Download - Thermal Spray Coatings
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SURFACING BYTHERMAL SPRAYING
Dr. M. KAMARAJ
DEPT OF METALLURGICAL AND MATERIALS ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY MADRASCHENNAI 600 036
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OUTLINE
Classification of Surface damages
Industrial Components : problems
Need for Surface Modification
Coating Techniques:
Hardfacing by welding methods
Thermally spray processes
Applications
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LOSS OF USEFULNESS OF MATERIAL OBJECTS
The cause of loss of usefulness of material objects
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LOSS OF USEFULNESS OF MATERIAL OBJECTS
The cause of loss of usefulness of material objects
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CLASSIFICATION OF WEAR
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An accelerated corrosion of metal surfaces
The combined effect of oxidation and reactionswith sulfur compounds and other contaminants,such us chlorides, to form a molten salt on ametal surface that fluxes, destroys, ordisrupts the normal protective oxide.
HIGH TEMPERATURE CORROSION
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HIGH TEMPERATURE CORROSION
Engineering components:
- Gas turbines of aircraft/Marines engines
- Fossil-fired power plants: Boilers, superheaters,
and heat exchangers
-Refineries and Petro-chemical processing
Components: Fluid-catalttic cracking units (FCCU),catalytic steam reformersand Ethylene cracking
- Fluidized-bed coal combustion
performance : materials
Corrosion resistance or
high temperature mechanical properties
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HIGH TEMPERATURE CORROSION DAMAGE
High temperature corrosion may have variousDetrimental effects:
i. Due to the loss of materials
ii. Corrosion reduces the load-bearing capability ofconstructoin materials
iii. Significant amount of material may be lost or wastedin hot working of metals
iv. Reaction chambers to which a particular reaction orprocess is confined may break down etc.
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MODES OF HIGH PEMPERATURE CORROSION
Principal modes of high –temperature corrosion in industrialenvironments, as well as the interaction between oxygen activity andprincipal corrodent activity
Applications above 650 C
-More than one mechanismacts to degrade the material
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SURFACE
TREATMENTMETHODS
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INTRODUCTION
Surface Engineering :
- is a discipline that combines chemistry, physics,Mechanical engg with Metallurgy and Materialsscience
- provides the weapon for deterioration processes
product SE Enhanced Product
Raw materials Thermal Energy Product
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PERFORMANCE OF COMPONENTS
Chemical : acids, alkalis, salts, solvents, etc
Thermal : High temperatures, direct flames,oxidising gases, etc
Atmospheric: humidity, rain, snow, sunlight, sea water,erosion, etc
Mechanical : Abrasion, vibration, adhesion, galling, etc
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SIGNIFICANCE OF SURFACE ENGINEERING
Main purpose may be :
i. To minimize corrosionii. Reduce frictional energy lossesiii. Reduce weariv. Act as a diffusion barrierv. Provide thermal insulationvi. Exclude certain wave length of radiationvii. Promote radiation electronic interactionsviii. Electrically insulateix. Improve asthetic appearance for the surface
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SURFACE TREATMENT METHODS
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THERMAL SPRAYING
- A group of processes in which a finely divided material (metallicor nonmetallic) is heated rapidly in a hot gaseous medium &simultaneously projected at high velocity on to a preparedsubstrate surface where it builds up the desired coating”
Definition of Thermal Spraying: ASMDefinition of Thermal Spraying: ASM
- powder, rod, cord or wire
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PRICIPLE OF TS
1 - Spraying particles transport2 - Impact on the surface3 - Thermal transfer from particles to substrate4 - Particles Solidification and5 - Mechanical bond6 - Local Fusion
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THERMAL SPRAY METHODS
Air/Vacuum/Low pressure plasma
D - Gun
High velocity oxygen fuelHigh velocity flameHigh velocity air fuel
Electrical
Oxyfuel
Oxyfuel
High energy processa)Plasma spraying
b)Detonation flame spraying
c)High velocity oxyfuelspraying
Oxyfuel gas-powder sprayingOxyfuel gas-wire sprayingMetallising
Electrical arc sprayingTwin arc sprayingMetallising
Oxyfuel
Electrical
Low energy processa)Flame spraying
b)Arc spraying
Other names for the processEnergy sourceProcess
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CLASSIFICATION
Thermal spray classificationThermal spray classification
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FLAME SPRAYING
Flame spraying is the oldest of the thermal sprayingprocesses
Flame spray uses the chemical energy :
- combustion of gases to generate heat
Oxyacetylene torches are the most common :
- acetylene as the main fuel in combination ofoxygen to generate the highest combustiontemperature
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FS process : Powder, wires or rods are
introduced axially through the
rear of the nozzle into the flame
at the nozzle exit
- the feedstock materials are melted and the
particles/droplets accelerated toward the substrate
surface by the expanding gas flow and air jets
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OXY FUEL GAS WIRE SPRAYING
Schematic diagram of Oxy-fuel gas wire spraying
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MATERIALS USED
• Zn & Al anti-corrosion cathodic coatings on
steel.
