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7-Mar-09 11111 1
COATINGS FOR HIGH TEMPERATURE
APPLICATIONS
Department of Metallurgical and Materials EngineeringIndian Institute of Technology Madras
Chennai 600 036
DR. M. KAMARAJ
7-Mar-09 11111 2
OUTLINE
Reasons for Surface damage
Need for Surface Modification
Classification of wear & Corrosion
Surface modification processes:
- case studies
(i) Exhaust valves
(ii) Boilers
(iii) Gas turbine blades
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REASONS FOR SURFACE DAMAGE
LOSS OF USEFULNESS OF MATERIAL OBJECTS
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CLASSIFICATION OF WEAR
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Uniform or general corrosionGalvanic corrosionPitting corrosionCrevice corrosionErosion-corrosion (Cavitation erosion & Frettingcorrosion)Intergranular corrosion including sensitization andexfoliationDealloyingEnvironmentally assisted cracking (SCC, corrosionfatigue, & hydrogen damage)
FORMS OF CORROSION
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Oxidation
Carburization and metal dusting
Nitridation
Halogen corrosion
Sulfidation
Ash/salt deposit corrosion
Molten salt corrosion
Molten metal corrosion
MODES OF HIGH PEMPERATURE CORROSION
Principal modes of high –temperaturecorrosion in industrial
environments, as well as the interactionbetween oxygen activity andprincipal corrodent activity
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Engineering components
Gas turbines of aircraft/Marines engines
Boilers, superheaters, heat exchangers in Fossil -fired power plants
Petro-chemical processing
Fluidized-bed coal combustion
performance : materials
wear/Corrosion resistance or
high temperature mechanical properties
APPLICATIONS
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• Increasing component life and hence reducingmaintenance and replacement cost
• Avoiding excessive component breakdown andthereby limiting consequential losses because oflost production
• Reducing investment costs through increasedmachinery life
NEED FOR SURFACE PROTECTION
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THERMAL SPRAYING
- A group of processes in which a finely divided material (metall icor 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 th e substrate
surface by the expanding gas flow and ai r 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
7-Mar-09 11111 21
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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7-Mar-09 11111 24HVOF spraying Tungsten Carbide / Cobalt Chromium Coating(WC/10Co4Cr) onto Roll for the Paper Manufacturing Industry
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SURFAC MODIFICATION BY
PLASMA PROCESSES
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PLASMA ARC PROCESSES
Two different arc Modes:
Non-transferred arc : Plasma spraying
- An arc is established between the electrode and theconstricting orifice : working piece is essentially kept outof the electrical circuit
The heat imparted to the job is obtained from the plasma jetonly
Transferred arc :PTA
The arc is struck between the cathode and the work piece(anode)
This results in greater energy transfer to the workpiece
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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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COLS 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
7-Mar-09 11111 32
COMPARISION
Temp/velocity regimes for common thermal spray processes compare d tocold spray
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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 .
Coating thickness generally range from 0.001 to 0.100 inch
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TEXTILE COMPONENTS
Thread overrun roller and thread guidescoated with ceramic by plasma spraying
to protect against frictional wear
7-Mar-09 11111 36
APPLICATION
Engine Wear
Adhesive wear (cylinder linersand piston rings)
Abrasive wear (cams, rockerarm, and tappets)
Scuffing (cylinder liners andpiston rings )
Corrosive wear (piston ring,cylinder)
Fatigue wear (connecting rod &valve seat)
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APPLICATION : Valves
Function :- control the gas flowing into and out of the
engine cylinder
- it must form a gastight combustionchamber seat at the valve seat ring inthe cylinder head
- high-velocity reciprocating motion withlow friction along valve guides
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WORKING CONDITIONS
Valves: - exposed to severe loads they are lifted up to 3000
times in a minute & force back firmly on to
their seats by valve spring
The valve typicaly receives an acceleration of
2000 m/s2 under high temperature
Inlet valve is kept cool by regular inward flowof fresh mixture, but it can still reach atemperature as high as 500 C
Stem end
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WORKING CONDITIONS
Exhaust valve is heated to between 650 – 850 C, high temp corrosion
Temperature distribution of valves during operation:An air cooled 200cm3 engine (a) Inlet valve , (b) Exhaust valve
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During operation :
