A THICK MULTILAYER THERMAL BARRIER COATING:

DESIGN, DEPOSITION, AND INTERNAL STRESSES

Hamed Samadi

A thesis submitted in Conformity with

the requirement for the degree

Doctor of Philosophy

Department of Materials Science and Engineering

University of Toronto

© Copyright by Hamed Samadi 2009

II

A Thick Multilayer Thermal Barrier Coating:

Design, Deposition, and Internal Stresses

Hamed Samadi

Doctor of Philosophy

Department of Materials Science and Engineering

University of Toronto

2009

Abstract

Yttria Partially Stabilized Zirconia (Y-PSZ) plasma-sprayed coatings are widely used in turbine

engines as thermal barrier coatings. However, in diesel engines Y-PSZ TBCs have not met with

wide success. To reach the desirable temperature of 850-900˚C in the combustion chamber from

the current temperature of 400-600˚C, a coating with a thickness of approximately 1mm is

required. This introduces different considerations than in the case of turbine blade coatings,

which are on the order of 100µm thick. Of the many factors affecting the durability and failure

mechanism of TBCs, in service and residual stresses play an especially important role as the

thickness of the coating increases. For decreasing the residual stress in the system, a multi-layer

coating is helpful. The design of a multilayer coating employing relatively low cost materials

with complementary thermal properties is described. Numerical models were used to describe

the residual stress after deposition and under operating conditions for a multilayer coating that

exhibited the desired temperature gradient. Results showed that the multilayer coating had a

lower maximum stress under service conditions than a conventional Y-PSZ coating. Model

validation with experiments showed a good match between the two.

III

Acknowledgments

In His name…,

A journey is easier when you travel together. Interdependence is certainly more valuable than

independence. This thesis is the result of five years of work whereby I have been accompanied

and supported by many people. It is a pleasant aspect that I have now the opportunity to express

my gratitude for all of them.

Firstly, I would like to thank my Supervisor, Prof. Tom Coyle. I could not have imagined

having a better advisor and mentor for my PhD, and without his common sense, knowledge and

perceptiveness, I would never have finished. Thank-you to my dissertation committee, Prof.

Utigard, Prof. Hibbard, Prof. Kesler, Prof. Mostaghimi and Dr. Marple for managing to read the

whole thing so thoroughly, and for a surprisingly enjoyable viva. I would also like to thank all

the rest of the academic and support staff of the Centre for Advanced Coating Technologies,

particularly those who have put up with my drifting a long way away from my original title.

Much respect to my officemates, and hopefully still friends, Hanif, Fardad, Reza, Hamid,

Mehdi, Ala, Amir, Bob, Ben, Ken, Arash, Tommy, Rajeev, Babak, Sanaz and Nikoo for putting

up with me for more than five years. Thanks to Prof. Mostaghimi, Dr. Larry Pershin, Dr.

Eugenio García, Dr. Salimijazi and everyone else for all that serious discussion and helps in

running the equipment. In addition, thanks to Sal Boccia, who trained and helped me during use

of SEM facilities in the Department of Materials Science and Engineering. Also thanks to those

people and groups who provided me with so much whether they know it or not.

On a different note, I would like to thank the coffee club of CACT for keeping me thinking (you

have to ask why?), medical researches for making me feel okay about drinking coffee and

traditional Persian alongside classical music for keeping me sane.

Finally, I have to say 'thank-you' to: all my friends and family, wherever they are, particularly

my Mom and Dad and my in-laws; and, most importantly of all, to Narges and Amir Mahdi, my

beloved family, for everything. Without their encouragement and understanding, it would have

been impossible for me to finish this work.

Is that everyone?

Hamed, July 1st 2009.

IV

To my wife, Narges, who made me a

very happy man…

Her love has made me wealthy beyond

my dreams.

V

Table of Contents

Abstract..........................................................................................................................................II

Acknowledgments ....................................................................................................................... III

List of Tables .................................................................................................................................X

List of Figures.............................................................................................................................. XI

1. Chapter I: Introduction .......................................................................................................... 1

1.1. Coating techniques.......................................................................................................... 1

1.2. Thermal spraying............................................................................................................ 1

1.3. Application of coatings in diesel engines ....................................................................... 2

1.4. Diesel engines and turbine engines: similarities and differences ................................... 2

1.5. Thesis objective .............................................................................................................. 3

1.6. Structure of the thesis document .................................................................................... 4

2. Chapter II: Thermal Spray Process........................................................................................ 5

2.1. Introduction .................................................................................................................... 5

2.2. Plasma spraying.............................................................................................................. 6

2.3. Process parameters.......................................................................................................... 7

2.3.1. Plasma current ................................................................................................................ 8

2.3.2. Primary gas flow rate: argon........................................................................................... 8

2.3.3. Auxiliary gas flow rate: hydrogen .................................................................................. 9

2.3.4. Carrier gas flow rate ..................................................................................................... 10

2.4. Feedstock injection ....................................................................................................... 13

2.5. Particle-particle interactions ......................................................................................... 14

2.6. Particle trajectory.......................................................................................................... 15

2.7. Particle- substrate interactions...................................................................................... 16

VI

2.8. Coating formation......................................................................................................... 20

2.9. Summary....................................................................................................................... 22

3. Chapter III: Coating Materials............................................................................................. 23

3.1. Introduction .................................................................................................................. 23

3.2. Failure mechanisms in TBCs........................................................................................ 24

3.3. Alternatives................................................................................................................... 27

3.3.1. Zirconates ..................................................................................................................... 28

3.3.2. Garnets.......................................................................................................................... 28

3.3.3. Al2O3.SiO2.MgO system............................................................................................... 29

3.3.3.1. Cordierite ................................................................................................................... 30

3.3.3.2. Forsterite .................................................................................................................... 31

3.3.3.3. Spinel ......................................................................................................................... 32

3.3.3.4. Mullite ....................................................................................................................... 34

3.3.4. Multilayer system ......................................................................................................... 35

3.3.5. Phase stability ............................................................................................................... 37

3.4. Summary....................................................................................................................... 40

4. Chapter IV: Coating Deposition .......................................................................................... 41

4.1. Introduction .................................................................................................................. 41

4.2. Feedstock powder ......................................................................................................... 41

4.2.1. Morphology .................................................................................................................. 41

4.2.2. Size distribution............................................................................................................ 43

4.2.3. X-ray diffraction and phase analysis ............................................................................ 45

4.3. Optimization of deposition parameters......................................................................... 48

4.3.1. Measurement of in-flight particle temperature and velocity ........................................ 50

4.3.2. Statistical design of experiments .................................................................................. 52

VII

4.4. Analysis of the results of the Taguchi design of experiments ...................................... 55

4.5. Coating deposition........................................................................................................ 60

4.6. Characterization of deposits ......................................................................................... 63

4.6.1. Sample preparation ....................................................................................................... 63

4.6.2. Coating microstructure ................................................................................................. 64

4.6.3. Porosity measurement................................................................................................... 67

4.6.4. Crystallinity index ........................................................................................................ 70

4.6.5. Thermal diffusivity and conductivity ........................................................................... 72

4.6.6. Elastic modulus measurement ...................................................................................... 72

4.7. Single splat collection................................................................................................... 74

4.8. Results and discussion .................................................................................................. 77

4.8.1. Deposition efficiency.................................................................................................... 77

4.8.2. Porosity ......................................................................................................................... 78

4.8.3. Crystallinity index ........................................................................................................ 80

4.8.4. Single splat collection................................................................................................... 81

4.8.5. Optimization of plasma parameters .............................................................................. 84

4.8.6. Thermal diffusivity and conductivity ........................................................................... 85

