sulzer Metco F210 ID plasma spray gun (Courtesy Sulzer Metco). ........................................................................................... 66

Figure 30. HVOF spraying of WC-CoCr on landing gear with TAFA gun (Courtesy Praxair-TAFA). ............................................................... 66

Figure 31. Northwest Mettech Axial III tri-electrode plasma system (Courtesy Northwest Mettech).. ...................................................... 66

Figure 32. Stellite Jet-Kote HVOF gun (Courtesy Deloro Stellite).......... 66 Figure 33. Comparison of porosity at 200x and 1000x magnification. ... 73 Figure 34. Relationship between velocity and porosity.......................... 74 Figure 35. Comparison of Carbide Distributions in 88-12 WC-Co (left) vs.

83-17 WC-Co (right) at 500X (Courtesy Praxair/TAFA)................... 74 Figure 36. Microstructure of WC-CoCr 1000X....................................... 75 Figure 37. Transverse cracking in plasma sprayed carbide coatings.25. 76 Figure 38. X-ray diffraction plot of powder(lower curve) and coating

(upper curve). ................................................................................. 77 Figure 39. Comparison of carbide content in as-sprayed A-12 (12%

cobalt ). Hybrid 2600 gun (left), air-cooled DiamondJet (right). ...... 79 Figure 40. Tensile assembly from ASTM C-633.................................... 85 Figure 41. Stress as a function of coating thickness for HVOF WC-CoCr.

....................................................................................................... 90 Figure 42. Almen strip stress measurement.......................................... 91 Figure 43. Average residual stress as a function of spray distance. ...... 91 Figure 44. Average residual stress as a function of powder feed rate. . 92 Figure 45. Bend test technique, evaluation criteria, results. .................. 94 Figure 46. Typical stress profile for modified layer removal technique. . 95 Figure 47. Air Plasma Spray residual stress pattern. ............................ 97 Figure 48. Wire Arc Spray residual stress pattern. ................................ 98 Figure 49. HVOF Spray residual stress pattern..................................... 98 Figure 50. B117 Appearance Rankings for coatings on 4340 high

strength steel, PH13-8Mo stainless steel, and 7075 Al. ................ 112 Figure 51. GM9540P/B Appearance Rankings for coatings on 4340 high

strength steel, PH13-8Mo stainless steel, and 7075 Al. ................ 112 Figure 52. 4340 steel 18-month beach exposure tests, with and without

scribing. ........................................................................................ 114


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Figure 53. 7075 Al 18-month beach exposure samples with and without scribing. ........................................................................................ 115

Figure 54. Corrosion Current for an Aerated 0.1 N HCl Solution.40 ..... 117 Figure 55. Typical hourglass-shaped fatigue bar................................. 121 Figure 56. Typical smooth fatigue bar. ................................................ 121 Figure 57. Flat Kb fatigue bar.............................................................. 122 Figure 58. Comparison of Fatigue Data on Smooth Bars for 4340 ...... 125 Figure 59. Comparison of fatigue data on Kb bars for 4340 ................ 125 Figure 60. Fatigue of coated 4340 steel - hourglass samples. ............ 126 Figure 61. Fatigue Results for HVOF and Chrome on 7075 Aluminum127 Figure 62. Fatigue Curve for 300M with .24�dia. hourglass tested in air �

coating thickness 0 .003�. ............................................................. 129 Figure 63. Fatigue Curve for 300M with .24�dia. hourglass comparing air

results with samples tested in NaCl and .003� Coating Thickness. 130 Figure 64. Fatigue Curve Comparing Thickness Effects 0.003� (.250�

dia.) vs. 0.010� (.500� dia.) on 4340 using hourglass configuration tested in air. .................................................................................. 131

Figure 65. Fatigue of HVOF-coated and chrome plated high strength steels, Kt=1.5, Boeing qualification testing. (Courtesy Engelhard Surface Technology)..................................................................... 133

Figure 66. Comparison of fatigue for chrome and HVOF WC-CoCr deposited with Jet Kote and Diamond Jet guns. (Courtesy Southwest Aeroservice.)............................................................... 133

Figure 67. Comparison of Residual Stress and Resistance of Coating to Crack Initiation.............................................................................. 136

Figure 68. Comparison of Final Fatigue Life with Residual Stress ...... 137 Figure 69. Typical Set-up of ASTM G76 erosion test. ........................ 141 Figure 70. ASTM G 76 set-up. ............................................................ 142 Figure 71. Schematic of Sliding Wear Apparatus for hydrualics ......... 143 Figure 72. Side view of fretting apparatus. .......................................... 143 Figure 73. Erosion Results As Conducted By Stony Brook/Sulzer Metco

..................................................................................................... 144 Figure 74. Comparison of HVOF Processes and WC-Co Powders ..... 148 Figure 75. Fretting wear of hard chrome, HVOF WC-17Co, and HVOF

T400. (Note � the zero wear measurement resulted from material transfer from the uncoated block to the coated shoe, protecting it from wear.) ................................................................................... 150

Figure 76. Average creep measured by direct micrometer readings. .. 157 Figure 77. Cumulative hydraulic fluid leakage in rig tests.................... 161

Rowan Technology Group

Project #: 3105JSF3 ; Report #: Final Page xxii

Figure 78. Seal wear during hydraulic rig tests..................................... 162 Figure 79. F/A-18E/F main landing gear, showing locations of HVOF-

coated pins....................................................................................... 164 Figure 80. Landing gear components HVOF sprayed for flight testing by

Delta Airlines (sprayed areas numbered). (Courtesy Delta Airlines.)......................................................................................................... 168

Figure 81. Boeing 737 nose landing gear inner cylinder undergoing flight test inspection at Delta Airlines (Courtesy Delta Airlines). ............. 168

Figure 82. Boeing 757 axle sleeves HVOF-sprayed with WC-CoCr (Courtesy Delta Airlines).................................................................. 169

Figure 83. Canadian F-18 main landing gear polygon repair (Courtesy Messier-Dowty)................................................................................ 171

Figure 84. Thermal spray coatings used in a typical gas turbine engine. (Courtesy GE Aircraft Engines)50 .................................................... 178

Figure 85. Boeing 767-400 with HVOF coated landing gear. ............... 181 Figure 86. Boeing 767-400 main landing gear (Courtesy Sulzer Metco).

......................................................................................................... 181 Figure 87. Boeing 767 main landing gear axle (part # 2207-85-10),

showing HVOF areas (engineering note 3). (Courtesy Sulzer Metco.)......................................................................................................... 182

Figure 88. Boeing 767-400 main landing gear inner cylinder (Part # 2207-4-10) with asterisks showing locations of HVOF coatings (Courtesy Sulzer Metco.).................................................................................. 183

Figure 89. Canadian F-18 main landing gear axle (Courtesy Messier-Dowty).............................................................................................. 183

Figure 90. Repair area of F-18 main landing gear polygon (Courtesty Messier-Dowty)................................................................................ 184

Figure 91. HVOF WC-Co repair of Boeing 737 nose landing gear inner cylinder (Courtesty Southwest Aeroservice). .................................. 186

Figure 92. Thermal spray actuator coating system developed by Praxair.............................................................................................. 187

Figure 93. CH-53 helicopter (Sikorsky). ................................................ 188 Figure 94. Bombardier Q-400 (Courtesy Bombardier.)......................... 189 Figure 95. Typical flap track - Bombardier Dash 8 (Courtesy Vac Aero,

Canada). .......................................................................................... 190 Figure 96. HVOF-sprayed Dash 8 flap track. Coated areas are dark.

(Courtesy, Vac Aero, Canada.) ....................................................... 191 Figure 97. Flap and slat track repair by HVOF (Southwest Aeroservice).

......................................................................................................... 191 Figure 98. Boeing 737 nose landing gear lower bearing shock strut, Part

# 69-76508. HVOF WC-Co coated and super finished. (Courtesy Sulzer Metco.).................................................................................. 192


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TABLE OF DOCUMENTS Document 1. Hard Chrome Coatings - Advanced Technology for Waste

Elimination, DARPA Grant MDA972-93-1-0006, Keith O. Legg, Jerry Schell, George Nichols, Robert Altkorn............................................. 2

Document 2. Lung Cancer Among Workers in Chromium Chemical Production, Herman J. Gibb et. al., American Journal of Industrial Medicine, 38, 115-126 (2000). (Courtesy of the authors and American Journal of Industrial Medicine.) ......................................... 2

Document 3. Clinical Findings of Irritation Among Chromium Chemical Production Workers, Herman J. Gibb et. al., American Journal of Industrial Medicine, 38, 127-131 (2000). (Courtesy of the authors and American Journal of Industrial Medicine.) .................................. 2

Document 4. JSF Phase 1 Report: Chrome Replacements for Internals and Small Parts. (Rowan Technology Group). ................................ 14

Document 5. JSF Phase 2 Report: Optimal Chrome Replacement Technologies for Internal Diameters and Heat-Sensitive Parts. (Rowan Technology Group)............................................................ 15

Document 6. Common thermal spray powder types.............................. 25 Document 7. Test standardization: a Key Tool in Coating System

Implementation (Courtesy Sauer Engineering, Gorham Advanced Materials)........................................................................................ 34

Document 8. Training in Coating Evaluation Techniques: a Unique Approach for Discussion (Courtesy Sauer Engineering). ................ 34

Document 9. The Use of Metallographic Standards in Calibration of the Polishing Process (Courtesy Sauer Engineering). .......................... 34

Document 10. Metallographic Preparation of Thermal Spray Coatings: Coating Sensitivity and the Effect of Polishing Intangibles (Courtesy Sauer Engineering)......................................................................... 35

Document 11. Tensile Bond Variance of Thermally Sprayed Coatings with Respect to Adhesive Type. ..................................................... 35

Document 12. Almen Strips and Temperature Measurement During HVOF Processing (Courtesy, Sauer Engineering). ......................... 35

Document 13. Design of Experiment for HVOF WC-Co process (Courtesy HCAT, www.hcat.org)..................................................... 35

Document 14. Summary of DOE results for optimization of HVOF WC-CoCr (Courtesy NRC Montreal and C-HCAT). ................................ 35

Document 15. NTS Stripping Report,PDF. ........................................... 52 Document 16. NDCEE Evaluation of Stripping Methods. ..................... 52 Document 17. Stripping of WC Coatings from Aermet 100, Southwest

Aeroservice, Menasco, Carpenter Technology (Courtesy Southwest Aeroservice). ................................................................................. 52


Page xxiv

Document 18. Surface Metrology Guide (Courtesy Precision Devices, Inc.). .............................................................................................. 57

Document 19. Superfinishing of Hard Chrome and HVOF Coated Workpieces (Courtesy Supfina and Gorham Advanced Materials). 57

Document 20. Surface Finishing of Tungsten Carbide Cobalt Coatings, J. Nuse, J. Falkowski. ........................................................................ 57

Document 21. Barkhausen Noise as a Quality Control Tool (Courtesy Stresstech Inc., Finland). ............................................................... 57

Document 22 Evaluation of Four High Velocity Thermal Spray Guns Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault, C. Moreau, V. Bouyer, L. Leblanc). ............................ 70

Document 23. Fracture Toughness of HVOF Sprayed WC-Co Coatings (Courtesy of S. De Palo, et al). ...................................................... 70

Document 24. Tungsten Carbide-Cobalt Coatings for Industrial Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan). 70

Document 25. A Critical Evaluation of the Employment of Microhardness Techniques for Characterizing and Optimizing Thermal Spray Coatings 2000 (Courtesy of M. Factor and I. Roman, Hebrew University). .................................................................................... 82

Document 26 Behaviour of Tungsten Carbide Thermal Spray Coatings 1995, J. Wigren et al....................................................................... 86

Document 27 An ASM Recommended Practice for Modified Layer Removal Method (MLRM) to Evaluate Residual Stress in thermal Spray Coatings 2000, Ed Rybicki and ASM TSS Committee. ........ 86

Document 28 Properties of WC-Co Components Produced Using the HVOF Thermal Spray Process 2000, J. Stokes and L. Looney. ..... 86

Document 29 X-ray diffraction residual stress techniques, P.S. Prevey. ....................................................................................................... 86

Document 30 Processing Effects on Residual Stress in Ni+5%Al Coatings-Comparison of Different Spraying Methods 2000, J.Matejicek et al. ............................................................................ 87

Document 31 Residual Stress Measurement in Plasma Sprayed Coatings by X-Ray Diffraction (Courtesy of J. Matejicek et al) 1997. ....................................................................................................... 95

Document 32 HCAT Test Protocol for Initial Work 1996 (Courtesy of HCAT Team) ................................................................................ 100

Document 33 Joint Test Protocol (JTP) for Landing Gear 1998 (Courtesy of HCAT and CHCAT Teams) ...................................... 100

Document 34 Joint Test Protocol (JTP) for Propeller Hub Components 2000 (Courtesy of HCAT, JG-PP, and C-HCAT Teams) .............. 100

Document 35 Joint Test Protocol (JTP) for Gas Turbine Engines 2000 (Courtesy of HCAT and PEWG Teams......................................... 101


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Document 36. Report of Replacement of Chromium Electroplating Using HVOF Thermal Spray Coatings AESF Plating Forum 1998 (Courtesy of Bruce Sartwell and HCAT Team). ............................................ 103

Document 37. Replacement of Chrome Plating by Thermal Spray � Results of Corrosion Testing of HVOF Coatings 1996 (Courtesy of Lufthansa). .................................................................................. 103

Document 38. Replacement of chrome plating by thermal spray coatings � Summary of tests (Courtesy of Lufthansa). ............................... 103

Document 39 Performance of HVOF Sprayed Carbide Coatings in Aqueous Corrosive Environments 2000 (Courtesy of S. Simard (NRC) et al). ................................................................................ 103

Document 40 Summary of 4340 Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut) ..................................................... 118

Document 41 Summary of 7075 Al Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut) ..................................................... 118

Document 42 Summary of 13-8 Stainless Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut) ..................................................... 119

Document 43. HCAT landing gear JTP fatigue data - HVOF WC-Co on 4340, 300M, AerMet 100 in air and NaCl solution......................... 119

Document 44 Advanced Thermal Spray Coatings for Fatigue Sensitive Applications (Courtesy of John Quets Praxair).............................. 119

Document 45 Compressive Creep Tests of Hard Chrome and HVOF coatings 1998, J. Schell, GE Aircraft Engines. .............................. 155

Document 46. Evaluation of Chrome Rod Alternative Coatings, Tony Degennaro, Green Tweed, 1999................................................... 159

Document 47. F/A-18E/F Main Landing Gear HVOF-coated Pin Testing and Evaluation.............................................................................. 163

Document 48. Table of contents of BAC 5851 Thermal Spray Specification, 2000 (Courtesy Boeing Aircraft Corp.). .................. 172

Document 49. Standards for the Thermal Spray Industry, Bhusari and Sulit. ............................................................................................ 172

Document 50. HVOF WC aerospace applications for OEM and rebuild (Courtesy Southwest Aeroservice). .............................................. 177

Document 51. Thermal Spray Applications at GE Aircraft Engines (Dorothy Comassar, Courtesy GE Aircraft Engines). ................... 177

Document 52. OEM Approval for HVOF Wear Resistant and MCrAlY Coatings (Gary Naisbitt and Gorham Advanced Materials). .......... 177

Document 53. Replacement of Chromium Electroplating on Gas Turbine Engines. ....................................................................................... 177

Document 54. List of Boeing thermal sprayed parts (Courtesy, Boeing Aircraft Corp). ............................................................................... 180


Project #: 3105JSF3 ; Report #: Final Page xxvi

TABLE OF ACRONYMS AFRL Air Force Materials Lab (Dayton, OH)

ALC Air Logistics Center (Air Force maintenance depot)

AMS Aircraft Materials Specification (a specification of the Society of Automotive Engineers)

APS, VPS Air Plasma Spray, Vacuum Plasma Spray

BFG B.F. Goodrich

C-HCAT Canadian Hard Chrome Alternatives Team

Cr6+ Hexavalent chrome

DARPA Defense Advanced Research Projects Agency

D-Gun Detonation gun (also Super D-Gun) – high velocity thermal spray method based on fuel detonation (proprietary to Praxair)

DND Department of National Defence (Canada)

DoD Department of Defense (US)

DOE Design of Experiment (statistically designed matrix of experiments used for process optimization)

EPA Environmental Protection Agency

ESTCP Environmental Security Technology Certification Program (funding HCAT)

GEAE General Electric Aircraft Engines

GTE Gas turbine engine

HCAT Hard Chrome Alternatives Team

HVOF High Velocity Oxy-Fuel thermal spray

ID Inside diameter

JG-PP Joint Group – Pollution Prevention (DoD environmental group assisting with qualifying clean processes)

JSF Joint Strike Fighter

JSF IPT Joint Strike Fighter Integrated Product Team

JTP Joint Test Protocol

NADEP Naval Aviation Depot (Navy Maintenance depot)

NAWC Naval Air Warfare Center

NDCEE National Defense Center for Environmental Excellence

NRC National Research Council of Canada


Project #: 3105JSF3 ; Report #: Final Page xxvii

NTS National Technical Systems, Inc.

O&R Overhaul and Repair

OD, ID Outside diameter, inside diameter

OEM Original Equipment Manufacturer

OSHA Occupational Health and Safety Administration

PC Personal computer

PEWG Propulsion Environmental Working Group (turbine engine environmental issues)

PVD Physical Vapor Deposition (vacuum coating deposition process)

P & W Pratt and Whitney

QPL Qualified Provider List

R&O Repair and Overhaul

SERDP Strategic Environmental Research and Development Program (funding ID chrome replacements)

TPC Technology Partnerships Canada (funding C-HCAT)

WC Tungsten Carbide

WC-Co, WC-CoCr

Cobalt cemented WC (usually WC-17Co or WC-12Co) and cobalt-chrome alloy cemented WC (usually WC-10Co4Cr). (Percentages by weight.)

Rowan Technology Group Page 1 Project #: 3105JSF3 ; Report #: Final

1.1.1.1. Introduction There is currently intense activity in the area of replacing chrome plating in aircraft, both for original equipment (OEM) use and for overhaul and repair (O&R) use. The most commonly-used alternative to chrome plating is thermal spray, which has now replaced chrome plating in many aircraft OEM and repair applications. While there is a great deal of information on the performance of thermal spray coatings, it tends to be scattered across a large number of disparate documents, few of which are publicly available. This report brings this data into one place for ready access. Its aim is to provide the underlying technical data, as well as information on specifications and qualified components needed by engineers in charge of component design, coating specification, or process and material qualification. In its electronic format, it is intended to be a living document, providing a source of information that can be constantly updated as new data become available. The report is split into four parts:

Part 1. Aerospace Usage of Chrome � Types of components and applications in which hard chrome is currently used in the aircraft industry.

Part 2 Overview of Thermal Spray � Types and principles of thermal spray, with emphasis on the primary method used for chrome replacement, HVOF and APS. This Part includes information on thermal spray producibility and quality control, stripping, and finishing.

Part 3. Thermal Spray Data � Compilation of data on structure, properties, and performance of thermal spray coatings. This includes hardness, adhesion, corrosion, fatigue, wear, hydraulic and landing gear rig testing and flight testing.

Part 4. Specifications and Qualified Components � This Part summarizes the primary specifications used for thermal spray, as well as the aircraft and components on which thermal spray coatings are presently qualified.

The report is extensively hyperlinked so that the reader can jump directly to sections, tables, figures, references, etc. Data and reference materials are summarized in this document, while the underlying documents, where they can be made available, may be accessed directly by double-clicking on the yellow boxes adjacent to their Document Captions.


1.1. Documents Document 1. Hard Chrome Coatings - Advanced Technology for Waste Elimination, DARPA Grant MDA972-93-1-0006, Keith O. Legg, Jerry Schell, George Nichols, Robert Altkorn. 1 This document is the final report for an initial DARPA-funded program which showed that HVOF is the most reasonable dry alternative to chrome plating. HVOF, PVD, and laser clad coatings were tested.

Document 2. Lung Cancer Among Workers in Chromium Chemical Production, Herman J. Gibb et. al., American Journal of Industrial Medicine, 38, 115-126 (2000). (Courtesy of the authors and American Journal of Industrial Medicine.)2 This is the most recent report at time of writing that documents in detail the lung cancer risks of hexavalent chrome for workers in industries where they are exposed.

Document 3. Clinical Findings of Irritation Among Chromium Chemical Production Workers, Herman J. Gibb et. al., American Journal of Industrial Medicine, 38, 127-131 (2000). (Courtesy of the authors and American Journal of Industrial Medicine.)3 This study is the most recent report at time of writing that documents other health effects of hexavalent chrome for workers in industries where they are exposed.

1.2. Recent data on health effects of Cr6+ Recently, new data on the health effects of hexavalent chrome have been developed by the EPA and John Hopkins University. Document 2 and Document 3 detail the health effects of hexavalent chrome exposure on worker health in general and incidence of lung cancer in particular. Document 2 concludes that even the lowest suggested level (0.5 µg Cr6+/m3) produces a measurable increase in lung cancer rate. This study is expected to be used by OSHA in lowering the permissible exposure limit (PEL) from its current 100µg/m3, and may well lead to its being lowered very significantly � almost certainly into the range 0.5-5 µg/m3, and quite possibly into the lower part of that range. The effect of the lowest limit would be a large increase in cost associated with providing adequate worker protection, while the increased liability risks would be likely to drive many vendors out of the chrome plating business. No matter what the details of the final outcome, there is every reason to believe that the environmental and health pressures on chrome plating will increase in the coming years, and that the move toward chrome alternatives will accelerate.

"DARPA chrome FinalReport.PDF"

"Lung Cancer - Chromium Chemical W

"Clinical findings - Chromium among wo


1.3. Progress in chrome replacement The initial reason for considering alternatives to chrome plate was the increasing regulation of chrome (and especially hexavalent chrome) processes. This regulation was designed to combat the environmental and worker health and safety problems inherent in the use of Cr6+ in plating baths, as well as chrome generation in stripping and grinding operations. However, as our experience with alternatives has grown and more data has become available, users are increasingly adopting chrome alternatives because they nearly always have better performance, and frequently have lower cost. Thermal spray coating is the principal technology that has long been used for coating high performance gas turbine engine components (See Document 49, for example). As a result the engine industry has many years of experience with thermal spray processes, such as plasma spray and High Velocity Oxy-Fuel (HVOF), which have been qualified on hundreds, if not thousands, of turbine engine components. Over the past 5 years or so, thermal spray coatings have been qualified for numerous airframe components. In this case, the primary replacement for chrome plating is HVOF, which is now being used in the manufacture and repair of aircraft landing gear. Some examples of the use of thermal spray on airframe components include:

• Thermal spray coatings (primarily HVOF) are now qualified and used on over 100 airframe components made by Boeing (see Section 15.4).

• The landing gear on the new Boeing 767-400 is now specified for HVOF or chrome, depending on customer wishes. Several airlines now require the HVOF version. (See Section 15.5.1.)

• Boeing has specified HVOF tungsten carbide coatings as a qualified replacement for chrome plating for overhaul and repair of landing gear (see Section 15.5.4).

• HVOF tungsten carbide coatings are used for new flap and slat tracks, and are now qualified and widely used for repair of older tracks (see Section 15.7).

Qualification of thermal spray coatings to replace chrome plating is now the subject of extensive laboratory, rig, and flight testing in the defense and commercial sectors. Hard Chrome Alternatives Team (HCAT) � This binational integrated team is the primary program for chrome replacement in the Department of Defense (HCAT in the US) and the landing gear industry (C-HCAT in Canada). The team comprises members from the aircraft industry in the US and Canada, military depots, DoD (US) and DND (Canada) offices, Industry Canada, various and laboratories. This program is run by Bruce Sartwell of the Naval Research Laboratory and is funded by the Environmental Security Technology Certification Program (ESTCP) and other DoD organizations in the US, and by Technology Partnerships Canada (TPC), the Canadian Department of National Defence (DND),

http://www.hcat.org/



http://www.estcp.com/

http://strategis.ic.gc.ca/sc_mangb/tpc/engdoc/homepage.html

http://strategis.ic.gc.ca/sc_mangb/tpc/engdoc/homepage.html

http://www.dnd.ca/


and the landing gear makers (BF Goodrich, Messier-Dowty, and Heroux) in Canada. The program is validating thermal spray coatings (primarily HVOF WC-Co and WC-CoCr) for chrome replacement on landing gear, propeller hubs, and helicopter head components. A PEWG/HCAT program is qualifying thermal spray chrome replacements for gas turbine engine overhaul, while an HCAT/JG-PP program is qualifying HVOF for aircraft hydraulics. The HCAT team includes major aerospace manufacturers, overhaul and repair companies, thermal spray companies, and DoD repair depots. Team members are shown in Table 1.

Boeing � Boeing has been introducing HVOF and D-gun coatings on airframe components for several years and now has over 100 parts specified for thermal spray coatings (see Section 15.4). These components include slat tracks, landing gear, and pins. The new Boeing 767-400 aircraft now has HVOF WC-CoCr specified for its landing gear axles and inner cylinders (Section 15.5.1). Boeing has now approved HVOF coatings in place of chrome as a repair procedure for landing gear (Section 15.5.4). Other manufacturers � Thermal spray coatings have been approved by other manufacturers, including Sikorsky (helicopter landing gear,

Table 1. Hard Chrome Alternatives Team members (full list available on HCAT web site). B.F. Goodrich Messier-Dowty PEWG

Boeing Aircraft Corp Metcut Research Pratt and Whitney

Corpus Christi Army Depot

NADEP Cherry Point Praxair Surface Technologies

Delta Airlines NADEP Jacksonville QuesTek Innovations

Engelhard Surface Technologies

NADEP North Island Rolls Royce

GE Aircraft Engines NTS (McClellan) Rowan Technology Group

Green-Tweed and Co. National Research Council (Canada)

Southwest Aeroservice

Hamilton-Sundstrand Naval Research Lab Sulzer Metco

Heroux NAWC PAX Technology Partnerships Canada

Industry Canada OC_ALC Vac Aero

JG-PP OO-ALC Westaim Corp

Lockheed Martin Orenda Aerospace

http://www.pewg.com/

http://www.pewg.com/


actuators), Hamilton Sundstrand (actuators), Bombardier (flap tracks), Messier-Dowty (landing gear), and Parker-Hannifin (actuators) (See Section 15). Airlines � Delta Airlines has been flight testing HVOF coatings on landing gear of Boeing 737, 757, and 767 aircraft, and has now qualified HVOF WC-CoCr coatings for overhaul and repair in place of chrome on a number of parts. Lufthansa has also flight tested and qualified HVOF WC-CoCr coatings for use on landing gear. (See Section 13.3.2 and 13.3.1.) United Airlines has been using HVOF coatings in place of chrome for some years (Section 15.8). The use of thermal spray in place of chrome for OEM parts and for repair is therefore spreading from its initial use in gas turbine engines to components throughout the aircraft.

Rowan Technology Group Project #: 3105JSF3 ; Report #: Final Part 1 � Aerospace Usage of Chrome

Page 6

PART 1. AEROSPACE USAGE OF CHROME

2.2.2.2. Typical Chrome Plated Components Hard chrome plating is used in many areas of new, Original Equipment Manufacturer (OEM) components, as well as for many rebuild applications.

2.1. New equipment usage

Table 2. Some typical OEM chrome plated components. System Component Notes Landing gear Inner cylinder OD Dynamic seal

Outer cylinder ID Thin dense Cr or flash Cr often used for IDs

Uplock and downlock hydraulics

Axles

Pins High-load rotation

Lug faces

Hydraulic actuators Rods Dynamic seal

Outer cylinder ID

Pins Mostly OD, some ID

Turbine engines Power shafts Wear and press-fits

Bearing holders Press-fits

Seals

Actuators

Gears Not gear teeth

Propeller/rotor Propeller hubs

Rotor head components

Gears Not active profile of gear teeth

Shafts

Dampers Hydraulic rods and outer cylinder IDs


Page 7

Hard chrome is used wherever wear is known or expected to be a problem. Table 2 shows some of the many aircraft components that are typically chrome plated. The primary OEM usages of chrome plating are landing gear components and hydraulic actuators. These account for the largest chrome plated areas. Most OEM applications are for areas that are subject to wear. However, some applications, especially on shafts and bearing holders, use chrome to provide a press-fit interface that will minimize galling on assembly. Other applications are for locations where high precision is required and a ground chrome surface is used to provide a better surface finish or more accurate dimensions. OEMs also use chrome plating for restoring dimensions on mismachined parts. OEM usage is typically quite thin � 0.003� is common. Where corrosion resistance is needed and wear is not a serious issue, thin dense chrome (typically 0.0003� thick) is commonly used. Thin dense chrome is frequently used, for example, on hydraulic outer cylinder IDs.

2.2. Overhaul and repair usage During overhaul and repair chrome is frequently replaced on originally-chromed areas that have been worn, pitted, or are otherwise out-of-specification. Hard chrome is also used for general rebuild of many components that may be worn or damaged, but were never originally chromed. Most, if not all, DoD maintenance depots and aircraft O&R shops are equipped with hard chrome plating tanks. Rebuild usage is typically thicker than OEM usage � 0.010� � 0.020� being quite common. Both externals and internals may be plated, although ID plating is a more specialized process that is often contracted out.

2.3. Landing gear components Landing gear are primarily made of 300M high strength steel, Aermet 100 steel, and in some cases aluminum or titanium alloys.


Page 8

Landing gear inner cylinders are the largest aircraft components commonly chrome plated. The Boeing 737 landing gear inner cylinder of Figure 1 is relatively small � about 24� high and wide. The Boeing 767 main landing gear, on the other hand, is far larger, with

four sets of wheels on two removable axles, each almost 60� long (Figure 2). As with other landing gear, the axle journals and the piston are chrome plated.

Figure 1. Boeing 737 nose landing gear inner cylinder. Shiny areas are chromed - piston and four axle journals (Courtesy Sulzer Metco).

Figure 2. Boeing 767 main landing gear (Courtesy Sulzer Metco).


Page 9

Items such as pins are much smaller � generally 1-2� in diameter, and a few inches long. These pins link hydraulic actuators to landing gear and airframe attachment points. Most are coated on the outside, while some are ID coated to reduce corrosion and wear from end caps.

2.4. Hydraulic actuators Since hydraulic actuators are used throughout the aircraft, they constitute the second most important application for aerospace chrome plating. Almost all actuator rods are chrome plated, while on the actuator shown in Figure 4 the bore of the outer cylinder is also chromed.

Chrome plating prevents wear of the metal by the seals and also serves to hold hydraulic fluid to lubricate the seals and reduce seal wear.

Figure 3. F-18 main landing gear Oleo Attach Pin (Courtesy Boeing).

2.5”

5.7”

Figure 4. F/A-18 E/F aileron servocylinder, manufactured by HR Textron (Courtesy Boeing).


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3.3.3.3. Chrome replacement options and requirements

The requirements for OEM and O&R chrome replacement are somewhat different. One of the primary differences, although difficult to quantify, is that at the OEM the processing environment is quite well defined and there is a limited number of different components to be plated, whereas in O&R operations many different components must be processed, each with its own unique problems resulting from its field history. Some of the major differences are summarized in Table 3.

One issue to consider very carefully is whether a process for OEM can also be used for O&R. Although in principle there is no reason that O&R processes cannot be different from OEM processes, it makes validation and acceptance doubly expensive and time-consuming if two different processes must be validated, rather than a single one accepted by OEMs, DoD stakeholders, and depots alike. This is an especially important issue in view of the wide range of coating requirements for chrome replacement, from 0.0003� to 0.015� thick. Some processes are good for OEM use but cannot be used for rebuild, while others cannot reliably deposit a coating thinner than about 0.001�. Since most hard chrome applications are 0.003� or more in thickness, the requirements for thin dense chrome and flash chrome are not critical for the vast majority of cases. Inability to replace these specialized thin chrome coatings is not a critical issue in a general hard chrome replacement since they are not strictly hard chrome applications.

Table 3. Some differences between OEM and O&R chrome replacement.

OEM O&R Limited numbers of different components, processed in significant quantities on a regular basis. Standard production lines

Many different components, sporadic work loads

New substrate material; only new coatings need be stripped in case of processing errors

Old, dirty substrates; old coating must be stripped prior to recoat. Coating must withstand component cleaning and servicing

Coating thickness typically 0.0003� (thin dense chrome) to 0.004�

Coating thickness up to 0.020� as-coated, 0.010� finished, for rebuild

Approvals from OEM engineers Approvals from OEMs, NAVAIR, ATCOM, Single Item Managers, Program Managers


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3.1. Hard chrome replacement criteria The following is drawn from the Brooks AFB Statement of Need describing the basic requirements for a chrome replacement4: “A coating process (or processes) is needed that will meet the requirements of chromium without the environmental and health hazards associated with chromic acid. Ideally, the process would not use any EPA 17 chemicals. The process must not cause hydrogen embrittlement. Fatigue loss should be no worse than electro-deposited chromium. The deposit should exhibit adhesion to steel equivalent to electro-deposited chromium. The deposit must be machinable or grindable to produce surface finishes of approximately 8 rms. The deposit must be easily strippable. Good corrosion protection would be a plus. The finished surface must have low friction characteristics and must not gall. The process should be relatively easy to control. It should not require large amounts of capital to install and should fit into existing space.”

Replacing hard chrome involves a great deal more than meeting the technical requirements for wear, corrosion, fatigue, etc. To be viable in the aerospace community the replacement must fit into the way the industry works at both the OEM and O&R level (commercial shops and military depots). Technical performance cannot be considered in isolation from other technical issues such as stripping and finishing, from environmental and safety issues, from issues of complexity and cost, or from the more �political� issues of acceptance and validation. The most important criteria that an alternative must meet to replace chrome successfully on IDs are summarized in Table 4. There are many more detailed issues, but these will in general be different for each different application. The Hard Chrome Alternatives (HCAT) team started out with the approach that, to be viable, a hard chrome alternative must meet the same performance standards as hard chrome in all critical areas (as we have indicated in Table 4). As a practical matter, however, the team has found that the alternative must exceed the performance of hard chrome. If it does not, there is no strong driver to specify the replacement, since all changes of this type involve both cost and risk. In general, environmental drivers are very weak, especially in an industry as complex as aerospace, where responsibility for change is diffused and decision makers are often not directly affected by their decisions. However an alternative will be strongly and rapidly embraced by engineers and other stakeholders if the replacement provides a clear technical and cost benefit, and especially if it provides some critical capability that chrome lacks.


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Table 4. Hard chrome replacement criteria.

Issue Criteria Notes ID coating requirements

Coating thickness OEM: 0.003�

OEM thin dense Cr: 0.0003�

O&R: 0.003-0.015�

Note: Widely different OEM and O&R needs

Smoothness 16µ� Ra typical, some replacements may need to be 4µ� Ra

Note: Thermal spray coatings generally need to be smoother than Cr for the same application.

Deposition temperature High strength steels: <250C

Aluminum alloys: <150C

Critical issue is time-at-temperature. Overheating may cause fatigue reduction due to changed surface microstructure.

Technical issues Wear resistance and

hardness Match performance of chrome on actual components. (Cr is 800-1000HV)

Critical issue is wear life (wear rate x thickness) in service, and avoidance of seal wear in hydraulics

Hydrogen embrittlement

None (Note: Cr requires bakeout for embrittlement relief)

This is a critical flight safety issue

Corrosion resistance Must match chrome - primarily B117 salt fog

Microcracks make chrome a poor corrosion inhibitor � good corrosion resistance requires sealer or Ni underlay.

Fatigue Fatigue debit must not exceed chrome

Navy particularly concerned with NaCl and SO2 atmospheres for corrosion and fatigue - critical flight safety issue.

Producibility Reproducibility Process must be stable Both OEM and O&R environments

Process window Within day-to-day operating parameters

Simple, reasonable QC needed

Cost Total production cost comparable to chrome.

Life-cycle cost < chrome

Reasonable capital cost

Production cost needs to include cleaning, masking, finishing, heat treating, waste disposal, etc.

OEM and O&R fit Stripping Must be able to be stripped - safe

chemicals, water jet, etc Strippability is crucial to O&R.

Field and O&R chemical stability

Must withstand O&R cleaning, chemicals, hydraulic fluid, etc.