The carbon soot formed by combustion can stick to the valve,hindering valve closure and consequently causing leakage
To prevent this, the valve revolves during reciprocatingmotion
The rotation rubs off the soot and prevents uneven wear ofthe valve face and seat
The face is exposed to high temperature combustion gasand rubbing occurs without oil lubrication
PROBLEMS
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PROBLEMS/PROPERTIES
Problems :
- wear occurs on the stem and at its end (valve seat)
- high temperature strength and corrosion
Properties :
- the valves must be of light weight to allow reciprocating motio n
- High rigidity, high hardness and good wear resistance
-high temperature fatigue strength and
- high temperature corrosion resistance
7-Mar-09 11111 42
MATERIALS USED
Exhaust valves: Materials Used : Single or Bimetallic metals1. Precipitation-hardening austenitic steel
( 0.5 C-9Mn-3.75Ni-21Cr-0.4N)- best properties of this alloy: Solution -treated at 1180 C,quenched and aged for 12 h at 760 C
2. Ni-base alloys: Nimonic alloysNimonic 80A (wt.%): C = 0.02, Cr=19, Co=1.6, Ti = 4.42
Al = 1.05, Fe = 2.08, Mn = 0.4%,Si =0.44, P & S = 0.005
3. Co-base heat resistant alloy ( stellite No.6)1.2%C, 1.1%Si, 0.5%Mn, 3%Ni, 28%Cr, 1%Mo, 3%FeRole Cr: forms thick oxide
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7-Mar-09 11111 44
Schematic of dilution definition
WELD DILUTION
DilutionDilution
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RECENT METHODS
HardfacingHardfacing Techniques :Techniques :
Plasma Transferred Arc (PTA)Plasma Transferred Arc (PTA)
Laser beamLaser beam hardfacinghardfacing
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The PTA process is a technological evolution of GTAW welding. T he maindifferences are :
GTAW Open ArcGTAW Open ArcGTAW Open ArcPPAW ConstrictedArc
PPAW ConstrictedPPAW ConstrictedArcArc
constricted arc with columnar shape
use of metallic powders as filler material
GTAW PAW
45° 6°
10 ll
Variation of Thermalcross section = 20 %
Arc Length – ArcDivergence
Arc LengthArc Length –– ArcArcDivergenceDivergence
Plasma Transferred Arc (PTA)
7-Mar-09 11111 47
PLASMA PROCESSES
PLASMA TRANSFERRED ARC WELDING (PTAW) PROCESS
The arc is struck between the cathodeand the work piece (anode)
This results in greater energy transferto the workpiece
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From Metallurgical and Mechanical property standpoint
-Precise control of weld parameters-Powder feed rates, gas flow rates,-curreent and voltage (heat input)-Controlled heat input helps Less dilution,-less HAZ (otherwise, grain coarsening, martensitetransformation, or strain aging)
PTA weld deposits :- low levels of inclusion, oxides and discontinuities make
good toughness and general corrosion resistance
REASONS TO CHOOSE PTA
7-Mar-09 11111 49
Carbide-strengthened alloys
Boride-strengthenedd alloys
Silicide-strengthenedd alloys
Intermetallic laves-phase alloys
Solid solution alloys
Cermet composites
MATERIALS FOR PTA HARDFACING
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SOLUTION
Solution : - hardfacing improves the wear resistance
The valve face is coated with melted stellite powderCo-based heat resistant alloy:
Materials Used :
2. Co-base heat resistantalloy ( stellite No.6) (57HRC)
1.2%C, 1.1%Si, 0.5%Mn,3%Ni, 28%Cr, 1%Mo, 3%Fe
Role Cr: forms thick oxide
7-Mar-09 11111 51
MATERIALS FOR PTA HARDFACING
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LASER BEAM HARDENING PROCESS
• The energy supply can be well controlled;
• Very local treatment is possible;
• The total heat input is low, resulting in minimal
distortion;
• The heating and cooling rates are high, resulting in a fine
microstructure and/or metastable phases;
• The treatment is a non-contact process. There is no tool
wear, nor act mechanical forces on the workpiece;
• The process depth is well defined.
The use of a laser beam in surface treatments offers several advantagesover conventional heat sources:
7-Mar-09 11111 53
Laser surface treatment processes
Laser PVD Laser CVD Laser assisted electroplating
Thin coating processes
Shock hardening Laser engraving and marking Paint stripping Laser cleaning
Laser treatments with partialevaporation
Laser melting Laser alloying Laser cladding Laser nitriding
Liquid phase laser surfacetreatments
Transformation hardeningSolid state laser treatments
7-Mar-09 11111 54
Processing parameters compared to other laser processes
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LASER CLADDING PROCESS
Laser cladding is a process
-by which a powdered/ wire
material is melted by use of a
LASER in order to coat part of a
substrate.
7-Mar-09 11111 56
The powder/wire is injected into the system by either coaxial orlateral nozzles.
The interaction of the metallic powder stream and the lasercauses melting to occur, and is known as the melt pool.
This is deposited onto a substrate; moving the substrate or lase rbeam allows the melt pool to solidify and thus produces a t rack ofsolid metal.
IMPORTANT STEP: LASER CLADDING
Laserbeam
Laserbeam
ShieldinggasPowder
andconveyin
g gas
Shieldinggas
Powderandconveyinggas
Workpiece
Workpiece
7-Mar-09 11111 57
COATING AND WELD PATTERNS
Cross-sections and bead appearances by diode laser and TIGcladding.