4.8.7. Modulus of elasticity .................................................................................................... 87

4.9. Summary....................................................................................................................... 88

5. Chapter V: Residual Stress .................................................................................................. 89

5.1. Introduction .................................................................................................................. 89

5.2. Quenching stress........................................................................................................... 89

5.3. Thermal stresses............................................................................................................ 92

5.4. Total residual stress ...................................................................................................... 93

5.5. Stress relaxation............................................................................................................ 94

VIII

5.6. Residual stress determination ....................................................................................... 96

5.6.1. Hole drilling.................................................................................................................. 96

5.6.2. X-ray diffraction ........................................................................................................... 99

5.6.3. Synchrotron ................................................................................................................ 101

5.6.4. Neutron diffraction ..................................................................................................... 101

5.6.5. Curvature and layer removal ...................................................................................... 102

5.6.6. Magnetic method ........................................................................................................ 104

5.6.7. Ultrasonic methods ..................................................................................................... 104

5.6.8. Piezo-spectroscopic (Raman) ..................................................................................... 105

5.6.9. Comparison between techniques ................................................................................ 105

5.7. Curvature measurement .............................................................................................. 108

5.7.1. Setup ........................................................................................................................... 108

5.7.2. Calculating curvature from the displacement ............................................................. 110

5.7.3. Results and discussion ................................................................................................ 111

5.8. Summary..................................................................................................................... 115

6. Chapter VI: Numerical Modeling of Curvature and In-plane Stresses.............................. 116

6.1. Introduction ................................................................................................................ 116

6.2. Previous models.......................................................................................................... 116

6.3. The numerical model .................................................................................................. 119

6.3.1. Model description ....................................................................................................... 120

6.3.2. Material properties...................................................................................................... 121

6.3.3. Heat transfer boundary conditions.............................................................................. 122

6.3.4. Mechanical boundary conditions................................................................................ 123

6.3.5. Meshing ...................................................................................................................... 124

6.4. Results and discussion ................................................................................................ 124

IX

6.4.1. Thermal behaviour...................................................................................................... 124

6.4.2. In-situ curvature calculations...................................................................................... 127

6.5. Comparison with existing models .............................................................................. 129

6.6. Internal stress under service conditions...................................................................... 133

6.7. Summary..................................................................................................................... 137

7. Chapter VII: Conclusion and Suggestions for Further Work ............................................ 138

7.1. Summary......................................................................................................................... 138

7.2. Original contributions..................................................................................................... 138

7.3. Suggestions for future work............................................................................................ 139

8. Appendices ........................................................................................................................ 140

8.1. Appendix A: Cost calculation..................................................................................... 140

8.2. Appendix B: Effect of in-flight particle properties on deposition of air plasma sprayed

forsterite................................................................................................................................. 141

8.2.1. Introduction ................................................................................................................ 141

8.2.2. Experimental procedure.............................................................................................. 142

8.2.2.1. Powder ..................................................................................................................... 142

8.2.2.2. Coating deposition................................................................................................... 143

8.2.2.3. In-flight particle properties ...................................................................................... 145

8.2.3. Results and discussion ................................................................................................ 145

8.2.4. Conclusion .................................................................................................................. 150

References.................................................................................................................................. 151

X

List of Tables

Table 2-1. Comparison of different spraying methods [2]. ............................................................. 6

Table 4-1. The L9 DOE matrix for spinel .................................................................................... 54

Table 4-2. The L9 DOE matrix for mullite and forsterite ............................................................ 54

Table 4-3. ANOVA of in-flight particle velocity for forsterite ................................................... 56

Table 4-4. ANOVA of in-flight particle temperature for forsterite............................................. 56

Table 4-5. ANOVA of in-flight particle velocity for spinel........................................................ 56

Table 4-6. ANOVA of in-flight particle temperature for spinel.................................................. 57

Table 4-7. ANOVA of in-flight particle velocity for mullite ...................................................... 57

Table 4-8. ANOVA of in-flight particle temperature for mullite................................................ 57

Table 4-9. Grinding preparation steps. ........................................................................................ 63

Table 4-10. Physical Properties of the Coatings.......................................................................... 80

Table 4-11. Chosen parameters for multilayer material .............................................................. 85

Table 4-12. Modulus of elasticity................................................................................................ 88

Table 5-1. Practical issues with different technique materials [119]. .......................................... 106

Table 5-2. Materials issues with different techniques [119]. ....................................................... 107

Table 5-3. Physical Characteristics [119]..................................................................................... 107

Table 6-1. Material properties used in the model ...................................................................... 122

Table 6-2. Values used in heat transfer physics. ....................................................................... 127

Table 8-1. Cost calculations for duplex using SG-100 and multilayer coatings using SG-100 (for

spinel and mullite) and CACT gun (for forsterite). ................................................................... 140

Table 8-2. Spray parameters ...................................................................................................... 144

XI

List of Figures

Figure 2.1. Effect of plasma current on: (a) plasma gas temperature, (b) plasma gas velocity [21].

....................................................................................................................................................... 8

Figure 2.2. Effect of Ar flow rate on: (a) plasma gas temperature and (b) plasma gas velocity [21].

....................................................................................................................................................... 9

Figure 2.3. Effect of hydrogen flow rate on: (a) plasma gas temperature , (b) degree of Ar and H

ionization and (c) plasma gas velocity [21]. .................................................................................. 10

Figure 2.4. Particle trajectories expected for low and high powder carrier gas flow rates [23]. ... 11

Figure 2.5. Effect of carrier gas flow rate on (a) density and (b) deposition efficiency [23]. ....... 12

Figure 2.6. Schematic picture of particle trajectory for different particle sizes [13]..................... 15

Figure 2.7. Dispersed trajectories of alumina particles of different sizes in a turbulent free

plasma argon jet [2]....................................................................................................................... 16

Figure 2.8. Schematic deformation of a splat at impact [2]. ......................................................... 17

Figure 2.9. Microstructure of fractured as-sprayed partially stabilized zirconia showing

columnar grains [28]. ..................................................................................................................... 17

Figure 2.10. Morphology of different ceramic splats on AISI304 substrate [29].......................... 18

Figure 2.11. Dependence of fraction of disk shape splats on substrate temperature (substrate:

AISI 304, particle Al2O3) [29]. ...................................................................................................... 19

Figure 2.12. Dependence of Al2O3 splat morphology on substrate (AISI 304) temperature [29]. 19

Figure 2.13. Schematic rendering of the chaotic structure of plasma sprayed coating: 1. Thin

molten shell; 2. Unmelted core; 3. Liquid splash; 4. ‘Pancake’ splat; 5. Interlocked splat; 6.

Oxidized particle; 7. Unmelted particle; 8. Pore; 9. Void; 10. Roughened substrate; 11.