Must not deteriorate when put through O&R process, including plating

Environment/safety Must be environmentally benign and safe for workers

Note that O&R operations are more diverse and less easily controlled

Acceptance issues Specifications AMS and/or aircraft company

specifications needed Cannot be specified and put on drawings without specs.

Proprietary technology Cannot be proprietary to one company

If possible, should be able to be done at general O&R site to avoid outsourcing.


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3.2. Thermal spray for hard chrome replacement Thermal spray in general, and HVOF in particular, is being accepted in the aerospace industry as the most widely-used chrome alternative for a number of reasons:

• Thermal spray coatings are already widely used in the industry for turbine engines � as a result aerospace engineers are familiar with the technology, and most O&R shops and DoD repair depots are equipped with some kind of thermal spray.

• Thermal spray is a relatively simple technique that avoids the complications of vacuum coatings.

• Thermal spray processes are environmentally benign. They do, however generate particulates in the overspray (material that does not melt properly and bounces off the substrate), which are caught in a standard bag-house dust collector.

• The HVOF method is not exceptionally capital-intensive, especially for a shop already set up for other types of thermal spray.

• HVOF is a high-quality coating that can be made with very low porosity and can be machined, ground, and superfinished as necessary without chipping or delaminating.

• The main process limitation is that HVOF cannot be used on deep IDs (such as hydraulic or landing gear outer cylinders), except the very largest in normal use (about 11� ID). Plasma spray can be used down to 3� ID, and perhaps smaller, but the material quality is not as good. These methods are under development.5

HVOF thermal spray is now being used (both on OEM equipment and for O&R), on landing gear, flap and slat tracks, hydraulic actuators, and an increasing number of components. Users include airframers, landing gear manufacturers, airlines, and repair shop

Rowan Technology Group Project #: 3105JSF3 ; Report #: Final Part 2 � Overview of Thermal Spray

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PART 2. OVERVIEW OF THERMAL SPRAY Thermal spray is a coating process that is widely used in the aerospace industry. Its advantages are that it is clean, very flexible, can be done in air, and can be used both for OEM and for O&R, since it can produce coatings from 0.003� typically for original equipment, and 0.020� or more for rebuilding worn parts.

4.4.4.4. Principles of thermal spray

4.1. Summary This section is intended to be introductory, providing a quick overview of thermal spray, with details provided in subsequent sections. We describe

• the basic principles of thermal spray,

• the types of thermal spray equipment,

• what controls the properties and performance of thermal spray coatings, and

• the limitations of the process. The thermal spray process applies coatings to the substrate via a combination of thermal and kinetic energy, which makes the particles soften and �splat� onto the surface. Many thermal spray processes exist to produce a dense structure that will meet the needs of a wide variety of applications including wear, corrosion, erosion, electrical conductivity, etc. Since thermal spray coatings can vary in thickness from 0.003� to >0.020�, the method can be used both for new OEM components and for rebuilding worn or damaged items. Emphasis is placed upon the carbide and metallic coatings since these materials are the primary thermal spray alternatives for many of the current chrome applications.

4.2. Documents Document 4. JSF Phase 1 Report: Chrome Replacements for Internals and Small Parts. (Rowan Technology Group).6 This document is a technology analysis carried out for the Joint Striker Fighter Program office evaluating the various options for chrome plating in internal replacement diameters and heat-sensitive items.

"JSF Final ID Report Phase 1.pdf"


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Document 5. JSF Phase 2 Report: Optimal Chrome Replacement Technologies for Internal Diameters and Heat-Sensitive Parts. (Rowan Technology Group).7 This report evaluates the critical issues and costs for bringing the best technologies found in Phase 1 to production in aircraft.

4.3. General

Thermal spray is a widely-used coating process in which materials ranging from metals and ceramics to plastics are melted into individual droplets and fired onto the substrate being sprayed to form a final coating structure. The thermal spray coating technique is a �line of sight� procedure, meaning that the coating material is deposited only on the surfaces directly open to the spray stream. The coating �overlays� the substrate and is �mechanically � bonded to the part. To ensure a good bond the surface is usually roughened by grit blasting before spraying. There are three major components to the thermal spray process:

1. Coating material in either powder or wire form. 2. Kinetic energy � used to accelerate the particles to high speed,

usually by injecting them into a high-velocity gas stream. 3. Thermal energy � usually an electric arc plasma or a high

temperature flame, to soften or melt the coating material.

"JSF Final ID Report Phase 2.pdf"

Figure 5. Thermal spray process schematic (left); close-up view of surface (right).8


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As illustrated in Figure 5, the molten particles individually �splat� against the substrate via this combination of thermal/kinetic energy, resulting in a solidified surface deposit.

4.4. Thermal spray processes As with other manufacturing processes, there are many techniques to achieve the final coating structure. Many people commonly interchange the use of plasma spray and thermal spray; however, plasma spray is only one type of thermal spray. Broadly, thermal spray technologies are categorized by the heat source:

• Combustion (flame or detonation)

• Plasma (intense electric discharge)

• Electric Wire Arc (sparking between wires) Figure 6 illustrates the typical commercial processes, broken down under these broad categories.

Figure 6. Types of thermal spray processes. Types covered in this report shown in green.8


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Table 5 summarizes the typical characteristics of the main types of thermal spray coatings.

Table 5. Typical characteristics of thermal spray coating processes. Attribute Flame Spray

(Combust-ion)

HVOF* (Combust-

ion)

D-Gun** (Combust-

ion)

Wire Arc Air Plasma

Gas Jet

Jet Temp. (K) 3500 5500 5500 >25000 15000

Jet Velocities (m/s)

50-100 500-1200 >1000 50-100 300-1000

Gas Flow (slm) 100-0200 400-1100 N/A 500-3000 100-200

Gas Types 02, Acetylene CH4 ,C3 H6 ,H2, O2

02, Acetylene

Air, N2, Ar He, N2, Ar, H2

Power Input (kW equiv)

20 150-300 N/A 2-5 40-200

Particle Feed

Particle temp. max, (deg C)

2500 3300 N/A >3800 >3800

Particle velocities,

(m/s)

50-100 200-1000 N/A 50-100 200-800

Material feed rate,

(g/min)

30-50 15-50 N/A 50-100 200-800

Deposit/Coating

Density Range 85-90 95-98 95-98 80-95 90-95

Bond Strength (MPa)

7-18 82 82 10-40 40-68

Oxides high small small Moderate to high

Moderate to coarse

* High Velocity Oxy-Fuel ** Detonation Gun-A proprietary process from Praxair


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4.5. Factors determining coating properties When evaluating a coating, the characteristics can primarily be divided into two major categories:

a) Properties and structure of the material, and b) How the coating affects the performance of the component

Figure 7 illustrates a typical structure of a plasma spray coating. All coatings will possess theses characteristics to varying degrees, which affect final coating performance. The specification for a thermal spray coating usually defines, among other

things, the coating method to be used, and the allowable number of imperfections such as porosity, oxide particles, and unmelts (see Table 6). The quality control of coating structure is monitored by tests such as metallography, macro/microhardness, and tensile testing, to name a few. However, care must be taken with these evaluations since the evaluator is analyzing a composite structure (coating/substrate) in lieu of just a metallic coupon, and response to testing can change dramatically with different combinations of substrate and coating material. Careful control of QC procedures is necessary for consistent and repeatable results.

Figure 7. Structure of Thermal Spray Deposit at 100-500X.8


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4.6. Applications of common thermal spray coatings Performance measures how the coated system (component, with its particular substrate material, plus coating) performs in various environments. Performance in turn determines what applications the system is suited to. With the wide variety of materials that can be sprayed by thermal spray techniques, the applications for these coatings are widespread. Some typical applications and performance characteristics of some coating materials are shown in Table 7.

Table 6. Some fundamental terms that define the quality of thermal spray coatings. Coating Characteristic Cause and Background Splat Molten particles that have hit the surface and

solidified as elongated shapes parallel to the substrate (the ideal form of deposition).

Porosity (Voids) With individual particles �splatting� as irregular shapes in the deposit, porosity or voids may be formed on solidification if the thermal and kinetic energy are not sufficient to minimize this effect.

Oxide With many thermal spray processes conducted in air, oxides will form as the molten particles travel to the substrate.

Unmelt Dependent upon the process used, not all particles will see the same heat input and therefore some will not have enough energy to splat into an elongated shape and will instead retain the shape of the starting stock.

Layer lines Dependent upon the coating material, lines between splats will be evident.


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Materials can range from pure metals/alloys to cermets, carbides and plastics. Because the starting material is usually in powder or wire form, it can usually be formulated to meet the needs of general applications or of special requirements if so desired.

4.7. Limitations of thermal spray As with any process, limitations exist for application and usage. The major limitations of thermal spray are:

• Thermal spray is a line-of-sight process

• Substrate heating by the thermal spray process

• Thickness (maximum and minimum).

Table 7. Some common thermal spray coatings, their structure, performance, and applications.

Coating Structure and Performance

Characteristics

Application

Nickel Graphite CompositePowder Abradable Coating

(NiG)

Structure Metallography

Hardness Tensile

Performance Erosion

Abradability

Sprayed on cowling parts to allow blades to �cut into� coating and form seal, preventing air bypass.

Chrome Carbide Nickel Chrome

Powder Wear coating (Cr3C2 Ni Cr)


Hardness Performance

Erosion Corrosion

Sprayed in areas on pump housings where high velocity flow causes erosion and corrosion.

Tungsten Carbide Cobalt Powder Wear Coating

(WC-Co)


Hardness Performance

Erosion Corrosion

Sprayed on blade tips to resist erosion of material being ingested as plane goes down runway.

Zinc Wire Structure Metallography Performance

Corrosion

Used for protection of iron and steel against corrosion in fresh/salt water.


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4.7.1. Line of sight issues The issues of line-of sight processing are covered at length in two prior reports for the Joint Strike Fighter Program Office.

Document 4. JSF Phase 1 Report: Chrome Replacements for Internals and Small Parts. (Rowan Technology Group)

Document 5. JSF Phase 2 Report: Optimal Chrome Replacement Technologies for Internal Diameters and Heat-Sensitive Parts. (Rowan Technology Group The area to be sprayed must be accessible to the thermal spray gun, which can limit ID applications or use in tight areas. This leaves two options:

1. Use an ID gun, able to reach into the ID and deposit directly onto the wall

• HVOF guns cannot be used inside an ID < 11� diameter, which is the diameter of the largest landing gear outer cylinders

• Plasma spray guns can be used down to 3� ID, although the coating quality is generally inferior to HVOF

• Some miniature plasma guns can be used down to 1.5 � 2� ID, but these are less well characterized.

2. Spray from outside the hole

• The general rule of thumb is that a hole can be sprayed from outside if the ratio depth/diameter < 1, which corresponds to an impact angle of no more than 45° from the vertical. It is possible, in some cases, to obtain good coatings at 60° from the vertical, while in other cases the properties of the coatings diminish at 20° from the vertical.

4.7.2. Heating issues Since the thermal spray process incorporates both thermal and kinetic energy, the part being sprayed will experience some temperature rise, especially at the surface of the part. With the temperatures of the particles shown in Table 5, temperatures can be reached that will degrade the properties of the substrate � especially fatigue properties. The solution to overheating is generally to apply sufficient cooling air (by air jets surrounding the component, and proper matching of the rotation of the component and movement of the gun to prevent the gun spending too long on any area. This issue is discussed at some depth in Section 7.3.6.

4.7.3. Coating thickness There is a practical lower limit to the coating thickness, which is set by the size of the particles and the requirement for a continuous coating. The minimum thickness possible as a practical matter is about 10 splat


Page 22

thicknesses, or 0.001� (25µm). This is because the size of most carbide particles is about 2.5µm (0.0001�), while typical 25 - 60µm spray particles tend to splat into a ratio of about 10:1 when they hit the substrate. For coatings that are to be ground, it is found that a minimum thickness to avoid spalling during grinding is about 0.003� (see Section 13.3.2). In principle there is no upper thickness, provided the coating is not deposited under conditions that that build up a high level of stress with thickness. Boeing has specified an upper thickness limit for landing gear repair by HVOF tungsten carbide as 0.010� (see Section 15.5.4). However, much thicker repairs have been developed, either with a single coating, or a combination of metallic build-up with WC cermet hard overcoating (Section 15.5).


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5.5.5.5. Thermal spray coatings

5.1. Summary This section provides an overview of the general materials, properties, performance, and usage of thermal spray coatings. It is intended to provide an overview of the general applications of thermal spray coatings, and the materials used to produce them. The most common coating materials for chrome replacement are carbides (such as cobalt-cemented tungsten carbide, WC-Co, which is the same material as that commonly used in carbide cutters and inserts) and alloys, such as Tribaloy. A critical requirement for high quality coatings is the proper definition of the starting material (i.e. the powder in plasma, HVOF, and D-gun processes). Examples are given of typical powders.

5.2. Thermal spray materials

5.2.1. General A wide variety of materials is used in the application of thermal spray powders. The general classifications are shown in Table 8. The starting stock is a critical part of the thermal spray process. Some important characteristics of the starting stock are summarized in Table 9.


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These characteristics must be considered when choosing the appropriate application technique to maximize production and ease of producibility. Specific examples of how this information is supplied for varied material

Table 8. Some major thermal spray powder classifications. Description Uses Application Methods

Abradables and Plastics

• Aluminum Base Abradables

• Cobalt Base Abradables

• Copper Base Abradables

• Nickel Base Abradables

• Plastics

Abradables provide clearance control in high-speed applications where near-zero clearances between moving parts are required. Typically used by jet engine manufacturers to improve engine performance by reducing the clearance between the blades of the compressor and the surrounding casing.

Air plasma and flame spray

Pure Metals, Alloys, Cermets, Composites, and Blends

• Pure Metals • Aluminum Base

Powders • Cobalt Base Powders • Copper Base Powders • Iron Base Powders • Molydenum Base

Powders • Nickel Base Powders • Cermet Powders

For surface enhancement, corrosion, oxidation and abrasion resistance, bestowing electrical conductivity/shielding characteristics, superalloy repair and everything else in between.

Air plasma, flame spray, and HVOF

Carbides

• Chromium Carbides

• Tungsten Carbide

Selected primarily for their wear resistance, abrasion resistance and erosion resistance, coatings of these materials are especially suitable for use in many harsh environments.

Plasma spray and HVOF

Ceramics

• Aluminum Oxide Base Powders

• Chromium Oxide Base Powders

• Titanium Oxide Base Powders

• Zirconium Oxide Base Powders

Ceramic coatings exhibit properties such as wear resistance, high dielectric strength, hot corrosion resistance, chemical resistance and even bioactivity

Plasma spray

HVOF

(High Velocity OxyFuel)

These materials are properly sized for High Velocity Oxy-Fuel spray. For applying dense, strongly bonded coatings with reduced oxides, porosity and excellent wear resistance properties

HVOF

Thermal Spray Wires(Arc and Combustion)

Used for a broad range of corrosion control and machine element repair applications, these wires are specially manufactured to tight tolerances for thermal spraying. Used particularly for on site applications.

Electric Arc


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types is shown in the next sections on carbides and pure metals

The following are links to the web sites of some of the major manufacturers of thermal spray materials:

Two material classes have been chosen to illustrate materials, applications, and general properties: Tungsten Carbides and Pure Metals.

Document 6. Common thermal spray powder types.9 Examples of different types of carbide and metal powders are given in this document.

Table 9. Important parameters defining thermal spray powders and electric arc wire.

Characteristic Description Form Wire/Powder

Size Wire-diameter Powder-size distribution of particles from fine to coarse sizes

Shape Spherical, nears spherical, and regularly shaped powders with smooth surfaces (preferred) vs. non-spherical, irregular and �cuspy� particles where surface irregularities exist

Manufacturing Method Water atomized, gas atomized, sintered and crushed, agglomerated, etc.

Surface Finish Characteristics for feeding powder or melting wire

Physical properties Melting temperature, density, composition, thermal properties, flow characteristics, etc.

PRAXAIR-Thermal Spray: Powders TAFA-Thermal Spray Powders Sulzer Metco Wire and Powder Product Portfolio Powder Alloy Corporation-Powders

Stellite Powders

"Common powder types.pdf"

http://www.praxair.com/Praxair.nsf/1b1a158c246dbf3e8525654300683b77/3e397933ca800681852566db005367f5?OpenDocument

http://www.tafa.com/TPowders.htm

http://www.sulzermetco.com/products/material/index.html

http://www.powderalloy.com/

http://www.stellite.com/powprop.html


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5.2.2. Powders frequently used for chrome replacement Carbides are a major material type as listed in Table 8 and can be further subdivided into categories defining the particular carbide phase such as tungsten or chrome carbides. The most common application of these powders for wear resistance, which is why tungsten carbides are commonly used to replace chrome. Examples of the most common powders used in chrome replacement are summarized in

An example of how data is supplied for such a powder is shown in Figure 8. Note that these powders are generally referred to by the type of carbide powder (tungsten carbide, WC), the type of binder metal holding the carbide grains together (cobalt), and the percentage by weight of the constituents. Hence, with 12% by weight of Co, this material is referred to as WC-12Co or 88/12 WC-Co. Powder size is given in mesh size (lines per inch) or microns. Hall Flow is a measure of how well the powder flows, measured by a Hall flowmeter funnel according to ASTM B855-94.

Table 10. Examples of thermal spray powder used in chrome replacement operations. Powder type Manufacturer,

product Powder size Comments

WC-17Co Sulzer Metco Diamalloy 2005

-53 +11µm Specified for BMS 10-67 Type 1

WC-12Co Praxair WC 106 -45 +5µm BMS 10-67

WC-10Co4Cr Sulzer Metco 5847 -53 +11µm Chosen by C-HCAT for chrome replacement (Section 7.4.3)

Tribaloy 400, 800

Stellite Softer than Cr, but better wear in hydraulics (Section 13.2.1), useful on Al alloys (Section 12)

Ni5Al Metco 450NS Used for build-up

Note: This is a very limited listing. See links in Section 5.2.1 for more details.


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Even for the same chemical composition, powder can come in a variety of different types (depending on how it was made) and sizes. This makes it necessary either to define the allowable powder characteristics to be sure of reliable coatings, or to require that a consistent powder be used to ensure reproducibility between test items and production items.

5.3. Typical structural properties of thermal spray coatings With the wide variety of materials available, the applications for thermal spray coatings run across many industries and applications. Two material classes (carbides and pure metals) have been highlighted thus far, and applications will be summarized detailing the critical properties of each material. Table 11 and Table 12 give a general summary of properties and process expectations with varied spray

TYPICAL POWDER PROPERTIES Chemical Composition Cobalt (Co) ��11.0 to 13.0%

Tungsten Carbide (WC) ��.�..Remainder

Sieve Analysis

-325 Mesh + 10 µm ��(-45µm+10µm)

Melting Temperatures Cobalt (Co) ��..5031°F (2777°C)

Tungsten Carbide (WC) ��2723°F (1495°C)

Hall Flow 16 grams per minute

Apparent Density 0.177 lbs/cu in ��.��.(4.9g/cc)

Starting powder at 500X

Figure 8. Example of powder definition for sintered irregularly shaped 88/12 Tungsten Carbide Cobalt powder.


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techniques and coating materials.

Table 11 Comparison of thermal spray coating processes – general properties.

Coating Type

Wire Flame Spray

Powder Flame Spray

HVOF(1) Spray

Electric Wire Arc

Spray Plasma Spray

Gas Temperature 103 ºC (103 ºF)

All Coatings 3 (5.4)

3 (5.4)

2 - 3 (3.6 - 5.4) N/A 12 - 16

(21.6 - 28.8)

Bond Strength MPa (103 psi)

Ferrous metals 14 - 28 (2 - 4)

14 - 21 (2 - 3)

48 - 62 (7 - 9)

28 - 41 (4 - 6)

22 - 34 (3 - 5)

Non-ferrous metals 7 - 34 (1 - 5)

7 - 34 (1 - 5)

48 - 62 (7 - 9)

14 - 28 (2 - 7)

14 - 28 (2 - 7)

Self-fluxing alloys --- 83+ (12+) 62 (9)** --- ---

Ceramics --- 14 - 32 (2 - 5) --- --- 21 - 41

(3 - 6)

Carbides --- 34 - 48 (5 - 7)

83+ (12+) --- 55 - 69

(8 - 10)

Density, % of equivalent wrought material

Ferrous metals 85 - 90 85 - 90 95 - 98+ 85 - 95 90 - 95

Non-ferrous metals 85 - 90 85 - 90 95 - 98+ 85 - 95 90 - 95

Self-fluxing alloys --- 100* 98+** --- ---

Ceramics --- 90 - 95 --- --- 90 - 95+

Carbides --- 85 - 90 95 - 98+ --- 90 - 95+

Hardness

Ferrous metals 84Rb-35Rc 80Rb-35Rc 90Rb-45Rc 85Rb-40Rc 80Rb-40Rc

Non-ferrous metals 95Rh-40Rc 30Rh-20Rc 100Rh-55Rc 40Rh-35Rc 40Rh-50Rc

Self-fluxing alloys --- 30 - 60Rc 50 - 60Rc --- ---

Ceramics --- 40 - 65Rc --- --- 45 - 65Rc

Carbides --- 45 - 55Rc 55 - 72Rc --- 50 - 65Rc


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5.4. Typical applications of thermal spray coatings Thermal spray coatings are used for thousands of applications in aircraft, machinery, infrastructure, and elsewhere. Table 13 summarizes some a few of these applications.

Table 12. Comparison of thermal spray coating processes - permeability, thickness.

Coating Type

Wire Flame Spray

Powder Flame Spray

HVOF(1) Spray

Electric Wire Arc

Spray Plasma Spray

Permeability

Ferrous metals High Medium Negligible Medium Medium

Non-ferrous metals High Medium Negligible Medium Medium

Self-fluxing alloys --- None* Negligible* --- ---

Ceramics --- Medium --- --- Low-Medium

Carbides --- Medium Negligible --- Low-Medium

Coating Thickness Limitation mm (in.)

Ferrous metals 0.5 - 2.0 (0.02 - 0.08)

0.5 - 2.0 (0.02 - 0.08)

0.6 - 2.5 (0.025 - 0.1)

0.5 - 2.5 (0.02 - 0.1)

0.4 - 2.5 (0.015 - 0.1)

Non-ferrous metals 0.5 - 2.0 (0.02 - 0.08)

0.5 - 2.0 (0.02 - 0.08)

0.6 - 2.5 (0.025 - 0.1)

0.5 - 2.5 (0.02 - 0.1)

0.4 - 2.5 (0.015 - 0.1)

Self-fluxing alloys --- 0.4 - 2.5 (0.02 - 0.2)

0.4 - 3.8 (0.015-0.15) --- ---

Ceramics --- 0.4 - 0.8 (0.015 - 0.1) --- --- (0.4 - 5.0

(0.015 - 0.2)

Carbides --- 0.4 - 0.8 (0.015 - 0.1)

0.4 - 5.0+ (0.015-0.2+) --- (0.4 - 5.0

(0.015 - 0.2)

* Fused Coating (1) High Velocity Oxygen Fuel

** Unfused Coating


Page 30

Table 13. Some applications of thermal spray coatings.. Coating type Application Notes Carbides (WC-Co, Cr3C2-NiCr, etc.)

Wear Easy to spray. Most common chrome replacement

Zn, Zn-Al, Al Corrosion, conductivity (aluminum)

Used on landing gear, bridges

Zirconium oxide Thermal barrier Turbine blades

Ni5Al, In 718, stainless steels

Build-up Repair

Aluminides High temperature oxidation

Gas turbines

Chrome oxide Anilox print rolls Laser engraved for very high resolution flexographic printing

Babbit and other soft alloys

Bearing journals Ships, machinery

Hasteloys, Inconels, Stellites, MCrAlYs

High temperature oxidation and wear

Turbine engines

Mo Wear, lubricity Piston rings for trucks, cars

Abradables Clearance coatings Turbine engines


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6.6.6.6. Types of thermal spray processes There are four general types of thermal spray in widespread commercial use, in order of increasing coating quality � flame spray, arc spray, plasma spray, and high velocity oxy-fuel (HVOF) spray. There are other technologies in development or specialized use, including

• Cold spray (or gas dynamic spray) � in which there is no combustion, but the particles are accelerated to high speed by an ultra-high velocity gas stream. When they strike the surface they soften and melt by a combination of conversion of kinetic to thermal energy, and cold forging of each coating layer by the high velocity incoming particles that form the subsequent layers. At this point the method is in the early development stage and is only suitable for depositing low melting point metals such as Cu and Al.

• High velocity air-fuel spray (HVAF) � similar to HVOF, but with air as the oxidizer. These guns are made for use in areas of the world where oxygen is not easily obtained.

• Spray forming (the Osprey process) � in which an alloy is melted and atomized into a gas jet so as to spray the molten particles. It is not generally used for coating, but for spray-casting three-dimensional objects.

This report is concerned only with plasma spray and HVOF, as they are used for aerospace components. We describe these technologies in the following sections.

6.1. Flame spray This is the simplest thermal spray method, which is used for lower-quality alloy coatings. Powder is entrained in a gas jet and fed through a flame. The coating is generally of quite poor quality (porous and low adhesion), but the method is used for some aircraft components. It is not generally suitable as an alternative to hard chrome.

6.2. Arc spray In this method an electric arc serves both as the source of heat and as the source of molten metal droplets. The arc is struck between two feed wires (or a feed wire and an electrode), and the molten droplets are driven to the component surface by a gas jet. The method is used to spray metal and alloy coatings. Commercial arc guns are used to spray interiors of automobile engine cylinders, and they are gaining increased currency for aircraft applications. At this point, they are not generally suitable as an alternative to hard chrome. However, arc spray technology is constantly improving and the applications for arc spray coatings are growing.


Project #: 3105JSF3 ; Report #: Final Part 2 – Overview of Thermal Spray Page 32

6.3. Plasma spray Plasma Spray is perhaps the most flexible of all of the thermal spray processes as it can develop sufficient energy to melt any material. Plasma spray is usually the best choice for facilities where many different surfaces must be applied, and it is the only technique able to spray most high-quality ceramic coatings. The components of a typical system are shown in Figure 9.

Since it uses powder as the coating feedstock, the number of coating materials that can be used in the plasma spray process is almost unlimited. The plasma gun incorporates a cathode (electrode) and an anode (nozzle) separated by a small gap forming a chamber between the two. DC power is applied to the cathode and arcs across to the anode. At the same time, gases are passed through the chamber. The arc temperature is sufficient to strip the gases of their electrons, and the state of matter known as plasma is formed. As the unstable plasma recombines back to the gaseous state thermal energy is released. Because of the inherent instability of plasma, the ions in the plasma plume rapidly recombine to the gaseous state and cool. At the point of recombination, temperatures can be 6,600 ºC to 16,600 ºC (12,000 ºF to 30,000 ºF),which exceeds surface temperatures of the sun. By injecting the coating material into the gas plume, it is melted and propelled towards the target component.

Typical plasma gases are Hydrogen, Nitrogen, Argon and Helium. Various mixtures of these gases (usually 2 of the 4) are used in combination with the applied current to the electrode to control the amount of energy produced by a plasma system. Since the flow of each

Figure 9. Components of a typical plasma spray system.


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of the gases and the applied current can be accurately regulated, repeatable and predictable coating results can be obtained. In addition, the point and angle that the material is injected into the plume as well as the distance of the gun to the target component can also be controlled. This provides a high degree of flexibility to develop appropriate spray parameters for materials with melting temperatures across a very large range. The distance of the plasma gun from the target components, gun and component speeds relative to each other and part cooling (usually with the help of air jets focused on the target component) keep the part at a comfortable temperature that is usually in the range of 38 ºC to 260 ºC (100 ºF to 500 ºF). Several commercial plasma spray guns can coat IDs down to 3�, while a few can coat to IDs as small as 1.5�.

6.4. High velocity oxy-fuel (HVOF) spray and detonation gun We include under the definition of HVOF, the Detonation gun (D-gun) and Super D-gun, which are proprietary to Praxair. HVOF uses a fuel and oxygen, which burn in a combustion chamber and exit the gun nozzle at supersonic speed. The powder is injected into the combustion chamber and is accelerated to high (but not supersonic) speed by the gas. In the detonation guns the fuel and oxygen are burned in distinct detonations (similar to a machine gun with blank cartridges), while in other HVOF guns the combustion is continuous. The high speed of the particles makes HVOF coatings the highest quality thermal sprays. The most common fuels are hydrogen, propylene, acetylene, and kerosene. Natural gas is beginning to be used because of its lower cost, but hydrogen, propylene and kerosene remain the primary fuels used for aircraft applications.

Sulzer Metco High Velocity Oxy-Fuel Thermal Spray Process This link contains data on the Diamond Jet HVOF System

Praxair HV-2000.pdf This link contains data on the HV 2000 HVOF gun from Praxair

http://www.sulzermetco.com/tech/hvof.html

http://www.praxair.com/Praxair.nsf/d63afe71c771b0d785256519006c5ea1/a5db0022812fa3ce85256672005c52d4/$FILE/Hv-2000.pdf


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7.7.7.7. Thermal spray producibility

7.1. Summary This section is concerned with issues involved in production thermal spray processing.

7.2. Documents Document 7. Test standardization: a Key Tool in Coating System Implementation (Courtesy Sauer Engineering, Gorham Advanced Materials).10

Document 8. Training in Coating Evaluation Techniques: a Unique Approach for Discussion (Courtesy Sauer Engineering).

Document 9. The Use of Metallographic Standards in Calibration of the Polishing Process (Courtesy Sauer Engineering).

Table 14. Producibility summary and links. Item Item Quality control Stripping

Metallography WC-Co

Hardness WC-CoCr

Adhesion Tribaloy

Temperature monitoring Finishing Residual stress monitoring Superfinishing

Process optimization Hydraulic rig test experience

Inspection Flight test experience

"Gorham Conf.Pres.pdf"

"Training in coating evaluation.pdf"

"Metallographic standards in polishing


Page 35

Document 10. Metallographic Preparation of Thermal Spray Coatings: Coating Sensitivity and the Effect of Polishing Intangibles (Courtesy Sauer Engineering).

Document 11. Tensile Bond Variance of Thermally Sprayed Coatings with Respect to Adhesive Type. 11 This article by K. A. Evans summarizes a comparison of epoxy types, and the strength level/degree of variation experienced between liquid and film epoxies. Actual tensile results may be found in Section 11.4.3.

Document 12. Almen Strips and Temperature Measurement During HVOF Processing (Courtesy, Sauer Engineering). This document, by John Sauer, is a discussion of practical problems of temperature measurement and almen strip set-up for an actual case history problem.

Document 13. Design of Experiment for HVOF WC-Co process (Courtesy HCAT, www.hcat.org). This document summarizes the design of experiment used to transfer an HVOF process developed at one location to another spray shop. This was an 11-run DOE (including 3 duplicate runs at the center points).

Document 14. Summary of DOE results for optimization of HVOF WC-CoCr (Courtesy NRC Montreal and C-HCAT).12 This document describes an extensive DOE that includes the effects of different gases and powders.

7.3. Quality Control Of the Thermal Spray Process

7.3.1. Choice of powder This is a very important issue, which has been covered in Section 5.

7.3.2. General An important first step in thermal spray producibility is understanding the tools used to control the process. Many of the tests used to evaluate the thermal spray process are conventional methods specified for the analysis of metallic products such as metallography, tensile, and hardness testing. However, in the case of coatings, the evaluation is now being performed on a composite structure consisting of the coating on top of the substrate. The response to testing variation can therefore become very significant given this �dual� structure and the category of coating

"Comparison of tensile adhesives.pdf"

"Metallographic standards in polishing

Almen-Temp.pdf

HitemcoDOE.PDF

Jean-Gabriel.pdf



Page 36

material being investigated. A thorough understanding of the testing process is therefore critical to properly provide results that lead to optimization and control of the thermal spray process.

Many methods are used to evaluate coating properties and performance. This section will present the varied tests used primarily in daily quality control of the thermal spray process. Methods such as fatigue, corrosion testing, wear testing, etc. are sometimes referred to as �capability� or �performance� tests that validate the process but are not performed on a daily basis. Sections 11and 12 present more detailed

information on these methods and summarize actual results for the varied analysis techniques. The commonly used methods are shown in Table 15. The remainder of this section will provide greater detail on the application of these test methods. Currently, no general specification test methods with the exception of ASTM C-633 on Tensile Testing exist for usage. All other test methods are covered under general metal testing. Efforts are now in progress to standardize the methodology used in evaluation of coatings. Document 7 summarizes some of the groups and actions involved in process standardization .

Table 15 Commonly Used Quality Control Tests Method General Description

Metallography Test coupons are sprayed and then sectioned, mounted, and polished for evaluation.

Hardness Coupons are coated and then macrohardness performed. Some microhardness evaluation on mounted cross sections from above.

Tensile Buttons or tensile adaptors are coated, bonded together with adhesive, and then pulled in tensile machine.

Residual stress This method is primarily used for HVOF coatings where Almen Strips (same as shot peening) are coated on one side and the stress from coating application measured as a deflection.

Temperature monitoring This method is primarily used for HVOF coatings where monitoring temperatures effects is critical to minimize affect on fatigue properties.

Surface roughness For many applications such as wear or thermal transfer, surface roughness is monitored via use of profilometers.



7.3.3. Metallography Metallography is a destructive test which provides information about the structure of the coating. It is commonly performed on coupons that are sprayed at the same time as the part in question. Common metallographic techniques can be applied but consideration must be given to the composite structure of coating and substrate. Some commons issues to consider and carefully monitor in coating metallographic preparation are:

• Hardness of substrate vs. coating

• Format being used to grind-polish: disc vs. papers

• Cutting method

• Hardness of polishing abrasive vs. coating

• Type of polishing cloth: nap vs. no nap

• Mounting material hardness vs. coating

• Impregnation of coating with mounting material if porous

• Concentration and type of polishing abrasive

After the microstructure is prepared and ready for evaluation, the test lab technician will commonly review the structure for the characteristics in Table 16 (Section 4.5 for explanation of how characteristics evolve as part of the thermal spray process).


Page 38

Figure 10 represents typical photomicrographs of thermal spray coatings, cross-sectioned, metallographically mounted, and polished. The photograph on the left shows a NiAl coating with varied features such as oxides, porosity, and unmelts. The photograph on the right shows a WC-Co coating with a distribution of dark carbide phase in a matrix of �white� or cobalt material. Note the manner in which the material forms layers

Table 16 Common Characteristics Evaluated in Metallographic Specimens.

Characteristic Description Interface contamination Grit that is embedded at the interface form

the grit blasting operation-usually compare to a photostandard to assess degree.

Porosity Assess distribution of voids/holes in coating against photostandard from incomplete splat bonding

Oxides May be in form of stringers or clusters from powder traveling through the air-again compare to photos

Unmelted particles All particles do not obtain sufficient thermal energy in flame to deform when accelerated toward the substrate-usually count particles and a shape is defined for characterization as unmelt

Phase content Distribution and content of phases is critical for some coatings such as carbides in a wear coating-compare to photos

Delaminations and cracks Can be located at interface and within coatings-a definition on length and frequency critical to evaluator

Figure 10. Typical HVOF coating cross sections; Ni-Al left (200x), WC-Co right (500x). (Courtesy Praxair-TAFA)


Page 39

parallel to the surface (top of pictures). This layering is typical of thermal spray coatings. As mentioned earlier, a mounted cross section such as this is commonly characterized against photostandards for acceptance rejection criteria. There is work currently in progress to use a rapid growing technique called image analysis to quantify the phases in coatings. Due to the heterogeneous nature of some coating types, it has been difficult to apply this method but the tool has been used successfully on material that are more homogeneous in nature. A more detailed description of coating microstructure analysis is found in Section 10.4. Further information on metallography can be found at the websites shown below or in the referenced PDF files Document 8 to Document 10, and in the web sites below. Buehler web site: http://www.buehler.com/ Struers website: http://www.struers.com/

7.3.4. Hardness Hardness testing on coatings is very similar in methodology to that performed on metallic materials. Macro and microhardness are covered by specifications ASTM E-19 and ASTM E-384 respectively. Coupons used for metallography are sometimes tested for hardness first and then cut and polished for microstructure analysis. Macro or Rockwell hardness for coatings is most often performed using the R15N superficial scale which applies a small load of 15 kg. With the usual thickness range of .002-.008�, the use of conventional Rc or Ra scales with loads of 150 and 60 kg respectively would result in penetration of the indentor thru the coating thickness. The hardness reading would then be a composite of the substrate/coating which is not representative of true hardness. The surface is usually lightly sanded to remove irregularities and a series of readings distributed randomly across the face of the coupon are taken to determine hardness. The readings are averaged to obtain a composite value since values may vary dependent upon the phases present under the indenter due to the heterogeneous nature of some coatings. Microhardness can be performed using either a Knoop (elongated impression) or Vickers (diamond impression). For most coating applications, the Vickers method is used. Since phase distribution is critical for many coatings, microhardness can identify any segregation or absence of important constituents in the microstructure. The pattern for impressions may be random as described for macrohardness but some specifications require a stair step pattern thru the thickness. This can check for variations in the spray process as passes are applied to the coupon. If something changes in spraying, a hardness change should be identified in the progression of readings. For more information on hardness, see Section 11.3 or click the link below for the ASTM website and pertinent specifications.

http://www.buehler.com/

http://www.struers.com/


Page 40

ASTM website: http://www.astm.org/

7.3.5. Tensile/Adhesion The tensile strength of coatings is monitored by use of either buttons or tensile adaptors/loading fixture sprayed with the material in question. The typical tensile assembly used for determining strength is shown in Figure 11 from the ASTM C-633 specification on tensile testing.