• Ni/Al composite wire bond coats, heat
& oxidation resistance.
• Mo bond coats, excellent resistance to
adhesive wear.
• High Chromium steel hard and wear resistantcoating.
• SS, Ni and monel anti-corrosion and wear.
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WIRE ARC SPRAYING
Schematic diagram of Wire Arc Spraying
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Detonation Thermal Spraying Process
Schematic diagram of D-Gun wire spraying
Barrel 1-1.5m long and 20-30 mm Internal dia into which thegas mixture is injected and ignited by a spark plug
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ADVANTAGES
• Dense microstructure with 0.1-2 % porosity.
• Smooth surfaces finish (1-4 Ra).
• Better impact wear /fretting wear /erosion/corrosion/resistance.
• Controlled residual compressive stress
• Negligible thermal degradation of powder butalso to preserve bulk microstructure
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Materials and Applications
• WC/Co coatings :
- D-Gun coatings have higher degree of retainedcarbides
(Due to the reducing atmosphere of the confined
combustion zone in the barrel and the shorter dwell
time)
- D-Gun coatings : densest and hardest
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HIGH VELOCITY OXYFUEL SPRAYING
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11-Feb-10 11111 30HVOF spraying Tungsten Carbide / Cobalt Chromium Coating(WC/10Co4Cr) onto Roll for the Paper Manufacturing Industry
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Materials Used
(i) Cr3C2- NiCr
(ii) NiCrMo (wire process)
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SURFAC MODIFICATION BY
PLASMA PROCESSES
Non-transferred arc :Plasma spraying
- An arc is established between theelectrode and the constrictingorifice
: working piece is essentially keptout of the electrical circuit
The heat imparted to the job isobtained from the plasma jet only
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PLASMA SPRAYING (APS)
Schematic diagram of Plasma Spraying
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PLASMA SPRAYING (APS)
Schematic diagram of Plasma Spraying
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AIR PLASMA SPRAYING
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SPRAYING MATERIALS
Pure metal Mo,Ni,Ta,Al ,Zn.
Alloys NiCr, NiCrAlY, FeCrBSiC ,steels ,Bronzes.
Pseudo alloys CuW, AlMo.
Ceramics/Carbides Al203 ,Cr203 ,Ti02 ,Zr02 ,WC.
Cermets Cr3C2/NiCr ,WC/Co ,Zr02/NiAl
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COLD SPRAY PROCESS
A coating technology: in which spray particles ranging in size from1 to 50m in dia, in the solid state are accelerated to high velocity(above 700-1200m/s, supersonic velocity) and subsequently develop adeposit or coating on a substrate by an impaction process
Various terms: Kinetic energy metallization,Kinetic spraying,High velocity powder deposition &Cold gas-dynamic spray method
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COLD SPRAY PROCESS
)
Schematic diagram of Cold spraying
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ADVANTAGES
TS may be used to deposit non-weldable coating materialssuch as plastics or ceramics
No distortion
No post treatment Reduced cost
Low Heat Input -No HAZ.
Versatility: Almost any M, C or P.
Thickness Range: 0.001 inch to more than 1 inch thick,depending on the material and spray system.
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PROTECTIVE COATINGS FOR HIGHTEMPERATURE APPLICATIONS
Superalloys : Designed - to operate at high temp - good high temp mechanical, - heat resistance, & - good corrosion resistance
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- aircraft gas turbine engines
- Industrial land-based turbines
SUPERALLOYS: APPLICATIONS
The result of 2500 h low altitude sea flightservice on an uncoated and NiAl coated bladeturbine blade
Two general types of environmentaleffects:
i. Oxidation ii. Hot corrosion
iii. Combination of these effects
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COATING PROCESSES
Air-plasma sprayed coatingcontain porosity & micro-cracks :- help to redistribute thermal stresses but providecorrosion paths through the coating
Low-pressure plasma spray coating :
-Provide high coating purity and essentially eliminatesoxides and porosity
HVOF, EBPVD
Or combinations methods oxides and porosity
11-Feb-10 11111 43Typical coating thickness/depth of penetration for various coatingand surface hardening processes
COATING THICKNESS/DEPTH OF PENETRATION