Substrate[2]. .................................................................................................................................. 20

Figure 2.14. Dependence of deposition efficiency on various plasma spray parameters [2]........ 21

Figure 3.1. Air Plasma-Sprayed (APS) coating [42]. .................................................................... 23

Figure 3.2. SEM image of plasma sprayed mullite coatings ....................................................... 25

XII

Figure 3.3. Burner rig showing four-specimen rotating carousel [23]. ......................................... 27

Figure 3.4. Surface temperature of burner rig specimens during rig cycle [23]. ........................... 27

Figure 3.5. Experimentally measured thermal conductivity of different garnets [55]................... 29

Figure 3.6. Al2O3.SiO2.MgO phase diagram [56].......................................................................... 30

Figure 3.7. SEM micrograph of plasma sprayed forsterite on 80Ni-20Cr bond coat and mild

steel substrate after 719 thermal cycles[57]................................................................................... 31

Figure 3.8. As-sprayed forsterite [57]............................................................................................ 32

Figure 3.9. Crystal structure of spinel [64]. ................................................................................... 33

Figure 3.10. Phase diagram of MgO-Al2O3 [56]. .......................................................................... 33

Figure 3.11. Spinel coating after 719 thermal cycles [57]. ............................................................ 34

Figure 3.12. Binary phase diagrams of Al2O3-SiO2 [56]. .............................................................. 34

Figure 3.13. DTA of plasma sprayed mullite; the recrystallization temperature is shown as an

exothermic peak [78]. .................................................................................................................... 35

Figure 3.14. Calculated surface stresses of zirconia and mullite during cooling following the

establishment of steady state temperature and stress distributions under exposure to a heat flux

of 270 kW.m-2 [15]......................................................................................................................... 36

Figure 3.15. XRD of mullite - forsterite mixture (JCPDS card numbers 85-1358, 79-1454 and

02-0646)....................................................................................................................................... 38

Figure 3.16. Mullite - forsterite sample after heating.................................................................. 38

Figure 3.17. XRD pattern for forsterite-spinel heated sample (JCPDS card numbers 85-1358,

73-1959 and 87-0653).................................................................................................................. 39

Figure 3.18. XRD pattern for forsterite-spinel heated sample (JCPDS card numbers 79-1454,

73-1959 and 11-0607).................................................................................................................. 39

Figure 3.19. Spinel-forsterite (left) and spinel-mullite (right) samples after heating.................. 40

Figure 4.1. SEM micrographs of the starting powders: forsterite (top left), spinel (top right),

mullite (bottom). .......................................................................................................................... 42

XIII

Figure 4.2. SEM micrographs of cross sections of starting powders: forsterite (top left), spinel

(top right) and mullite (bottom)................................................................................................... 43

Figure 4.3. Particle size analysis of mullite sample. X axis is logarithmic. ................................ 44

Figure 4.4. Particle size analysis of powders. X axis is logarithmical. ....................................... 44

Figure 4.5. Binary phase diagrams of Al2O3-SiO2 (top left), MgO-SiO2 (top right) and MgO-

Al2O3 (bottom) [56]. ...................................................................................................................... 46

Figure 4.6. XRD result for forsterite powder (JCPDS card number 85-1358)............................ 47

Figure 4.7. XRD result for spinel powder (JCPDS card number 73-1959)................................. 47

Figure 4.8. XRD result for mullite powder (JCPDS card number 79-1454)............................... 48

Figure 4.9. Argon concentration isocontours (current = 450 A)[80]............................................. 49

Figure 4.10. Alternatives in tailoring the properties................................................................... 49

Figure 4.11. (a) DPV-2000 set up and (b) DPV-2000 particle detector mechanism................. 51

Figure 4.12. Typical DPV-2000 results....................................................................................... 52

Figure 4.13. Factors with the highest influence on forsterite in-flight particles: (a) velocity, (b)

temperature. ................................................................................................................................. 58

Figure 4.14. Results of ANOVA analysis of the most influential process parameters on the in-

flight characteristics of spinel particles: (a) velocity, (b) temperature. ....................................... 59

Figure 4.15. Factors with the highest influence on mullite in-flight particles: (a) velocity, (b)

temperature. ................................................................................................................................. 60

Figure 4.16. Spraying setup. ........................................................................................................ 61

Figure 4.17. Spraying pattern. ..................................................................................................... 62

Figure 4.18. Substrate temperature measured at the back surface of the substrate vs. time

(mullite sample). The lower curve is for a non-preheated substrate, while the upper curve was

collected for a substrate preheated to ~300°C before deposition. ............................................... 62

Figure 4.19. SEM micrograph of a cross section of a forsterite coating. .................................... 64

Figure 4.20. SEM micrograph of a cross section of a spinel coating. ......................................... 65

XIV

Figure 4.21. SEM micrograph of a cross section of a mullite coating. ....................................... 65

Figure 4.22. XRD pattern for spinel. Powder (bottom), coating (top) (JCPDS card numbers 73-

1959 and 88-0826)....................................................................................................................... 66

Figure 4.23. XRD pattern for mullite. Powder (bottom), coating (top) (JCPDS card numbers 79-

1454 and 88-0826)....................................................................................................................... 66

Figure 4.24. Effect of magnification and number of fields of view on (a) porosity and (b)

measurement variability of plasma sprayed Al2O3 [97]. ............................................................... 68

Figure 4.25. A typical SEM micrograph ..................................................................................... 69

Figure 4.26. A typical micrograph used for image analysis. ....................................................... 69

Figure 4.27. Grey threshold ......................................................................................................... 70

Figure 4.28. Main peak of a mullite sample. ............................................................................... 71

Figure 4.29. XRD patterns of the starting powder (left) and a typical coated sample (right)

(JCPDS card 79-1454)................................................................................................................. 71

Figure 4.30. Four-point bending test assembly for modulus of elasticity measurement. Strain

gauge is located on the bottom of the sample.............................................................................. 73

Figure 4.31. Shutter system for collecting single splats within the deposition footprint of the

plasma plume. .............................................................................................................................. 75

Figure 4.32. Upper half of a mullite footprint produced in front of the stationary gun as

described in the text. .................................................................................................................... 76

Figure 4.33. In-flight temperature and velocity maps for mullite sample including single splat

collection. Point c is out of the left side map............................................................................... 77

Figure 4.34. The thickness of coatings deposited with different deposition parameters: (a)

forsterite, (b) spinel, (c) mullite................................................................................................... 78

Figure 4.35. Porosity of different layers: (a) forsterite, (b) spinel, (c) mullite. ........................... 79

Figure 4.36. Crystallinity Index for mullite coatings. ................................................................. 80

Figure 4.37. Tp-Vp map for forsterite........................................................................................... 81

Figure 4.38. Forsterite single splats for samples 9 (left) and 2 (right) ........................................ 82

XV

Figure 4.39. Tp-Vp map for spinel. ............................................................................................. 82

Figure 4.40. Spinel single splats for sample 3 (left) and sample 4 (right)................................... 83

Figure 4.41. Tp-Vp map for mullite. ........................................................................................... 83

Figure 4.42. Mullite single splat for 8 (left) and 5 (right). .......................................................... 84

Figure 4.43. Thermal diffusivity and thermal conductivity of the coating materials versus

temperature. ................................................................................................................................. 86

Figure 4.44. XRD patterns of the layers, before and after thermal conductivity test. In each

image, top pattern is the sample after heating to 1000°C and the bottom is the one as-sprayed. 87

Figure 5.1. Schematic depiction of impact, spreading and cooling of a single splat [46]. ............ 90

Figure 5.2. Specimen setup for quenching stress measurement [46]............................................. 91

Figure 5.3. Quenching stress dependency on substrate temperature [46]...................................... 91

Figure 5.4. Quenching stress of various powders on different substrates [115]. ........................... 92

Figure 5.5. Schematic diagram of the variation of the final residual stress in the coating with

substrate temperature Ts during spraying. σr (T0) is the final residual stress after the sprayed

deposit and the substrate cool down to T0 [46]. ............................................................................. 93

Figure 5.6. Surface stress of mullite and zirconia [117]................................................................. 94

Figure 5.7. Experimental averaged interface crack length (ai) versus coating thickness (lc)[118]. 95

Figure 5.8. Residual stress measurement distribution according to techniques [119]. The numbers

represent the industries which use the specific measurement method in the above mentioned

study............................................................................................................................................. 96

Figure 5.9. Hole-drilling apparatus and residual stress strain gauge rosette design [119]. ............ 97