Figure 11 shows a button as part of the assembly; if one fixture is sprayed the adaptors are bonded together with a single adhesive application. Currently, both liquid and film adhesive are being used for this purpose. The assemblies are cured at temperatures between 300-450 degrees F for 1-3 hours and then cooled before pulling. A sample with no coating is usually placed in a furnace run to verify proper epoxy curing. Epoxy only values should exceed 10,000 psi and normally pull in the 12,000 psi range. When the test is required, the material is sprayed with the part in question and a set of three samples is usually processed. Test values can range from 100 psi for very soft abradable coatings to epoxy-only failures at over 12,000 psi for HVOF materials (see Document 11 ). Thisis due to the limited strength value of the epoxy and is not a true test of the coating strength. Research is in progress to develop stronger epoxiesor alternative testing methods for coating strength. See, for example Document 11.

7.3.6. Temperature monitoring As described in Section 4.7.2, heat can be transferred to the substrate as the material is coated. Although the part does not see the 1,000°+ F of the thermal plume, sufficient heat can be transferred to degrade the

Figure 11 Tensile Assembly from ASTM C-633

http://www.astm.org/


Page 41

surface material properties. This can be controlled by either choice of coating process or process modification to allow cooling time during the spray cycles. Temperature monitoring is therefore very critical if surface properties such as fatigue are important. An obvious mechanism to monitor temperature would be a contact pyrometer after the part has been sprayed. However, this is not practical for the thermal spray process due to booth and safety constraints. Most spraying is performed in a booth with robotics for part manipulation and air circulation to insure a proper environment for worker health and safety. With cooling air also placed on the part for temperature reduction, temperature monitoring must be instantaneous to monitor the true effects of the spray process. Infared (often called non-contact) temperature measurement equipment is currently being used to achieve this performance. Infrared instruments can be set-up inside the booth and data transmitted remotely to a PC for later analysis. A peak hold capability allows the operator to view maximum temperature and modify the spray process if data indicates values above the upper material limit. A typical arrangement for non-contact temperature measurement is shown in Figure 12. Note that the substrate (a landing gear cylinder or hydraulic rod, for example) rotates while the gun traverses up and down on a robot arm. The pyrometer is set near the center of the part (or at the most critical area, or the area most likely to be overheated), aimed such that the deposition spot never passes directly into its field of view (so that it is not affected by the high radiation from the flame itself). Cooling air jets are usually arranged around the part in strategic locations to give thorough cooling.

Figure 12. Typical non-contact temperature arrangement for HVOF.


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A typical plot of temperature vs time is shown in Figure 13. This plot was taken as the HVOF gun was moved up and down a rotating sample. Each peak corresponds to the gun crossing the level of the pyrometer measurement spot. Note how rapidly the temperature rises and falls. Clearly, by the time the deposition was stopped, the booth opened, and the temperature measured with a simple contact probe, the surface temperature would have fallen about 100°F, making the measurement almost meaningless. Two potential problem issues with infared pyrometry are emissivity and spot size. Emissivity is the measurement of how the radiation emitted form the target in question compares to that of a blackbody. It is therefore a ratio and will be different between say a coated and coated piece of steel. The instrument must therefore be set to the proper emissivity for the specific material. There are some published values but a value for a specific material can be obtained by comparing the infared values from the instrument to contact pyrometer on the same samples temperature values and then dialing in the proper emissivity value on the calibration for the unit. Spot size is related to how focused the infared beam in relation to the part being measured. If a small part is being measured and the beam is bigger thab the piece, compensations must be made for the background also being measured by the unit.

Figure 13 Typical Temperature Plot From a Spray Cycle (J. Schell, GEAE, Courtesy HCAT).

050

100150200250300350

11:56

11:57

11:58

11:59

12:00

12:01

12:02

12:03

12:04

12:05

12:06

12:07

12:08

12:09

12:10

12:11

Time

Tem

pera

ture

[F]



More detailed information on temperature measurement is given in Document 12. Web sites for a number of instrument manufactures are also listed.

Some infrared measurement system manufacturers:

Raytek: http://www.raytek.com/

Ircon: http://www.ircon.com/

Omega: http://www.omega.com/

7.3.7. Monitoring residual stress Residual stress can be an important factor in the performance of coatings and the substrate being used. If a compressive residual stress can be introduced at the substrate surface, this will greatly enhance the fatigue properties of the material. There are many techniques to measure residual stress that are mentioned in Section 11.5, such as hole drilling, X-ray diffraction, and possibly neutron diffraction. These methods are not quality control tools and cannot be applied on a daily basis to qualify spray runs.

At the present time, the technique of using Almen strip deflection is the method being used as a quality control tool. The Almen strip concept has been used in shot peening for many years to measure the residual stress

(1) SAE 1070 Cold Rolled Spring Steel

(2) Edge Number One (on 3 inch edge)

(3) Blue Temper (or Bright) Finish

(4) Uniformly hardened and tempered to Rockwell hardness C44 to C50

(5) Flatness tolerance is ± 0.0015 inch arc height as measured on an Almen gauge

(6) Dimensions in inches

Figure 14. Almen “N” Test Strip.

http://www.raytek.com/

http://www.ircon.com/

http://www.omega.com/toc_asp/section.asp?book=temperature&section=j


Page 44

effect of varied peening media by blasting only one side and inducing a deflection in the coupon as shown in Figure 14. For HVOF quality control Almen �N� strips are commonly used. The Almen strip is attached to the fixture shown in Figure 15 and then sprayed with the production lot. After removal from the holding fixture, the deflection is measured in a fixture similar to Figure 16. Almen strip measurements and temperature monitoring are critical items for process control especially in HVOF coatings. The link below highlights current processing and techniques for use of Almen strips in HVOF spraying. Work is currently in progress to refine this methodology as more spraying on components is performed (see Document 12. Almen Strips and Temperature Measurement During HVOF Processing (Courtesy, Sauer Engineering).)

7.4. Process optimization and control

7.4.1. General Many coating providers simply deposit thermal spray coatings with general parameters supplied by the equipment or powder manufacturer, or use a process optimized for some other use. Past experience shows that such coatings are seldom optimized for fatigue-critical applications. The deposition conditions for a particular thermal spray powder with a particular gun should be optimized with respect to the most critical end-use parameters (fatigue, corrosion, wear, etc.). Optimization of the process involves controlling the many parameters which are part of the thermal spray process. Table 17 details the large number of parameters/variables that are possible in the thermal spray process. However, the process can really be broken down into two major components:

Figure 15. Almen holding fixtures (Electronics Inc.).

Figure 16. Almen measuring instrument (Electronics Inc).


Page 45

1. How the material is fed into the system 2. Jet temperature/particle velocity distributions

Control of these items affects deposit quality because our final structure depends upon how much thermal energy is absorbed by the particles while in the flame and the kinetic energy imparted as the material impacts on the substrate.

Table 17. Thermal spray process parameters. Jet Formation

Process type (combustion, arc, plasma), gas composition, gas flow rate, power/current, power/voltage, power/gas heat content, nozzle diameter, nozzle length, nozzle cooling, chamber/nozzle ambient pressures, nozzle internal pressure

Materials Composition, melting range, thermal properties, size, shape, morphology/manufacturing method, form (wire, rod powder), coefficient of thermal expansion

Substrate Size, geometry, surface preparation, surface texture, temperature, thermal properties, coefficient of thermal expansion

Material feed Feed rate, location of feed relative to jet, angle of particle injection, diameter of feed port, carńer gas flow, mass-to-particle ratio, particle shape, particle specific gravity

Deposition Angle of deposit/particle impact angle, particle velocities, distance to substrate, pressure, environment composition, speed of traverse, pattern of traverse, relative motion of pattern


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Figure 17 illustrates the relative contributions from both the thermal/kinetic energy. With the use of statistical process control, a process can be optimized cost-effectively using Design of Experiment or DOE methods. This methodology combines knowledge and experience with the process in conjunction with statistics to determine what parameters are significant. When the important parameters for the technique in question are identified, experiments are run that then zeroes in on the best settings to optimize the spray results.

7.4.2. Example 1 – Optimization of WC-Co This is a basic DOE optimization where the process is fairly well known and the powder defined. It was used by Jerry Schell of GE Aircraft Engines to transfer a known HCAT WC-Co HVOF process to a new vendor. The illustration below, Table 18, shows the eight-run experiment in optimizing the HVOF process with 4 variables identified as significant issues. Prior work was already performed to arrive at these parameters as most important to the process. The factors identified as most important were:

1. Surface feet per minute How fast was the thermal spray gun traversing across the substrate surface

2. Combustion gas How much combustion gas was flowing in the process

3. Stoichiometry Ratio The ratio of combustion gas to oxygen during

Figure 17. Kinetic versus thermal energy for the main thermal spray technologies.


Page 47

the combustion process. This ratio will is based upon a complete chemical reaction and combustion between the gases which in production never occurs. This variable is critical to jet temperature and particle melting.

4. Spray Distance How far away is the nozzle from the substrate. Other factors of the process are fixed, either from prior DOE work or previous experience in the process. When this work is performed, the response to these variables must be monitored to determine how a change affects coating quality. The responses monitored in this case are shown in Table 19. Note that, since Almen number relates directly to coating residual stress, which in turn is directly related to fatigue, Almen number was especially important in this DOE. A full DOE review of this particular analysis can be reviewed in Document 13.

Table 18. Design of Experiment analysis tool.

Design 1: Use L8 design plus Center Points, 11 runs total FIXED:

Levels 54 grit alumina grit blast at 40 psi, 6 inches

FACTORS: -1 +1 C Pt Substrate is 4340 steel, 260-280 ksi

A Surf Speed,Feed Rate 1335, 5.1 1835, 3.5 1585 ipm, 4.3 Powder size/type is WC-17Co, Diamalloy 2005, Lot 54480

B Combustion Gas 1525 scfh 1825 scfh 1675 scfh Powder Feed Rate** 8.5 lbs/hr

C Stoic Ratio 0.405 0.485 0.445 Spray angle is 90 degrees

D Spray Distance 10 inch 13 inch 11.5 inch 100 psi cooling air, 4 AJs @ 6 inch spaced over coupon area

Carrier gas N2 at 148 psi, 55 flow, air vib @ 20 psi

Turntable Robot Spd Robot % @ Spray pattern length Approximately 13 inch

A Factor: RPM ipm mm/sec 750 mm/sec Spots/Rev Fixture diameter 2 inch

(-1) 212 25 10.6 1.41% 5.1

C Pt 252 35 14.8 1.98% 4.3

(+1) 292 50 21.2 2.82% 3.5

RESPONSES: RELATED CTG FUNCTION:

(B,C) Factor Combinations: 1) Part temperature Fatigue

Comb Gas Stoic Ratio Hyd SCFH Oxy SCFH Air SCFH Point (CG,SR) 2) Almen strip Fatigue, ctg residual stress

1675 0.445 1159 332 920 ( 0, 0) 3) Hardness, HV300 Wear

1525 0.405 1085 258 920 (-1,-1) 4) Coating dep/pass Cost

1525 0.485 1027 314 920 (-1,+1) 5) Porosity Ctg quality, corrosion

1825 0.405 1299 342 920 (+1,-1) 6) Oxides Ctg quality

1825 0.485 1229 412 920 (+1,+1) 7) Carbides Ctg quality, wear

8) Tensile bond Adhesion/cohesion


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7.4.3. Example 2 – Optimization of WC-CoCr Another example of optimization was conducted by NRC-IMI of Canada12. This was a highly detailed DOE that included not only the best parameters for spraying with hydrogen but also the use of propylene and powder sprayability involving 7 different powder morphologies. Table 20 compares the parameters between hydrogen and propylene.

The response characteristics evaluated for these DOE�s were:

• Residual stress

• Porosity

Table 19 Response vs. Coating Property Response Coating property Part temperature Fatigue

Almen strip Fatigue; coating residual stress

Hardness (HV300) Wear, abrasion

Deposition/pass Cost

Porosity Coating quality, corrosion

Oxides Coating quality

Carbides Coating quality, hardness, wear

Table 20. Comparison of hydrogen versus propylene DOE. DOE Using Propylene … DOE using hydrogen

! Parameters that were varied. Parameters that were varied.

– Powder – Powder

– Flow rates – Total flow rates

– Propylene – Stoichiometry

– Air – Stand off distance

– Carrier – Powder feed rate

– Coating build up rate

! Constant parameters ! Constant parameters

– Oxygen flow rate – Powder feed rate

– Stand off distance – Surface temperature

– Step size – Step size

! Parameters adjusted ! Parameters adjusted

– Gun displacement speed – Cooling

– Cooling


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• Carbide degradation � degradation index measurement is discussed in section 10.5.2.2

The optimum results to the response characteristics are shown in Figure 18 (propylene) and Figure 19 (hydrogen).

Note that even the optimized materials are imperfect. Optimization minimizes the imperfections, and specifications for the process should therefore define the maximum permitted unmelted grains, oxide particles, porosity, etc.

Figure 18. Best propylene results. Degradation index = 4.25.

Figure 19. Best hydrogen results. Degradation index = 3.46.


Page 50

Temperature and velocity were also optimized as part of the fuel comparison. Figure 20 shows that use of propylene results in both higher temperature and velocity at the optimum stand-off.

This optimization concluded that hydrogen was the optimum fuel to produce the best microstructure, with special emphasis on carbide degradation. Note that this DOE primarily optimized structure, whereas Example 1 optimized stress, which is directly related to fatigue performance. The other portion of this DOE analyzed seven different powders with respect to a number of different characteristics to select the best materials. The characteristics evaluated as described earlier in Section 4 were:

• Particle size

• Carbide grain size

• Morphology/microstructure of the coating

• Porosity/phase distribution of the coating

1000

1200

1400

1600

1800

2000

2200

2400

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Position from gun (cm)

Tem

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(°C

)

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500

600

700

800

900

1000

1100

1200Temperature propTemperature HydrogenVelocity propVelocity Hydrogen

Velo

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(m/s

)

Figure 20. Graph showing the temperature/velocity profile with varied fuel types.


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Three powders (SM 5847, 1350VF, and WC-636-3) out of the 7 tested were selected with characteristics as shown in Figure 21 and Figure 22. Note that each of these powders looks somewhat different, sprays

differently, and would be expected to perform somewhat differently as a coating.

• SM-5847 is composed largely of hollow spherical particles that would be expected to accelerate faster and heat evenly.

• WC-636-3 has a very narrow grain size distribution, which should ensure that each grain is heated and accelerated in much the same way.

• 1350 VF has a much broader size distribution, which means that particles will strike the substrate with a broad distribution of

Figure 21. Microstructure/morphology of selected powders.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

0 10 20 30 40 50 60 70 80 90 100

Microns

% F

requ

ency

5847

636-31350-VF

Figure 22. Particle size distribution for the three best powders.

velocities and temperatures.

From the results obtained in this optimization, the SM-5847 powder was selected based on:

– Higher compressive residual stresses (related to fatigue)

– Lower porosity level

– Lower carbide degradation

However intrinsically this material produced an heterogeneous coating, containing large Cr rich areas.

7.5. Stripping Stripping of thermal spray coatings is critical for O&R, since new coatings cannot generally be sprayed over old, just as new chrome cannot be reliably plated over old chrome. Plasma spray coatings can often be removed by water jet stripping. However, HVOF coatings adhere more strongly than plasma sprayed coatings, and most attempts to remove them with water jets have not succeeded, although there has been more success with ultra-high pressure water jets. There are, however, standard electrochemical stripping solutions used by the industry and approved for aerospace use.

The following is a summary of information from Sulzer Metco, Southwest Aeroservice, and Lufthansa, as well as reports from NTS (McClellan AFB) and National Defense Center for Environmental Excellence (NDCEE).

7.5.1. Documents Document 15. NTS Stripping Report,PDF. 13 This report is authored by Elwin Jang of NTS (National Technical Systems, formerly Sacramento ALC) and Robert Kestler of NADEP Cherry Point, 1998. It describes the results of a project to evaluate the electrolytic stripping of HVOF HVOF WC-Co and WC-CoCr from steels by use of the Rochelle Salt solution.

Document 16. NDCEE Evaluation of Stripping Methods. 14 TT

DSS

R

"NTS stripping report.pdf"

"NDCEE evaluation of stripping methods.pdf"

"Stripping of Aermet 100 Southwest Aeroservice.p

his is a report of a study funded by NRL and run by Concurrent



echnologies Corp (NDCEE), evaluating various stripping methods, andincludes data on water jet stripping experiments run at NDCEE.

ocument 17. Stripping of WC Coatings from Aermet 100, outhwest Aeroservice, Menasco, Carpenter Technology (Courtesy outhwest Aeroservice). 15

This document is authored by Southwest Aeroservice, Menasco (now B.F. Goodrich, BFG), and Carpenter Technology. It reports stripping of HVOF WC-17Co and WC-CoCr using the standard Southwest Aeroservice

ochelle salt bath and an alkaline non-electrolytic bath.

df"


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7.5.2. Stripping of WC-Co The most common stripping solutions are electrochemical solutions based on Rochelle Salt (sodium potassium tartrate), and are fairly innocuous.

7.5.2.1. Southwest Aeroservice Specification # SASP.025A

Summary of method: 1. Solvent clean 2. Strip, checking every 10 min to ensure no dissolution of base

metal. 3. Rinse 4. Dry 5. Inspect 6. Embrittle relieve 7. Inspect.

Table 21. Electrolytic stripping method for HVOF WC-Co (Courtesy Southwest Aeroservice). Component Value Notes Anhydrous sodium carbonate

20 - 30 oz/gal water

150 - 225 gm/l

Sodium potassium tartrate (Rochelle Salt)

8 - 12 oz/gal water

60 - 90 gm/l

Temperature 104 - 150°F

40 - 66 C

130 -150 °F optimal

pH 11 - 12

Voltage 4 - 6 V DC

Current density 4 - 8 A/sq in

62 - 124 A dm2

Parts are anodic (positive)

Dissolution rate Approx 0.006�/hr

Applicable substrates High strength steels

A similar method used for Ti and Al alloys by BFG


Page 54

7.5.2.2. Sulzer-Metco Process specification #E-2. This process is for stripping WC-Co from steel (dated 1979).

Note: The same solution dissolves tungsten (W) at a rate of about 3 mils/hr.

Table 22. Electrolytic stripping method for HVOF WC-Co (Courtesy Sulzer Metco). Component Value Notes Sodium carbonate 20%

Tartaric acid 5%

Water 75%

Temperature 160 - 180°F 70 - 80 C

Voltage 6 V DC

Current density 4 - 8 A/sq in 62 - 124 A dm2

Parts are anodic (positive)

Dissolution rate Approx 0.006�/hr 0.18 mm/hr

Applicable substrates Steels


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7.5.2.3. Lufthansa Summary of method:

Note: This method is used for �old� WC-Co � i.e. material to be stripped from parts during O&R. For new WC-Co (i.e. material just deposited, but that must be stripped to correct misapplication), Lufthansa uses a simple chemical immersion method that does not work for �aged� (i.e. oxidized) WC-Co, as shown in Table 24.

7.5.2.4. Other specifications GE Aircraft Engines, Pratt and Whitney, and Praxair specify a rather

Table 23. Electrolytic stripping method for “aged” HVOF WC-Co (Courtesy Lufthansa). Component Value Notes Citric acid Various

Sodium hydroxide Various

Sodium carbonate Various

Temperature 104 - 140°F 40 - 60 C

Voltage 6 +- 0.5 V DC

Current density 1 - 3 A/dm2 Parts are anodic (positive)

Time 3 - 10 hrs 10 hrs max for HSS

Applicable substrates HSS, IN 718, Ti6Al4V

BFG uses a similar method for steels.

Table 24. Electrolytic stripping method for "new" HVOF WC-Co (Courtesy Lufthansa). Component Value Notes Citric acid

Hydrogen peroxide

Temperature 30 C

Time 3 - 10 hrs 10 hrs max for HSS

Applicable substrates HSS, IN 718, Ti6Al4V


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similar electrochemical stripper for removing WC-Co from titanium alloys, comprising a solution of Rochelle Salt, sodium hydroxide, and sodium carbonate.

7.5.3. Stripping of WC-CoCr NTS and NADEP Cherry Point have tested the use of the standard Rochelle salt stripping method for HVOF WC-Co and WC-CoCr. The results were similar with both materials. This is reported in Document 15. These tests were done to evaluate the standard Rochelle salt stripping method for stripping both WC-Co and WC-CoCr. Both stripped at about the same rate.

Southwest Aeroservice has tested their standard Rochelle salt WC-Co stripper (Table 21) for WC-CoCr, and it appears to work in the same way as for WC-Co. They report similar results for Aermet 100 substrates coated with WC-Co and WC-CoCr in Document 17.

7.5.4. Stripping of Tribaloy 400 Tribaloy is far more difficult to strip than WC-Co. The simple Rochelle salt method is not effective. Southwest Aeroservice has tested 50% nitric acid for T-400 on �Custom 450� steel. A 0.010� coating of T-400 breaks down in 4 - 5 hours sufficiently to remove by glass bead blasting. This method is approved and used by GE Aircraft Engines for T-400.

Table 25. Electrolytic stripping method for HVOF WC-Co (NTS/NADEP Cherry Point). Component Value Notes Anhydrous sodium carbonate

20 - 30 oz/gal water

150 - 225 gm/l

Sodium potassium tartrate (Rochelle Salt)

8 - 12 oz/gal water

60 - 90 gm/l

Temperature 130 - 150°F

54 - 66 C

pH 11 - 12

Voltage 4 - 9 V DC Parts are anodic (positive)

Current density 40 - 80 A/sq ft Note low current density

Dissolution rate Approx 0.001 0.002�/hr

Note low rate

Applicable substrates 4340, PH13-8 Mo, 1010


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7.5.5. Water-jet stripping Although standard water jet stripping works quite well for most plasma sprayed coatings, it seldom works well for most HVOF coatings, since their adhesion strength is too high. CTC has reported some success with ultra-high pressure water jet stripping of tribaloy from mild steel, but the method is not well defined (see Document 16).

7.6. Finishing

7.6.1. Documents Document 18. Surface Metrology Guide (Courtesy Precision Devices, Inc.). 16 This document summarizes the various surface profile parameters, their definitions, and their uses. (See also http://www.predev.com/smg/parameters.htm)

Document 19. Superfinishing of Hard Chrome and HVOF Coated Workpieces (Courtesy Supfina and Gorham Advanced Materials). 17 This paper by Norbert Klotz of Supfina gives a good overview of the effects of Superfinishing on the surface of a component.

Document 20. Surface Finishing of Tungsten Carbide Cobalt Coatings, J. Nuse, J. Falkowski. 18 This paper is based on work done by Jim Nuse of Southwest Aeroservice, and John Falkowski of Boeing. It describes flight test experience with surface finish and a laboratory evaluation of different finishing methods.

Document 21. Barkhausen Noise as a Quality Control Tool (Courtesy Stresstech Inc., Finland). 19 This document is a brief review of the origin of Barkhausen noise and the way in which it is used as a QC method. (See also http://www.stresstech.fi/)

7.6.2. General requirements Chrome plating is usually finished by grinding with a standard carbide wheel. Specifications for chrome plate usually define the finish in terms of Ra (the arithmetical mean deviation of the surface profile from the average). Typical Ra values for chrome plate are

"Surface Metrology Guide - Profile Param

"Klotz - Supfina superfinishing.pdf"

"Nuse Falkowski - AESF 2000 Paper.pdf

Barkhausen.pdf

http://www.predev.com/smg/parameters.htm

http://www.stresstech.fi/


Page 58

Axle journals 32 µ� Ra (0.8µm)

Hydraulic seal surfaces 16 µ� Ra (0.4µm)

Metallic HVOF coatings, such as Tribaloy, can be ground in a similar manner. However, HVOF WC-Co and WC-CoCr coatings can only be ground with a diamond wheel. As with chrome, the specifications for surface finish depend on the application. Although finishes are specified for aerospace-qualified thermal sprayed components, a definitive specification for HVOF coating finish is not yet available. However, in general it found that WC-Co coatings must be finished to a significantly smoother surface than one would use for chrome. The reason for this appears to be that, since WC-Co is so much harder than chrome and contains many small particles of hard carbide, rough HVOF WC-Co acts almost like a file against soft materials such as seals and bushing alloys, causing rapid seal failure or excessive transfer of bushing material. Furthermore, in actuators and landing gear, typical 5µm WC particles come off rough surfaces and become suspended in the hydraulic fluid, turning it into an abrasive cutting fluid.

For this reason HVOF coatings are usually specified as <8 µ� Ra, and preferably as <4 µ� Ra. However, some users specify much finer superfinishes (down to 1 µin Ra), and others specify the surface finish more thoroughly than with just an Ra number. Although this emphasis on very smooth finishes runs counter to the generally accepted belief that some roughness is necessary for oil retention, testing clearly shows that smoother HVOF surfaces usually perform better.

7.6.3. Specifying the surface finish Methods of specifying surface finish are discussed in Document 18 and at the following web sites: http://intranet.siu.edu/~cafs/surface/file10.html and http://www.predev.com/smg/parameters.htm A review of standards for surface finish measurement can be found at http://www.predev.com/smg/standards.htm A typical surface after grinding is covered in grinding marks and loose debris. The profile of the surface (usually measured with a stylus that traverses the surface in a line) shows sharp �hills� and �valleys� about a mean. In most cases the mean is also wavy on a larger distance scale.

http://intranet.siu.edu/~cafs/surface/file10.html

http://www.predev.com/smg/parameters.htm

http://www.predev.com/smg/standards.htm


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The surface roughness, Ra (see Figure 23), is simply the arithmetic average of the absolute deviations from the mean line (which can be defined from the area under the curve between the profile and the mean). This is the most common surface finish parameter, specified in most drawings. However, it is becoming clear that Ra is a very unsatisfactory measure of surface finish as it relates to performance, since many different surface profiles can have the same Ra, but will perform very differently. For example, the surfaces in Figure 24 all have the same Ra, but the top one will tend to cut into seals, while the middle one will tend to hold lubricant at the surface. For this reason some users of thermal spray coatings are resorting to the use of additional parameters to describe the surface more completely. For example, Figure 25 shows several other surface parameters that better show the shape of the surface.

• Rt is a measure of the total peak-to-valley height

• Rp shows how high the peaks rise above the centerline

• Rv shows how deep the valleys fall below the centerline

Figure 23. Typical surface profile.


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In principle, a smooth surface would have a very small Rt. However, one might actually prefer a surface with a small Rp and large Rv, since this would have few damaging peaks that would gouge into seals and mating surfaces, while having fissures that would hold lubricant.

Some users are adopting the Surface Bearing Ratio, tp, as a measure of surface finish. This parameter is usually defined as �the cutting depth for an X% bearing ratio�. This parameter answers the question �How deeply do I have to cut into the surface to get an area of flat surface X% of the entire surface?� The surface profile is integrated from the topmost peak (Figure 26). As the graph at the right of the figure shows, as you move inward from the peak you intersect a larger and larger area of the surface. This ratio provides a good idea of how �mountainous� the surface is, and how rapidly it will wear (or can be superfinished) to a flat surface.

Figure 24. Three different surfaces with the same Ra.

Figure 25. Other surface roughness parameters.


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Delta has adopted superfinishing (see below) for landing gear inner cylinders, and a set of surface finish specifications that include Ra, Rt, and tp

7.6.4. Superfinishing Superfinishing is being increasingly recognized as critical to ensuring the optimum performance of HVOF-coated hydraulic cylinders of all kinds. Document 19 is an excellent overview of superfinishing. There are two basic types of Superfinishing

• Stone finishing, in which a fine honing stone is held against the surface and vibrates axially while the cylinder rotates.

• Belt finishing, in which an abrasive belt is vibrated against the workpiece instead of a stone.

A simple review of superfinishing may be found at: http://www.supfina.com/english/html/press115.html Superfinishing is usually done after the surface has been ground to a 4 or 8 Ra finish. This ensures that the surface is true and avoids the need to remove large amounts of material. The superfinishing tool removes material uniformly from the surface, leaving a surface that is not only smooth (often <1 µin Ra), but it removes any surface lay (grinding marks), and leaves the surface free of embedded grinding materials and loose debris. With WC-Co thermal sprays, small WC particles are often found loosely held on the ground surface. Superfinishing removes these particles.

Figure 26. Definition of bearing ratio.

http://www.supfina.com/english/html/press115.html



It is generally thought that a very smooth surface will be a poor surface for hydraulics since it cannot retain fluid to lubricate the seal. For HVOF coatings, however, superfinishing can be used to remove the surface “hills”, leaving the “valleys” and a small amount of porosity to hold fluid.

Figure 27. Effect of various finishing methods on an HVOF coating at 175x (Courtesy Supfina).


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Boeing, Delta, and Southwest Aeroservice have evaluated superfinishing for landing gear inner cylinders. The results, shown in Figure 28, clearly illustrate that one obtains different finishes on chrome and HVOF coatings by the same finishing techniques. Grinding leaves significant debris and striations on the HVOF surface that polishing does not remove. However, superfinishing eliminates the debris and the surface lay, leaving a clean, smooth surface finish with some porosity to hold fluids.

7.6.5. Rig test experience The issue of surface finish has been evaluated by Green-Tweed for its effect on seal life in hydraulic rig tests designed to simulate flight surface

Figure 28. Surface finishes obtained on Chrome and HVOF WC-CoCr by various finishing methods.18


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actuators. With a surface finish of 4-8µ� Ra, the HVOF coatings performed much better than hard chrome (i.e. less seal leakage) when run against PTFE seals, but much worse (seal failure) when run against elastomeric seals. This work is described in Section 13.2.1 and Document 46.

7.6.6. Flight experience There is now a great deal of flight experience with different finishes on HVOF coatings on landing gear, and on slat and flap tracks. In particular, Boeing and Delta Airlines have evaluated surface finish quite closely (see Document 20.) In their flight tests, Lufthansa�s HVOF WC-CoCr-coated landing gear inner cylinders were 4-6µ� Ra. However, in early field testing by Boeing

and Delta Airlines, HVOF-coated landing gear shock struts had a 16µ� finish � the same as that called out for chrome. Their elastomeric seals failed rapidly. Damaged seals showed the pock-mark degradation typical of seals run against a rough surface, and analysis of the hydraulic fluid showed the presence of 5µm WC particles, evidently released by wear or polishing of the HVOF-coated surface. These seal failures were eliminated by refinishing the HVOF surface to 2µ� Ra, and then performed better than standard chrome plated gear. Boeing, Delta, and Lufthansa flight experience with seal life is summarized in Table 26. The surface finish of stationary components is less critical than sealing surfaces. Delta Airlines now specifies a finish for HVOF coatings of 4µµµµin Ra maximum on hydraulic seal surfaces, and 8µµµµin Ra maximum on axles.

Table 26. Seal life in HVOF-WC-Co sprayed landing gear.18,20 Aircraft/HVOF coating

Finish Seal type Life (cycles)

Boeing 757/WC-CoCr 13µ� Ra Elastomer 936

Boeing 737/ WC-CoCr 9-12µ� Ra Teflon 855

Boeing 757/ WC-CoCr 2µ� Ra Elastomer No failures in 1 year (approx 2,000 cycles)

Boeing 737/WC-Co (Lufthansa)

2µ� Ra - 1910

Boeing 737/WC-Co (Lufthansa)

16µin Ra - 1100 average


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7.7. Inspection Detection of cracks in HVOF surfaces is not as easy as it is with chrome plate. It is made more difficult since HVOF coatings are generally in compression (which closes cracks), rather than in tension (which opens them). In most chrome plated surfaces, cracks in the underlying steel are evident at the surface, and tend to be opened by the chrome�s tensile stress (hence its fatigue debit). Substrate cracks tend to be closed by the compressive stress of HVOF coatings and are often not evident at the surface. Since most HVOF coatings are non-magnetic, magnetic particle inspection (MPI) is not useful. Lufthansa has found that simple dye penetrant inspection is also not useful. The only simple inspection method for detecting cracks in the coating is fluorescent penetrant inspection (FPI). This method has been adopted by Delta Air Lines. Eddy current methods must be more sensitive than normal to detect cracks under HVOF coatings. For this reason, Bombardier increased the inspection frequency when adopting HVOF for flap tracks in place of electroless nickel. Boeing successfully uses Barkhausen Noise methods of evaluating conditions beneath the coating. However the method is not widely available, the results are difficult to interpret, and there is a lack of recognized standards. Information on the Barkhausen method can be found at http://www.stresstech.fi/BarkNoisAnal.html. The method is reviewed in Document 21. To provide better inspection methods, various techniques including ultrasonic and eddy current methods are currently being tested and improved by the HCAT and by Heroux.

http://www.stresstech.fi/BarkNoisAnal.html


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8.8.8.8. Thermal spray equipment Most thermal spray equipment (flame, wire, plasma, and HVOF) is supplied by a few major international vendors, who supply guns, parts, and complete turnkey deposition systems. The following are links to their web sites:

• Sulzer Metco o Plasma, HVOF, ID

plasma systems, powder

• Praxair (including the TAFA division)

o Plasma, HVOF, ID plasma systems, powder

• Deloro Stellite o HVOF Jet-Kote systems,

powder Other vendors include

• Northwest Mettech o 3-electrode high power

plasma gun This is not an exhaustive summary of equipment vendors, but it contains most of those companies selling equipment commonly used for aerospace applications.

Figure 29. Sulzer Metco F210 ID plasma spray gun (Courtesy Sulzer Metco).

Figure 30. HVOF spraying of WC-CoCr on landing gear with TAFA gun (Courtesy Praxair-TAFA).

Figure 31. Northwest Mettech Axial III tri-electrode plasma system (Courtesy Northwest Mettech)..

Figure 32. Stellite Jet-Kote HVOF gun (Courtesy Deloro Stellite).

http://www.sulzermetco.com/

http://www.praxair.com/

http://www.stellite.com/

http://www.mettech.com/


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9.9.9.9. Thermal spray services There are numerous aerospace-qualified companies offering thermal spray services. Table 27 shows companies qualified by Boeing for deposition of thermal spray coatings on airframe parts. Information can be accessed by clicking on the Processor #. These are not the only aerospace-qualified vendors, however, since other aerospace companies have their own qualified vendor lists.

Other aerospace-qualified vendors include

• Cincinnati Thermal Spray

• Ellison Surface Technology

• Colonial Coatings

Table 27. Qualified providers for Boeing 5851 thermal spray coatings, as of June 2000 (Source, Boeing Aircraft Corporation21).