Figure 5.10. Dataset collected for stress in shot peened Ni based alloy using hole drilling

method alongside the suitable arrangement of the strain gauges [124].......................................... 98

Figure 5.11. Practical example of a diffractometer, showing the X-ray source, sample stage,

detector and goniometer [119]........................................................................................................ 99

Figure 5.12. Stress distribution in an 8 mm wide tungsten inert gas weld in 3 mm thick plate [124]. ............................................................................................................................................ 100

XVI

Figure 5.13. A stress map collected for welded plate using neutron diffraction method [124] ... 102

Figure 5.14. Curvature resulting from coating deposition......................................................... 103

Figure 5.15. Stress distribution in a tungsten carbide coating on Ti substrate using layer

removal[124]. ............................................................................................................................... 103

Figure 5.16. The Almen strip system [139].................................................................................. 104

Figure 5.17. Schematic illustration of the continuous curvature measurement set-up.............. 108

Figure 5.18. Schematic diagram showing spray pattern of the gun........................................... 109

Figure 5.19. Schematic diagram showing geometry of the curved sample ............................... 110

Figure 5.20. Scale drawing of the fixture and mask for curvature measurement (values in mm).

................................................................................................................................................... 112

Figure 5.21. Curvature for forsterite single layer coating. ........................................................ 113

Figure 5.22. Curvature of multilayer coating after deposition of each layer: forsterite (top left),

spinel (top right) and mullite (bottom). ..................................................................................... 114

Figure 6.1. Substrate dimensions............................................................................................... 121

Figure 6.2. Mechanical boundary condition for (a): side view of in-process sample and (b) in-

service sample (From left to right: multilayer and duplex). ...................................................... 123

Figure 6.3. Zoom-in of the meshed area.................................................................................... 124

Figure 6.4. Temperature of the back of the substrate during spraying: (a) forsterite, (b) spinel,

and (c) mullite............................................................................................................................ 126

Figure 6.5. Curvature calculation versus measurement for the multilayer coating: (a) forsterite ,

(b) spinel, and (c) mullite. ......................................................................................................... 128

Figure 6.6. Quenching stress for forsterite ................................................................................ 129

Figure 6.7. Comparison between numerical and analytical solutions with experiment in: (a)

forsterite and (b) mullite. ........................................................................................................... 131

Figure 6.8. Comparison between numerical and analytical solutions in multilayer coatings. .. 132

Figure 6.9. Comparison between this study and the work done by Zhang et al.[158]. ................ 133

XVII

Figure 6.10. In-plane stress distribution of coatings after deposition and cooling to room

temperature: (a) duplex coating (b) multilayer coating. ............................................................ 134

Figure 6.11. Geometry and boundary condition of multilayer and duplex systems.................. 135

Figure 6.12. The steady state temperature distribution in the two systems............................... 136

Figure 6.13. Stress distribution in coatings at service temperature: (a) duplex coating, (b)

multilayer coating. ..................................................................................................................... 136

Figure 8.1. Cross sectional SEM micrographs of as-received (left) and calcined (right) powders.

................................................................................................................................................... 142

Figure 8.2. Particle size distribution of as-received, calcined and calcined-sieved powders. ... 143

Figure 8.3. Spraying pattern. ..................................................................................................... 144

Figure 8.4. Temperature measurement of the back of the substrate during deposition............. 145

Figure 8.5. In-flight particle properties of SG-100. From top left: temperature, velocity, particle

size and particle number distributions. Particles are injected 5 mm inside the gun. The oval is the

highest particle density trajectory.............................................................................................. 146

Figure 8.6. In-flight particle properties of CACT gun. From top left: temperature, velocity,

particle size and particle number distributions. Particles are injected 5 mm inside the gun. The

oval is the highest particle density trajectory. ........................................................................... 146

Figure 8.7. In-flight particle characteristics for CACT gun with sieved powder injected 5 mm

inside the gun. From top left: temperature, velocity, particle size and particle number

distributions. The oval is the highest particle density trajectory. .............................................. 147

Figure 8.8. Cross sections of (a) calcined powder deposited with SG-100 (b) calcined powder

deposited with CACT (c) calcined and sieved powder deposited with CACT. All injected 5 mm

inside the gun............................................................................................................................. 149

Figure 8.9. XRD result for forsterite powder (JCPDS card number 85-1358).......................... 150

XVIII

Nomenclature

c

•

ε Strain rate

1/R Curvature

a Surface area of a particle

A Constant

a Substrate displacement

b Width of four-point bend sample

b Width of coating

CI Crystalinity index

cp Specific heat capacity

dc Thickness of coating

ds Thickness of substrate

Ē1 Average modulus of elasticity

Ec Modulus of elasticity of coating

Ed Modulus of elasticity of the coating material

Es Modulus of elasticity of the substrate

Es(Ts) Modulus of elasticity of the substrate at Ts

F Initiated force

GF Gage factor

h Thickness of four-point bend sample

H Thickness of substrate

h Heat transfer coefficient

hd Thickness of the coating

XIX

hs Convective heat transfer coefficient

hs Thickness of the substrate

I Moment of inertia

Ia Intensity of the background

Ic Intensity of the main peak

l Length of substrate

M Moment

m. Plasma flow rate

n Constants

P Applied load

qc Convective heat flux

qr Radiative heat flux

Rf Lead wire resistance

Rg Gage resistance

S1 Outer span of four-point bend setup

S2 Inner span of four-point bend setup

T∞ Free-stream plasma temperature

Ta Ambient temperature

Tmd Melting point of the coating material

TP Particle surface temperature

TS Substrate initial temperature

Ts* Transition temperature

Tsurf Surface temperature

XX

V Voltage

w Thickness of coating

y Distance from the neutral axis

z Layer number

zNI Neutral axis

α Thermal diffusivity

αc Linear coefficient of thermal expansion of coating

αd Coefficient of thermal expansion of deposit

αs Linear coefficient of thermal expansion of substrate

β Moduli ratio

δ Distance from the bending axis to the substrate/coating interface

ΔH Activation energy

Δh Thickness of coating

ΔT Temperature gradient

Δκ Curvature difference

ε Particle emissivity

εi In-plane strain of layer i

θ Curved angle

κn Curvature of layer n

ν Poisson’s ratio

ρ Density

ρ% Percent influence

σ Von Mises equivalent stress

XXI

σc In-plane stress of coating

σdj In-plane stress at the midpoint of jth layer

σi In-plane stress of layer i

σq Quenching stress

σsb In-plane stress at the bottom of substrate

σst In-plane stress at the top of substrate

σT Stefan-Boltzmann constant

υc Poisson’s ratio of coating

υs Poisson’s ratio of substrate

1

1. Chapter I: Introduction

1.1. Coating techniques

Coatings are applied to surfaces of materials for various reasons. Corrosion, oxidation and wear

resistance are only a few examples. There are different techniques for applying a coating onto a

substrate, such as chemical and physical vapour deposition, sol-gel, electro-deposition, dip

coating, and thermal spray. For different applications, a specific method should be chosen

according to deposition rate, desired microstructure, cost, final surface finish and so on. Among

these methods, thermal spray process sales are increasing regularly. Thermal spraying

techniques may continue to grow as they are more environmentally friendly than some

competitive coating techniques and full of undiscovered potentials [1].

In this work we used the air plasma spray technique (APS), which is the most commonly used

thermal spray process, and studied the generation of residual stresses in a multilayer coating.

1.2. Thermal spraying

Thermal spray is the name of a category of coating deposition techniques which use thermal

energy to melt and/or pyrolyze the precursor (powder, solution or wire) and accelerate it

towards the substrate. At the substrate, the individual molten particles flatten and solidify.

One of the first techniques invented in this category was flame spaying, which uses a flame

(usually oxy-propane) to melt the powder and gravity or compressed air to accelerate the

droplets towards the substrate. The temperature generated (on the order of 3000°C) may be

sufficient to melt metals, but in most cases it is insufficient to melt ceramics [2].

Another type of thermal spray is air plasma spray (APS), in which a DC arc is formed between

an anode and cathode, which generates a plasma [3]. The precursors are injected into the high

temperature plasma and accelerated towards the substrate as a result of the high velocity gas

flow. This process can also be performed in an inert atmosphere or vacuum, in which case the

process is named vacuum plasma spray (VPS). Due to the high temperatures attained, plasma

spray is a good application method for ceramic coatings, as it can ensure that the precursors

have been melted before hitting the substrate.

2

Plasma spray process parameters govern the resulting coating characteristics and when changing

these parameters (gas flow rate, current, standoff distance, feeding rate, etc.), coating

characteristics change. Thus, we need to optimize the plasma parameters to have a coating with

the desired characteristics.

1.3. Application of coatings in diesel engines

Diesel engines are commonly used in buses, trucks and in passenger cars. In a diesel engine, the

fuel is compressed to a very high pressure and it automatically ignites and burns. The idea of

thermally insulating the engine is as old as the internal combustion engine itself and is based on

the knowledge that only 30-40% of the entering fuel energy is converted to useful work on the

output shaft [4]. In the 1980s there was an effort to use thermal barrier coatings (TBCs) in diesel

engines in pursuit of advantages, including higher power density; fuel efficiency; reducing

specific fuel consumption; emissions and noise; improving the engine life and cold start

reliability; and multi-fuel capacity due to higher combustion chamber temperatures (900°C vs.

650 °C) [5-8]. The goal was to have an engine with 48% efficiency rather than the ordinary 33% [9]. Seker and Kamo developed an adiabatic engine for a passenger car which showed an

increase of 12% in performance [10]. Prasad and Samria coated the piston crown with partially

stabilized zirconia (PSZ) and reported a 19% reduction in heat loss through the piston [11].

Studies done by Hejwowski and Weronski showed that using TBCs in a diesel engine could

increase engine power by 8%, decrease the specific fuel consumption by 15-20%, and increase

the exhaust gas temperature by 200K [12]. A significant problem was still unsolved: durability.

1.4. Diesel engines and turbine engines: similarities and differences

Thermal barrier coatings (TBCs) have been in use for three decades in the hot sections of

turbines [13]. High performance turbine blades consist of single crystal super alloys, which

cannot tolerate the high temperature (more than 1000°C) and corrosive environment

experienced in current turbine engines. A thin layer of yttria stabilized zirconia (YSZ), with a

thickness of approximately 200 µm, decreases the alloy surface temperature by 100-300°C,

which largely fulfills current requirements [14].

The service environment of the coating in the turbine is markedly different from that in the

diesel engine. In the former, the service temperature is high (1000-1100°C). The superalloy

substrate’s maximum service temperature is about 800°C [14]. The thickness of coating is a few

3

hundred microns and the coating is applied to protect it against oxidation, hot corrosion,

thermomechanical fatigue, and creep. Due to the high substrate temperature, oxidation of the

bond coat and creep play major roles in coating failure.

On the other hand, in the diesel engine the gas temperature, currently less than 650°C, would

ideally approach 900°C[5, 6]. The substrate temperature is limited to approximately 200°C, and

therefore a thick coating (at least 1mm) is required, which leads to a large thermal gradient. In a

thick thermal barrier coating (TTBC), the bond coat temperature is too low for severe oxidation

and creep [15]. Another major difference between the two systems is that due to the on/off nature

of the diesel engine operation, the thermal barrier coating in a diesel engine experiences more

transient mode conditions than in a turbine engine. These differences in service conditions and

coating thickness result in different failure mechanisms, primarily thermocyclic fatigue and

thermal shock in the TTBC.

Numerical modeling done by Kokini et al. [15] suggested that in a thick thermal barrier coating,

when the surface of the yttria-stabilized zirconia coating is heated, a large compressive stress is

developed in the surface, which may be relaxed after two hours of steady state heating. Upon

cooling, the stress may become tensile and initiate cracks which grow with each thermal cycle.

Modelling also indicated that during cyclic operation, rapid cooling can result in the

development of a transient tensile stress at the surface of the coating, leading to crack initiation

and thermal shock failure [15]. Due to the mismatch in thermomechanical properties of the top

coat and bond coat, this interface is a source for cracking and delamination [16].

1.5. Thesis objective

The objective of the thesis research is to design and fabricate a relatively low cost multilayer

ceramic thick thermal barrier system, which is physically and chemically stable under the

operating conditions typical of a diesel engine and which minimizes the internal stress

experienced when subjected to a thermal gradient typical of that experienced in service. In this

regard, traditional refractory ceramic material compositions have been chosen instead of

expensive synthetic oxides. Two paths have been pursued simultaneously: optimization of the

coating process for each material and development of a model of the stress in the coating during

processing and service. The model was validated using an in-situ curvature measurement

4

technique, and then used to predict the stress distribution that would be experienced under

service conditions.

1.6. Structure of the thesis document

The structure of the thesis is as follows. The thermal spray process and the most significant

process parameters are described in Chapter 2. The third chapter discusses the selection of the

coating materials for the specified application. In Chapter 4 the experimental work involved in

establishing the deposition process parameters for each material is presented. This includes

characterization of the feedstock powder and deposited coatings. Porosity, crystallinity, thermal

conductivity and modulus of elasticity are measured for coatings of each material. Chapter 5

reviews the nature of residual stress in such coatings, and direct and indirect methods for its

measurement, before concentrating on the technique of curvature measurement. The

experimental implementation of this technique is described, and in-situ measurements of

curvature during coating deposition presented. Numerical modeling of the build-up of internal

stresses during deposition, the prediction of the resulting curvature, and the associated residual

stresses are included in Chapter 6. To validate the model, the curvature predictions are

compared with the experimentally measured behaviour. In Chapter 7 the results of the study are

discussed, conclusions drawn, and suggestions for further studies presented.

5

2. Chapter II: Thermal Spray Process

2.1. Introduction

Thermal spraying is a process in which a stream of molten and/or semi-molten particles is

sprayed onto a substrate. A high temperature flame or plasma jet is used to melt the feedstock

powder and form a coating on the substrate [2]. The molten particles impact, flatten, and form

splats on the substrate.

Coating properties depend on five subsystems on which the operator can have some control [1]:

1. Flame or jet formation: linked to the torch design, gas composition and flow rate, power,

and other process parameters.

2. Powder and its injection: depends on powder chemical composition, particle size

distribution, structure and morphology, injector design and positioning.

3. Deposition atmosphere: spraying can be applied in air, inert gas or vacuum.

4. Substrate material and preparation: roughness, oxidation state, preheating time and

temperature during and after spraying.

5. Relative motion of the torch and substrate: traverse speed controls coating thickness per

pass and partially the heat transferred to the coating and substrate.

There are different ways of categorizing spraying methods, based on principal energy source,

maximum achievable temperature [2], particle impact velocity and temperature [17], and even

popularity in the industry [18].

Based on achievable temperature, we may use either plasma spraying or combustion based

spraying techniques. In combustion based spraying techniques, the maximum flame temperature

is governed by the enthalpy of the chemical reaction between combustion gases, which is no

greater than 3300K [2]. On the other hand, when using plasma, the only limitation is the amount

of electrical energy supplied, which in turn is a function of the cross section of the power leads [2]. Plasma temperatures of 5000 K to 25000 K are reported for different types of plasma

spraying methods (Table 2-1) [2, 17, 18].