Country State Processor Processor Name

CANADA MANITOBA 646547 NATIONAL COATING TECHNOLOGIES

CANADA QUEBEC 561323 VAC AERO INTERNATIONAL

ENGLAND 641824 PRAXAIR SURFACE TECHNOLOGIES LTD

JAPAN SAITAMA PREF 627739 PRAXAIR SURFACE TECHNOLOGIES

USA CA 560994 FLAME SPRAY INCORPORATED

USA CA 561023 PLASMA COATING CORPORATION

USA CA 537751 PLASMA TECHNOLOGY INC

USA CA 305611 PRAXAIR SURFACE TECHNOLOGIES

USA CA 031185 SERMATECH TECHNICAL SERVICES

USA CT S01081 ENGELHARD SURFACE TECHNOLOGIES

USA CT 627738 PRAXAIR INCORPORATED

USA IN 621420 PRAXAIR SURFACE TECHNOLOGIES

USA KS 632859 THERM-O-COAT INCORPORATED

USA NY 409600 HITEMCO

USA OK 597139 SOUTHWEST AEROSERVICE INC

http://active.boeing.com/doingbiz/d14426/getprocesscodes.cfm?SupplierCode=646547









http://active.boeing.com/doingbiz/d14426/getprocesscodes.cfm?SupplierCode=S01081






http://www.cincinnatithermalspray.com/ctsaero2.htm

http://www.thomasregister.com/olc/ellison/

http://colonialcoatings.com/aerospace.html


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• Aerospace Welding

• GKN Aerospace

• Turbine Metal Technology

• Vac Aero International

• Sermatech International

• Thermal Spray Technologies

• Plasma-Tec This is by no means an exhaustive list. Praxair is the only major supplier of detonation gun (D-gun) coatings, since this is a proprietary Praxair technology. In addition several airlines and turbine engine manufacturers have their own in-house thermal spray facilities.

http://www.aerospacewelding.com/aerospace.html

http://www.chem-tronics.com/

http://www.tmtechnology.com/

http://www.vacaero.com/

http://www.sermatech.com/

http://www.tstcoatings.com/

http://www.plasmatec.com/


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Part 3 Thermal Spray Data

PART 3. THERMAL SPRAY DATA There is a great deal of data available on thermal spray coatings. However, much of it is not well-defined in terms of substrates, coatings, or test methods. We have included in this section a limited amount of data that is well-defined and illustrative of typical coating properties and performance.


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10.10.10.10. Coating structure

10.1. Summary The structure of a coated material can be defined by both microstructure and phase. It is not always possible to determine the presence of varied phases by visual examination of the microstructure even with Scanning Electron Microscopy (SEM). A combined approach is sometimes necessary due to the effect on final coating performance caused by the presence of varied phases. Microstructural work can currently identify both porosity content and primary WC carbide distribution. X-Ray diffraction can identify the phases present but the relationship between quantifiable content and performance properties is still under investigation.

10.2. Documents Document 22 Evaluation of Four High Velocity Thermal Spray Guns Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault, C. Moreau, V. Bouyer, L. Leblanc). 22 This summary document details the effect of gun type on the microstructural porosity and abrasive wear characteristics of the coating.

Document 14. Summary of DOE results for optimization of HVOF WC-CoCr (Courtesy NRC Montreal and C-HCAT). This document summarizes the optimization work performed with varied powders and fuel gases using the HVOF system on WC-CoCr material.

Document 23. Fracture Toughness of HVOF Sprayed WC-Co Coatings (Courtesy of S. De Palo, et al). 23 Summary of testing concerning fracture toughness and erosion resistance on WC-Co coatings.

Document 24. Tungsten Carbide-Cobalt Coatings for Industrial Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan). 24 Summary of varied spray processes for deposition of coatings and study on abrasive wear/microstructure/phase content interactions

"Evaluation of 4 HVOF guns, Legoux.p

"Frac troughness of HVOF WC-Co DePalo

"Tung carb coatings for ind apps.pdf"


Page 71


10.3. General The coating structure is obviously the ultimate factor for determination of coating properties and the performance of the HVOF alternatives in service. Microstructure (seen by optical or scanning electron microscopy) describes the visual appearance or structure of the coating for characteristics like porosity, oxides, and distinguishable phases such as carbides. The presence and distribution of features determines the coating performance in most applications. Phase determination supplements the identification via light microscopy and strives to quantify all phases that are present in the coating structure. This analysis measures the dissolution of the primary WC, for example, and what phases form in the structure. Both methods are critical and necessary to adequately characterize the coating morphology. This section will summarize the test methods for both microstructure and phase determination and explain the significance of the results for each structural property.

10.4. Microstructure

10.4.1. General Description and Test Methods HVOF Tungsten Carbide Cobalt (WC-Co) and Tungsten Carbide Cobalt Chrome (WC-CoCr) are the primary thermal spray materials used for replacement of hard chrome. As referenced earlier in Section 7.3.3, the major characteristics evaluated in the microstructure are listed in Table 28, with a description of those commonly found with carbides. The most common methods used to characterize the microstructure are metallography and microstructural interpretation via light microscopy. These techniques are described in Section 6.


Page 72


Table 28. Common microstructural Characteristics Observed in Tungsten Carbide Materials. Characteristic Description Carbide Coatings Interface contamination

Grit that is embedded at the interface from the grit blasting operation � usually compare to a photostandard to assess degree.

This characteristic is present for all coatings. The degree of contamination will be closely related to the hardness of the substrate rather than any coating material characteristics.

Porosity Assess distribution against photostandard of voids/holes in coating resulting from incomplete splat bonding

Present with carbide coatings but minimized in the HVOF process due to high particle velocities.

Oxides May be in form of stringers or clusters from powder traveling through the air � again compare to photos

Usually not observed in cobalt-based coatings (minimized oxidation tendency) but can be present if temperature is extremely high.

Unmelted particles

All particles do not obtain sufficient thermal energy in flame to deform when accelerated toward the substrate � usually count particles, defining a shape for characterization as unmelt

Powder morphology and size usually results in very complete melting and thus lack of unmelts in cobalt based carbide coatings.

Phase content Distribution and content of phases is critical for some coatings such as carbides in a wear coating � compare to photos

Distribution and size/morphology of carbide phase are very important in cobalt based materials. Carbides can dissolve in matrix

Delaminations and cracks

Can be located at interface and within coatings � crack length and number of cracks is critical

Tendency in carbide coatings for transverse (parallel to surface) cracking, especially if dissolution of carbide phase occurs. Delaminations at interface possible with poor grit blasting.


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10.4.2. Microstructural Features 10.4.2.1. Porosity/Voids

Porosity levels in properly sprayed HVOF materials are typically very low � in the 0.5% to 5% range. Since cobalt is a soft phase as compared to tungsten carbide, there can be a tendency in metallographic preparation to smear the cobalt into the existing porosity of the structure, which presents what appears to be a fully dense structure. This is illustrated to a degree in Figure 33, which shows the cross section at two different magnifications. Although appearing essentially porosity-free at the lower magnification, a degree of porosity does exist as illustrated by the higher-power picture. True porosity and void content is a controversial subject in the thermal spray industry. Evaluations have been performed with mounting materials containing fluorescent dyes to determine the true level of porosity. This technique works on the basis of first using either vacuum or pressure impregnation to force the mounting media into the pores of the coating. The polished mount is excited using a Xenon light source and porous areas filled with �glowing � mounting media are characterized as �true� porosity. With HVOF coatings, no porosity could be identified with this technique. This is due to the lack of �interconnected� porosity in HVOF materials as compared to their plasma sprayed counterparts. The connected porosity of plasma sprayed materials allows the mounting media to flow through the coating and into all void areas. It is therefore easier to ascertain the �true� porosity level. Work is continuing with such techniques as �cryogenic fracturing� and the application of NDT methods such as �CAT� scans to determine the exact porosity levels. Material will be made available on the HCAT Home Page as research data is available.

10.4.2.1.1. Comparison of Porosity vs. Particle Velocity

Document 22 Evaluation of Four High Velocity Thermal Spray Guns Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault, C. Moreau, V. Bouyer, L. Leblanc) This documents relates void formation to process velocity. As shown in Figure 34, the porosity is reduced with increasing particle velocity. This is related to the high kinetic energy in the HVOF process that results in better bonding of the particles and reduced void formation. The Mettech Axial III is a plasma spray gun, and shows little obvious dependence of porosity on velocity.

Figure 33. Comparison of porosity at 200x and 1000x magnification.



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10.4.2.2. Matrix/Carbide Phases/Composition WC-Co and other cermet coatings are essentially two phases: a substantial amount of hard carbide particles held together by a matrix of softer material such as a cobalt or cobalt-chrome. The distribution and size of the carbide particles can obviously affect many properties as described below. Figure 35 shows a comparison of carbide content between two different compositions. The white phase is the background matrix of cobalt while the darker particles are the carbides. Note the obvious change a 5% difference in cobalt content makes in the apparent carbide distribution.

Figure 18 is a higher magnification illustration of a WC-CoCr coating showing the varied distribution in carbide size in the microstructure. The various features of Figure 18 are summarized in Table 29. These phases will be present in all carbide/cermet coatings to some degree. Optimization obviously involves providing a uniform distribution

Figure 34. Relationship between velocity and porosity.

Figure 35. Comparison of Carbide Distributions in 88-12 WC-Co (left) vs. 83-17 WC-Co (right) at 500X (Courtesy Praxair/TAFA).


Page 75


of the desired carbide particles and minimization of both carbide dissolution (thus less secondary phases) and segregated matrix phase. (Details may be found in Document 22.)

Figure 36. Microstructure of WC-CoCr 1000X25

Table 29. Features seen in Figure 35. Feature Description Small carbides and large carbides

Results from initial distribution of carbide particles in the starting powder. Must be controlled for varied guns to optimize properties and prevent areas of segregation

Chrome/cobalt rich areas Softer matrix phase can become segregated during powder manufacture or improper mixing during the spray processing

Carbide/amorphous phase Results form dissolution of primary WC carbide into secondary phases such as W2C, amorphous phase, and other W?C? compositions. Desire is to minimize these secondary phases (see Section 11.4.3.2)


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10.4.2.3. Transverse Cracks Carbide coatings can be prone to transverse cracking, especially if the coatings are sprayed at too high a temperature so that substantial carbide dissolution (carbide in solution) is prevalent. Cracks can also appear in the microstructure from excessive force in clamping during cutting of the test sample and overheating during the sectioning process. Properly sprayed materials will not show cracking indications in the 200-500X magnification range. Cermet materials at 1000x will show some degree of transverse separation but this can be observed with all coated materials and does not adversely affect performance. Figure 37 shows cracking in a plasma sprayed coating.

10.4.3. General trend of microstructural features Characterization of microstructural features is used to measure porosity and carbide phase content and distribution. Levels of porosity below 2% are easily obtainable. However, the best method to determine this content is still the subject of debate. Carbide distribution is relatively easy to determine on a correctly polished sample. This content is dependent upon the primary carbide content and composition. These characteristics will ultimately be reflected in coating performance, such as corrosion and wear, due to the dependence of these characteristics on structural morphology.

10.5. Phase Determination and Effect

10.5.1. General Description and Test Methods Carbide coatings primarily consist of a distribution of carbides within the soft cobalt or cobalt-chrome matrix. During the spray process, some breakdown or decomposition of the WC particles can occur forming a variety of secondary phases. X-ray diffraction (XRD) is commonly used to quantify and identify these phases.

Figure 37. Transverse cracking in plasma sprayed carbide coatings.25


Project #: 3105JSF3 ; Report #: Final

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A typical plot comparing the varied phases found in powder vs. final sprayed product is shown in Figure 38.

This change in crystalline phases or “carbide degradation/dissolution” is critical to coating performance in properties such as wear and abrasion. It is therefore an important aspect of coating evaluation and is highlighted in the results section.

10.5.2. Phase Determination and Effect Results

10.5.2.1. Carbide Phase Comparison vs. Process Type Document 24. Tungsten Carbide-Cobalt Coatings for Industrial Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan) This document shows the relationship of retained carbon content and carbide phase when comparing data from two different spray guns. An air cooled DiamondJet system vs. a Hybrid 2600 were compared using A12 (12% cobalt) and A17 (17% cobalt) materials. Table 30 summarizes the physical properties of the coatings.

Figure 38. X-ray diffraction plot of powder(lower curve) and coating (upper curve).26


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Table 31 to Table 32 summarize the phase data. The amount of carbide and therefore carbon retained in the process is higher with the air cooled

system, as would be expected due to the lower operating temperatures of the system and lower degradation rate. Although not quantitatively shown, the authors also indicated the A-12 material showed more degradation, possibly due to the finer carbide size and increased surface

area for reaction. This can also be seen in comparing the carbide content of the A-12 material. The microstructure of the hydbrid-gun material (Figure 39) shows significantly fewer carbides, as Table 30 would lead us to expect.

Table 30. Physical properties of coatings produced by different guns. 27

Set 1 Propylene Set 2 Hydrogen Set 1 Propylene Set 2 Hydrogen

A-12 (Air-cooled) A-12 (Hybrid) A-17 (Air-cooled) A-17 (Hybrid)

Surface Texture

(aa, micro inch)

(Ra, micro meter)

120-170

3.1-4.3

90-140

2.3-3.6

150-250

3.8-6.4

120-160

3.1-4.1

Macrohardness Rc 60-65 65-70 60-65 65-70

Microhardness HV0.3 950±65 1030±46 900±76 1100±74

Carbide Content Vol% 40-45 23-27 47-55 30-35

Carbide Size (um) 1-2 1-2 4-6 4-6

Porosity (Vol%) 2-4 <2 2-4 <2

Deposit Efficiency % 55-60 40-45 55-60 40-45

Table 31. Effect of Gas Flows and Cooling Gases on Retained Carbon. Spray Set

Material Fuel Gas Type

Oxygen/Fuel Ratio

Cooling Gas Retained Carbon

% Carbide Out of

Solution 1 A-17 Propylene 4.7:1 Air 3.3 47-55 5 A-17 Propylene 3.7:1 Nitrogen 3.5 60-65 3 A-17 Hydrogen .34:1 Nitrogen 2.7 45-50 4 A-17 Hydrogen .27:1 Nitrogen 3.3 52-57 2 A-17 Hydrogen .45:1 Air 2.4 30-35 2 A-17 Hydrogen .45:1 Air 2.4 23-27 1 A-17 Propylene 4.7:1 Air 3.0 40-45

Table 32. Retained C and XRD phases. A-12 A-17

Powder Diamond Jet

Hybrid Powder Diamond

Jet

Hybrid

Carbon Content 5.6 3.0 2.4 5.1 3.3 2.4

X-ray Diffraction Phases

WC,Co WC,W2C,W WC,W2C,W WC,Co WC,W2C,W WC,W2C,W


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Carbide dissolution is obviously dependent upon gun type, process temperature, and starting carbide morphology.

10.5.2.2. Carbide Degradation Indexing The most important phase in the carbide/cermet coatings is the primary WC which should be retained through the spraying process. However, due to carbide dissolution in the Co at high temperature in the flame, a combination of varied secondary phases from W2C to amorphous phase may be present in an as-sprayed structure. It can be difficult, costly, and time consuming to identify the discrete crystallographic morphologies that exist in a typical spray deposit. A semi-quantitative method of indicating the degree of dissolution of the carbide is being used by NRC, who term it the carbide degradation index (see Document 14). This method involves the use of XRD to determine crystallographic phases and then calculates a ratio of primary WC to all other phases present. The method, developed by Jean-Gabriel Legoux at NRC, is not intended to be quantitative, but rather to be a simple qualitative method of gaining insight into the degree of degradation or dissolution of the carbides, which is useful in optimizing carbide coatings. The method consists of taking the ratio of the sum of the degradation products (W2C, W-Co alloys, etc., which can be seen in the region between the (100) and (101) WC peaks in Figure 38) to a well-separated WC peak (such as the WC (001) peak in the same figure). The ratio is an arbitrary number, but is directly dependent on the extent to which the WC has broken down.

10.5.3. General Trend of Carbide Phase Distribution

In general, an optimized carbide thermal spray should retain the carbide. In practice, however, some of the carbide is likely to dissolve in the cobalt matrix or degrade into sub-carbides, lowering the hardness of the coating.

Figure 39. Comparison of carbide content in as-sprayed A-12 (12% cobalt ). Hybrid 2600 gun (left), air-cooled DiamondJet (right).


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Work is currently in progress to understand the relationship between carbide dissolution and the phases present after spraying. This is critical to understand the effects of phases present on performance properties such as fatigue, wear, and impact. A more time efficient, precise, cost effective method must be developed to address these needs.


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11.11.11.11. Coating properties

11.1. Summary This section contains data on the physical characteristics of thermal spray coatings. The properties covered are:

• Hardness (Macro/micro)

• Adhesion/Tensile strength

• Residual stress (both coating/substrate) With hardness, direct comparison of data with hard chrome is easily done. Hardness values can be obtained with HVOF alternatives that provide superior wear resistance as described in Section 12.6. For properties such as tensile and residual stress, no direct comparison to hard chrome exists or data is difficult to obtain. Tensile and residual stress information are not critical for chrome plating since the process does not allow control of these properties. The HVOF coatings exhibit excellent repeatability in tensile and tests are being developed to better understand this property. A controlled level of residual stress has been identified as a need in the coatings to improve resistance to cracking of the coating and minimize (and in some cases eliminate) fatigue debit in the substrate.

11.2. General Background Coating properties involve the physical and quantifiable characteristics of the materials other than the structural and phase data covered in Section 10. These properties include, but are not limited to, hardness, adhesion/tensile, and residual stress. Hardness is critical for the wear performance of coatings. Adhesion/tensile is required to insure both adequate cohesion of the deposit and adhesion of the coating to the substrate. Residual stress is critical to the ability of the coating to resist cracking. The magnitude of the stress in the coating must also be balanced by the values which are present in the substrate; this degree/magnitude/sign of substrate residual stress affects fatigue life and the tensile magnitude must be minimized. This section will describe test methods and results for these properties.


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11.3. Hardness

11.3.1. Documents Document 25. A Critical Evaluation of the Employment of Microhardness Techniques for Characterizing and Optimizing Thermal Spray Coatings 2000 (Courtesy of M. Factor and I. Roman, Hebrew University). 28

11.3.2. General Description and Test Methods Hardness testing on coatings is very similar in methodology to that performed on metallic materials. Macro and microhardness are covered by specifications ASTM E-19 and ASTM E-384 respectively. Coupons used for metallography are sometimes tested for hardness first and then cut and polished for microstructure analysis. Macro or Rockwell hardness for coatings is most often performed using the R15N superficial scale which applies a relatively small load of 15 kg. With the usual thickness range of .002-.008�, the use of conventional RC or RA scales with loads of 150 and 60 kg respectively would result in penetration of the indenter through the coating thickness. The hardness reading would then be a composite of the substrate/coating, which is not representative of true hardness. The surface is usually lightly sanded to remove irregularities and a series of readings distributed randomly across the face of the coupon are taken to determine hardness. The readings are averaged to obtain a composite value since values may vary dependent upon the phases present under the indenter due to the heterogeneous nature of some coatings. Microhardness can be performed using either a Knoop (elongated impression) or Vickers (diamond impression). For most coating applications, the Vickers method is used. Since phase distribution is critical for many coatings, microhardness can identify any segregation or absence of important constituents in the microstructure. The pattern for impressions may be random as described for macrohardness, but some specifications require a stair step pattern through the thickness. This can check for variations in the spray process as the coating builds up on the coupon. If something changes in spraying, a hardness change should be identified in the progression of readings.

11.3.3. Hardness Results The goal with HVOF materials is generally to meet or exceed the value for hard chrome. (Note, however, that hardness does not always relate directly to wear rate, and softer materials can perform better than harder ones depending on the surface finish, wear mechanism, etc.) The generally recognized value for hardness of hard chrome is 800 � 1000, or an average of about 900 HV(300g). With HVOF materials, the process parameters can be varied to obtain a

"Factor - Crit Eval of Microhardness.pdf"


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wide variety of hardness values normally ranging from 1000 to 1450 HV for the carbides. Expected values for macrohardness can range from R15N 88-92. The hardness values are also obviously dependent upon coating composition and the percentage of carbide vs. matrix present in coatings such as WC-Co. The ultimate hardness target in HVOF coatings is dependent upon the final coating/substrate properties desired with regard to wear, fatigue, etc. When a process parameter set is developed, the hardness values can be consistently obtained on a repeatable basis. Since the carbides are so much harder than chrome, hardness can sometimes be traded for other desired properties, such as fracture toughness or ease of deposition. (In fact, WC-17Co is often used instead of the harder WC-12Co for its greater ease of deposition.) The presentation of HVOF hardness results is primarily a summary of quality control results obtained during spraying. Hardness is used with HVOF sprayed materials for the following purposes:

• A quality control tool to provide process control (both macro/micro)

• A means of determining phase distribution in the coating structure (primarily micro)

• A measure of the carbide degradation that may occur (primarily micro).

Microhardness Table 33 lists the microhardness values obtained in a comparison of several coating processes and compositions. The values obtained for different materials can be similar as process parameters are varied for each material. As compared to hard chrome, there are issues that must be recognized when performing hardness testing on HVOF materials. HVOF materials are heterogeneous in structure. It should therefore be expected that the carbide HVOF materials will show a larger degree of variability in the results when 10 readings are taken across the sample, since the indenter may hit various concentrations of carbide particles or soft binder. A variation from 1000 to 1400HV is not unusual. Document 25 is a study of this variation. Porosity in the coating can also affect results, but this should be mitigated by the relatively high density of HVOF materials.

Coating Chemistry

Coating Process

Density g/cc

Average Hardness

VH.3

Chrome Cr Plate 6.9 900

WC-Co HV 2000 13.2 1200

WC-Co Jet Kote 13.2 1100

WC-Co JP 5000 13.2 1125

WC-Co D-Gun 13.2 1075

WC-Co SDG 13.2 1100

WC-Co-Cr JP 5000 12.4 1100

WC-Co-Cr SDG 12.4 1270

WC-Cr-Ni SDG 10.5 1100

Table 33. Microhardness for various HVOF coatings and equipment (Courtesy Praxair Surface Technology).


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Table 34 lists results from a comparative study of varied materials all sprayed with the same parameter set. This data shows:

• Variability in microhardness data (1032 to 1435 for Coating D )

• The effect of composition (Coatings C and D with more cobalt are softer)

• Different materials with approximately the same soft matrix percentage will produce similar results (Coatings A/B and E/F)

Dependent upon the specification, absolute minimum/maximum single values are sometimes specified and/or minimum/maximum average values are listed. Current work shows that values of approximately 1100-1200 HV produce optimum fatigue results for WC cermets Section 12; wear results (Section 12.6) favor values towards the 1400 range. The variability of carbides is related to the dissolution of carbides discussed in Section 10.5. As with WC-Co, the hardness of Tribaloy 400 coatings varies with deposition conditions, and values of 580-670 HV are typical. This is significantly less than the hardness of chrome plating. However, T400 is better in fatigue on aluminum alloys than WC-Co (see Section 12.6.3), and, although it does not have the wear performance of WC-Co, it is still better than chrome in hydraulic rig tests (Section 13.2.1). More actual data for the varied processes is available and usually incorporated in the documents which are referenced in both Sections 12 and 13 on coating properties and performance respectively. Macrohardness The values generally obtained for R15N readings on carbide coatings can vary from 88-92 dependent upon the spray process parameters. The requirement for macrohardness varies from supplier to supplier, and it is not always required. It is normally used as a quick indication of process

Table 34. Comparison of Microhardness Values and Resultant Variation (Courtesy Sulzer Metco and SUNY Stony Brook).

Sample Code

Nominal Composition

(wt%)

Starting Powder

Size (µm)

Manufacturing technique

DPH300g St. Dev. Min. Max.

A WC-12Co -45,+5 Sintered/ Crushed

1334 110 1139 1448

B WC-12Co -53, +11 Agglomerated/Sintered

1261 66 1176 1373

C WC-17Co -53, +11 Spray Dried 1068 37 1020 1124

D WC-17Co -53, +11 Spray Dried 1187 116 1032 1435

E WC-10Co-4Cr -45, +11 Sintered/ Crushed

1389 80 1292 1581

F WC-10Co-4Cr -53, +11 Agglomerated/Sintered

1209 75 1124 1340



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control and followed by microhardness measurement as a more accurate gauge of coating properties.

11.3.4. General Trend of Hardness Results The general HVOF hardness results vs. hard chrome indicate that higher values can be obtained with the HVOF alternatives. For each application the optimum hardness values should be balanced against the other properties of fatigue, resistance to cracking, and wear performance.

11.4. Adhesion

11.4.1. Documents Document 11. Tensile Bond Variance of Thermally Sprayed Coatings with Respect to Adhesive Type This article compares the results obtained for tensile bond testing using both liquid and film adhesives.

11.4.2. General Description and Test Methods The tensile strength of coatings is generally monitored by use of either buttons or tensile adaptors/loading fixture sprayed with the material in question. The typical tensile assembly used for determining strength is shown in Figure 40 from the ASTM C-633 specification on tensile testing.

Figure 40 shows a button as part of the assembly; if one fixture is sprayed the adaptors are bonded together with a single adhesive application. Currently, both liquid and film adhesive are being used for this purpose. The assemblies are cured at temperatures between 300-450 degrees F for 1-3 hours and then cooled before pulling. A sample with no coating is usually placed in a furnace run to verify proper epoxy curing. Epoxy only values should exceed 10,000 psi and normally pull in the 12,000 psi range. When the test is required, the material is sprayed with the part in question and a set of three samples is usually processed.

Document 11 summarizes the advantages and disadvantages of both epoxy types.

Figure 40. Tensile assembly from ASTM C-633.


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Test values can range from 100 psi for very soft abradable coatings to epoxy only failures at over 12,000 psi for HVOF materials. This is due to the limited strength value of the epoxy and is not a true test of the coating strength. Research is in progress to develop stronger epoxies or alternate testing methods for coating strength.

11.4.3. Tensile/Adhesion Results Tensile results for HVOF carbide/cermet coatings are rated as epoxy pulls because the strength of the HVOF materials exceeds the 12,500 psi limit of the epoxy. The true strength of the coatings is therefore not truly known. Research is underway by 3M to develop a higher strength epoxy but his work in the developmental stages.

11.4.4. General Trend of Tensile Results For properly-bonded HVOF coatings, the tensile results should exceed 12,500 psi, which is the upper limit of the testing epoxy. All failures should be in the epoxy rather than in the coating or at the substrate-coating interface. This indicates both good cohesion within the coating and excellent adhesion to the substrate material.

11.5. Residual Stress

11.5.1. Documents Document 26 Behaviour of Tungsten Carbide Thermal Spray Coatings 1995, J. Wigren et al.29 Study involving application of WC-Co to mid-span dampers on aircraft blades with fatigue, bend testing, and residual stress analysis.

Document 27 An ASM Recommended Practice for Modified Layer Removal Method (MLRM) to Evaluate Residual Stress in thermal Spray Coatings 2000, Ed Rybicki and ASM TSS Committee. 30 Method for determining residual stress where subsequent layers are removed and change in stress measured by strain gages.

Document 28 Properties of WC-Co Components Produced Using the HVOF Thermal Spray Process 2000, J. Stokes and L. Looney. 31 This paper reviews residual stress intensity as a function of spray distance and powder feed rate.

Document 29 X-ray diffraction residual stress techniques, P.S. Prevey. 32 Describes principles and theory of XRD methods for residual stress measurement. Includes a number of examples.

"Volvo WCCo Damper Application.p

"MLRM paper Ed Rybicki.pdf"

"Stokes - properties of HVOF WCCo Spray

"Prevey - XRD stress measurement.pdf"


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Document 30 Processing Effects on Residual Stress in Ni+5%Al Coatings-Comparison of Different Spraying Methods 2000, J.Matejicek et al. 33 The neutron diffraction method is used to calculate through-thickness residual stress values in a substrate thickness of .100� and a coating thickness of .080�.

11.5.2. General Description and Test Methods Residual stress in coatings is important for two reasons:

1. The stress state of the coating must be high enough to prevent spalling or delamination from occurring in service, due to thermal or mechanical changes to the system.

2. The stress state in the coating can have a direct effect on the residual stress state of the substrate, and thus on the final material properties, particularly fatigue.

Section 7.3.7 discusses the Almen Strip as the current quality control tool for rapid verification of the degree of compressive residual stress present in HVOF coatings. Mechanical property data indicates that compression in the coating is beneficial for fatigue. Techniques for residual stress measurement fall into two different categories: qualitative and quantitative. Qualitative techniques give a relative value of a specimen at a fixed set of parameters. Examples of these techniques are listed in Table 35. This helps to control the process but the values are relative to other evaluations that have established acceptance or rejection criteria via quantitative residual stress measurements. Industry procedures for qualitative techniques are currently being formulated. The measurements can be very technique sensitive. It is currently difficult to compare values from location to location for consistency although in-shop consistency is excellent when written guidelines for spraying and evaluation are established.

"Matejicek - Proc effson resid stress Ni5Al.


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Quantitative techniques are the same as substrate techniques with some allowance for the presence of an interface between coating and substrate. Some commons methods are listed in Table 36. The quantitative techniques provide absolute values of the residual stress in the material. These techniques are usually more time consuming and costly, but provide the background data that allows acceptance or rejection criteria to be established for the qualitative methods used in QC. As with qualitative techniques, industry procedures for quantitative techniques are currently being formulated. The measurements can be very technique sensitive and coating material properties are required for the calculations. It is currently difficult to compare values from laboratory to laboratory for consistency although in-lab consistency is excellent when written guidelines for evaluation are established. Work is in progress to establish consistent usage of established material properties. The need for compressive stresses in HVOF carbide coatings has been established but values will vary with coating material and thickness.

Table 35. Qualitative techniques for measuring residual stress. Name Description

Almen Thin strip measured for deflection before spraying, held in fixture during spraying, and then deflection/bowing of strip measured after spraying. Difference in deflection is indication of residual stress. Method most commonly used for HVOF QC.

In-process Almen Strip

Same principle as Almen above except deflections are measured during spraying by means of very sensitive deflectometers

Bend Testing Coupons are sprayed with a coating and then bent to a specified dimension. Number of cracks and lack of spalling correlated to quantitative residual stress values and accept/reject limits established


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11.5.3. Residual Stress Results

11.5.3.1. Almen strip The simplest method of stress measurement is the Almen strip, which is now frequently specified as a QC parameter, using Almen strips coated at the same time as the component. The Almen data gives a good indication of whether the stress is compressive or tensile, within the limitations we have already described. Almen measurements have been found to be a good predictor of fatigue life,34 since as a general rule fatigue is enhanced by compressively stressed coatings and reduced by coatings with tensile stress (such as chrome).

Table 36. Common quantitative residual stress measurement techniques. Name Description Modified Layer Removal Technique

Samples are polished to remove a given thickness of material and change in stress is measured by strain gages attached to specimens.

X Ray Diffraction

Strain in the crystal lattice is measured, and the residual stress producing the strain is calculated. Applicable to all crystalline materials (metallic or ceramic). Surface residual stress measurement is non-destructive. Subsurface measurements are obtained by removing material via electropolishing methods. Penetration of x-rays relatively shallow compared to neutron.

Neutron Diffraction Based on the same general principals as x-ray technique. The main difference is the size and location of the diffracting volume in the sample as a result of the lower absorption of neutrons. Subsurface measurements can be obtained non-destructively. Knowledge of the unstressed lattice spacing is required in order to calculate the residual stress from the measured strains.


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Stress tends to build up with coating thickness. This becomes very important when using thermal spray coatings for rebuild, because the stress can become so high in very thick coatings that it causes delamination, especially under high stress. In other situations, however, it appears that the stress is self-limiting. Figure 41 shows data for a series of HVOF WC-CoCr coatings deposited

under the same conditions but at varying thicknesses. Clearly, the total Almen number rises (the coating becomes more compressive) with thickness, but is limited and reaches a maximum by about a 0.007� thickness. When normalized to a 0.005� coating (i.e. divided by thickness/0.005�), the normalized number drops above 0.005� showing that the stress does not build up linearly with thickness. The reasons for these differences in stress behavior are unknown, but are doubtless related to process conditions and powder material. For this reason, coating specifications generally require the Almen stress to be within a specified range for a specific thickness, and one cannot simply make the assumption that doubling the thickness will double the stress.

11.5.3.2. Almen/Residual Stress Comparison Document 28 Properties of WC-Co Components Produced Using the HVOF Thermal Spray Process 2000, J. Stokes and L. Looney This paper presents a study of the residual stress and microstructural properties of thick spray formed components produced using HVOF thermal spray processing. It is generally an analysis of the dependence of residual stress and resultant material characteristics on spraying distance and powder feed rate conditions. This investigation takes the quality control tool of Almen strips, and using the equation below calculates actual residual stress values.

Residual stress, σσσσ = ECY/R

0

1

2

3

4

5

6

7

8

2.3 4.5 5.9 7.1 9.5Total thickness

Alm

en Total AlmenNormalized Almen

Figure 41. Stress as a function of coating thickness for HVOF WC-CoCr.35



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where E C is coating modulus of elasticity, Y is Neutral axis of composite strip, R is the bending radius.

This is a method to estimate residual stress magnitude. It tends to underestimate the actual stress in a component due to factors such as heat sink, thickness, etc.

Residual stress is closely related to spraying conditions – especially spray distance and powder feed rate. Figure 43 and Figure 44 show the relationships between residual stress, spray distance, and powder feed rates. The zoning of the plots is an excellent representation of how the HVOF process produces varying results as the parameters are changed within a range. The available thermal energy must be used efficiently to melt the particles but a portion of that energy is also transferred to residual stresses in the final deposit.

Figure 42. Almen strip stress measurement.

Figure 43. Average residual stress as a function of spray distance.



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The following information can be ascertained from the data:

• Almen Strips can be used to obtain a quantifiable indication of the numerical magnitude of residual stress

• For spray distance, there is a target range where the particle temperatures result in both good coating microstructure and acceptable residual stress.

• For powder feed rate, there is a target range where the amount of material being deposited results in both good coating

Figure 44. Average residual stress as a function of powder feed rate.

Table 37. Zone analysis of thermal spray coatings. Spray distance analysis Powder feed rate analysis

Zone 1 The coating is too hot because there is little time for cooling in such a small spray distance and you would expect substantial residual stress which is not acceptable. Structure is acceptable

Zone A The feed rate in this range in too low to produce particle cohesion and the resulting microstructure is unacceptable. With a porous deposit, residual stress is also low.

Zone 2 An acceptable combination of coating microstructure and reasonable residual stress.

Zone B An acceptable combination of coating microstucture and reasonable residual stress.

Zone 3 The residual stress drops due to the reduced thermal energy from the longer spray distance and the microstructure degrades the particles lack sufficient energy for spalt formation.

Zone C The feed rate in this range cannot melt all the particles but a substantial amount is till deposited resulting in high residual stress.


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material being deposited results in both good coating microstructure and acceptable residual stress.

• The input of thermal energy and powder material must be balanced to provide enough heat for melting the particles and still result in acceptable residual stress upon cooling.

11.5.3.3. Modified Layer Removal Technique Document 26 Behaviour of Tungsten Carbide Thermal Spray Coatings 1995, J. Wigren et al. Document 27 An ASM Recommended Practice for Modified Layer Removal Method (MLRM) to Evaluate Residual Stress in thermal Spray Coatings 2000, Ed Rybicki and ASM TSS Committee Document 26 evaluates WC-Co, WC-Ni, and WC-CoCr (Table 38) sprayed with the HVOF process at a variety of different spray parameter settings. A wear problem was identified on engine fan blades and the goal of the evaluation was to balance compressive residual stress in the coating to prevent spalling in service with a minimal degradation of substrate fatigue life. The following tests were made:

• Three point bend testing for crack resistance

• Residual stress measured by Modified Layer Removal Technique

• Low cycle fatigue testing

Table 38. WC coating system designations for Document 24. Coating composition

Application process

HVOF Plasma

A B C D E F G H

1. (WC-Co)

2. (WC-Ni)

3. (WC-Co/Cr)

●

●

● ● ●

●

●

●

● ● ●



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Residual stress and bend test results will be discussed here. Fatigue results from this evaluation are summarized in Section 12.6.3.5 in Coating Performance.