6

Since in thermal spraying the particles must be in a molten or semi-molten state [19] before

deposition, according to Table 2-1, plasma spraying tends to be a suitable choice for ceramic

materials, which generally have higher melting points than metals.

Table 2-1. Comparison of different spraying methods [2].

Flame D-Gun HVOF APS IPS VPS RFS

Gas Temperature (°C)

p=1 atm 2700 3200 3000 14000 14000 --- ---

p=0.25 atm --- --- --- --- --- 10000 8000

Particle velocity (ms-1)

Al2O3- 30 μm

p= 1 atm 70 500 350 230 250 --- ---

p= 0.25 atm --- --- --- --- --- 380 30

Plume length (cm)

p= 1 atm < 10 --- < 20 < 7 < 10 --- ---

p= 0.25 atm --- --- --- --- --- < 15 < 15

Particle injection Axial Axial Axial Orthogonal or Axial Axial

APS: air plasma spray, IPS: inductive plasma spray, VPS: vacuum plasma spray, RFS: radio frequency spraying.

2.2. Plasma spraying

As discussed earlier, plasma spraying is a suitable method for applying ceramic coatings. Due to

the high temperature, most ceramic materials can be sprayed using this method. However, to

have satisfactory deposition efficiency, the melting temperature should be at least 300 K lower

than the vaporization or decomposition temperature [2].

There are some unique features in plasma spraying such as [18]:

7

1. A wide range of materials can be deposited, from polymers to metals to ceramics and their

combinations.

2. Combinations of ceramics and alloys having different vapour pressure can be deposited

without significant changes in final composition.

3. Homogenous coatings can be produced with no significant difference in composition through

the thickness parallel to spray direction.

4. Fine, equiaxed grains without columnar defects can be produced.

5. Functionally graded coatings can be generated.

6. High deposition rates of the order of mm.s-1 are achievable.

7. Near-net shape free-standing thick coatings can be sprayed.

8. The process can be carried out in different environments such as air, reduced pressure,

controlled atmosphere, and underwater.

2.3. Process parameters

The microstructure and mechanical properties of plasma sprayed coatings are determined by the

size, temperature and velocities of the droplets in the jet at deposition [20] and the substrate

temperature. The temperature and velocity of the particles are determined by the plasma jet

characteristics [21]. A common way to achieve a satisfactory deposit is to apply a design of

experiment (DOE) to find a combination of process parameters that produce acceptable coating

properties[22]..Understanding the relationships between the process parameters, plasma

characteristics, and the in-flight particle characteristics is vital to assure reproducible, high-

quality coatings.

In plasma spraying, the relationship between the processing variables and the resulting plasma

and particle characteristics is complex because of the large number of processing parameters,

including plasma current, plasma gas composition and flow rate, powder size, powder feed rate,

carrier gas flow rate, standoff distance, and plasma gun traverse speed [21]. For example, Zhao et

al. tried to interpret plasma characteristics in an A2000 vacuum plasma spray system (Sulzer

Metco AG, Winterthur , Switzerland) by modeling the influence of different plasma parameters

on the energy balance of the jet [21]. Some of these results are shown below.

8

2.3.1. Plasma current

Figure 2.1 shows variations of plasma gas temperature and velocity, with plasma current in the

range of 400-1000 A, at a fixed Ar flow rate of 35 l.min-1 and H2 flow rate of 8 l.min-1 [21]. The

increases shown are due to increasing the plasma power, plasma energy and plasma gas

ionization [21].

Figure 2.1. Effect of plasma current on: (a) plasma gas temperature, (b) plasma gas velocity [21].

2.3.2. Primary gas flow rate: argon

Figure 2.2 illustrates the effect of Ar flow rate on plasma gas temperature and velocity found by

Zhao et al.[21]. Ar flow rate was in the range of 15-100 l min-1 at a constant current of 700 A, H2 flow rate of 8 l min-1[21]. The decrease in temperature, in this range, is because the amount of gas

to be heated increases faster than the energy input to the plasma. In contrast, velocity increases

almost linearly with increasing the Ar flow rate, which is due to more moles Ar in the jet.

9

Figure 2.2. Effect of Ar flow rate on: (a) plasma gas temperature and (b) plasma gas velocity [21].

2.3.3. Auxiliary gas flow rate: hydrogen

Zhao et al.[21] found that H2 flow rate has a dual effect on plasma gas characteristics (Figure

2.3). The H2 flow rate was in the range of 0-20 l min-1, at a fixed plasma current of 700 A and

Ar flow rate of 35 l min-1. H2 is often used as the secondary gas in APS and VPS, because its

dissociation and ionization contribute strongly to the overall plasma enthalpy [2]. As shown in

Figure 2.3, increasing H2 up to 2 l min-1 increases both the plasma gas temperature and the

degree of Ar and H ionization (Figure 2.3 (a), (b)). However, when gas flow rate exceeds 2 l

min-1, the volume of gas to be heated per unit time increases at a faster rate than the plasma

energy input rate [21].

10

Figure 2.3. Effect of hydrogen flow rate on: (a) plasma gas temperature , (b) degree of Ar and H

ionization and (c) plasma gas velocity [21].

2.3.4. Carrier gas flow rate

Miller et al.[23] showed that the carrier gas flow rate affects coating properties via changing the

particle dwell time and trajectories for radially injected particles. Figure 2.4 illustrates the effect

of radially injected carrier gas flow rate on particle trajectory [23]. With a low carrier gas flow

rate, fine powders bounce off the plasma jet, and most are not melted. Heavier (coarser)

particles, though, can penetrate to the centreline of the jet, heat up and accelerate towards the

substrate. This situation results in a porous coating containing unmelted particles, which

deposits above the centreline of the torch.

Using a high carrier gas flow rate in radial injection, finer particles may travel along near the

centreline, while coarser particles pass the centreline, spending more of their residence time in

11

the hottest section of the jet. The coating forms mainly beneath the centreline with this condition [23].

Figure 2.4. Particle trajectories expected for low and high powder carrier gas flow rates [23].

12

(a)

(b)

Figure 2.5. Effect of carrier gas flow rate on (a) density and (b) deposition efficiency [23].

13

Increasing the carrier gas flow rate increases the density and to some extent increases the

deposition efficiency by melting more particles (Figure 2.5) [23]. If the carrier gas flow rate is too

high, the plasma temperature and velocity can be significantly affected and many particles may

pass completely through the plasma.

2.4. Feedstock injection

The ideal situation for a high density plasma-sprayed coating would be when all particles

injected reach the substrate with a temperature well above their melting point (and below the

decomposition or vaporization point) with velocities as high as possible but compatible with

achieving a fully melted state. High velocities decrease the particle residence time and thus their

heating [19]. This “ideal situation” is impossible to achieve because of the following reasons [1].

(a) Particles resulting from milling and atomization processes have a relatively wide size

distribution: typically between 22 and 45 μm in diameter. For example if the diameter

distribution has a maximum ratio of 2, the mass distribution has a ratio of 8, which leads

to particles with very different thermal history.

(b) Particles, at least in radial injection in DC arc plasma spraying, should be injected with a

momentum similar to that of the plasma jet. This is achieved by using a carrier gas and

an injector. At the exit of the injector, particles are not all parallel to the injector axis.

Thus, some of the injected particles do not penetrate the plasma jet. Furthermore,

depending on their trajectories and masses, not all particles which penetrate the plasma

will melt. The best situation is to optimize the carrier gas flow rate for the mean of the

size distribution.

(c) Particle melting depends on plasma gas velocity (which is related to the residence time)

and the plasma gas composition. The plasma gas composition determines the plasma

enthalpy and controls the heat transfer to the particle. For materials with a low thermal

conductivity (e.g, thermal barrier coating materials such as zirconia and mullite), high

plasma temperatures and velocities may result in larger particles not being uniformly

heated and melted.

(d) For low melting point materials, the particle size distribution is still a problem, as the

question of whether the radially injected particles penetrate into the jet or not remains.

With smaller particles, the carrier gas velocity has to be increased drastically

14

(proportional to the negative third power of the particle diameter) and, for example,

below 5-10 μm the carrier gas flow rate disturbs the plasma jet.

2.5. Particle-particle interactions

Typically powder, 20-100 μm in diameter, is injected radially into the plasma jet and while

accelerating toward the substrate, melts rapidly. The velocity of particles ranges from 100 to 400

ms-1 and the thermal conditions are such that even materials with a high melting point melt in

the residence time [24].

Powder particles in the plasma jet are heated through convection by the plasma gas and

conduction inside the powder particle and at the same time release energy through radiation to

the ambient [2]. The amount of heat gained by the powder from the plasma is:

)( Sc TThaq −= ∞ ( 2.1)

and the amount of heat lost by radiation to the surroundings is:

)( 44 aSTr TTaq −= εσ ( 2.2)

where:

h: plasma/particle convective heat transfer coefficient

a: surface area of the particle

T∞: free-stream plasma temperature

TS: particle surface temperature

Ta: temperature of the ambient

σT: Stefan-Boltzmann constant

ε: particle emissivity

For a particle to gain energy in a plasma jet, the net energy change must be positive:

0>>−= rcn qqq ( 2.3)

Since qr is generally small compared to qc because of the small value of the Stefan-Boltzmann

constant, which is in the range of 5.67×10-8 Wm-2K-4, the net energy is much greater than zero

and the particles heat up in the jet [2].

15

Solid particles tend to bounce off the substrate or previously formed coating. Therefore, the first

essential task in successful plasma spraying is to melt the majority of powder fed into the jet. In

practice, due to large temperature and velocity gradients inside a plume, particles with different

thermal and velocity histories exist [24]. In the case of porous or low thermal conductivity

materials, there is always a thermal gradient between surface and the core of the particles, which

might exceed 1000 K [2]. These particles are the ones that can have the most significant effect on

the coating structure and properties.

2.6. Particle trajectory

Radially injected particles in a plasma jet have different thermal histories even for a constant

carrier gas flow rate. Particle density is one factor affecting the trajectory. Low density particles

having lower momentum cannot easily penetrate into the plasma jet and may bounce off it [13].

These particles spend little or no time in the hot section of the jet and might not melt. On the

other hand, larger size particles may penetrate and travel through the plasma jet, and may not

make it to the substrate. Larger particles need more dwell time in the jet to melt or they may end

up as non- or semi-melted particles (Figure 2.6).

For a given set of plasma parameters and particle structure, there is only one particle size which

will be correctly heated, accelerated and deposited on the substrate. Therefore, a narrow particle

size distribution should be used [13].

Figure 2.6. Schematic picture of particle trajectory for different particle sizes [13].

16

Figure 2.7 illustrates the dispersed particle trajectories of alumina particles with three different

mean diameters in a turbulent-free argon plasma jet [2]. Smaller particles disperse more and

dispersion becomes more prominent when moving downstream from the injection point due to

accumulated random walk influence [2].

Figure 2.7. Dispersed trajectories of alumina particles of different sizes in a turbulent free

plasma argon jet [2].

2.7. Particle- substrate interactions

At impact onto the substrate, the molten or semi-molten/plastic state particles flatten, solidify

and form splats (Figure 2.8). Splats usually have columnar structures with grain size between 50

and 200 nm (Figure 2.9Error! Reference source not found.). Different coating properties are

highly linked to the quality of contacts between splats [19].

Computer simulations show that the most important reason for a change in splat shape is a

change in the thermal contact resistance between the splat and the substrate[25, 26]. The growth of

an oxide layer on metal substrates is one way in which the contact resistance can change

significantly. The thickness of the oxide film on a surface depends on the heating temperature

and time [27].

17

Figure 2.8. Schematic deformation of a splat at impact [2].

Figure 2.9. Microstructure of fractured as-sprayed partially stabilized zirconia showing

columnar grains [28].

Commonly it is believed that splat morphology should have an important effect on adhesion and

coating quality. Tanaka and Fukumoto found that splat shape is highly dependent on substrate

temperature. The transition temperature (Tc) has been introduced as the substrate temperature at

which half of the splats were disk type [29]. The transition temperature is found to depend more

on the coating material than on the substrate material [30]. The most widely accepted explanation

18

for the transition temperature is that Tc is the temperature at which adsorbates and condensates

at the substrate surface are eliminated [31] (Figure 2.10 to Figure 2.12). As generally known,

clean surfaces energetically attract foreign species, resulting in adsorption and condensation of

these molecules. At a given temperature these condensed species begin to vaporize and the

evaporation rate increases with temperature. By heating the substrate the condensed molecules

would be removed from the substrate and the splats deposit on a clear surface. Increasing the

substrate too much and metallic substrate may oxidize which forms an entirely new layer on the

substate.

On the other hand, Pasandideh-Fard et al. found that the rate of solidification is much more

sensitive to values of thermal contact resistance than to substrate temperature [27]. Thermal

contact resistance can be affected by gases or solid impurities trapped at the particle-substrate

interface [30].

Figure 2.10. Morphology of different ceramic splats on AISI304 substrate [29].

19

Figure 2.11. Dependence of fraction of disk shape splats on substrate temperature (substrate:

AISI 304, particle Al2O3) [29].

Figure 2.12. Dependence of Al2O3 splat morphology on substrate (AISI 304) temperature [29].

20

Studies by Pershin et al. showed that heating the substrate while applying a plasma spray

coating can significantly increase adhesion strength and reduce porosity [30]. Similar results were

reported by Salimijazi et al., showing a 40% decrease in porosity after substrate preheating [32].

2.8. Coating formation

Coatings are built up by successive splats. A thermally sprayed coating has a complex

microstructure containing splats with different thermal histories, pores, voids, and microcracks

(Figure 2.13). In this regard, optimizing the coating structure for a specific application is of

great importance.

There has been a great deal of work done to find correlations between coating properties and

spray parameters, but attempts to link at a more fundamental level coating properties to in-flight

particle characteristics, particle properties at impact, and substrate properties are less than a

decade old [19].

Figure 2.13. Schematic rendering of the chaotic structure of plasma sprayed coating: 1. Thin

molten shell; 2. Unmelted core; 3. Liquid splash; 4. ‘Pancake’ splat; 5. Interlocked splat; 6.

Oxidized particle; 7. Unmelted particle; 8. Pore; 9. Void; 10. Roughened substrate; 11.

Substrate[2].

21

The coating structure depends on many parameters in addition to the degree of melting and

velocity of the particles, such as s [1, 18, 19]:

(a) Surface preparation: cleaning, grit blasting or surface roughening has an important role

in increasing the adhesion of the coating. The main adhesion mechanism is splat

shrinkage while cooling around peaks of the roughened substrate. Chemical adhesion

may exist at some points in metal coating/metal substrate interfaces.

(b) Splat layering: this depends on the particle parameters at impact, the shape and topology

of already deposited layers, the ability of the flattening particle to accommodate the

existing pores, asperities, etc, and, finally, of their temperature at the moment of impact.