Figure 45 shows the bend test information. This must be compared with the residual stress results shown in Figure 46 and obtained with the Modified Layer Removal Technique summarized in (Document 27).The average residual stress in the substrate represents locations close to the coating/substrate interface. As expected, the ranking in the bend test shows F-1, A-1, and G-1 as most resistant to cracking due to the compressive stresses as measured in the coating. This follows the trend in all work thus far towards compressive stresses. Coating A-1 was chosen for implementation based upon these results and fatigue data. Coating F-1 showed spallation in engine testing due to the higher compressive stresses as compared to A-1.

Coating system

Ranking Coating system

Ranking

F-1 1 D-1 7

A-1 2 D-2 8

G-1 3 H-1 9

C-1 4 E-3 10

B-1 5 E-1 11

A-2 6

Figure 45. Bend test technique, evaluation criteria, results.



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The residual stress and its distribution as a function of depth through the coating and into the substrate are shown in Figure 46. Compressive stresses are negative, and tensile stresses are positive. Note that if the average stress in the coating is compressive, the average stress in

the substrate is negative to balance out the overall stress.

Note that this stress measurement technique is quite accurate, and yields the stress variations with depth. As a QC method it would be far too time-consuming and expensive. For this reason, QC relies on the average stress, as measured by the almen strip method.

11.5.3.4. Residual Stress by X-ray Diffraction

Document 31 Residual Stress Measurement in Plasma Sprayed Coatings by X-Ray Diffraction (Courtesy of J. Matejicek et al) 1997. 36

Document 29 X-ray diffraction residual stress techniques, P.S. Prevey.

X Ray diffraction (XRD) is one of the most common quantitative methods used to determine residual stresses in coatings and many other metallic materials. Features of the method are:

• Ability to measure residual stress in a very thin surface layer (i.e. a few splat thicknesses), thus allowing assumptions of plane stress

• Phase distinctive – can determine stress in specific phases, rather than only in the material as a whole

• A through-thickness stress distribution can be measured by electropolishing a small area together with continuous XRD measurements

o This method can be somewhat non-destructive if the

Figure 46. Typical stress profile for modified layer removal technique.

"Matejicek - Residual stress in plasma spray.pdf"


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electropolishing can be performed in a non-critical area. The referenced documents contain data on XRD methodology and analysis. To date, the majority of XRD information on coatings has been obtained using the surface only method (Document 31) in lieu of through thickness analysis. Work is in progress to better understand XRD analysis and will be available on the HCAT Home Page by December 2000. Document 31 is a summary of the XRD method for near surface residual stress in Ni, NiCrAlY, and YSZ coatings. The effects of surface finish and material anisotropy are discussed. A very critical detail of this summary is the comparison of data from both neutron diffraction and blind hole techniques to XRD for Ni coatings as shown in Table 39. Although the comparison of methods shows qualitative agreement, the magnitude of the values is substantially different. Unfortunately, this is a result of the many assumptions required in determination for each method and the lack of standardization and established material constituents for coating materials. This is an important area of quality assurance and is being addressed by industry research groups.

11.5.3.5. Residual Stress by Neutron Diffraction Document 30 Processing Effects on Residual Stress in Ni+5%Al Coatings-Comparison of Different Spraying Methods 2000, J.Matejicek This study uses neutron diffraction to determine the residual stress patterns on thin specimens sprayed with Ni+5Al% coatings. Three different spray methods with significantly different particle temperatures and velocities were used to apply the coatings. The experimental set-up is listed in Table 40. The neutron diffraction method, like X-ray diffraction, is based upon the accurate measurement of changes in the spacing of different crystal planes (i.e. strain) based on shifts in positions of diffraction peaks. From the strain, the stress can

Table 39. Comparison of Residual Stress by Varied Techniques. Specimen Ni-APS Ni-VPS

Method σxx (MPa)

σxx (MPa)

X-ray diffraction

62 -116

Hole drilling 241 -55

Neutron diffraction

186 -37

Table 40. Experimental set-up for neutron diffraction. Spray equipment Sample HVOF 0.1�X1�X2�

Air plasma spray Ni + 5wt% Al alloy

Wire arc spray .080� thick




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be calculated with the use of appropriate elastic constants (which can sometimes be difficult to determine).

Figure 47, Figure 48, and Figure 49 summarize the data for the three spray processes, and show the following:

• In air plasma spray, the overall stress is slightly tensile with a gradient towards more tension near the deposit surface as a result of successive buildup of deposit layers with tensile quenching stresses. The substrate stress is therefore slightly compressive.

• In wire arc spray, the stress gradient is somewhat higher, while in the substrate, it is negligible. This difference between air plasma and wire arc may be caused by the difference in deposit density which will obviously contribute to final coating residual stress. Higher porosity will allow for stress relaxation which may be occurring in the wire arc deposit.

• In HVOF, the stress is mainly compressive as a result of the peening action of the high velocity particles, whose impact plastically deforms the previously-deposited layers. This compressive coating stress causes a tensile residual stress in the substrate to balance stresses. These stress gradients and levels can be varied with different spray parameters.

The important concept is the differentiation between the cause of the residual stress: quenching stresses in air plasma vs. a peening action in HVOF. This supports the use of HVOF processes with the compressive residual stresses needed to enhance coating performance characteristics.

Figure 47. Air Plasma Spray residual stress pattern.



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11.5.4. General Trend of Residual Stress Results The general trend for HVOF coatings is to have compressive stress in the coating, which even though it results in tensile stress in the substrate, has the overall effect of minimizing or eliminating the fatigue debit. However, simply maximizing the coating stress is not a good approach, as this reduces coating fracture strength and can even lead to coating delamination if compressive stresses become too high. Compressive stress measured by Almen strips is a simple quality control that gives an indication of the residual stress pattern, but the actual stress in the coating on the part will be different due to size, heat sink, etc. Work is in progress to develop a better correlation between Almen strips and actual residual stress coating/substrate values.

Work to date has shown that Almen strip stress measurements are highly dependent on the temperature reached by the Almen strip, which in turn depends on the way the strip is held, and the speed and direction in which it is sprayed. When using Almen strips these factors must be built in to the test procedure to ensure consistency.

Figure 48. Wire Arc Spray residual stress pattern.

Figure 49. HVOF Spray residual stress pattern.


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12.12.12.12. Coating performance

12.1. Summary

Coating performance measures how the coated substrate performs in laboratory tests. Performance depends on coating properties, substrate

Table 41. Summary of performance tests. Measurement Typical test Notes Corrosion

Landing gear steels B117 Lufthansa

4340, PH13-8Mo B117 HCAT

4340, PH-13-8Mo, 7075Al GM 9540P/B

HCAT

Atmospheric 4340, PH-13-8Mo, 7075Al

Atmospheric beach exposure

HCAT

HVOF WC cermets vs stainless steel

Electro-chemical

NRC

Fatigue Air, corrosive, various temperatures

4340 high strength steel, PH13-8Mo stainless, 7075 Al

ASTM E466-96

HCAT general

300M, 4340, Aermet 100 ASTM E466-96

HCAT landing gear

WC-Co, WC-CoCr, WC-Ni Fan blade midspan evaluation, Volvo

Wear Abrasion, erosion, sliding, fretting

Erosion Various guns, coatings

Abrasion

Sliding, fretting

Impact

Hydrogen embrittlement ASTM F-519

Lufthansa results and HCAT planned tests

Creep GE Aircraft Engines data


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properties, and how the coating and substrate interact. Much, but not all, of this data is derived from a number of test protocols carried out by the HCAT/C-HCAT cooperative effort. The performance data covered in this section are listed in Table 41. Go directly to the relevant section by clicking on the blue links. The majority of the data is a direct comparison between hard chrome and the HVOF alternatives. Most, if not all, performance data to date show that the HVOF carbides have performance equal or superior to hard chrome plating. Some of the testing listed is currently in progress and should be available on the HCAT Home Page in the next 6-12 months.

12.2. General Background Chrome has been used for many years in a variety of environments with success, and coating performance must either meet or exceed current design allowables. A substantial amount of work is currently being performed in the industrial sector to evaluate coating performance, but the most extensive and detailed efforts are being conducted by the Hard Chrome Alternatives Team (HCAT) and Canadian Hard Chrome Alternative Team (C-HCAT). The HCAT effort has been in place since 1995 to demonstrate and validate the replacement of hard chrome plating with HVOF coatings. The Canadian counterpart began work in 1999. As mentioned in Section 1.2, the HVOF carbides (WC-Co and WC-CoCr) have been identified as primary replacements for chrome plating, and extensive test protocols have either been carried out or are in process, as will be highlighted in this section.

12.3. Documents Document 32 HCAT Test Protocol for Initial Work 1996 (Courtesy of HCAT Team) Summarizes the testing protocol initially developed to validate HVOF as a viable chrome alternative.

Document 33 Joint Test Protocol (JTP) for Landing Gear 1998 (Courtesy of HCAT and CHCAT Teams) Protocol developed for validation of WC-Co and WC-CoCr as chrome replacements for landing gear applications.

Document 34 Joint Test Protocol (JTP) for Propeller Hub Components 2000 (Courtesy of HCAT, JG-PP, and C-HCAT Teams) Protocol developed for validation of HVOF coatings as chrome replacements for propeller hub applications

"HCAT Test Plan - original.pdf"

"HCAT JTPPart I Landing Gear.pdf"

"HCAT JTP Propeller hubs.pdf"



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Document 35 Joint Test Protocol (JTP) for Gas Turbine Engines 2000 (Courtesy of HCAT and PEWG Teams) Protocol developed for validation of HVOF coatings as chrome replacements for gas turbine applications � this protocol is under development at time of writing.

12.4. Test Protocol Summaries

12.4.1. Start-up test Protocol Document 32 HCAT Test Protocol for Initial Work 1996 (Courtesy of HCAT Team) Initial work was planned in 1996 to provide a generic evaluation of possible HVOF thermal spray alternatives. The emphasis was placed upon implementation of the chrome alternative on components undergoing repair at military depots. As a generic plan, the purpose was to evaluate a broad range of materials to assess the potential of HVOF coatings. This work centered around two coating types:

1. WC-17Co. The most commonly used HVOF coating is WC-Co (either WC-17%Co or WC-12%Co. Of these two materials the easiest to spray is the WC-17Co. The WC-12Co is somewhat harder, but is more brittle and more difficult to spray. Given that both materials are superior to chrome in wear resistance, WC-17Co was chosen.

2. Tribaloy1. Tribaloy 400 and 800 are wear-resistant Co-based alloys frequently used in engine and other wear applications. Their compositions are given in Table 42.

Three alloys were chosen: 1. 4340 steel. 4340 is the most

common steel used for hydraulics. It is very similar to 4340(M) and 300M, the most common landing gear steels. For landing gear the 4340 would be heat treated to 260-280 ksi.

2. PH13-8-Mo steel. This is a common precipitation-hardened alloy frequently used for helicopter components.

3. 7075-T73 aluminum. This alloy is commonly used for landing gear and other large components

Performance of the coating on component alloys was evaluated in the following tests:

• Friction and wear 1 Tribaloy is a trade name of Stellite Corp.

"PEWG GTE JTP.pdf"

Table 42. Chemistry of Tribaloys. Co Mo Cr SiT400 62 28 8 2

T800 52 28 17 3


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• Fretting

• Fatigue

• Corrosion Data from this work is summarized below under the appropriate heading of this section. Status � This work is complete.

12.4.2. JTP for Landing Gear Document 33 Joint Test Protocol (JTP) for Landing Gear 1998 (Courtesy of HCAT and CHCAT Teams) After the initial protocol work validated the potential of HVOF coatings, a plan was formulated to address specific components for full scale validation. This joint effort between HCAT and C-HCAT is currently in progress. Two coatings have been chosen for evaluation:

U.S. HCAT: WC/Co (83%/17%) HVOF coatings Canadian HCAT: WC/CoCr (86%/10%4%) HVOF coatings

Three substrate materials are being evaluated to cover the wide range of landing gear applications, as shown in Table 43. Performance tests being conducted are:

• Fatigue

• Corrosion

• Wear

• Impact

• Hydrogen Embrittlement Data currently available from this work are summarized under the appropriate heading of this section (below). Status � Close to completion. This work should be complete by the end of 2000.

12.4.3. Other Protocols There are other types of components for which coating performance protocols are currently being formulated.

• Propeller Hubs: Document 34 Joint Test Protocol (JTP) for Propeller Hub Components 2000 (Courtesy of HCAT, JG-PP, and C-HCAT Teams)

• Gas Turbine Engines: Document 35 Joint Test Protocol (JTP) for Gas Turbine Engines 2000 (Courtesy of HCAT and PEWG Teams).

Table 43. Materials and heat treats for HCAT Landing Gear JTP. Material Heat Treat (tensile

strength) 4340 260-280 ksi

300M 280-300 ksi

Aermet 100 280-290 ksi


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• Hydraulic actuators � in process of formulation.

• Rotary wing (helicopter rotor head) components � in process of formulation..

Progress and current copies can be obtained in the Joint Test Protocols section of the HCAT Home Page.

12.5. Corrosion

12.5.1. Documents Document 36. Report of Replacement of Chromium Electroplating Using HVOF Thermal Spray Coatings AESF Plating Forum 1998 (Courtesy of Bruce Sartwell and HCAT Team). 37 Report details the results of property and corrosion testing from protocol referenced in Document 30.

Document 37. Replacement of Chrome Plating by Thermal Spray – Results of Corrosion Testing of HVOF Coatings 1996 (Courtesy of Lufthansa). 38 A summary is given of the ASTM B117 results which show superior performance of HVOF coatings vs. current hard chrome plating solutions. (Note: this is a very large document.)

Document 38. Replacement of chrome plating by thermal spray coatings – Summary of tests (Courtesy of Lufthansa). 39 This document summarizes the results of the testing used to qualify HVOF WC-CoCr for flight testing, as well as the results of initial flight tests.

Document 39 Performance of HVOF Sprayed Carbide Coatings in Aqueous Corrosive Environments 2000 (Courtesy of S. Simard (NRC) et al). 40 This paper summarizes corrosion studies performed using anodic polarization (Tafel plot) methodology to assess corrosion resistance of carbide coatings

"AESF Plating Forum Mar1998.pdf"

"Lufthansa Corrosion testing.pdf"

"Lufthansa testing.pdf"

"Simard - Performance of HVOF

http://www.hcat.org/bscw/bscw.cgi/0/35016



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12.5.2. Corrosion Test Methods Corrosion testing can be performed by many different techniques dependent upon the corrosion environment of the application being evaluated. Table 44 highlights some of the most common test methods.

12.5.2.1. Atmospheric Methodology The atmospheric corrosion tests generally follow ASTM G33 entitled, �Standard Practice for Recording Data from Atmospheric Corrosion Tests of Metallic-Coated Steel Specimens.� The size of the test panels is typically 6 inches by 4 inches by 3/16-inches in thickness and reasonable statistics require at least five panels of each base alloy with each coating. In order to evaluate through-thickness corrosion, which would occur if the coating is scratched through, the coating is scribed with an �X�, which penetrates through the coating to reveal bare surface. The atmospheric tests are typically run for at least 4000 hours, during which the panels are checked and data recorded at appropriate intervals. Two types of atmospheric corrosion tests may be performed.

1. The panels are mounted on racks which are exposed to sunlight and the marine air, but which are not subject to splashing with salt water.

2. Similar mounting and exposure except that the panels are subjected to periodic splashing with salt water (usually done manually for consistency).

12.5.2.2. Simulated Cabinet Testing Cabinet testing of this type is normally conducted in a salt spray chamber such as Q-Fog Model CCT600 or equivalent at ambient temperature.

Table 44 Common Corrosion Testing Methods Type Description Atmospheric Expose to atmosphere in environments such as

marine air and salt spray with recording per ASTM G 33-88

Simulated Chamber Testing Continual Salt Spray Exposure to ASTM B-117 Intermittent and Cyclic Salt Spray Exposure to GM 9540P/B Continual Salt Spray/SO2 Exposure to ASTM B-117

Electrochemical Potential Determination

Varied Methods such as Tafel Plot


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Salt Fog Corrosion Test The standard salt fog test follows ASTM B117:

(1) Position of the panels during exposure � The specimens are placed into a salt spray/fog chamber so that they are supported or suspended between 15 and 30° from vertical. The specimens are exposed to salt spray/fog continuously for 1,000 hours.

(2) Preparation of the salt solution � A typical test involves a 5 percent sodium chloride salt solution (5 ± 1 parts by weight of NaCl in 95 parts of water). The pH of this solution should be between 6.5 and 7.2.

(3) Specimen instpection � Visual inspection of the specimens for surface corrosion can occur at any interval, but common periods are after 48 hours, 96 hours, and every 100 hours between 200 and 1000 hours. It is customary to remove the samples at 500 and 1000 hours for photographing.

GM Cyclic Corrosion Test This test is conducted in accordance with the General Motors GM9540P/B protocol, which is indicated in Table 45. This test is intended to be less severe than B117, but to be similar to the types of exposures characteristic of automotive components.


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SO2 Salt Fog Test This test is conducted in accordance with ASTM G85-85. It is very similar to ASTM B-117 as summarizes earlier with the following exceptions: SO2 gas is introduced for one hour every 6 hours All other parameters with respect to inspection intervals, salt spray, etc. are identical. This test is frequently used to simulate exposure on aircraft carrier decks, where aircraft are exposed to NaCl spray and sulfur from jet exhaust.

Table 45 GM9540 Protocol for Corrosion testing.

GM9540P/B Cyclic Corrosion Tests

Solution: 0.9% NaCl, 0.1% CaCl2 and 0.25% NaHCO3

pH: 6.0 - 8.0

Test protocol: Step 1 Subcycle step 2-3 repeat 4

times

Step 2 Salt mist 25 C 15 min Step 3 Dry-off 25 C 75 min Step 4 Dry-off 25 C 120 min Step 5 RH 95-

100% 49 C 8 hours

Step 6 Dry-off 60 C 7 hours Step 7 Dry-off 25 C 1 hour Step 8 Final step, go to step 1 Note: RH = relative humidity

Test duration: 2000 hrs Examined every 125 hrs

At each 500 hours interval, the specimens can be removed and photographed.


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Specimens For cabinet testing, either flat or cylindrical specimens can be used. Groups of five are usually run and one specimen in each group will have an �X� scribed in the coated surface. The �X� should penetrate through the coating to evaluate through-thickness corrosion. The smaller angle of the �X� should be 30 to 45 degrees. Each line of the �X� should be approximately 2 inches long. On the uncoated edges and back of each specimen, an inert wax or epoxy is applied to ensure no galvanic couple between uncoated and coated areas. Based on visual inspection, a ranking is applied to each specimen at each interval of inspection and these rankings are tabulated and displayed graphically. The rankings are assigned in accordance with ASTM B537-70 as outlined in Table 46. Electrochemical Potential Electrochemical testing usually involves performing anodic polarization tests in electrochemical cells. Anodic polarization curves or Tafel plots provide information on

1. the relative rates of corrosion, 2. the type of corrosion (localized

versus general) occurring, and 3. the presence of connected

porosity in coatings (pores that provide a path from the electrolyte to the underlying substrate).

Anodic polarization curves can be obtained as described in ASTM G5. The electrolyte can be varied dependent upon the environment being encountered. For work on replacing hard chrome, an electrolyte of sodium chloride solution is a good choice because this represents many environments encountered by DoD weapons systems. A typical test might be a standard one compartment corrosion cell. The samples are typically immersed in a quiescent (open to the atmosphere without air sparging) chloride solution and allowed to stabilize in the solution for 1 hour prior to polarization. Ramping and sweep rate can vary but an example might be a potentiodynamic ramp from the open circuit potential to +1.6 volts at a sweep rate of 0.6 volts per hour. A saturated calomel electrode is usually used as the reference electrode.

Table 46. Visual ranking criteria (ASTM B537-70).

Defect area (%) Rank #

0 10 0 - 0.1 9

0.1 - 0.25 8 0.2 - 0.5 7 0.3 - 1.0 6 1.0 - 2.5 5 2.5 - 5.0 4 5 - 10 3 10 - 25 2 25 - 50 1

>50 0


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Anodic polarization curves should be obtained for the uncoated and coated samples for all coating/substrate combinations, with hard chrome coated samples as a control. Duplicate experiments for each sample type are a good experimental practice. The information obtained from the polarization curves can then be used to compare the protective nature of the coatings to that of hard chrome.

12.5.3. Corrosion Data

12.5.3.1. Simulated Cabinet Results from Lufthansa Document 37. Replacement of Chrome Plating by Thermal Spray – Results of Corrosion Testing of HVOF Coatings 1996 (Courtesy of Lufthansa) This report is a very extensive evaluation of the corrosion performance of thermal spray coatings compared with chrome plate. It summarizes in detail comparative salt spray corrosion tests according to ASTM B117 which were performed on different HVOF coatings and different chrome plates as shown in Table 47. The results of these tests were used by Lufthansa as the basis for their choice of HVOF WC-CoCr as a replacement for chrome plating on landing gear.


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The results of the testing are shown in Table 48. The investigation showed that several HVOF sprayed WC-CoCr coatings provide much higher corrosion resistance than chrome plate, which failed after three days of corrosion test exposure. In fact, some WC-Co and all WC-CoCr HVOF coatings survived 1000 hours of salt spray testing with no sign of corrosion either of the coating or the base metal. In all cases the HVOF WC-CoCr coatings performed the best. Cr-Ni-B-Si also performed very well, and were easy to spray, making them attractive alternatives. However, the lack of a chemical strip for these coatings led Lufthansa to reject them. WC-Co and WC-Ni showed some corrosion of the metal matrix, with surface discoloration and roughening. WC-CoCr of WC-CoNi showed no matrix corrosion. The use of an epoxy sealer did not change the corrosion of any of the HVOF coatings. Since HVOF WC-CoCr showed the best corrosion results, Lufhansa adopted this material for flight tests.

Table 47. Coatings tested (Lufthansa). Note: 25µm+0.001” Plating Specification Thickness [µm]

After grinding

Chrome QQ-C-320B 100 & 200

Chrome Platings Tested

� HEEF 25 100

Coating Spraying Equipment Thickness [µm]

After grinding

WC-Co

�

�

DJ 2600

JP-5000

Jet Kote II

100 & 200

100 & 200

100

WC-Ni JP-5000 200

WC-Co-Cr

�

JP-5000

Jet-Kote II

100 & 200

100

Cr3C2-NiCr

�

JP-5000

Jet-Kote II

100 & 200

100

HVOF-Coatings Tested

Cr-Ni-B-Si JP-5000 200

WC-Co Super-D-Gun 100

WC-Co-Cr Super-D-Gun 100

“Proprietary

Coatings”

Tested WC-Cr-Ni Super-D-Gun 100



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Interpretation of results The analysis showed the WC-Co or WC-Ni based coatings exhibited corrosion of the Co or Ni matrix materials. In contrast, the WC-NiCr and WC-CoCr had excellent matrix corrosion resistance. WC-CoCr showed no attack on the substrate metal after extended exposures of up to 1,000 hours. WC-CoCr was taken forward to flight testing since it provided the highest level of protection on ASTM B117 salt spray tests, with far higher performance than chrome plate.

Table 48. Summary of corrosion ratings for coatings tested by Lufthansa.

All Chrome Platings

HVOF WC-Co

HVOF WC-Co-Cr

HVOF Cr3C2-NiCr

SDG 2040G (WC-Co)

SDG 2020 (WC-Co-Cr)

SDG 2005 (WC-Cr-Ni)

All Chrome Platings

HVOF WC-Co (DJ2600[2])

HVOF WC-Co (JP-5000[3])

HVOF WC-Ni

HVOF WC-Co-Cr

HVOF Cr-Ni-B-Si

HVOF Cr3C2-NiCr[4]

[1] Please note that this graphic shows an “average” of days to occurrence of corrosion with regard to the coating type. Of each coating type, at least

four specimens had been tested which generally did not fail at the same testing time. For exact data, please refer to Figure 17 (100ìm thick coatings)

and figure 18 (200ìm thick coatings).

[2] This WC-Co coating (Diamalloy 2005, DJ2600 sprayed at LHT) passed the test, whereas all other WC-CO coatings failed.

[3] In contrast to [2], the WC-Co coating applied by the JP-5000 using powder AWN 3073 failed after approx. one week due to base material corrosion.

[4] This coating was incorrectly sprayed, i.e. too much powder was fed to the gun, resulting in a poor microstructure.

Please note that “thinner” coatings of the same chemistry revealed a distinct better corrosion performance.


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12.5.3.2. Cabinet and Atmospheric Testing - HCAT Document 36. Report of Replacement of Chromium Electroplating Using HVOF Thermal Spray Coatings AESF Plating Forum 1998 (Courtesy of Bruce Sartwell and HCAT Team) This report summarizes simulated cabinet work which included ASTM B117 salt spray and GM 9450P/B cyclic work plus atmospheric exposure studies carried out under the initial testing protocol (Document 32). Substrates and coatings are listed in Table 49. All coatings were nominally 0.004� (100µm) thick. Samples for the cabinet work were evaluated at 125 hour intervals.

12.5.3.2.1. ASTM B117 Salt Fog Testing Data from the 4340 substrate in ASTM B117 Salt Fog Testing is shown in Figure 50 and Table 51. The ASTM standard requires that the surface be measured and ranked without cleaning. The Appearance Ranking was therefore determined by observation of the surface on taking the samples from the chamber. However, cleaning of the surfaces permitted better evaluation of the degree of actual damage to the coating � including blistering and undercutting. The Protection Rating was therefore measured after the sample surface had been cleaned and the blisters and undercut portions of the coating removed. The Protection Rating was measured for the coating surface and edges separately. The data indicates that WC-Co actually performs better than hard chrome in a side by side comparison. Although the T-400 showed a better appearance rating, the surface was found to be severely blistered with over 50% of the surface affected. Clearly, using the ASTM standard test method, there was significant deterioration of the surface in all cases, with the WC-Co performing somewhat better than the hard chrome, and the T400 performing the best. However, the Protection Rating showed that the T400 provided less overall protection, and was significantly worse than chrome. The WC-Co, on the other hand, was comparable to, or a little better than, chrome. For stainless steel corrosion was significantly less, as one would expect. The WC-Co performed at the same level as the chrome, but the T400 was measurably worse. The situation for 7075 Al was significantly different, because chrome plating on aluminum requires a double-zincate process and copper and nickel strikes for adhesion, which appeared to protect the surface from corrosion better than chrome alone.

Table 49. Coatings and substrates - HCAT corrosion testing. Coatings Substrates Chrome 4340

WC-Co 7075 Al

T-400 PH 13-8 Stainless


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0

2

4

6

8

10

0

2

4

6

8

10

0 250 500 750 1000 1250

4340 Steel SubstrateASTM B117

T400 4340

WC-Co 4340

Cr 4340

Ran

king

WC-Co

T400

Hours

Chrome

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0 250 500 750 1000 1250

13-8 SS SubstrateASTM B117

Ran

king T400

WC-Co

Hours

Chrome

0

2

4

6

8

10

0

2

4

6

8

10

T400-Al

WC-Co-AlCr-Al

0 250 500 750 1000 1250

Ran

king

Aluminum Substrate

Chrome

Hours

T400

WC-Co

ASTM B117

Figure 50. B117 Appearance Rankings for coatings on 4340 high strength steel, PH13-8Mo stainless steel, and 7075 Al.

0

2

4

6

8

10

0 500 1000 1500 2000 2500

4340 Steel SubstrateGM9540P/B

T400CrWC-CoA

ppea

ranc

e R

anki

ng

Hours

T400

Cr

WC-Co

0

2

4

6

8

10

0 500 1000 1500 2000 2500

13-8 Stainless Steel SubstrateGM9540P/B

T400WC-CoCr

App

eara

nce

Ran

king

Hours

Cr

T400

WC-Co

0

2

4

6

8

10

0 500 1000 1500 2000 2500

Aluminum SubstrateGM9540P/B

T400WC-CoCr

App

eara

nce

Ran

king

Hours

Cr

WC-Co

T400

Figure 51. GM9540P/B Appearance Rankings for coatings on 4340 high strength steel, PH13-8Mo stainless steel, and 7075 Al.


Page 113


Table 50. Corrosion of 4340 steel with HVOF and Cr coatings - appearance and protection rankings. Coating Appearance

Ranking Protection Rating Face

Protection Rating Edge

4340 steel T400 5.0 1.6 1.0

WC/Co 4.0 3.4 3.2

Hard Cr 1.6 3.2 2.0

7575 Aluminum T400 5.8 9.0 3.0

WC/Co 4.8 10 10

Hard Cr 9.8 10 10

Table 51. GM9540P/B corrosion of 4340 steel with HVOF and Cr coatings - appearance and protection rankings. Coating Appearance

Ranking Protection Rating Face

Protection Rating Edge

4340 steel T400 8.0 9.6 2.4

WC/Co 8.0 10 8.8

Hard Cr 6.8 9.8 1.0

PH13-8Mo T400 9.0 9.6 9.8

WC/Co 8.0 10 10

Hard Cr 10 10 10

7575 Aluminum T400 7.5 9.2 1.8

WC/Co 7.6 10 1.6

Hard Cr 10 10 10


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12.5.3.2.2. GM 9540P/B Testing Data from the 4340 substrate in GM 9540P/B Testing is shown below in Figure 51 and Table 51. As for the B117 test, the samples were evaluated both for their appearance ranking and for the protection rating on coating surfaces and edges.

12.5.3.2.3. Atmospheric Testing Atmospheric work is currently still in progress at the writing of this report. In a first set of tests, the performance of the HVOF coatings was significantly better than chrome plating, as illustrated in Figure 52 and Figure 53. Additional testing is ongoing that incorporates WC-Co and WC-CoCr samples, and a final report will be posted on the HCAT Home Page as the corrosion cycles are completed.

Figure 52. 4340 steel 18-month beach exposure tests, with and without scribing.




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12.5.3.2.4. Interpretation of results In most corrosion evaluations, the HVOF coatings show equal, and in most cases superior, performance to hard chrome. Clearly, there is a difference in relative results between cabinet testing and beach exposure testing. In cabinet testing, the HVOF coatings appear to be in some cases a little worse and other cases a little better than chrome. The beach exposure tests show the largest variation between thermal spray and chrome plating, with the thermal spray coatings showing significantly less corrosion and no undercutting or blistering of the coating. The excellent performance of chrome plated aluminum apparently stems from the highly corrosion-resistant Ni strike used for chrome adhesion, rather than from the chrome plate. The fact that there is any substrate corrosion at all for the thermal sprays shows that there is some through-porosity, which permits penetration of liquid to the metal surface.

12.5.3.3. Electrochemical Testing of Carbide Coatings Document 39 Performance of HVOF Sprayed Carbide Coatings in Aqueous Corrosive Environments 2000 (Courtesy of S. Simard (NRC) et al.

Figure 53. 7075 Al 18-month beach exposure samples with and without scribing.


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This paper addresses the performance of various carbide coatings HVOF coatings (Table 52) in terms of corrosion rate and degradation mode in two corrosion environments � HCl and HNO3. Electrochemical tests allow a more precise evaluation of the corrosion resistance of a material in a variety of environments and concentrations as compared to the simulated cabinet tests. Corrosion potential can be measured by taking electrical current measurements when an amount of acid or electrolyte is in contact with the material being evaluated for corrosion. In this situation, 316 stainless steel, considered a very corrosion resistant material, was used as the comparative baseline. An example of the results for HCl exposure is shown in Figure 54. The graph has been corrected for metallic fraction (normalize % composition) so a more relevant comparison can be made between the materials. As expected, the materials containing the corrosion resistant materials like Cr, Ni, and varied alloy mixes show excellent corrosion results. The results of alloy H were surprising as compared to the overall ratings and further investigation revealed a higher porosity level in this coating as compared to the other materials. This allowed material to penetrate in to the coating and accelerate corrosion. Control of porosity is therefore critical to optimum corrosion resistance.

Table 52. HVOF coatings used for Comparison of Electrochemical Corrosion Potential.

Coating HVOF coating material A Wrought Stainless Steel (bulk)

B Stainless Steel Coating

C WC-12Co

D WC-10-Co-4 Cr

E WC-12Co + 25% Ni Superalloy

F WC-12Co + 35% (Cr3C2/NiCr)

G WC-17Ni

H WC-20Cr3C2-7Ni



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12.5.3.3.1. Interpretation of results Electrochemical rankings of the materials the presence of Cr or a combination of Ni/Cr greatly enhances the performance of HVOF carbide /cermets in HCl/ HNO3 environments. Porosity of the coatings must be controlled to prevent penetration of the corrosive media into the coatings which can accelerate corrosive attack.

12.5.3.4. Corrosion Work Planned in JTP Landing Gear Document 33 Joint Test Protocol (JTP) for Landing Gear 1998 (Courtesy of HCAT and CHCAT Teams)

The corrosion work in the Landing Gear Joint Test Protocol is currently in progress with estimated completion by December 2000. The matrix includes a variety of simulated cabinet testing with both cylindrical and flat

Figure 54. Corrosion Current for an Aerated 0.1 N HCl Solution.40

Table 53. HCAT/C-HCAT corrosion test matrix for landing gear steels and coatings. Tests Alloys Coatings Thickness

(inch) Notes

B117

GM9540P/B

B117+ SO2

4340

300M

Aermet 100

Uncoated

Hard chrome

HVOF WC-Co

HVOF WC-CoCr

0.003

0.010

Cr with and without Ni strike

HVOF with and without sealant

With and without shot peen


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specimens evaluating WC-Co (HCAT) and WC-CoCr (C-HCAT) on a number of substrates. Detailed test matrices are incorporated in the test plan, and include the materials and conditions summarized in Table 53. This work is being performed to validate the excellent performance of the carbides/cermets in the earlier HCAT protocol and further evaluate the role of coating thickness and sealants on the HVOF coatings, and the effect of hard chrome plating with and without Ni strike layers. The landing gear JTP beach exposure, B117, GM 9540, and SO2 tests are being evaluated at the time of writing this report. The data appear to be showing significant differences in relative performance of chrome and HVOF coatings, with the HVOF coatings performing much better than chrome in beach exposure and C-HCAT cabinet tests, but worse in HCAT cabinet tests. Tests and evaluations are ongoing, and it is too early at the time of writing to assess either the data itself or its meaning. Updated information will be available on the HCAT web site.

12.5.4. General Trend of Corrosion Results Testing in simulated cabinet (ASTM B 117, GM 9450P/B, Modified SO2), atmospheric, and electrochemical potential measurements show HVOF coatings to be usually equal in corrosion performance to hard chrome. Carbides with a matrix of CoCr, CoNi, or CrNi generally have better corrosion resistance than Co alone. Corrosion data also clearly demonstrate the need to minimize through-porosity. However, the reason for the differences between cabinet test and atmospheric test results is a matter of concern which is under investigation.

12.6. Fatigue

12.6.1. Documents Document 40 Summary of 4340 Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut) This document contains the HCAT data on fatigue of 4340 high strength steel (initial HCAT protocol).

Document 41 Summary of 7075 Al Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut) This document contains the HCAT data on fatigue of 7075 Al (initial HCAT protocol).

"4340 fatigue data.PDF"

"7075 fatigue.pdf"


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Document 42 Summary of 13-8 Stainless Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut) This document contains the HCAT data on fatigue of PH 13-8Mo stainless steel (initial HCAT protocol).

Document 43. HCAT landing gear JTP fatigue data - HVOF WC-Co on 4340, 300M, AerMet 100 in air and NaCl solution. This document contains the HCAT data on fatigue of 4340, 300M, and Aermet 100 high strength steels (HCAT landing gear protocol).

Document 44 Advanced Thermal Spray Coatings for Fatigue Sensitive Applications (Courtesy of John Quets Praxair)

Document 26 Behaviour of Tungsten Carbide Thermal Spray Coatings 1995, J. Wigren et al.

12.6.2. General Description and Test Method Fatigue is a very critical property in the aerospace industry, because of the repeated cyclic loading for landing gear, actuators, airframe parts, and gas turbine engine components. Since fatigue performance is driven by material strength and is especially related to near-surface effects, fatigue-critical applications require careful definition and control of the thermal spray process to

1. minimize surface heating so as to prevent loss of mechanical properties due to overheating, and

2. deposit thermal spray coatings with compressive stress to minimize of eliminate any fatigue debit.

Although plasma spray processes have been widespread in the aerospace industry for many years, they have tended to be limited to non-fatigue critical applications, largely due to the heat input of the process. The recent emphasis and work to understand the HVOF process, which relies more on kinetic than thermal energy for final coating properties, has started to move the design community towards thermal spray in fatigue-driven components. The evaluation of fatigue in a coated part is really the analysis of how the coating affects the known values of an uncoated component. Baseline data for the alloys used in most applications have already been established. There are established methods and specifications for determining fatigue properties. However, with coatings now applied, some of the guidelines used for bulk materials are somewhat different. For most chrome-replacement testing, axial fatigue testing (ASTM E466-96) provides the most useful data for evaluation (rather than bend

"13-8 stainless fatigue.pdf"

"Fatigue data - landing gear JTP.pdf"

"Quets - Adv Th Spray for fatigue.pdf"


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testing). In designing a fatigue testing protocol, some areas requiring definition are:

1. What is the load carrying capability of the coating and should this value be used in determining the applied stress? How will thickness of the coating affect this situation?

2. What is the best bar design for fatigue evaluation of coatings?

• Hourglass (smoothly varying cross section, thinnest at the center)

• Smooth section (constant cross section from some distance in center)

• Rectangular cross section (flat patch in center of bar for evaluation)

3. Will the testing be load (stress) or strain control? 4. Can grinding of the coating be repeatable on the fatigue bars to

produce a consistent thickness and surface for testing? As with all fatigue evaluations, considerations must be given to:

• Frequency or speed of testing o will determine time for testing, but to high a frequency can

cause overheating or a shift in results for sensitive materials

• Type of control (load or strain) o will application of force to bar be controlled purely by load

or by the deformation induced in the part?

• R ratio or A ratio o R ratio is defined as the ratio of minimum cyclic load to

maximum cyclic load. For example, an R ratio of �1.0 means the maximum and minimum loads are the same and the loading is fully reversed from positive to negative.

o A ratio is defined as: (maximum stress - minimum stress)/(maximum stress + minimum stress)

Typical fatigue bar shapes are shown in Figure 55, Figure 56, and Figure 57. The most common designs are the hourglass (which can only be used for load-control fatigue), and the smooth bar.



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Figure 55. Typical hourglass-shaped fatigue bar.

Figure 56. Typical smooth fatigue bar.


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Fatigue testing for coating comparison must be defined by the important parameters of the spray process and the critical controls for consistent property evaluation as shown in Table 54.

Corrosion fatigue must also be a consideration, given the corrosion environments for many of the hard chrome applications. This usually involves the same considerations as testing in air but the test area in question is exposed to the corrosive media for the entire test or for specific periods of time. There may also be pre-exposures for the purpose of initiating corrosion followed by constant exposure to the environment in question. When defining the protocol for testing, the frequency of exposure, and the degree of replenishment must be stipulated to best approximate the actual service conditions.

Figure 57. Flat Kb fatigue bar.

Table 54. Fatigue testing variables. Coating/Substrate Information Testing Information Peening of substrate Frequency of testing

machine

Thickness of coating Bar geometry

Surface finish (ground, unground, or superfinished)

Load vs. strain control

Location of coating (patch/full length)

R ratio, or A ratio

Almen (intrinsic stress) intensity


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Work in varied protocols and industrial applications will be discussed and the general trend for comparing hard chrome to HVOF coatings in fatigue established.

12.6.3. Fatigue Results

12.6.3.1. Comparison of Hard Chrome vs. HVOF WC-Co and T400

Document 40 Summary of 4340 Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut). Document 41 Summary of 7075 Al Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut). Document 42 Summary of 13-8 Stainless Data from Initial HCAT Protocol (Courtesy of Phil Bretz Metcut). This review summarizes the fatigue testing performed in the initial HCAT protocol of Document 32 for validation of HVOF materials as a viable hard chrome alternative. The materials and substrates are shown in Table 55. The required fatigue test parameters as defined in the test methodology section can be found in Table 56. A variety of test bar configurations were chosen to investigate the

Table 55. Materials and Substrates in Study Coating materials

Substrates

Hard chrome WC-Co T-400

4340 Alloy steel 7075-T73 Aluminum !3-8 PH Stainless

Table 56 Fatigue Test Parameters Coating/Substrate Information Testing Information

Peening of substrate Baseline polished Coated bars peened

Frequency of testing machine Load control 50 Hz Strain control 2 Hz

Thickness of coating Nominal .005� thickness

Bar type (hourglass vs. smooth) Combination of all types

Surface finish (ground/unground) As coated surface

Load vs. strain control Combination of both types

Location of coating (patch/full length) .5� patch in center of test section

R ratio Load control .025 Strain control 1.0

Almen intensity 8-11 compressive


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sensitivity of each configuration to changes in fatigue as caused by coating application. This step also addressed the concerns of many groups that had already chosen a bar type of their past verification work. Table 57 summarizes the number of bars tested for each condition on the three substrates selected.

Due to the substantial amount of data, representative results for only the 4340 and 7075 are presented. Data is currently available in referenced documents (Document 40, Document 41, Document 42) and a final report should be published and available on the HCAT Home Page by December 2000. All data for the HVOF materials on 4340 substrates show satisfactory results as compared to hard chrome in Figure 58, Figure 59, and Figure 60, regardless of the test bar type which was used. An interesting observation was the majority of failures for the T-400 coating were outside the coated patch (suggestive of, but not proving, a fatigue enhancement). The majority of failures for the WC-Co and hard chrome were underneath the coating patch as would be expected. Data for PH13-8Mo stainless steel were very similar to those for 4340. Regardless of sample geometry, the HVOF coatings performed better on 4340 and PH13-8Mo than hard chrome, showing little or no fatigue debit. For reasons that are not known the hourglass samples are particularly sensitive to the coating, and the chrome plated samples show a very marked fatigue debit.

Table 57. Fatigue Matrix for Initial Validation Low Cycle Fatigue (LCF) High Cycle Fatigue (HCF)

Baselines HVOF Coatings (per coating)

Baseline HVOF Coatings (per coating)

10 smooth bar tests uncoated 10 smooth bar tests hard chrome 6 hourglass bar tests hard chrome 6 Kb bar tests hard chrome

8 smooth bar tests HVOF coatings 6 hourglass bar tests HVOF coatings 6 Kb bar tests HVOF coatings

10 smooth bar tests uncoated baseline 10 smooth bar tests hard chrome baseline 6 hourglass bar tests hard chrome 6 Kb bar tests hard chrome

8 smooth bar tests HVOF coatings 6 hourglass bar tests HVOF coatings 6 Kb bar tests HVOF coatings

Totals:

Baselines: 1. 20 smooth bar tests per substrate per test condition (uncoated) 2. 20 smooth, 12 hourglass, 12 Kb bar tests per substrate per condition (hard chrome plated)

Coatings: 16 smooth, 12 hourglass, 12 Kb bar tests per coating, per test condition



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For 7075 aluminum, the situation was found to be more complex (Figure 61). While T400 gave a fatigue debit, it was much smaller than that caused by chrome. WC-Co, on the other hand, produced a worse fatigue debit than chrome. It was first assumed that the heat input from the process affected the substrate. However, extensive testing showed that heat input had no effect, and it is currently believed that the high debit derives from the extreme mismatch in mechanical properties (primarily elastic modulus) between the aluminum and the HVOF WC-Co. This mismatch is much less for T400.

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09

Cycles to failure (Nf)

Engi

neer

ing

Stre

ss M

ax (k

si)

Uncoated smooth

Cr plated smooth

T400 smooth

WC-17Co smooth

Figure 58. Comparison of Fatigue Data on Smooth Bars for 4340

100

110

120

130

140

150

160

170

180

190

200

210

220

1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Cycles to failure (Nf)

Engi

neer

ing

Stre

ss M

ax (k

si)

Cr plated Kb

WC-Co Kb Bar

T400 Kb Bar

Figure 59. Comparison of fatigue data on Kb bars for 4340


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In review of all the data, we can make the following conclusions:

• Data for the 4340 alloy steel and 13-8 stainless steel shows fatigue performance very close to baseline data and far superior to hard chrome.

• Test bar configurations do not affect the ranking of the coating materials. However, the hourglass configuration shows the most drastic variation between chrome and HVOF. The reason for this is not known.

• For the T-400, the majority of failures were outside the coating patch which has not yet been explained. All chrome failures were under the patch.

• The 7075 material showed a degradation in fatigue properties as compared to the hard chrome baseline.

50

70

90

110

130

150

170

190

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Cycles to failure

Max

. stre

ss (k

si)

WC-CoT-400EHC

Figure 60. Fatigue of coated 4340 steel - hourglass samples.


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12.6.3.2. Comparison of Hard Chrome vs. HVOF WC-Co for Landing Gear

This data is close to completion at time of writing, but is not yet quite completed. These data were taken to address specific requirements for fatigue in landing gear applications, as defined by NAVAIR, the US Air Force, Boeing, and the landing gear manufacturers. The data from the initial HCAT work (Document 40 to Document 42) was analyzed, and areas identified where further information and clarification was needed. The resulting Landing Gear Joint Test Protocol (JTP) is a joint effort between HCAT (WC-Co coating) and C-HCAT (WC-CoCr coating). The fatigue data presented here is only for the WC-Co material; the C-HCAT testing is currently in progress and data will be made available on the HCAT Home Page. Since the C-HCAT data is for WC-CoCr, it is discussed in the following section.

Low Cycle Fatigue Data7075-T73 Smooth Gage

0

0.2

0.4

0.6

0.8

1

1.2

1.0E+3 1.0E+4 1.0E+5 1.0E+6

Cycles (Nf)

Stra

in M

ax(%

)

Baseline S280/WCCo/Jet-Kote S110/T-400/Jet-KoteS110/Chrome

Figure 61. Fatigue Results for HVOF and Chrome on 7075 Aluminum




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Table 58 and Table 59 summarize the materials and test conditions for the landing gear JTP data presented in this section.

This work was very similar to the initial HCAT evaluation, with key differences as listed below:

• The coatings were ground to a 16µ� Ra finish before testing (prior tests used as-sprayed specimens)

• The majority of the tests were run with the hourglass configuration (smooth bars and flat bars were used in the prior work)

• The testing was performed with fully reversed loading R = -1.0

• A .010� thick coating was also sprayed to evaluate thickness dependence, in order to evaluate O&R usage

o (NOTE: Test bar size for 0.010� coatings was increased to 0.50� dia. to keep the percentage of total coating area similar to the .25�dia. bar and 0.003� coating, i.e. coating thickness << specimen diameter)

• The full data set as of the time of writing is summarized in Document 43. Corrosion fatigue tests were performed to evaluate the effect of salt exposure on cyclic life.

Table 58. Substrate and coating materials - landing gear JTP. Coating materials Substrate materials Hard chrome 4340 high strength steel

WC-Co 300 M landing gear steel

AerMet 100 landing gear steel

Table 59. Test conditions for landing gear JTP. Coating/Substrate Information Testing Information Peening of substrate: Baseline and coated bars peened

Frequency of testing machine: Load control 50 Hz

Thickness of ground coating: Nominal thickness .003� and 0.010�

Bar type: Primarily hourglass, some smooth

Surface finish: Ground surface, 16µ� Ra

Load vs. strain control: Load control

Location of coating: 0.5� patch in center of test section

R ratio: Load control -1.0

Almen intensity (stress): 8-11 compressive


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As an example of the data, curves for 300M (a very damage intolerant material) are shown in Figure 62 (.003� thickness-air), and Figure 63 (.003� thickness-NaCl). Thermal spray coatings on peened materials have equivalent performance to their chrome plated counterparts. The data correlates with the initial HCAT work on 4340 steel, which showed superior or equal performance of the HVOF alternatives in comparison with hard chrome. In NaCl, the performance of the HVOF coating is marginally better than the chrome. (Note that failure to peen the material before chrome plating leads to a very large fatigue debit, which is not the case for the HVOF coatings.)

300M, SMALL HOURGLASS SPECIMEN(0.003" COATING) R = -1, AIR

100

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140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

Bare/UNPeenedEHC/UNPeenedEHC/UNPeened/AverageEHC/PeenedEHC/Peened/AverageWCCo/UNPeenedWCCo/UNPeened/AverageWCCo/PeenedWCCO/Peened/Average

Figure 62. Fatigue Curve for 300M with .24”dia. hourglass tested in air – coating thickness 0 .003”.


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Figure 64 shows the effect of coating thickness on 4340 samples, showing increased fatigue debit as thickness increases. This effect is quite commonly seen with coated materials. Experience shows that a thicker coating or a larger bar will show a higher fatigue debit, and it cannot yet be determined what percentage of the reduction may be attributed to the larger bar diameter and what to the coating thickness. Again, the HVOF material is significantly better than the same thickness hard chrome.

300M, R = -1, SMALL HOURGLASS SPECIMENAIR VS. NaCl ENVIRONMENT

100

110

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130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08


EHC/AirEHC/Air/AverageEHC/NaClEHC/NaCl/AverageWCCo/AirWCCo/Air/AverageWCCo/NaClWCCo/NaCl/Average

Figure 63. Fatigue Curve for 300M with .24”dia. hourglass comparing air results with samples tested in NaCl and .003” coating thickness.


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The following observations were made during the testing: HVOF Coating

• Dependent upon the magnitude of loading (165-180 ksi), cracks could sometimes be observed in the coating after only a small number of cycles (10-1000). However, fatigue remained better than chrome.

• For the larger .50� dia. bars and some of the smaller diameter bars, spalling/delamination of the coating has been observed at specimen failure.

• The specimen failure initiation sites were primarily subsurface, especially at low loads, with limited surface sites, primarily at high loads.

Hard Chrome

• �Chicken wire� cracking can be observed on the failed bars which is typical of hard chrome plating. No spalling/delamination has been observed.

• The failure initiation sites were primarily surface with limited subsurface sites.

4340, R = -1, AIRLARGE (0.010"CTNG) VS. SMALL (0.003"CTNG) HOURGLASS

100.0

110.0

120.0

130.0

140.0

150.0

160.0

170.0

180.0

190.0

200.0

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08


LgHG/EHC/PeenedLgHG/EHC/AverageLgHG/WCCo/PeenedLgHG/WCCo/AverageSmHG/EHC/PeenedSmHG/EHC/AverageSmHG/WCCo/PeenedSmHG/WCCo/Average

Figure 64. Fatigue curve comparing thickness effects 0.003” (.250” dia.) vs. 0.010” (.500” dia.) on 4340 using hourglass configuration tested in air.


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These observations with regard to initiation sites are consistent with the previously reported data that cracks in chrome plating are directly transferred in to the substrate. In contrast, cracks in the HVOF alternatives do not correlate with substrate fatigue initiation sites; instead they are subsurface and not process related. This again confirms that substrate materials properties drive fatigue performance in HVOF coatings, whereas hard chrome coatings are detrimental to fatigue. The data concerning cracking of the coatings indicates more surface discontinuities with the HVOF alternatives. Clearly, the cracks in the coating do not affect fatigue, since the HVOF coatings have longer fatigue lives than chrome. The other possible effect of cracks might be increased corrosion and corrosion-fatigue, since one might argue that cracks would permit penetration of liquids to the underlying steel. The data from the NaCl fatigue testing clearly shows that this is not so; the HVOF samples show improved corrosion fatigue as well as dry air fatigue � in fact the differential performance between HVOF and chrome is generally larger under corrosive conditions. Conclusions The following conclusions can be drawn:

• In air, the performance of HVOF coatings is equal or superior to hard chrome.

• In NaCl, the performance of HVOF coatings is superior to hard chrome.

• The cracking observed in the HVOF alternatives during cyclic loading does not substantially reduce the ability of the coating to provide protection against corrosion as evidenced by the corrosion fatigue data.

• The thicker coatings show a higher fatigue debit for chrome and HVOF, which appears to be a function of coating thickness and bar diameter.

12.6.3.3. Comparison of Hard Chrome vs HVOF WC-CoCr WC-10Co4Cr is now used commercially on landing gear for the Boeing 767-400 as well as for Boeing-approved O&R of landing gear (see Section 15.5). All vendors on the Boeing Qualified Provider List (QPL) are qualified on the basis of fatigue performance (among other criteria). All fatigue testing is carried out on Boeing standard axial 4340 steel fatigue test specimens, which are hourglass-shaped. Testing is done using an R-ratio of 0.1 and loads up to 170 ksi. The HVOF coating must perform better than chrome. An example of this type of qualification testing is shown in Figure 65. (As defined by Boeing in BMS 10-67 (Section 14.2.2), Type I coatings are WC-18Co, while Type XVII coatings are WC-10Co4Cr.) Note that this figure plots the range of values obtained for the fatigue life with each coating material at a given stress level. Since runout was defined as 1 million cycles, most of the WC-CoCr samples went to runout.



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1.00E+04 1.00E+05 1.00E+06

Cycles to failure

Uncoated

GPX2800 (Type XVII)

GPX2700 (Type I)

UncoatedGPX2800 (Type XVII)

GPX2700 (Type I)

Cr Plate

Figure 65. Fatigue of HVOF-coated and chrome plated high strength steels, Kt=1.5, Boeing qualification testing. (Courtesy Engelhard Surface Technology).

Figure 66. Comparison of fatigue for chrome and HVOF WC-CoCr deposited with Jet Kote and Diamond Jet guns. (Courtesy Southwest Aeroservice.)


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Figure 66 shows the fatigue life of HVOF WC-CoCr compared with hard chrome using 0.25� diameter 4340 smooth fatigue bars. Two different HVOF guns and powders gave equivalent performance coatings that were somewhat better than chrome. (Note that the hourglass specimen shape tends to separate the performance of different coatings, moving the S/N curves apart, while the smooth bar shape tends to move the S/N curves together.) Although all prior work had shown excellent performance for WC-CoCr, on the basis of which it is now a production process for commercial aircraft, work by the C-HCAT team under way at the time of writing is showing that for their specimens the WC-CoCr coating tends to rumple and delaminate at high loads. These specimens use the same specimen geometry as that of Figure 66. This data is under evaluation at the time of writing to determine whether this is a materials or a testing issue, and whether it is relevant to military usage of WC-CoCr coatings, where higher service loads might be expected.

12.6.3.4. Other Other fatigue data is available in the document as listed below. Document 44 Advanced Thermal Spray Coatings for Fatigue Sensitive Applications (Courtesy of John Quets Praxair).

12.6.3.5. Comparative Study of Compressive Stress Effects on Fatigue for HVOF

Document 26 Behaviour of Tungsten Carbide Thermal Spray Coatings 1995, J. Wigren et al. VolvoAero identified a wear problem on engine fan blades and the goal of this study was to balance compressive residual stress in the coating to prevent spalling in service, with a minimal degradation on substrate fatigue life. This study primarily evaluates WC-Co (Table 60) sprayed with the HVOF process at a variety of different spray parameter settings. The following tests were performed:

• Three point bend testing for crack resistance

• Residual stress measurement by Modified Layer Removal Technique

• Low cycle fatigue testing


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Fatigue results will be discussed here. Residual stress work from this document was discussed earlier in Section 11.5.3.2. The fatigue evaluation involved both work to identify crack initiation and full fatigue performance. The test parameters are as shown in Table 61. The specimens were loaded initially by applying incremental load steps, each corresponding to an increase of 0.1% in strain, with 1000 cycle cycles between each step until a crack was initiated in the coating. Fluorescent penetrant inspection was used prior to load application and after every 1000 cycle load increment to check for crack initiation. Since the purpose of cracking the coating was to determine the fatigue life of the substrate subsequent to this event, the load was increased directly after crack initiation to a stress of 700 Mpa for 100, 000 cycles or failure of the substrate.

Table 60. Tungsten Carbide Coating System Designations (Volvo)

Coating composition

Application process

HVOF Plasma

A B C D E F G H

1. (WC-Co)

2. (WC-Ni)

3. (WC-Co/Cr)

●

●

● ● ●

●

●

●

● ● ●



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The results for crack initiation are shown in Figure 67. As expected, the F-1, A-1, and G-1 are most resistant to cracking due to the compressive stresses as measured in the coating. This follows the trend in all work thus far towards compressive stresses.

Figure 68 summarizes the final fatigue performance of the substrate. As expected, the H-1, E-1, and E-3 samples show the longest lives based upon the highest levels of compressive residual stress in the substrate.

Table 61. Fatigue Test Parameters for Volvo Evaluation Coating/Substrate Information Testing Information

Peening of substrate

Coated bars peened

Frequency of testing machine

Load control 50 Hz

Strain control 2 Hz

Thickness of coating

Nominal .008’ thickness

Bar type (hourglass vs. smooth)

Smooth

Surface finish (ground/unground)

As coated surface

Load vs. strain control

Stain control till crack initiation-switch to load to runout

Location of coating (patch/full length)

Full length of test section

R ratio

Strain control 0

Residual stress intensity

Varied

Figure 67. Comparison of Residual Stress and Resistance of Coating to Crack Initiation



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The goal of the Volvo study was to optimize the coating with respect to both fatigue and residual stress. Engine tests were run to validate the laboratory test results.

The following observations were made:

• Although coating H-1 showed superior substrate fatigue life due to substrate compressive residual stress, the tensile residual stress in the coating caused spallation during engine evaluations.

• Coating F-1 also showed spallation in engine testing. Although the compressive residual stress would be expected to preclude this occurrence, it is hypothesized that the high compressive residual stress may be close to that of the WC-Co material, thus resulting in failure.

Based upon these observations, A-1 was chosen for implementation, balancing both residual stress and fatigue data.

12.6.3.5.1. Interpretation of results A balance must be obtained between residual stress and substrate fatigue to balance the effect of the coating on substrate performance and the ability of the coating to remain intact during application in the environment. Properties of the coating such as ability to resist compressive stresses are also critical and must be understood.

12.6.4. General Trend of Fatigue Results Hard chrome always causes a significant fatigue debit because the coating is under tensile stress, which tends to open up cracks in the substrate. There is also the possibility that some of the cracks in the chrome may propagate into the substrate. The HVOF carbide materials can exhibit cracking early in the fatigue testing process, but current evidence, from the extended fatigue life of HVOF-coated alloys and cross-sectional microscopy, shows those cracks do not propagate into the substrate.

Figure 68. Comparison of Final Fatigue Life with Residual Stress


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In general, therefore, HVOF coatings can largely avoid the severe fatigue debit that chrome plate creates, and often have no measurable fatigue debit. Hourglass-shaped fatigue bars are particularly sensitive for fatigue evaluation of coated materials, although the reason for this is unknown. HVOF WC-Co shows superior performance to hard chrome on both the 4340 alloy steel and 13-8 stainless steels. On the 7075 Al, however, the WC-Co is significantly worse than chrome, while T400 is significantly better than chrome. There are several important issues to take into account when replacing chrome plating with HVOF coatings:

• Deposition temperature � It is important to ensure that the coating process does not overheat the substrate.

• Intrinsic stress in the coating � The coating should be under compressive stress, but not under very high compressive stress, which can delaminate the coating. Coating stress can be measured by Almen strip � a satisfactory Almen number is 0.003 � 0.012� Almen �N� for a 0.005� thick tungsten carbide coating41.

• Coating mechanical properties � Large mismatches between the elastic moduli of the substrate and the coating should be avoided, as this appears to lead to reduced fatigue life. While WC-Co and WC-CoCr work well on steels, softer coatings with lower elastic modulus (such as Tribaloy) work better on aluminum alloys.

Corrosion fatigue is being measured in the Landing Gear JTP. Current data shows no significant effect on material performance, even with initial cracking in the HVOF materials. There is an outstanding issue, brought up by initial C-HCAT data, that WC-CoCr may be more brittle than WC-Co and therefore more prone to crack or delaminate at high load or coating thickness. This issue is in process of being resolved at the time of writing.

12.7. Wear – Erosion, Abrasion, Sliding, Fretting 12.7.1. Documents Document 23. Fracture Toughness of HVOF Sprayed WC-Co Coatings (Courtesy of S. De Palo, et al) Summary of testing concerning fracture toughness and erosion resistance on the referenced materials. Document 24. Tungsten Carbide-Cobalt Coatings for Industrial Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan) Summary of varied spray process for deposition of coatings and study on abrasive wear/microstructure/phase content interactions. Document 22 Evaluation of Four High Velocity Thermal Spray Guns Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault, C. Moreau, V. Bouyer, L. Leblanc)


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Study of spray process parameters with regard to velocity, temperature, efficiency, etc. and relation to microstructure, wear resistance and carbide degradation.

12.7.2. General Description and Test Methods Wear resistance has been one of the prime characteristics that warrants the use of hard chrome in a many applications. Real-life applications frequently involve sliding of two mating surfaces (sometimes in combination with abrasive particles). Wear tests are usually simpler, and there are several philosophies of wear testing:

1. Carry out simple tests to measure how materials perform compared to each other. Typical tests are Taber abrasion, pin-on-disk, ring-on-block, etc. These tests are simple, but often fail to rank even relative wear correctly as it is measured in service.

2. Carry out tests designed to evoke the wear mechanisms expected in real applications. This can be quite difficult since it requires a very good understanding of the actual wear situation.

3. Carry out tests that simulate service conditions as accurately as possible. For aircraft this includes landing gear and hydraulic rig tests. Because these tests are accelerated they permit investigation of complex interactions and widely varying conditions.

4. Evaluate in service (i.e. field testing, or for aircraft, flight testing). This method is obviously the most true-to-life, but of course it measures only the wear resulting from the conditions experienced by that particular test aircraft. It is uncontrolled, takes a long time, and is expensive. Because of the cost and risks of failure it is usually preceded by rig testing.

Because no one method is satisfactory, most engineers depend on a variety of test methods, but ultimately rely on rig and flight testing. This section covers only coupon wear testing. Rig and flight testing are found in Sections 13.1 and 13.3. Wear can be defined by a variety of mechanisms primarily involving loss of material and substrate integrity. For the purposes of this section, there are four primary wear mechanisms as detailed below in Table 62.


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To define a wear environment, a number of factors must be considered such as:

• Lubrication � amount (if any), frequency, contamination

• Type/size of abrasive � if involved

• Wear stroke � relative motion, length, contact surface

• Type of loading during cycle � side, axial, combination

• Removal of wear debris � frequency, mechanism

• Similarity to service geometry and conditions. These factors are very important to assess and define, but sometimes impossible to reproduce with reasonable accuracy in a laboratory evaluation. This therefore can make defining wear methodology very difficult. Even if a �standard� procedure has been formulated such as ASTM or SAE specifications, each laboratory may have a different interpretation of the requirements. Comparative ranking of results from varied sources can then sometimes present incorrect conclusions because of these differences. With this in mind, many companies and laboratories develop wear procedures unique to the application/environment in which the wear problem exists. However, problems can arise if the formulator of the test methodology designs a test that is too severe and fails most if not all of the potential candidates. It is easier to implement new materials when a current baseline material such as hard chrome is being replaced by possible alternatives. The current product provides not only a benchmark for

Table 62. Four Primary Wear Mechanisms Wear mechanism

Description Primary Test Method

Erosion Loss of material due to impact of abrasive material carried in a liquid or gas stream.

ASTM G 76

Abrasion Displacement of material from a surface in contact with hard projections on a mating surface or with hard particles that are moving relative to the wearing surface.

ASTM G 65

Sliding/Adhesive Two surfaces rubbing together over a defined distance where adhesion can occur in some areas and wear debris generated can cause further degradation.

Varied methods

Fretting Occurs between two mating surfaces; it is adhesive in nature and vibration is its essential causative factor. Relative motion in design is usually not expected.

Varied methods


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comparison, but also serves to validate the test methodology if it is unique for the application in question. Actual usage in the environment is the final test but wear testing can hopefully rank varied materials for consideration. With hard chrome as the baseline, this section will define both specification (ASTM) and industry-unique procedures designed for comparison of hard chrome vs. alternatives in specific applications. Results, examples, and general trends will be discussed.

12.7.3. Test Methods

12.7.3.1. Erosion Testing per ASTM G 76 This test involves firing of abrasive particles at a test surface using a gas stream. Parameters to control in this test are:

• Length and diameter of gun nozzle

• Carrier gas type

• Velocity

• Size and type of abrasive (the erodent)

• Distance from nozzle to gun

• Angle of impingement

• Area of impingement on sample

• Ability to catch used erodent

• Duration of testing

Results are usually expressed in a weight loss per unit area format. Problems can arise if the flow and mixing of gas/erodent is not uniform. The test is therefore sometimes normalized for the amount of erodent used in a given time frame of testing if more or less material is used for a particular test. It is also critical to control the size/shape and distribution of abrasion particles as shifts in these characteristics can substantially

Fl owm e te r(20 s lpm)

P ow der F eede r(20 g / min)

N ozz le( 1/ 16 � ID , 2 � long )Fil ter

C om pr es s ed D ry Air

(50 ps i)

R ot a te(30 °- 90° )

C oatedS ample

60 m /sA l2O 3

(50 um, angular)

Sol id Pa rtic le E ro sion Te st Ap pa ra tus ( mod i fi ed A STM G 76- 89)

10 mm s tandof f

Figure 69. Typical Set-up of ASTM G76 erosion test.


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effect the final result and ranking. A typical set-up is shown in Figure 69.

12.7.3.2. Abrasion Testing ASTM G 65 This technique involves the use of dry sand abrading against the testing surface with pressure applied by means of a rotating rubber wheel. Rounded Ottawa silica sand is normally used and is introduced between the coated block and the chlorobutyl rubber wheel/rim. Rotation of the rim is in the direction of abrasive flow. The test specimen is pressed against the rotating wheel by means of a lever arm. The sample is measured before and after the test. Weight loss is converted to volume loss by dividing by the coating density. Results are expressed as volume loss per 1,000 revolutions. Other There can be many variations for abrasion testing involving use of some abrasive media in the wear process. These specific test setups will be highlighted in the results section for that particular test.

12.7.3.3. Sliding/Fretting Wear Methods For many applications in landing gear, hydraulic, and actuator designs, wear involves either sliding of a seal against the coating over a certain stroke length (e.g. landing gear uplock or downlock hydraulics), or situations where the piston dithers or vibrates at a single position for extended time frames (e.g. fly-by-wire actuators). These situations describe perfect circumstances for occurrence of sliding and fretting wear respectively. Actual hydraulic tests as reviewed in Section 13.2.1 are the best option but are expensive and time consuming. Application-driven test methods provide an option for more economical and rapid evaluations, provided they simulate service conditions properly. Methods for each type of test have been devised under the HCAT/CHACT protocols. Document 33 Joint Test Protocol (JTP) for Landing Gear 1998 (Courtesy of HCAT and CHCAT Teams). Other methods could be acceptable but these tests are currently being used. A short summary of each methodology is shown below.

Figure 70. ASTM G 76 set-up.


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Sliding Wear An oscillating piston test can simulate piston actuation which is typically

encountered. This test can be used to reflect typical conditions of use under a side load. The design is shown schematically in Figure 71. Standard ASTM tests are not applicable since they do not reflect conditions of use in hydraulics. The piston consists of 4340 high strength steel 1" in diameter and 9� in length, typical of hydraulic rods. The rod coating and finish, the bushing material or seal, and

the wear test conditions are as detailed in the test protocol. The rod and bushing wear specimens are dimensioned and finished as required per protocol specifications. Fretting Wear The fretting wear test is used to reflect typical actuator piston dithering or vibration movement. This equipment is shown schematically in Figure 72. It is a test system commonly used by GE Aircraft Engines for measuring fretting wear of engine components, and comprises a flat block and a shaped shoe which move rapidly with respect to each other. Standard ASTM tests are not used since they do not reflect conditions of use in hydraulics.

12.7.4. Wear Results

12.7.4.1. ASTM G 65 Erosion Testing Document 44. Advanced Thermal Spray Coatings for Fatigue Sensitive Applications (Courtesy of John Quets Praxair)

Guide

Oscillating piston

Load

Testbushing Guide

Figure 71. Schematic of Sliding Wear Apparatus for hydrualics

B loc k

O sc i l la t in g M otio n

S h oe

L O A D

Figure 72. Side view of fretting apparatus.



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Testing was performed to ASTM G65 standards according to the description summarized in 12.7.1.

There is currently very little data available on carbides/cermets concerning erosion. The data from Praxair is a direct comparison of hard chrome with WC-Co produced by a number of different spray processes. The Stony Brook/Sulzer Metco analysis is a side-by-side review of varied WC-Co compositions/powder morphologies sprayed with the same gun. Although the data is reported in different units, review shows the following information:

• Erosion is higher for all WC-Co materials in the 90° mode for both studies.

o This occurs because the harder thermal spray coatings fracture and are eroded at this high angle of incidence. (HV 900 for hard chrome vs. HV 1150 for WCCo.) In general ceramics are avoided in these situations.

Table 63. Erosion Results as Conducted By Praxair.

Erosion

Rate ìm/g

Coating Chemistry

Coating Process

30° 90°

Chrome

WC-Co

WC-Co

WC-Co

WC-Co

WC-Co-Cr

WC-Co-Cr

WC-Cr-Ni

HCP

Jet Kote

JP 5000

D-Gun

SDG

JP-5000

SDG

SDG

> 130

10

10

17

20

10

17

20

50

70

65

100

85

60

85

105

Figure 73. Erosion Results As Conducted By Stony Brook/Sulzer Metco

Sample Code

Nominal Composition

(wt%)

Starting Powder Size(ìm)

Manufacturing Technique

A WC-12Co -45, +5 Sintered/ Crushed

B WC-12Co -53, +11 Agglomerated/ Sintered

C WC-17Co -53, +11 Spray Dried

D WC-17Co -53, +11 Spray Dried

E WC-10Co-4Cr -45, +11 Sintered/

Crushed

F WC-10Co-4Cr -53, +11 Agglomerated/


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• At the lower 30° angle, the carbide materials substantially outperform the hard chrome materials � again, as one would expect.

• As expected, the WC-17Co material showed higher erosion loss due to the higher content of the softer matrix Co.

12.7.4.1.1. Interpretation of results The HVOF coatings show good performance in erosion testing at low incidence angles. Reduction in hardness closer to hard chrome values may be required for thermal spray materials to show parallel performance at angles up to and equal to 90 degrees.

12.7.4.2. ASTM G 76 Abrasion Testing Document 44. Advanced Thermal Spray Coatings for Fatigue Sensitive Applications (Courtesy of John Quets Praxair) Document 22. Evaluation of Four High Velocity Thermal Spray Guns Using WC-10Co-4Cr Cermets (Courtesy of J.G. Legoux, B.Arsenault, C. Moreau, V. Bouyer, L. Leblanc) These documents summarize the testing of coatings using the ASTM G 76 rubber wheel (Taber) abrasion test as described in 12.7.1. There is currently very little data available on carbides/cermets concerning abrasion. The data from Praxair is a direct comparison of hard chrome with WC-Co based materials produced by a number of different spray processes. The NRC analysis is a side by side review of varied spray HVOF guns with the same WCCoCr compositions/powder morphology. A hybrid plasma gun is also analyzed. Analysis of the data shows the following:

• WC-Co and WC-CoCr based materials show better abrasion resistance as compared to hard chrome sometimes by a factor of 8:1.

• Although the NRC study is not normalized to revolution data, the trend corresponds to Praxair data for the JP 5000 study, supporting lower abrasive wear for higher hardness HVOF coatings.