(c) The substrate and coating temperature: this controls the inter-lamella contacts and the

residual stress distribution.

To address the coating growth rate, deposition efficiency is used. It is defined as the ratio of the

weight of coating formed on the substrate to the weight of powder consumed. The effect of

different plasma parameters on deposition efficiency is shown in Figure 2.14 [2].

Figure 2.14. Dependence of deposition efficiency on various plasma spray parameters [2].

The absolute value of deposition efficiency will depend on the substrate size, shape, and the

spray pattern. For comparison purposes, with the same feeding rate, number of gun passes,

traverse speed, and coating densities, the thickness of the deposited layers will reflect the

22

deposition efficiency. If coating densities differ by a small amount, coating thickness could still

be a useful parameter for qualitatively comparing the deposition efficiencies. For example, if

two coatings had the same thickness but a 5 vol% difference in porosity, the actual deposition

efficiency would be different by only 6%.

2.9. Summary

The plasma spray process is governed by a large number of process parameters. To reliably

obtain an acceptable coating, the effect of the spraying parameters on the coating properties

must be understood. These parameters can then be optimized within the technical limitations of

the system in use.

23

3. Chapter III: Coating Materials

3.1. Introduction

Thermal Barrier Coatings (TBCs) in diesel engines lead to advantages including higher power

density, fuel efficiency, and multifuel capacity due to the possibility of higher combustion

chamber temperature (900°C vs. 650°C) [5, 6]. The increased operating temperature possible

when using TBCs can increase engine power by 8%, decrease the fuel consumption by 15-20%

and increase the exhaust gas temperature 200K [12]. Although several systems have been used as

TBCs for different purposes, yttria-stabilized zirconia with 7-8 wt% yttria has received the most

attention [33, 34]. Several factors play important roles in overall TBC performance including

thermal conductivity, microstructural and chemical stability at the service temperature, creep

resistance, and the coefficient of thermal expansion (CTE) [5, 6, 16, 34-40].

Plasma spraying is the most common method of depositing TBCs for diesel applications. It

creates a splat structure with 10-20 % volume fraction of voids and cracks [41] (Figure 3.1). The

high porosity of this structure makes it an ideal choice for TBCs. Although the potential

advantages of utilizing thermal sprayed TBCs in diesel engines were recognized over 30 years

ago, widespread application has been limited by insufficient lifetimes and unacceptable cost.

Premature failure by spallation has been a frequently observed failure mode for YSZ thermal

barrier coatings. This issue has again become very important in the context of future engines

designed for improved efficiency, durability and reliability [37].

Figure 3.1. Air Plasma-Sprayed (APS) coating [42].

24

In contrast to the case of TBCs for diesel engines, TBCs have become almost universally

employed within the hot sections of gas turbine engines over the course of the past three

decades. Partially stabilized zirconia (PSZ) coatings on turbine blades enhance the oxidation and

corrosion resistance of the Ni- based alloy blades and allow operation of the turbine engines at

higher gas temperatures. The coating increases the lifetime of the blades and limits the

maintenance costs [43].

There are two major differences in the two cases. The cost of the coating represents a much

smaller portion of the total cost of a turbine engine than it does of the cost of a diesel engine.

From a technical perspective, the service environment of the coating in the turbine is markedly

different than in the diesel engine. In the former, the service temperature is high (1000-1100°C).

The superalloy substrate’s maximum service temperature is about 800°C. The thickness of

coating is a few hundred microns and is applied to protect against oxidation, hot corrosion,

thermo-mechanical fatigue, and creep. Due to the high substrate temperature, oxidation of the

bond coat plays a major role in coating failure. In contrast, in the diesel engine the gas

temperature, currently less than 650°C, would ideally approach 900°C. The substrate

temperature is limited to approximately 200°C, and therefore a thick coating (at least 1mm) is

required, which leads to a large thermal gradient. In a thick thermal barrier coating (TTBC) the

bond coat temperature is too low for significant oxidation and creep [15]. As a consequence the

dominant failure mechanisms are different in the two cases.

3.2. Failure mechanisms in TBCs

Mechanical behaviour of a material is highly dependent on its microstructure, which in turn is

dependent on the processing technique. Plasma sprayed coatings have complicated

microstructures (Figure 3.2). Plasma sprayed TBC coatings have a high density of microcracks,

isolated large pores, and weak interfaces between splats. These defects can be sources of

mechanical failure in thermal barrier coatings. Another characteristic of plasma spray coatings

is anisotropy in microstructure and mechanical properties. Cracks can propagate more easily in

the plane parallel to the coating-substrate interface than in the perpendicular plane [44].

Horizontal (parallel to the coating-substrate interface) and vertical (perpendicular to the coating-

substrate interface) cracks in the TBC perform different roles. Horizontal cracks, mostly located

at splat boundaries, may considered non-detrimental to the coating, and helpful to reduce the out

25

of plane heat transfer in it, making the TBC more effective. However, these cracks can grow

during thermal cycling, link together, and cause coating spallation [45]. Vertical cracks, which

may propagate through the coating thickness (then often referred to as segmentation cracks) can

increase the coating compliance and extend its lifetime [45].

Figure 3.2. SEM image of plasma sprayed mullite coatings

Coating failures in diesel engines are known to occur due to either loss of cohesion in the

ceramic layer or loss of adhesion at the coating/bond coat or the bond coat/substrate interface.

Loss of adhesion may occur at high service temperatures due to the growth of an oxide layer

between the bond coat and top coat, known as a thermally grown oxide (TGO) layer. This

mechanism is not significant in water-cooled diesel engines, as the maximum service

temperature does not exceed 1000°C. At the lower service temperatures, thermo-mechanical

fatigue and residual stresses play a more important role in coating failure [5].

Thermal stresses generated by the difference in the coefficient of thermal expansion between the

substrate and coating are one of the major factors contributing to failure in plasma sprayed

coatings [46]. The residual stresses which are induced in the fabrication process of the TBCs are

26

associated with many mechanical failures of the coating. For example, delamination may occur

along the interface of the pre-tensioned coatings [47] while compressive residual stress may cause

spalling inside the coating [48]. The mechanism whereby residual stresses are generated within

thermal barrier coatings will be discussed in chapter 5.

The failure of plasma-sprayed TBCs under thermal cycling is a highly complex process

involving an interplay between several general phenomena listed below: (i) thermal expansion

mismatch stress; (ii) growth of thermally grown oxide (TGO) at the interface; (iii) cyclic creep

of the bond coat; (iv) depletion of Al in the bond coat leading to the formation of brittle oxides

other than α- Al2O3; (v) sintering of the porous TBC and the attendant deterioration of strain

tolerance and thermal resistivity; (vi) degradation of the metal ceramic interface toughness; (vii)

delamination and cracking; (viii) crack coalesces. The TBC failure mechanisms are highly

system and application specific, where one or more of the above phenomena dominate [37]. For

example, in thick thermal barrier coatings (for diesel engine applications), service temperature is

not high enough for TGO formation, top coat sintering or cyclic creep of the bond coat. Thermal

stresses are the most important factors in this application [20].

In thick coatings, additional thickness and low thermal conductivity result in a higher thermal

resistance. These coatings are prone to cracking and delamination near the interface due to the

mismatch in thermomechanical properties between the top coat and the substrate [16]. Kokini et

al showed that the stress relaxation process occurring in thick TBC systems at high temperature

is a significant cause of crack initiation and propagation [15]. Hence, developing a multilayer

thick thermal barrier coating may be a solution that reduces the driving force for TBC-substrate

interface fracture [16].

Plasma sprayed coatings start out with microcrac