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Table 64. ASTM G76 data from Praxair. Coating

Chemistry Coating Process

Density

g/cc

Average Hardness

VH.3

Abrasion

Wear

mm3/1krev

Chrome

WC-Co

WC-Co

WC-Co

WC-Co

WC-Co

WC-Co-Cr

WC-Co-Cr

WC-Cr-Ni

Cr Plate

HV 2000

Jet Kote

JP 5000

D-Gun

SDG

JP 5000

SDG

SDG

6.9

13.2

13.2

13.2

13.2

13.2

12.4

12.4

10.5

900

1200

1100

1125

1075

1100

1100

1270

1100

8

3.0

1.6

0.6

2.9

1.4

0.6

1.3

1.1

Table 65. ASTM G76 Data from NRC. Sample Dep efficiency Substrate temp. Particle velocity Particle temp. Porosity Hardness Volume loss

% °C m/s °C % sdev VHN sdev mm3 sdev

MT1* 86 205 340 2300 6.2 0.6 1082 149 6.2 0.45

MT2 86 170 340 2300 8.6 0.9 1094 184 6.7 0.26

MT3 86 165 320 2350 8.8 1.5 1086 172 7.2 0.68

MT4 84 175 340 2400 5.3 0.5 1066 133 8.7 0.42

MT5 66 140 360 2150 16.1 1.5 949 172 5.6 0.44

MT6 81.5 190 300 2600 6.5 0.5 1059 158 13.7 0.89

MT7 87 155 320 2250 12.6 1.5 1025 149 6.5 0.38

JP1 -- 240 -- -- 2.1 0.4 1237 134 3.6 0.32

JP2 39 200 665 1805 3.1 0.8 1124 218 2.9 0.14

JP3 31 200 620 1742 2.8 0.6 1213 142 3.2 0.1

JP4 40 200 661 1850 2.7 0.5 1114 95 3 0.32

JP5** 7.8 -- 672 1465 0.6 0.1 1206 100 4.2 -

DJ1 -- -- 575 1975 5.3 1 1167 178 3.6 0.24

DJ2 -- -- 575 1975 3.7 0.8 1201 160 3.3 0.25

DJ3 57 -- 562 1840 8.4 1.64 1067 106 3.6 0.03

DJ4 -- -- 570 1980 5 0.39 1149 140 3.6 0.13

DJ5 63 -- 570 1980 4.6 0.93 1239 139 3.7 0.05

DJ6 62 -- 570 2005 4.5 0.39 1156 272 3.9 0.14

DJ7 62 -- 570 1975 4.7 0.73 1141 157 3.4 0.12

DJ8 65 -- 530 1915 7.7 1.2 1053 172 3.4 0.07

DJ9 67 -- 590 2025 5.6 0.91 1214 118 3.9 0.24

*presence of air jet ** used the ST-gun


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12.7.4.2.1. Interpretation of results The HVOF coatings show superior performance to hard chrome in abrasion testing. This can allow use of HVOF carbide materials in conditions where abrasive particle contamination has eliminated past consideration of hard chrome.

12.7.4.3. Other Abrasion Tests Document 24. Tungsten Carbide-Cobalt Coatings for Industrial Applications (Courtesy of M. Dorfman, J Defalco, J. Karthikeyan) Document 36. Report of Replacement of Chromium Electroplating Using HVOF Thermal Spray Coatings AESF Plating Forum 1998 (Courtesy of Bruce Sartwell and HCAT Team).

This summary describes abrasion testing other than the standard ASTM G-65. As can be seen from comparing the test parameters from Table 66, the methodologies are significantly different, which prevents direct correlation of the results.

Table 66. ASTM G76 Data from NRL and Sulzer Metco. Parameter Set-up #1-HCAT Calowear

Tester Set-up #2 Sulzer Metco

Abrasive media 4 micrometer diameter silicon carbide particles

-53+15 micron alumina

Carrier Distilled water Distilled water

How media applied

Drip feed -

Concentration of media

- 150 grams of alumina/500 milligrams of water

Mating wear surface

2.5 cm dia. hardened steel ball

Ground cast iron plate

Load-how applied

Sliding normal force of .27 N

200 g/cm2

How wear is measured

Measure volume of wear crater

Thickness and weight loss

How expressed Coefficient K expressed as volume removed per unit load and unit sliding distance

-

Wear sample size

- 25 mm diameter

Test duration 50, 000 revolutions 5 minute intervals with total test time of 15 minutes



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The HCAT-directed work contains a direct comparison to hard chrome baseline materials with both WC-Co and T-400 HVOF coatings. The Sulzer Metco work is a comparison of WC-Co powders sprayed with two different spray systems.

The data for both tests are shown in Table 67 and Figure 74. Review of the work indicates the following:

• The HVOF coatings show some improvement in Calowear Abrasion testing when run in side-by-side comparison with hard chrome, provided they are harder than chrome (WC-Co), but higher wear when they are softer (T400).

• Comparison of varied powders/processes for WC-Co indicates that, as expected, the higher hardness materials perform better in abrasion wear testing, but that there is little difference in wear rate between several guns and powders.

12.7.4.4. Sliding and Fretting Wear Results

12.7.4.4.1. DARPA program – GEAE/NU Document 1. Hard Chrome Coatings - Advanced Technology for Waste Elimination, DARPA Grant MDA972-93-1-0006, Keith O. Legg, Jerry Schell, George Nichols, Robert Altkorn.

A-12 = WC-12% Co Standard =Air cooled gun

A-17 = WC-17% Co Hybrid = Hybrid gun

Figure 74. Comparison of HVOF Processes and WC-Co Powders

Table 67. Average wear coefficients, K, expressed in units of 10-4 mm3/N-m, for the various coating/substrate combinations. Sample # of tests K

Cr-plate on 7075 Al 4 9.3

Cr-plate on 4340 5 9.9

Cr-plate on PH13-8 4 9.7

WC/Co on 7075 Al 5 6.7

WC/Co on 4340 5 5.7

WC/Co on PH13-8 5 6.4

T400 on 7075 Al 5 13.3

T400 on 4340 5 15.6

T400 on PH13-8 5 18.1


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12.7.4.4.1.1. Test conditions Fretting wear measurements were made as described in 12.7.3.3, above. Test conditions were as described in Table 68.

12.7.4.4.1.2. Results In these tests the shoe was coated and the block uncoated. The results are shown in Figure 75. The zero-values for wear of chrome plated materials against 4340 are due to the fact that the 4340 material wore off the bare block surface and formed a protective layer on the coated material.

12.7.4.4.1.3. Interpretation of results These results are in general agreement with the data obtained in hydraulic rig tests (see Section 13.2.1) and flight tests (see Section 13.3.2). It is important to note that the surface finish was 32µ�, which is standard for chrome plate, but very rough for a thermal spray. As a result the thermal spray coatings caused higher wear on the adjacent materials. The T400 coatings showed higher wear (and less wear of the adjacent material) than the WC-Co coatings. Again this is in agreement with rig and flight tests, which have shown similar results. Lessons learned in rig and flight testing are that HVOF coatings cause more wear than chrome if the surface Ra value is high, but less if the Ra value is less than about 6 µ�.

Table 68. Fretting test parameters. Surface finish Ground � 32 µ�

Ra nominal

Block Uncoated

Shoe Coated

Coating thickness

0.010�

Stroke 0.040�

Cycles 786,000

Frequency 10 Hz

Temperature 400°F

Load 2,000 psi


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12.7.4.4.2. JTP for Landing Gear Document 43. HCAT landing gear JTP fatigue data - HVOF WC-Co on 4340, 300M, AerMet 100 in air and NaCl solution. Wear testing in this JTP involves both sliding and fretting wear. The test set-up designs are shown in Section 12.7.1, which compares varied testing methodologies. The sliding wear test involves a bushing being oscillated along a rod with a downward side load. The fretting test involves a shoe being pressed against a block over a short dithering stroke. The execution of this testing is currently in progress and should be completed by June 2001. Initial work has centered around the identification of the important variables in the process using a design of experiment (DOE) methodology. Key variables were identified as shown in Table 69.

IN7184340EHC

IN718IN718EHC

4340 4340EHC

4340 IN718EHC

IN718IN718

WC-Co

4340 IN718

WC-Co

IN718IN718

WC-Co

IN718IN718T400

IN7184340T400

Shoe Wear Coeff

Block Wear Coeff1.00E-12

1.00E-11

1.00E-10

1.00E-09

Wea

r Coe

ff.

Shoe - coatedBlock - uncoatedCoating

Figure 75. Fretting wear of hard chrome, HVOF WC-17Co, and HVOF T400. (Note – the zero wear measurement resulted from material transfer from the uncoated block to the coated shoe, protecting it from wear.)


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12.7.5. General Trend of Wear Results Wear results show that the HVOF materials generally show superior performance to hard chrome in all types of wear evaluations from erosion to sliding/fretting mechanisms. There are exceptions, however:

• As we would expect, abrasive wear resistance is controlled by coating hardness, and so softer materials (such as T400) do not perform as well in abrasion tests as chrome or carbides.

• In erosion testing at 90 degree angles, the harder WC-Co does not perform as well as chrome, again, as would be expected.

• In fretting wear, HVOF coatings with rough surfaces (32µ� Ra) tend to cause increased wear on adjacent uncoated materials. In some cases chrome performs better in testing because of material transfer. However, rig and flight tests (using 1 � 6 µ� Ra surface finishes) show better wear performance for the HVOF carbides.

Table 69. Wear test variables for DOE factors. Variable Default value Alternatives or Ranges

Test Type Bushing wear Fretting wear Rod, Shoe Materials

4340 + Hard Chrome

4340 + HVOF WC-17% Co

4340 + HVOF WC-10Co-4Cr

Bushing, Block Matls

4340 AMS 4640 Anodized Al 4340 with nitrile or PTFE seals; 24 hr hydraulic fluid pre-soak

Finish, Ra microinches

8-12, (target 8) 2-6, (target 4)

Lubrication none MIL-H-83282 hydraulic fluid

Pressurized to 3000 psi for seal tests

oad, lbs. 72 lbs. 30 lbs. minimum 240 lbs. Maximum

capable of up to 1000 lbs

Stroke, inches 0.010 Fretting 0.5 Bushing

0.005 to 0.1 inch range for fretting

0 to 3” range for Bushing test

Frequency (Speed) 10 Hz, Fretting 90 cpm, Bushing

1 - 70 Hz Fretting test 1 - 90 cpm Bushing test

Duration, hrs (cycles)

20 hrs, (cycles = 720,000 fretting & 108,000 bushing)

0 - 48 hrs Note: Longer times can be run if required to get sufficient amounts of wear to distinguish between tests.

Temperature Room temp. 200oF seals 350 oF all other

No active cooling (Capable of higher temps)

Operating Environment

Ambient air


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12.8. Impact

12.8.1. General Description and Test Methods The purpose of this type of test is to assess the resistance to chipping or damage, primarily from runway debris thrown up on landing or takeoff. This debris will most commonly be in the form of small stones and similar objects, but may occasionally be larger, heavier objects. Two types of tests have been used to assess different damage mechanisms that are expected for landing gear:

1. Gravelometer testing (ASTM D 3170-87) - which impinges gravel in a high-velocity air stream onto the surface (high hardness projectiles, high speed, low mass, small impact area).

2. Ball impact testing - in which a 1lb hardened steel ball is dropped from varying heights (up to about 4 ft) onto the surface (relatively soft projectile, low speed, low mass, large impact area).

The gravelometer test is most commonly used to assess the chip resistance of paints, although it is also used to assess the brittleness and chip resistance of hard materials such as glass. The air velocity is approximately 100 m/s (200 mph), but the gravel velocity is considerably less, depending on its size and shape. Gravelometer test - ASTM D 3170-87, SAE J400 The test standard is a 5 - 10 second feed of 550ml of road gravel (size 9.5 - 16 mm) into an air stream passing at a rate of 100 cfm (47 l/s) and a pressure of 70 or 80 psi (480 or 550 kPa). The surface is then examined for chips. Since chrome and HVOF WC-Co materials will be more difficult to chip than paints, or even glass, the test must be increased in severity by increasing the air pressure or the exposure time. After each test the surface is examined visually and with a binocular microscope for evidence of chips or cracking of the coating. The number and size of chips and cracks are recorded photographically and compared between the coating and the control (chrome in this case). Ball drop test A 1lb ball of hardened steel is dropped down a tube onto the surface of the coated specimen. The extent of coating damage (cracked area and delaminated area) is recorded photographically as a function of drop height. Since this is only a relative test, the extent of damage (cracked or damaged area, or area of delamination) is recorded for the test coating and control.

12.8.2. Impact Test Results Limited testing ball drop and gravelometry testing has been performed (by NADEP Jacksonville and Boeing respectively) comparing hard chrome with alternative HVOF coatings, and has been reported at HCAT meetings. In each case the HVOF coating was reported to have


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performed better than chrome, but data is not presently available. Both ball drop and gravelometer testing is to be done by the HCAT under the Landing Gear Joint Test Protocol and results are expected to be available on the HCAT Home Page by December 2000.

12.9. Hydrogen Embrittlement Hydrogen embrittlement is a concern with the use of hard chrome electroplating on high strength steels as a result of hydrogen produced at the surface of the substrate during electrodeposition processes. A bake out for approximately 1-2 hours at 350 - 400oF is required for all hard chrome plating to remove any hydrogen generated in the plating process that has migrated into the substrate. HVOF does (or can) use hydrogen as the fuel gas. However, there is no evidence that the hydrogen can become incorporated into the steel. Partly this is because it is mostly burned in the flame, partly that it is in molecular form (not the highly active atomic, or evanescent, form found in plating solutions), and partly that the heat of the process itself would be expected to liberate any dissolved hydrogen, as does a standard hydrogen bake. Environmental embrittlement (or re-embrittlement) occurs when corrosion of the steel generates hydrogen, which diffuses into the bulk and causes failure. This type of failure could in principle occur because of corrosion through a damaged coating.

12.9.1. General Description and Test Methods

12.9.1.1. Embrittlement Testing: To address these concerns, a test methodology has been developed in the Landing Gear JTP (Document 33) which addresses the concerns with HVOF materials. The standard embrittlement test is ASTM F 519, in which a notched bar is held under load � typically 75% UTS for 200 hours. Specimen and notch geometries are described in detail in the standard. Environmental embrittlement is tested by submerging the specimen in a corroding solution, such as salt water during the test.

12.9.2. Lufthansa embrittlement tests Document 38. Replacement of chrome plating by thermal spray coatings � Summary of tests (Courtesy of Lufthansa). 42 This document summarizes Lufthansa�s testing of thermal spray coatings for landing gear, which formed the basis for their flight testing. Lufthansa has examined the possibility of hydrogen embrittlement during processing, by beginning ASTM F519 testing of 4340 steel less than 2 hours after coating with WC-17Co and WC-CoCr. All specimens



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passed the test. As part of their work in qualifying HVOF WC-CoCr for landing gear, Lufthansa also intends to test for possible embrittlement during stripping (which is electrochemical).

12.9.3. Hydrogen Embrittlement Tests Planned - HCAT

In manufacturing and maintenance the same component may be coated with both electroplates (e.g. Cd) and chrome coatings. Since hydrogen can easily diffuse through the cracked chrome, cadmium plating can be done subsequent to chrome plating provided the hydrogen can diffuse out through unplated or chrome plated areas. Whether or not hydrogen introduced during plating can diffuse through an HVOF coating could be important in properly sequencing coating operations. For this reason, the HCAT is testing embrittlement of high strength steels with HVOF coatings, addressing the following issues:

1. Demonstrating that the HVOF coating process does not contribute to hydrogen embrittlement.

2. Evaluating whether hydrogen that might be generated in a subsequent plate can pass through an HVOF WC-Co layer either during the HVOF process itself (which heats the surface) or on subsequent heat treating.

3. Evaluating whether HVOF-coated steels have any significant difference in re-embrittlement behavior than chrome plated steels.

No testing has been performed to date comparing hard chrome with alternative HVOF coatings. Results of testing are expected by December 2000 and will be made available on the HCAT Home Page.

12.9.4. General Trend of Hydrogen Embrittlement Results

All experience with thermal spray coatings to date is that they do not cause embrittlement, and the Lufthansa results confirm this. Issues that remain are (all are under testing by HCAT):

• Diffusion of hydrogen through HVOF coatings � However, the fact that these coatings have some permeability to liquids (as evidenced in the corrosion data) suggests that they should also be porous to hydrogen, permitting out-diffusion of hydrogen from subsequent plating operations.

• The possibility of embrittlement resulting from electrolytic stripping of thermal spray coatings.

• Possible differences in environmental embrittlement between chrome plated and thermal sprayed steels.



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12.10.Creep One function of hard chrome is to ensure proper interference fits between parts, such as a bearing journal surface and the bearing itself. Shafts are typical examples of this situation. An important aspect of this type application has been the ability of the hard chrome to maintain a tight fit without relaxation of the compressive interference loads due to creep. If the chrome creeps (especially at high temperature, as can happen in an engine), the holder will become loose, spin, and gall the shaft.

12.10.1. General Description and Test Methods There are no ASTM standard methods for creep evaluations for this particular situation and application. A test methodology has therefore been developed by GE Aircraft Engines as follows:

• The coatings should be deposited to an approximate thickness of .030� on flat substrates.

• The coated side can be ground flat with minimal coating removal. The substrate material is machined from the reverse side, leaving only coated material.

• Specimens are cut 0.25 inches square for testing.

• Testing is done using stacks of 3 specimens to allow for greater accuracy in measuring the height changes due to creep.

• Alumina platens are utilized for loading the coating specimens to assure the creep deformation is in the coating specimens and not the platens.

• Measurements can be obtained by two methods; 1) direct flat anvil micrometer measurements of the individual test specimen stacks before and after testing, and 2) dial gage extensometer readings during the test.

• Suggested test conditions are 800oF under varied loads and hold times. The 800oF temperature is the upper temperature limit where hard chrome might be used, so that these high load, high temperature tests represented a worst case set of conditions for compressive creep of a chrome replacement.

12.10.2. Documents Document 45 Compressive Creep Tests of Hard Chrome and HVOF coatings 1998, J. Schell, GE Aircraft Engines.43 This document was produced as part of an initial evaluation of HVOF coatings for chrome replacement.

"Compressive creep of Cr and HVOF coati


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12.10.3. Creep Testing Results

12.10.3.1. Results for HVOF WC-Co and T400 Document 45. Compressive Creep Tests of Hard Chrome and HVOF coatings 1998 This document summarizes creep testing for HVOF alternatives. Testing was done as described in 12.10.1 General Description and Test Methods, with the parameters shown in Table 70.

12.10.3.1.1. Test conditions

Table 70. Creep test parameters Description Parameter Hard chrome thickness .030�

HVOF thickness WC-17Co .025� T-400 .025�

Machining Acid etching used to remove final substrate from chrome due to fracturing problems

Test temperature 800oF

Test loads 50 ksi 100 ksi

Test Duration 300 hrs 1000 hrs


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12.10.3.1.2. Results Test results are shown in Figure 76. WC-Co showed no measurable creep at 50 ksi and gave the lowest compressive creep strain rate at 100 ksi, 0.000057 (10% that of chrome). The WC-Co showed such low creep

after the planned 300 hours at 100 ksi that the time was extended to 1000 hours and the low load test abandoned. T400 showed higher creep than WC-Co, but much less than hard chrome.

12.10.3.1.3. Interpretation of results Clearly, WC-Co is far less subject to creep than hard chrome, and therefore more suitable for most engine applications that involve high load at elevated temperature (such as shaft areas with press-fitted bearing holders, which are quite common on main power shafts). It is interesting to note that regardless of which set of measurement data was examined, the hard chrome strain rates were approximately doubled when the load was doubled while the Tribaloy 400 strain rates increased by about four times. The hard chrome is a homogeneous single phase material and gave linear behavior with load. However, the Tribaloy 400 material contains a dispersed hard phase (Laves phase) in an fcc alloy matrix. The Tribaloy 400 behavior probably occurred due to a creep mechanism change with load for the two phase structure not seen for the single phase structure of hard chrome.

12.10.4. General Trend of Creep Results Results show that creep behavior of HVOF alternative materials is superior to the current hard chrome materials. This is true for both WC-Co and T-400, although the harder carbide is far more creep-resistant than the Tribaloy.

WC-17Co

T400 Chrome

50ksi

100ksi

00.00010.00020.00030.00040.00050.0006

Avg

. str

ain

rate

(in

/in/h

r)

Figure 76. Average creep measured by direct micrometer readings.


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13.13.13.13. System performance

13.1. Summary This section summarizes the data obtained in various rig and flight tests. In most, if not all, rig and flight tests thus far HVOF coatings have proved superior to chrome. However, this level of performance in general requires that the surface finish of the HVOF coating is finer than that of chrome, achieved either by grinding to 4µ� Ra or less, or by superfinishing. Actuator tests have shown that the HVOF coatings have little or no wear or leakage when used with PTFE seals, but rapidly damage nitrile seals. In landing gear, however, either seal type can be used. As a result of rig and flight testing, HVOF coatings have been qualified as chrome replacements by a number of airlines and manufacturers.

Table 71. Summary of rig and flight testing data. Test Organization Notes Hydraulic actuator rig

Green, Tweed and Co

WC-Co

F-18 landing gear pin rig

Boeing Limited qualification test. HVOF-coated pins performed better than Cr.

Boeing 737, Airbus 320

Lufthansa Landing gear coated with WC-Co. Performance better than Cr

Boeing 737, 757, 767

Delta Airlines Landing gear coated with WC-CoCr. Performance better than Cr

F-18 landing gear repair

Messier-Dowty Canadian F-18 main landing gear polygon repair. Successfully flight tested. In approval cycle.


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13.2. Rig tests

13.2.1. Hydraulic Seals – Green, Tweed Phase 2 hydraulic rig test

13.2.1.1. Documents

Document 46. Evaluation of Chrome Rod Alternative Coatings, Tony Degennaro, Green Tweed, 1999. 44 This document describes the results of a 50 million cycle hydraulic actuator test that evaluated HVOF WC-CoCr and T400 versus hard chrome. The data from this test is also summarized on Green, Tweed�s web site at http://www.gtweed.com/Aerospace/ASTN/astnv11n7.htm.

13.2.1.2. Test Description The test is a hydraulic rig test using a fully-assembled hydraulic actuator in which the rods were coated with various chrome alternatives and the hydraulic cycled for a total of 50 million cycles. This test was designed to simulate typical actuator service conditions. Measurements were made of hydraulic fluid leakage rate as a function of time. Seal and rod wear and condition were measured on disassembly.

13.2.1.3. Test Conditions

Sample coatings and surface conditions are shown in Table 72. The stroke and frequency profile is shown in Table 73. The surface finish for

"Evaluation of Chrome Rod Alternat

Table 72. Hydraulic test conditions. Substrate material AISI-SAE 1566 steel, case hardened to 60-65 Rc

Coating material EHC HVOF T 400 HVOF WC-10Co4Cr

Coating thickness 0.002�

Surface finish EHC 4µ� Ra

T 400 9 µ� Ra

WC-Co 4 µ� Ra

WC-CoCr 6.5 µ� Ra

Seals ACT (elastomeric seal)

Enercap (capped-type PTFE seal)

http://www.gtweed.com/Aerospace/ASTN/astnv11n7.htm


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EHC was chosen to be comparable with the standard specified finish for hydraulic rods. Samples were ground only - no superfinishing was used.

13.2.1.4. Results Results for the testing are shown in Figure 77 and Figure 78. Figure 77 shows cumulative oil leakage as a function of time. Therefore a steadily rising curve represents an unchanging leak rate, while an increasing slope is representative of leakage that becomes worse with time. Where there is a peak, this means that the seal failed and had to be replaced, which restarted the leakage measurement. After the first failure with the WC-Co ACT seal and the second with the Tribaloy ACT seal, the elastomeric ACT seals were replaced with PTFE Enercap seals, which performed similarly to the other Enercap seals with the same rod coatings.

Table 73. Stroke and frequency profile for hydraulic tests. Pressure (psi) Stroke (�) Frequency (Hz) Cycles

3,000 3 0.5 1

2,200 1.5 1 2

1,800 .75 2 4

1,500 .3 4 10

1,000 0.09 5 33

800 0.06 5.5 50

500 0.03 6 100

200 total

Sequence repeated to 50,000,000 cycles


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Figure 78 shows the actual measured seal wear and its standard deviation. Failures were seen in all the coatings with the ACT seals, and none with the Enercap seals. After the failures in the ACT seals with WC-Co and Tribaloy, the ACT seals were replaced with Enercaps, and the leakage rate dropped to a low, steady value.

13.2.1.5. Interpretation of Results When HVOF coated rods are used in conjunction with PTFE seals, the performance of the rod and seal is markedly superior to EHC, with the WC coatings performing the best and providing seal life about an order of magnitude longer than EHC (presumably because of their hardness).

515 25 35 45

WC-Co, ACTTribaloy, ACT

Chrome, ACTWC-Co-Cr, Enercap

WC-Co, EnercapTribaloy, Enercap

Chrome, Enercap

0500

1000150020002500

Lea

kage

(g

ram

s)

Cycles (millions)

Figure 77. Cumulative hydraulic fluid leakage in rig tests.


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Both T-400 and WC-Co tend to damage elastomer seals and cause premature failure and leakage (about one third the seal life). For this reason, Green, Tweed does not recommend the use of elastomer seals on flight surface actuator rods with these coatings until more work has been done to determine whether superfinishing overcomes the problem.

13.2.1.6. Comments Note that this test was designed to represent control actuators, and does not represent landing gear or intermittent use actuators such as uplock and downlock hydraulics. Regarding elastomer seals, the Green, Tweed report states, �The leakage rate on WC-Co is similar to chrome but the seal life is roughly one-third that of chrome. Please note that this test simulated a flight control duty cycle. The WC-Co/elastomeric seal combination may be completely acceptable for landing gear and hydraulic system utility actuators but can not be substantiated under these tested conditions.� See Section 13.3 for information on landing gear seal wear, which appears to be acceptable with this combination of HVOF coating and elastomer seals.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

WC-Co-Cr,

Enercap

WC-Co,Enercap

Tribaloy,Enercap

Chrome,Enercap

Tribaloy,ACT

Chrome,ACT

WC-Co,ACT

Rod, Seal Combinations

Wea

r & S

tand

ard

Dev

iatio

n (i

nch) Avg

Standard Deviation

Figure 78. Seal wear during hydraulic rig tests.


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13.2.2. Landing Gear Pins – Boeing landing gear rig test

13.2.2.1. Documents

Document 47. F/A-18E/F Main Landing Gear HVOF-coated Pin Testing and Evaluation.45 This document summarizes rig testing on HVOF-coated landing gear run at Boeing St, Louis as part of a main landing gear fatigue test.

13.2.2.2. Test Conditions The following pins were stripped of chrome plate and recoated with HVOF WC-17Co per BAC 5851 specification:

• 74A400527 - Upper Sidebrace Attach Pin

• 74A400748 - Sidebrace to Universal Pin

• 74A400645 - Upper Oleo Pin The test was an FT66 fatigue test conducted on a full scale left side main landing gear. Loads were applied through hydraulics and strain was measured with strain gauges. The test involved subjecting the gear to a spectrum of loads and frequencies designed to simulate operating conditions over a period of 14,000 standard flight hours. Load requirements were designed to meet MIL-A-8867, and a pass required that each component complete two lifetimes without failure. The fatigue test was followed by a Constant Amplitude Cycling (CAC) test. Figure 79 shows the landing gear and the locations of the pins.

"Gaydos F-18 MLG rig test summary.pdf"


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13.2.2.3. Results The HVOF-coated pins showed no loss in dimensions, no wear damage, and passed magnetic particle inspection. There was some material transfer from the Cu-Be bushings onto the HVOF surfaces during the cycling test.

13.2.2.4. Interpretation of results The HVOF coated pins passed the FT66 fatigue test, with performance equal to or better than chrome. However, Boeing will require additional testing of HVOF-coated pins before the coating can be qualified on other components in the F-18 landing gear.

13.2.2.5. Comments There was some transfer of material from a Cu-Be bushing. This tends to occur quite frequently with HVOF-coated components running against soft bushings. Generally it appears to occur early in the wear process, and then stabilize with little or no further transfer. Delta Airlines has successfully eliminated material transfer by coating some bushings with Tribaloy 400 (see Section 13.3.2).

13.2.3. Rig tests under development – Messier-Dowty

Messier-Dowty, the manufacturer of the F/A-18E/F nose landing gear, plans to carry out the following Qualification Test Procedures in the period

Figure 79. F/A-18E/F main landing gear, showing locations of HVOF-coated pins.


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2000-2001: 1. QTP 1568: Fatigue testing of high velocity oxygen fuel (HVOF)

coated F/A-18/EF nose landing gear. A complete NLG assembly, will be tested in the ground load fatigue mode (excluding catapult and holdback loads).

2. QTP-1569: Endurance and fatigue testing of high velocity oxygen fuel (HVOF) coated F/A-18/EF nose landing gear drag brace assembly. A drag brace assembly will be tested for both ground load fatigue (including catapult and holdback loads) and retraction-extension endurance.

The point of contact for this work is Roger Eybel: mailto:[email protected].

13.3. Flight tests

13.3.1. Lufthansa Lufthansa was the first airline to place an HVOF WC-17Co-coated B737-300 nose landing gear inner cylinder into service in January 1996 for a two year flight test. The reason for moving toward thermal spray was both environmental and economic. The normal chrome plating process for this landing gear part is six days because of the need for special masking and anodes, and embrittlement-relief heat treating. The HVOF process can be done in less than 4 hours. Subsequently, Lufthansa has adopted HVOF WC-CoCr for its improved corrosion resistance.

13.3.1.1. Documents Document 37. Replacement of Chrome Plating by Thermal Spray � Results of Corrosion Testing of HVOF Coatings 1996 (Courtesy of Lufthansa This document details the corrosion test data underlying Lufthansa�s final choice of WC-CoCr for landing gear. (Note: this is a very large document.) Document 38. Replacement of chrome plating by thermal spray coatings � Summary of tests (Courtesy of Lufthansa). This document summarizes the Lufthansa data, including flight testing as of 1997.

13.3.1.2. Test Conditions Both the inner cylinder seal surface and the axle journals were coated. The aircraft was flown on standard commercial flights in Europe. The landing gear was inspected at 6-month intervals under full extension, and the wheels were removed to check the axles. The surfaces were



Page 166


inspected visually for chipping and flaking, and dye penetrant inspection was used to look for cracks.

13.3.1.3. Results The test, which began in January 1996, ended in April 1998 after completing 4701 flight cycles. No defects were found in the coating, although the surface roughness had risen to 10µin due to Teflon pick-up from the seal. Seal life was improved by about a factor of two, from a typical 900-1100 flight cycles for chrome to 1910 flight cycles for HVOF. A fluorescent penetrant inspection (FPI) showed crack-like indications on the aft of the cylinder that were not visible with dye penetrant. The cylinder was checked at Boeing with Barkhausen noise measurements, which measures stress under an oscillating magnetic field, and would indicate penetration of the cracks into the underlying steel. The test showed the cracks to be confined to the coating, and the part was returned to service. As of September 1999, no defects have been found.

13.3.1.4. Interpretation of results HVOF coatings perform better than chrome in flight testing. They do not show significant wear. Provided the surface is smooth (in this case 6µin Ra, but 2-6µin Ra is typical), seal life is improved.

13.3.1.5. Comments • It is necessary to use FPI rather than simple red dye penetrant

inspection.

• Some cracks may be seen in FPI tests, but thus far there is no evidence of any propagation of cracks into the substrate. (This agrees with extensive tests made on fatigue specimens � see Section 12.6.)

• Note that Lufthansa has now moved primarily to HVOF WC-CoCr for its better corrosion resistance.

13.3.2. Delta Delta Airlines has flight tested a number of landing gear inner cylinders, axles, and axle sleeves over the past three years.


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13.3.2.1. Test Conditions All components were coated per Specification BAC5851, Class 2 or 4 (HVOF or D-gun) using BMS 10-67 Type XVII powder (WC-10Co-4Cr). The pistons were ground to 8-16µin Ra (as specified for chrome) initially, and later superfinished to less than 2µin Ra to prevent seal damage. Axles were ground to 3-6µin Ra. The components and the areas coated are shown in Figure 80. In the case of the Boeing 757 and 767 main landing gear axles, the HVOF-coated axles were mounted on one side of the aircraft, with standard chrome plated landing gear on the other to permit direct comparison. The landing gear were inspected on the aircraft at 6-monthly intervals, visually inspecting for damage and delamination, measuring diameters for wear, measuring roughness, and using FPI for cracks. They were also tracked for seal leakage on a daily basis.

Aircraft Component Flight test status 2 each Boeing 737 Nose landing gear

piston; lower bearing Completed successfully

4 each Boeing 757 Main landing gear axles and axle sleeves

2 completed successfully; 2 still in test

4 each Boeing 767 Main landing gear axles and axle sleeves

1 completed successfully; 3 still in test


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Figure 81 shows a Boeing 737 undergoing inspection at Delta.

B 737 landing gear

B 757 landing gear axle

B 767 landing gear axle

Figure 80. Landing gear components HVOF sprayed for flight testing by Delta Airlines (sprayed areas numbered). (Courtesy Delta Airlines.)

Figure 81. Boeing 737 nose landing gear inner cylinder undergoing flight test inspection at Delta Airlines (Courtesy Delta Airlines).


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13.3.2.2. Results Pistons:

It was found very quickly that the nominal 16µin Ra finish was too rough for the pistons and caused seal failure twice in only 900 cycles. After the piston was superfinished to <2µin Ra, the seals lasted 1910 cycles � twice as long as a typical seal running on chrome (see Section 7.6.6). The pistons exhibited essentially no wear. FPI showed no indication of coating or cracking. Axles: As with the pistons, the axles showed neither wear nor cracking of the coating. The chrome plated axles, on the other hand, showed significant roughening due to wear. The only problem found was the result of an improper hardness test, which resulted in coating delamination at 18 months in several nearby locations on one of the B767 axles. It was determined that these delaminations occurred at the indents where the Rockwell hardness of the axle had been measured (improperly) after coating. The coating specification was revised to preclude hardness testing in the working area and on coated areas. (Note that coating hardness can be measured by Diamond Pyramid Hardness, but this should obviously not be done on working areas � see Section 7.3.4.)

It was found that the 15-5 PH sleeves in contact with the HVOF-coated axles exhibited higher fretting wear than against chrome (which has been confirmed by laboratory measurements at Boeing18). For this reason the sleeves were HVOF-sprayed inside and out with WC-CoCr and with T400. The effect of this coating on sleeve wear is still under evaluation. As a result of these successful flight tests Delta has converted overhaul procedures for a majority of existing landing gear components from chrome plating to HVOF WC-CoCr, including ODs, shallow IDs, and lugs.

Figure 82. Boeing 757 axle sleeves HVOF-sprayed with WC-CoCr (Courtesy Delta Airlines).



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13.3.2.3. Interpretation of results The Delta flight tests represent a total of 20 aircraft years of flight experience (2 years of experience with 10 aircraft). The HVOF coatings outperformed chrome plate in coating wear and seal life. However, it was found to be important to take steps to prevent wear of adjacent surfaces. Seal wear was greatly reduced by superfinishing, while sleeve wear was reduced with a similar hard coating (although this is still under evaluation).

13.3.2.4. Comments Jay Randolph of Delta Airlines notes a number of other issues to be borne in mind when using HVOF coatings on landing gear46:

• Identification – Since it is not easy to identify HVOF-coated parts versus chromed parts, an easily-visible marking system should be used.

• Finishing – Diamond grinding and (for pistons) superfinishing is important. Ra alone is insufficient as a roughness measure, and one should use peak-valley and bearing ratio as well. In general static surfaces (such as axles) may have an 8µin Ra finish, but hydraulic seal surfaces should have ≤ 4µin Ra. Most grinding fluids are not a problem, but some can leach the CoCr binder.

• Coating thickness – The coating thickness after grinding should be at least 0.003” to avoid the possibility of flaking during grinding.

• Inspection – Fluorescent penetrant inspection is necessary to detect cracks in HVOF coatings. Magnetic particle inspection will not pick up cracks since the coatings are non-magnetic.

• Subsequent coating and stripping – HVOF-coated components can be cadmium plated or nital etched (for surface examination) after HVOF coating. Chrome and nickel strips will etch the WC-CoCr, so it must be masked if these solutions are to be used to remove Cr or Ni from an HVOF-coated item.

13.3.3. F-18 landing gear repair A repair for the Canadian F-18 main landing gear axle polygon has been successfully flight-tested and is currently in the approval cycle with the Department of National Defence (DND), Canada. This application is described in Section 15.5.2.


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Since the polygon was showing excessive wear on its internal surface, a repair procedure was developed by Messier-Dowty, the landing gear manufacturer. Since the depth of the polygon is quite small (see Figure 83), an HVOF WC-CoCr coating is sprayed on the inside surface, by spraying at an angle so as to reach down into the hole. Masking is used to prevent overcoating of other areas. This repair has eliminated the wear problem and has been successfully flight tested. It is now in the approval cycle with DnD.

13.3.4. Flight tests under way or under development

Several flight tests are planned or in progress on HVOF-coated components of military aircraft, as summarized in Table 74.

In addition flight tests are still in progress on commercial landing gear with Lufthansa and Delta Airlines.

Figure 83. Canadian F-18 main landing gear polygon repair (Courtesy Messier-Dowty).

Table 74. Military flight tests of HVOF-coated components. Aircraft Component Agency Comments P-3 Main landing gear

piston NADEP Jacksonville

In progress, WC-Co

E-6A Main landing gear uplock hook shaft

NADEP Jacksonville

In progress, WC-Co

C-130 Nose and main landing gear pistons and axles

Canadian DND Planned, WC-CoCr


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Part 4 Specs, Qualified Components

PART 4. SPECIFICATIONS AND QUALIFIED

COMPONENTS

14.14.14.14. Specifications and standards for thermal spray

14.1. Documents Document 48. Table of contents of BAC 5851 Thermal Spray Specification, 2000 (Courtesy Boeing Aircraft Corp.). 47

This is the latest version of the Boeing 5851 specification, which is used as the industry standard. The document is still in the comment stage as of the time of writing, and has not been issued.

Document 49. Standards for the Thermal Spray Industry, Bhusari and Sulit. 48 This document gives a good overview of general industry standards for thermal spray, mostly for applications outside the aerospace industry.

14.2. Boeing thermal spray specs – method, powder, grinding

14.2.1. Boeing Thermal Spray Spec – BAC 5851 Title: �Application of Thermal Spray Coatings� Issued: October 1993; updated (greatly expanded) version to be issued in 2000. Availability: Available to Boeing contractors Scope: This specification covers Plasma, HVOF, D-gun, and Super D-gun coatings. Notes: This is the primary specification used in the aircraft industry for thermal spray coatings. Many thermal spray coatings use this specification, even when they are not done for Boeing. Thermal spray coatings are specified by Class, Grade, and Type. The Classes control the technology to be used, viz:

• Class 1 Plasma,

"Contents of Year 2000 Boeing 5851 Th

"Bhusari TS Standards.pdf"


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• Class 2 High Velocity Oxygen Fuel (HVOF),

• Class 3 Detonation Gun (D-Gun), and

• Class 4 Super D-Gun� (SDG) There are two Grades:

• Grade A General use coatings

• Grade B Coatings for fatigue applications There are 18 classes, covering various materials for applications such as wear, corrosion, thermal barriers, bearings and bushings. The classes are summarized in Table 75. Types XVI-XVIII were added to the original table of coating types defined in the 1993 version of BAC 5851.

The Year 2000 version of this specification covers the thermal spray process, testing, and evaluation in great detail. The contents of the document are summarized in Document 48.

Table 75. Boeing thermal spray coating types. Name Formula Use

Type I Tungsten Carbide-Cobalt � High Cobalt WC-18Co Wear

Type II Aluminum Bronze Cu-10Al Soft bearing

Type III Aluminum Oxide Al2O3-3TiO2

Type IV Chromium Oxide Cr2O3

Type V Zirconium Oxide ZrO2-5CaO Thermal barrier

Type VI Nickel-Chrome Ni-20Cr

Type VII C. P. Aluminum

Type VIIII AISI 316 CRES Fe-17Cr-12Ni-3Mo

Type IX Cobalt Alloy 31 Co-25Cr-10Ni-8W

Type X 7XXX Aluminum Al-5Zn-2Mg-2Cu

Type XI AISI 46XX Steel Fe-2Ni-0.3Mo

Type XII Nickel-Aluminum Ni-5Al Rebuild

Type XIII Nickel-Aluminum, prealloyed Ni-5Al Rebuild

Type XIV Copper-Nickel-Indium Alloy Cu-37Ni-5In

Type XV Cobalt Alloy T-400 Co-28Mo-8Cr-3Si Wear, corrosion - high temperature

Type XVI Chromium Carbide 80 Cr3C2-80Ni20Cr Wear

Type XVII Tungsten Carbide-Cobalt-Chrome WC-10Co-4Cr Wear, corrosion

Type XVIII Tungsten Carbide-Cobalt � Low Cobalt WC-12Co Wear


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14.2.2. Boeing Powder Spec – BMS 10-67 Title: �Thermal Spray Powders� Issued: August 1993. Availability: Available to Boeing contractors Scope: This specification covers the thermal spray powders to be used in thermal spray according to BAC 5851 (see Table 75). Notes: This specification is used in conjunction with BAC 5851. It defines the powder chemistry and particle size distribution.

14.2.3. Boeing Grinding Spec – BAC 5855 Title: �Grinding and Machining of Thermal Sprayed Coatings� Issued: April 1997. Availability: Available to Boeing contractors Scope: This specification covers the grinding and finishing of thermal spray coatings made according to BAC 5851, using the materials defined in BMS 10-67. Notes: This specification is used in conjunction with BAC 5851 and BMS 10-67. It covers grinding and honing feeds and speeds.

14.3. Hamilton-Sundstrand – HS 4412 Title: �Coating, Plasma Spray Deposition, Process Specification for� Issued: August 1992. Availability: Available to Hamilton Sundstrand contractors. Scope: This specification covers plasma spray, D-gun, and HVOF coatings. Notes: This specification defines two classes of coatings:

1. Coatings for wear and corrosion resistance 2. Coatings for buildup.

It defines 15 different coating chemistries, their hardnesses and bond strengths.

14.4. Society of Automotive Engineers - AMS 2447 Title: �Coating, Thermal Spray High Velocity/Fuel Process� Issued: May 1998, revised October 1998. Availability: Society of Automotive Engineers (SAE),


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http://www.sae.org/, Phone (724) 776-4970. Scope: Covers engineering requirements for applying thermal spray HVOF coatings, and the properties of HVOF coatings. Includes powders, substrate preparation, coating methods, temperatures, test methods and Quality assurance. Notes: The specifications for HVOF coatings deposited under AMS 2447 are summarized in Table 76.

14.5. American Welding Society – AWS C.2-19-XX Title: �Specifications for Thermal-Spray Coatings for Machine-Element OEM and repair Applications� Issued: This specification is in the latter stages of development by the AWS C2 Subcommittee on Machine-Element Coatings. The standard is expected to be issued in 2000.

Table 76. AMS 2447 HVOF Coating specifications. Coating designation

Name/

Chemistry

Hardness, min (HV)

Bond strength, min (psi)

Oxides, max %

Voids, max %

Unmelts, max (per

mm2)

AMS 2447-1 Stellite 31

57Co-25Cr-10Ni-7W

400 8,000 5 1 5

AMS 2447-2 Tribaloy 400

60Co-29Mo-8Cr-3Si

500 9,000 2 1 2

AMS 2447-3 75Cr3C2-25NiCr

69Cr-20Ni-11C

800 10,000 2 1 -

AMS 2447-4 Ni-Al Alloy

95Ni-5Al

275 8,000 2 1 3

AMS 2447-5 Ni-Cr-Al Alloy

76Ni-18Cr-6Al

350 8,000 2 1 3

AMS 2447-6 Inconel 718

60Ni-19Cr-18Fe-3Mo

375 9,000 5 1 3

AMS 2447-7 WC-17Co

78W-16Co-5.1C

1050 10,000 1 1 -

AMS 2447-8 WC-12Co

88WC-11Co-4C

1000 10,000 1 1 -

AMS 2447-9 WC-CoCr

82W-10Co-4Cr-3.5C

1050 10,000 1 1 -

AMS 2447-10 WC-Ni

86W-10Ni-3.5C

1000 10,000 1 1 -

http://www.sae.org/



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Availability: Not yet available. Will be available from AWS, probably in 2000.

Scope: This specification covers thermal spray coatings of coating materials such as stainless steel, Ni-Al, Aluminum –bronze, Babbitt, Ni-Cu, and alumina-titania for repair of machinery. It includes equipment, spray methods, and sealer use. It incorporates the following spray methods:

• Arc wire

• Flame wire

• Flame powder

• Plasma powder Notes: The specification is still in draft form.

14.6. AMS standards under development Several specifications are under development by the Society of Automotive Engineers (SAE) Aerospace Materials Committee. These Aerospace Materials Specifications (AMS) will cover the use of HVOF coatings on high strength steels. They will cover the following:

• Deposition methods

• Powders – two specifications, AMEC 99B and 99C will cover WC-Co and WC-CoCr powders

• Grinding methods

The standards are being championed by Don Parker, NASA mailto:[email protected] (coating standards), and Bruce Bodger, Sulzer Metco [email protected] (powder standards), Jon L. Devereaux, NADEP Jacksonville [email protected] (grinding standards).



[email protected]


Page 177

Part 4, Specs, Qualified Components

15.15.15.15. Qualified Thermal Sprayed Airframe Components

15.1. Documents Document 50. HVOF WC aerospace applications for OEM and rebuild (Courtesy Southwest Aeroservice). 49 This document, authored by Jim Nuse, lists many of the components currently HVOF sprayed with WC-Co for both OEM and rebuild. The list is dominated by landing gear components and flap and slat tracks.

15.2. Usage of thermal spray in Gas Turbine Engines Several documents give a good overview of usage of thermal spray in turbine engines, as well as what it takes to qualify thermal spray coatings for engine applications.

Document 51. Thermal Spray Applications at GE Aircraft Engines (Dorothy Comassar, Courtesy GE Aircraft Engines). 50 This paper is an excellent review of the general applications, specific materials, and specific types of components

Document 52. OEM Approval for HVOF Wear Resistant and MCrAlY Coatings (Gary Naisbitt and Gorham Advanced Materials). 51 This paper covers requirements for and progress toward approving HVOF coatings on various engine components.

Document 53. Replacement of Chromium Electroplating on Gas Turbine Engines.52 This paper by Jerry Schell and Mark Reichtsteiner of GE Aircraft Engines reviews a project begun in 2000 to replace chrome on aircraft engines that is being spearheaded by PEWG (the Propulsion Environmental Working Group).

"HVOF Applications Listing SWA.PDF"

"Comassar, GE Briefing.pdf"

"Naisbitt Gorham March 1999.pdf"

"AESF March 2000, GTEs, Schell.pdf"



Page 178


Thermal spray coatings have been used for many years on hundreds, if not thousands, of components in gas turbine engines (GTEs), both at the OEM and O&R levels. GTE coatings are most commonly plasma spray, HVOF, or D-gun. Some of these are indicated in Figure 84. Examples include

• McrAlY corrosion-resistant bond coats for thermal barrier coatings

• Zirconia thermal barrier coatings on hot section blades, combustion and exhaust components

• Al-Si Graphite abradable coatings for blade clearance control

• Co-based alloys such as Tribaloy to prevent fretting of hot section components

• Cr3C2-NiCr for wear

• WC-Co for shafts

The thermal spray process is therefore well-established for engines, and is essential for proper engine function. It is only over the past 5 years or so that thermal spray coatings have become more widely used in airframes. This section covers the use of thermal spray for coating airframe components.

Figure 84. Thermal spray coatings used in a typical gas turbine engine. (Courtesy GE Aircraft Engines)50

Row an Technology Group


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15.3. Summary of thermal spray coatings on non-engine components

Table 77. Summary of thermal spray-qualified non-engine components. (Click on links to access data directly.)

Component Coating Specific-ation

Notes

Steering collars HVOF, plasma, D-gun WC-Co

BAC 5851 Boeing OEM coating

Slat tracks Pan castings

Gimbals

HVOF, plasma, D-gun WC-18Co


Bolts

Wear rings

D-gun WC-14Co BAC 5851 Boeing OEM coating

Plungers

Fittings

Plasma Cr2O3 BAC 5851 Boeing OEM coating

Mufflers

Fairings

HVOF, D-gun, plasma Tribaloy 400


Landing gear sleeves, pins D-gun, plasma Cu-37Ni-5In, WC-18Co


Boeing 747-400 landing gear HVOF WC-CoCr BAC 5851 Boeing OEM coating

Boeing 737 landing gear HVOF WC-CoCr BAC 5851 Overhaul and repair

Airbus 320 landing gear HVOF WC-CoCr Overhaul and repair

F-18 Main landing gear axle HVOF WC-CoCr AMS 2447 Canadian Air Force – flight tested, in final approval

Sikorsky CH-53 blade damper

Plasma T400 In final approval for Navy CH-53 fleet

P&W F-119 Engine Convergent Nozzle Actuator

Super D-gun WC-Co and Plasma T400

HS 4412 Hamilton Sundstrand spec. Engine likely to be used on JSF

Slat tracks, Boeing Super D-gun WC-18Co

BAC 5851 OEM usage

Bombardier Q-400 flap tracks

HVOF WC-Co Bombardier PPS 24.04

OEM usage

Bombardier Q-400 and Global Express door stop pins

HVOF WC-Co Bombardier PPS 24.04

OEM usage

Slat and flap tracks HVOF WC-Co BAC 5851 Overhaul and Repair

Flap track repair – Bombardier Dash 8

HVOF WC-Co RD 8-57-1164 to 1169

Overhaul and Repair

L1011 flap tracks HVOF WC-Co Overhaul and Repair – Titanium alloy

Boeing 737, 757, 767 landing gear

HVOF WC-Co BAC 5851 Main and nose landing gear O&R at Delta Airlines

Parker-Hannifin actuators Various OEM usage – all new -design hydraulics


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15.4. Boeing – qualified thermal sprayed components Boeing has specified thermal spray coatings on over a hundred airframe part numbers. The majority of these components (86) are specified for WC-Co, primarily WC-14Co or WC-18Co.

Document 54. List of Boeing thermal sprayed parts (Courtesy, Boeing Aircraft Corp). Document 54 is Boeing�s list of components on which they have qualified thermal spray as of June 2000. A general summary of this listing is given in Table 78.

"Boeing Thermal Sprayed Parts List.pd

Table 78. Summary of Boeing components specified for thermal spray. Component types Coating method Coating material Steering collars Pins

HVOF, plasma, D-gun

WC-Co

Slat tracks Pins Pan castings Gimbals Sleeves

Plasma, HVOF, D-gun, Super D-gun

WC-18Co

Bolts Pins Wear rings

D-gun WC-14Co

Plungers Fittings

Plasma Cr2O3

Mufflers Fairings

HVOF, D-gun, plasma

Tribaloy 400

Landing gear sleeves, pins D-gun, plasma Cu-37Ni-5In, WC-18Co


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15.5. Landing gear

15.5.1. OEM Production - Boeing 767-400 landing gear

The landing gear of the new Boeing 767-400 are specified for HVOF WC-CoCr in place of hard chrome on axles, inner cylinders, and other components. These items are also specified for chrome plate, so that the two different coatings are interchangeable for different vendors. Some customers now specify HVOF explicitly for landing gear on their new aircraft. This is the first full-scale OEM production use of HVOF-coated landing gear on a commercial airliner in which the HVOF was designed on the component at the outset, rather than being used to solve a problem after the item had gone into production. The drawings permit either chrome of HVOF WC-CoCr to be used. The landing gear is among the largest used in commercial airliners, and is shown in Figure 86.

HVOF WC-CoCr is applied to the following 767-400 landing gear components:

• Main Inner Cylinder

• Main Axles

• Bogey Beam Pin

• Brake Rod Pins

Figure 85. Boeing 767-400 with HVOF coated landing gear.

Figure 86. Boeing 767-400 main landing gear (Courtesy Sulzer Metco).



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• Spindles

• Various Bushings

The major items in this list are the inner cylinder and axle. This landing gear has four sets of wheels, with eight journals, all of which are specified for chrome or HVOF WC-CoCr. In this landing gear the inner cylinders are separate from the axles. Figure 87 shows the main landing gear axle, on which the HVOF coating (or chrome plate) is indicated as engineering callout #3. The coating is used on the center section and journal areas.

Figure 88 shows the main landing gear inner cylinder, with asterisks showing the locations of HVOF-sprayed areas. Note that the OD of the cylinder is coated. The ID is also coated over a two-foot section, which is possible only because the ID is very large (10”). (This is about the smallest ID on which HVOF can be used, since it is limited by the gun size and standoff.)

Figure 87. Boeing 767 main landing gear axle (part # 2207-85-10), showing HVOF areas (engineering note 3). (Courtesy Sulzer Metco.)



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The specification for all Boeing landing gear repairs is BAC 5851.

15.5.2. Flight tested landing gear repair - Canadian F-18 MLG axle

A repair for the Canadian F-18 main landing gear axle polygon has been successfully flight-tested and is currently in the approval cycle with the Department of National Defence (DND), Canada. The axle is shown in Figure 89. The axle is approximately 18” in total length, while the polygon has a 4” equivalent diameter.

Figure 88. Boeing 767-400 main landing gear inner cylinder (Part # 2207-4-10) with asterisks showing locations of HVOF coatings (Courtesy Sulzer Metco.)

Figure 89. Canadian F-18 main landing gear axle (Courtesy Messier-Dowty).


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The repair was developed by Messier-Dowty, the landing gear manufacturer, for the polygon ID section of the axle, which was showing excessive wear (see detail in Figure 90). The figure shows the HVOF-repaired area, which is the white polygon. The coating material is HVOF WC-CoCr, which is sprayed into the repair area at an angle from the outside. The coating process specification for this repair is AMS 2447.

15.5.3. Other qualified landing gear applications Various other landing gear components are also qualified for HVOF, as indicated in Table 79.

Note that not all of these coatings are WC-Co. Tribaloy 400 is often used, e.g. where WC-Co is too hard and causes excessive wear on adjacent components.

15.5.4. Boeing overhaul manual revision In early 1999 Boeing issued a revision to its landing gear overhaul manuals to allow an alternate repair of HVOF, D-Gun, or SDG, in lieu of chrome plate for finished coating thicknesses of 0.010 inches or less. This makes it possible for airlines and landing gear O&R shops to

Figure 90. Repair area of F-18 main landing gear polygon (Courtesty Messier-Dowty).

Table 79. Other landing gear components qualified for OEM HVOF WC-Co (Courtesy Southwest Aeroservice). Item Coating F-18 Steering Covers T-400

A-340 Bushings T-400

F-18 Shock Absorber Piston Heads T-400

Boeing 737 Nose Torsion Link Pin WC-Co

Beech Premiere nose outer cylinder bearing lugs WC-Co

Various rotor drive keys for brake drums WC-Co


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substitute HVOF for chrome in the overhaul of most landing gear components. However, any such use of HVOF can only be done by a Boeing-approved facility. Approval involves validating the repair procedure, which includes spraying of metallographic and fatigue test samples. As of this point, there are a number of qualified thermal spray shops able to carry out HVOF for overhaul. As a result of this revision, HVOF WC-Co or WC-CoCr is being used on a number of aircraft landing gear, such as the Boeing 737 and the Airbus 32053.

15.5.5. Delta Airlines qualified landing gear repair – Boeing 737, 757, 767

Delta Airlines flight testing is summarized Section 13.3.2. After 24 months of successful flight testing involving 6-monthly inspections of several aircraft, Delta Airlines has converted the nose and main landing gear cylinders and axle journal repairs from chrome to HVOF WC-CoCr on the following aircraft:

• Boeing 737-200/300

• Boeing 757-200

• Boeing 767-200/300 Delta have also converted other line-of sight applications, such as shallow IDs and lug faces to HVOF. The airline has required HVOF WC-CoCr coated landing gear on its new Boeing 767-400 aircraft, and intends to convert most of its landing gear component repairs to HVOF WC-CoCr in the near future. During the course of flight testing, it was found that a smooth surface finish of the inner cylinder was essential to prevent damage to the seals and premature seal failure (see Section 13.3.2).



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15.5.6. Qualified landing gear repair With Boeing’s specification of HVOF as an alternative to chrome in landing gear repair, HVOF WC-Co and WC-CoCr are now being used extensively for repair of landing gear inner cylinders and axles (for example by Southwest Aeroservice). The specification for these coatings is BAC 5851.

Figure 91 shows a Boeing 737 nose landing gear inner cylinder being HVOF-coated. The HVOF gun on the right

traverses up and down the cylinder while the cylinder rotates about its axis. The combination of rotation speed and cooling air jets (not shown) is used to maintain a proper temperature.

Table 80 summarizes some of the landing gear components frequently coated with HVOF WC-Co. All of the commercial components are qualified repairs. The P3 component is currently in flight test.

15.6. Hydraulics Thermal spray coatings are not yet as widely used on hydraulics as they are on landing gear.

Praxair has developed a combination of coatings to replace chrome plating on hydraulic actuator rods, piston heads, and cylinder IDs. They supply the following combination of coatings as a general approach for hydraulic actuators:

Figure 91. HVOF WC-Co repair of Boeing 737 nose landing gear inner cylinder (Courtesty Southwest Aeroservice).

Table 80. Landing gear components commonly repaired with HVOF WC-Co (Courtesy Southwest Aeroservice).

737 Nose Inner Cylinder

Main Inner Cylinder

Nose Lower Bearing

757 Main Axles

Main Brake Sleeves

767 Main Axles

Main Brake Sleeves

Numerous Pins, Bolts and Hardware

P3 Main Inner Cylinder*

* In flight test



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• D-gun/HVOF WC-Co, WC-CoCr and WC-CrNi on rods

• Plasma (or low pressure plasma) sprayed Tribaloy 400 on cylinder IDs (>2.5" diameter)

• D-gun/HVOF WC-Co or T400 on piston heads.

Variations of this multiple-coating approach (see figure 92) have been qualified on several actuator systems by different manufacturers.

Parker-Hannifin has made the decision that the company will not use chrome plate on new designs54, and instead is moving to other materials and coatings, including thermal sprays. Some Parker flight control actuator pistons are already being HVOF-coated. Given the ubiquity of hydraulics in aircraft, and Parker’s position as the market leader in this area, this decision is likely ultimately to have a major effect on chrome usage in the aircraft industry.

15.6.1. P&W F-119 engine convergent nozzle actuator

The Pratt & Whitney F-119 engine is being produced for the F-22, and is expected to be used on the JSF. The convergent nozzles are hydraulically actuated using actuators produced by Hamilton Sundstrand. Both the actuator rod and cylinder ID are thermal sprayed. The specification for both of these coatings is HS 4412.

Plasma T400

D-gun/HVOF WC-Co, WC-CoCr, WC-CrNi

D-gun/HVOF WC-Co, T400

Figure 92. Thermal spray actuator coating system developed by Praxair.



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15.6.2. Flight test – Sikorsky CH-53 blade damper The CH-53 is a heavy-lift helicopter manufactured by Sikorsky. Currently the IDs of the blade dampers are coated with hard chrome. HVOF Tribaloy 400-coated outer cylinders are in the final stages of flight testing for eventual replacement of all dampers on the Navy’s CH-53 fleet. The damper has an ID of about 3.5” and a length of about 6”.

This application is expected to be qualified shortly.

Figure 93. CH-53 helicopter (Sikorsky).


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15.7. Production - Flap and slat tracks

15.7.1. OEM tracks - Boeing Boeing specifies Super D-gun WC-18Co on a number of flap and slat tracks, as summarized in Table 81.

Boeing 737 flap drive gimbals are also coated with HVOF WC-Co.

15.7.2. OEM tracks - Bombardier The flap tracks for the new Bombardier Q-400 are now in production with HVOF WC-Co. In this case the thermal spray replaces electroless Ni-B, which was found to have inadequate life. The aircraft has no slat tracks, but has 10 flap tracks, which are all coated to the same Bombardier Production Process Specification PPS 24.04.

Table 81. Flap and slat tracks specified for thermal spray coating with Super D-gun WC-18Co (Courtesy Boeing).

Part # Description

113A3941 TRACK ASSY - #2, OUTBD AFT FLAP

114A7511 MAIN TRACK ASSY - SLAT 1 AND 8 OUTBD, SS494.85

114A7512 MAIN TRACK ASSY - SLAT 1 AND 8 INBD, SS409.09

114A7521 MAIN TRACK ASSY - SLAT 2 AND 7 OUTBD, SS362.51

114A7522 MAIN TRACK ASSY - SLAT 2 AND 7 INBD, SS282.91

114A7531 MAIN TRACK ASSY - SLATS 3 AND 6 OUTBD, SS 240.95

114A7532 MAIN TRACK ASSY - SLATS 3 AND 6 INBD, SS 163.95

114A7542 MAIN TRACK ASSY - SLATS 4 AND 5 INBD, SS 47.69

Figure 94. Bombardier Q-400 (Courtesy Bombardier.)


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The basic shape of a flap track is shown in Figure 95. The track comprises two mirror-image components fastened together. The flap rides along the track on bearings attached to the sides of the flap. These

bearings ride on the surfaces of the track, and in the fully extended position the bearings tend to cause severe fretting wear at their stopping points on the track because of the vibration of the flaps. In the past, electroless nickel has been used,

but it has always been less than satisfactory because of its low thickness and limited wear life, hence the move to HVOF coatings. HVOF coatings may cause higher wear of flap bearings, but these are inexpensive and easily replaced on the aircraft, whereas the track is valued in excess of $100,000 and cannot be repaired in-place.

15.7.3. Flap track repair – Bombardier Dash 8 The Bombardier Dash 8-100, -200, and �300 aircraft all have qualified flap track repairs using HVOF WC-Co (see Table 82).

Repair involves building up the damaged track areas and then coating the entire track with HVOF WC-Co (Figure 96). Both sides of both tracks are coated. Because of the shape of the flap tracks, the coating of the inner surfaces must be made with the HVOF gun off normal incidence. The complexity of the shape also precludes a post-coating grind, so that the

Figure 95. Typical flap track - Bombardier Dash 8 (Courtesy Vac Aero, Canada).

Table 82. Bombardier Dash 8-100, -200, -300 flap tracks qualified for HVOF repair (Courtesy Vac Aero). Flap track # Part # 1 85780044 � 101/102-105-106-109-110

85780185-101/102/103/104

2 85780044 � 103/104/107/108/111/112/113/114 85780185 � 103/104/107/108/111/112/113/114

3 85780035 � 101/102/103/104/105/106 85780189 � 101/102/103/104/105/106

4 85780013 � 101/102/103/104/105/106 85780183 � 101/102/103/105/107/111/113/115

5 85780014 � 101/102/103/104 85780184 � 101/102/103/105/107


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coating must be smooth as-sprayed. Repair specifications are governed by Bombardier RD 8-57-1164 to 1169.

15.7.4. O&R of tracks – Boeing and other aircraft HVOF WC-Co is now qualified and used for O&R on slat and/or flap tracks on a number of aircraft. The HVOF coating replaces other hard coatings such as electroless nickel, providing improved performance and longer life (see Table 83).

In the UK, TWI (The Welding Institute) of Cambridge, England has developed a qualified repair procedure for the titanium alloy flap tracks of the L1011. The repair

procedure involves TIG welding to rebuild worn areas followed by spraying with HVOF WC-Co.

Figure 96. HVOF-sprayed Dash 8 flap track. Coated areas are dark. (Courtesy, Vac Aero, Canada.)

Figure 97. Flap and slat track repair by HVOF (Southwest Aeroservice).

Table 83. Common flap/slat track repairs using HVOF WC-Co (Courtesy Southwest Aeroservice). Boeing 707 Boeing 727 Boeing 737 Boeing 767 Douglas DC-9 McDonnel Douglas MD-80 McDonnel Douglas DC-10 McDonnel Douglas MD-11 Embraer EMB120 Lockheed L-1011 Bombardier Dash 8-200/300


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15.8. Other components Both the Bombardier Q-400 and the Bombardier Global Express specify HVOF WC-Co on the stop pins for the cabin and baggage doors. Figure 98 shows a Boeing 737 nose landing gear bearing made of Al-Ni-bronze that is now repaired on the ID using WC-Co. Because the part is 3.25� ID by 2.5� deep, and open at both ends, it can readily be sprayed from outside. The repair is superfinished to prevent damage to the mating surface. Boeing 747 Wear Plates are also repaired with HVOF WC-Co. A number of other assorted OEM applications are also approved, as outlined in Table 84. Since 1996 United Airlines has been replacing chrome plating with HVOF coatings, as well as replacing proprietary coatings (D-gun, etc.) with in-house HVOF. A number of components on which chrome plating has been replaced by HVOF for O&R are summarized in Table 85.

Figure 98. Boeing 737 nose landing gear lower bearing shock strut, Part # 69-76508. HVOF WC-Co coated and super finished. (Courtesy Sulzer Metco.)

Table 84. Other OEM HVOF WC-Co applications (Courtesy Southwest Aeroservice). Helicopter Hardware Swashplate supports Swashplate balls Bearing sleeves Radius rings Miscellaneous Hardware Boeing 767 Wear strips Boeing 777 Fireseal Depressor Boeing 777 Various Frames


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Table 85. United Airlines O&R components qualified for HVOF in place of chrome plate.55

Part Type of component Aircraft Butter Shaft Accessory Component Part Boeing 747

Disc Hi Stage Valve Accessory Component Part Boeing 747

HPSOV Actuator Cap Accessory Component Part Boeing 767

Shaft Disc Hi Stage Valve Accessory Component Part Boeing 767

Valve Body Accessory Component Part Boeing 767

Body Butterfly Accessory Component Part Boeing 737

Shaft IOG Worm Accessory Component Part Boeing 747, 757, 767

Hub Compressor Front Disc Engine Component Part JT8D engine



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17 Norbert Klotz, “Superfinishing of Hard Chrome and HVOF Coated Workpieces”, Gorham Advanced Materials Conference on Advanced Coating Systems for Gas Turbine Engines and Aircraft Components, San Antonio, March 2000. 18 J.D. Nuse and J.A. Falkowski, “Surface Finishing of Tungsten Carbide Cobalt Coatings Applied By HVOF for Chrome Replacement Applications”, AESF Aerospace Plating and Metal Finishing Forum, Cincinnati (March 2000). 19 “Barkhausen Noise as a Quality Control Tool”, Stresstech. Also see http://www.stresstech.fi/. 20 T. Seitz, Lufthansa Technik AG, “Summary of Tests Performed at Lufthansa Technik with Regard to HVOF Coatings as a Chrome Replacement”, 3rd Global Symposium on HVOF Coatings, March 1997. 21 Boeing Aircraft Corporation, 5851 QPL, see, for example, http://active.boeing.com/ 22 J.G. Legoux, B. Arsenault, C. Moreau, V. Bouyer, L. Leblanc, “Evaluation of Four High Velocity Thermal Spray Guns Using WC-10Co-4Cr Cermets”, Proc. 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 479-493, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 23 S. DePalo, M. Mohanty, H. Marc-Charles, M. Dorfman, “Fracture Toughness of HVOF Sprayed WC-Co Coatings”, Proc. 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 245-250, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 24 M. Dorfman, J. DeFalco, J. Karthikeyan, “WC-Co Coatings for Industrial Applications”, Proceedings 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 983-990, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 25 “HVOF Process Development, Evaluation and Qualification” C-HCAT Progress Report, J-G Legoux, NRC, March 2000. 26 “Fracture toughness of HVOF sprayed WC-Co coatings”, S. De Palo, M. Mohanty, H. Marc-Charles, M. Dorfman, Proceedings 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 245-250, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 27 “Tungsten carbide–cobalt coatings for Industrial Applications”, M. Dorfman, J. DeFalco, J. Kathikeyan, Proceedings of the 1st International Thermal Spray Conference, Montreal (2000), p 471-478. ASM International, Materials Park, OH 44073-0002. 28 M. Factor, I. Roman, “A Critical Evaluation of the Employment of Microhardness Techniques for Characterizing and Optimizing Thermal Spray Coatings”, Proc. 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 1345-1354, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 29 J. Wigran, D.J. Greving, J.R. Shadley, E.F. Rybicki “Behaviour of Tungsten Carbide Thermal Spray Coatings”, Volvo Technology Report, 1, 13 (1995). 30 E.F. Rybicki et al “An ASM Recommended Practice for Modified Layer Removal Method (MLRM) to Evaluate Residual Stress in Thermal Spry Coatings”, Proc. 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 377-383, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002.


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31 J. Stokes, L. Looney, �Properties of WC-Co Components Produced Using the HVOF Thermal Spray Process�, Proc. 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 263-271, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 32 P.S. Prevey, �X-Ray Diffraction Residual Stress Techniques�, Metals Handbook, 380 (1986). ASM International, Materials Park, OH 44073-0002. 33 J. Matejicek, S. Sampath, T. Gnaeupel-Herold, H.J. Prask, �Processing Effects on Residual Stress in NI+5%Al Coatings � Comparison of Different Spraying Methods�, Proc. 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 351-354, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 34 See J. Schell in ref 43. 35 J-G Legoux, HCAT report, August 2000. 36 M. Matejicek, S. Sampath, J. Dubsky, �Residual Stress Measurement in Plasma Sprayed Coatings by X-ray Diffraction�, Thermal Spray: A United Forum for Technological Advances, Ed. C.C. Berndt (pub by ASM, 1997). ASM International, Materials Park, OH 44073-0002. 37 B.D. Sartwell, P.D. Natishan, I.L> Singer, K.O. Legg, J.D. Schell, J.P. Sauer, �Replacement of Electroplating Using HVOF Thermal Spray Coatings�, AESF Aerospace/Airline Plating and Metal Finishing Forum (1998). 38 T. Seitz, �Replacement of Chrome Plating by Thermal Spray Coatings: Results of Corrosion Testing of HVOF Coatings�, 3rd Global Symposium of HVOF Coatings (1997). 39 T. Seitz, �Replacement of Chrome Plating by Thermal Spray Coatings: Summary of Tests Performed at Lutfhansa Technik�, 3rd Global Symposium of HVOF Coatings (1997). 40 S. Simard, B. Arsenault, K. Laui, M. Dorfman, �Performance of HVOF-Sprayed Carbide Coatings in Aqueous Corrosive Environments�, Proc. 1st International Thermal Spray Conference, Montreal, Canada, (2000) p. 983-990, Ed. C.C. Berndt. ASM International, Materials Park, OH 44073-0002. 41 D. Parker, �Application of Tungsten Carbide Coatings on Ultra High Strength Steels � HVOF Process�, Draft AMS Specification, 2000. 42 �Replacement of chrome plating by thermal spray coating�, T. Seitz, Lufthansa Technik AG, 3rd Global Symposium on HVOF Coatings, San Francisco (1997). 43 Keith O. Legg �Hard Chrome Coatings: Advanced Technology for Waste Elimination�, Final Report of DARPA Project # MDA972-93-1-0006, 1997. 44 Tony DeGennaro, Green, Tweed & Co., �Evaluation of Chrome Rod Alternative Coatings�, Report # GTE0644, September (1999). 45 Stephen Gaydos, Boeing St Louis �F/A-18E/F Landing Gear HVOF Testing and Evaluation�, presented at HCAT Meeting, Halifax (August 1999; and HCAT Meeting, Crystal City (December 1999). 46 Jay Randolph, �Service Evaluation Status and Impact of HVOF Coatings on Landing Gear at Delta Air Lines, Inc.�, Gorham Advanced Materials Conference on Advanced Coating Systems for Gas Turbine Engines and Aircraft Components, San Antonio, March 2000. 47 Boeing Aircraft Corp. Final version of this specification not yet complete at



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time of writing. 48 M.M. Bhusari and R.A. Sulit, “Standards for the Thermal Spray Industry”, Proceedings of the International Thermal Spray Conference, Montreal (2000), p 1313 49 J. Nuse, “HVOF Aerospace Applications for OEM and Rebuild”, private communication. 50 D. Comassar, “Thermal Spray Applications at GE Aircraft Engines”, presented at Executive Briefing, on Thermal Spray, 1st International Thermal Spray Conference, Montreal, Canada, (2000). 51 Gary Naisbitt, “OEM Approval for HVOF Wear Resistant and MCrAlY Coatings)”, Gorham Advanced Materials Conference on Advanced Coating Systems for Gas Turbine Engines and Aircraft, New Orleans, March 1999. 52 Jerry D. Schell & Mark Rechtsteiner, GE Aircraft Engines, “Replacement of Chromium Electroplating Using Advanced Material Technologies On Gas Turbine Engine Components”, AESF Aerospace Plating and Metal Finishing Forum, Cincinnati (March 2000). 53 Southwest Aeroservice (see, for example, http://www.netok.com/swa/therm.html) 54 Bob Cashman, Parker Hannifin Corp., JG-PP Hydraulic Actuator Program Kickoff Meeting, Dayton, April 19, 20, 2000. 55 Mark Buchedi, UAL “HVOF- Is this United’s Ticket into the 21st Century?”, 3rd Global Symposium on HVOF Coatings, San Francisco, 1997.