7-Mar-09 11111 58
Selection of alloys and principal alloying elements used for LAS ER Cladding
MATERIALS
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Industrial applications of laser cladding
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HIGH TEMPERATURE COATINGS
POWER PLANT APPLICATIONS
BOILERS TUBES
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PROBLEMS
Power plant components : Pipes and coal boilers
-failures of components due to
(i) Erosion
(ii) Corrosion (oxidation/hot corrosion) &
and combination of (i) and (ii)
- If not detected early stage:- leading to unwanted costly shut down
component replacement/repairing
7-Mar-09 11111 62
SOLUTION
Surface modification:
- enhances the life of the components
Which type of coatings????
Ex. Case-hardening :
- thin hardened surface
poor corrosion resistant and lose theireffectiveness at high temperatures
7-Mar-09 11111 63
COATING METHODS
- Electrodeposited intermetallic coatings
- Weld Overlay coatings
- Thermal spray coatings(HVOF/HVCC)
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HIGH VELOCITY OXY/FUELSPRAYING (HVOF)
-a high velocity flame spray process,
gas velocities exceeding Mach 1, 2300 C
7-Mar-09 11111 65
HIGH VELOCITY CONTINOUS COMBUSTION (HVCC)
In an HVCC process the spray material is introduced in wire form and thewires are melted by creating a supersonic air stream with a plas ma betweenthem.
The arc also heats an air stream that is accelerated through a n ozzle andremoves the molten wire particles.
• 4000° C arc temperature
• Supersonic air velocities creates fine particle atomization
• 300m/s particle velocities
• Finely layered homogeneous coating structure
• Coatings exhibit negligible porosity
• Coatings can be applied with < 5% oxides
• Applies metals, hardfacing alloys and high melting temperature alloys suchas tungsten and molybdenum.
7-Mar-09 11111 66
Materials Used
(i) Cr3C2- NiCr
(ii) NiCrMo (wire process)
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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
7-Mar-09 11111 68
- 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
7-Mar-09 11111 69
COATING REQUIREMENTS
• Coatings must withstand hot corrosion, oxidation, anderosion when placed into a flow of gas whoseparameters are similar to those of turbine gases
• It must safely withstand the static and alternatestresses applied to the blade surface;- to this end the coating must have the requisite combination ofstrength and ductility
• It must show good stability and not be destroyed byinteraction with the substrate
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SELECTION CRITERIA
• High melting point: less diffusion of ions
• No phase changes
• Low thermal conductivity
• Thermal expansion match with substrate
• Good adherence
• Low sintering rate
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Coatings are classified into two categories:
i. Diffusion coatings- coatings that alter the substrate outer layer by theircontact and interactions with certain metal species
ii. Overlay coatings- coatings that are formed by the deposition of protective
metallic species onto a substrate surface, with someelements inter diffusion providing coating adhesion
TECHNOLOGICAL PRINCIPLES OF COATINGS
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DIFFUSION COATINGS
Aluminized coatings: Al diffuses into the surface ofa material by diffusion
- Diffusion coatings are based on the formation ofintermetallics compounds such as NiAl and CoAlvia diffusion process
- During service, Al present in the coating reacts eithoxygen at high temp forms Alumina layer on thesurface of the coated alloy
Main types of coating : 1. Pack cementation2. Slurry-fusion3. Gas-phase (out-of-contact) process
7-Mar-09 11111 73
METHODS OF DIFFUSION COATINGS
Main types of coating : 1. Pack cementation
2. Slurry-fusion
3. Gas-phase (out-of-contact) process
7-Mar-09 11111 74
PACK CEMENTATION
- it is a type of vapor deposition process
- in this process, both the component to be coated and the react antsthat combine to form a vapor are contained in an airtight r etort.
- reactant : - consists of aluminiumcontaining powder,
a chemical activator andan inert filler Al2O3
: upon heating in an inert atmosphere, thereactant form a vapor that reacts with the
component surface enriching it with Al.(750-1150C)
Al penetrates into the substrate to form a zonethickness and morphology of which are a f unction of thetime and temperature of the process
7-Mar-09 11111 75
PACK CEMENTATION
Inwards diffusion : Formation of Ni2Al3 phase- brittle phase& low melting point ,
- not practical use
Heat treatment : Between 1040 – 1095 C, to convert NiAl- useful for service applications
In practice, inward diffusion coatings are applicable only to-Ni-base alloys
( because the coating initially formed by the inward diffusion of Al , substratemodification of of the coating is maximized as substrate element s are lockedin place)
7-Mar-09 11111 76
PACK CEMENTATION
Ni-base alloys : Aluminides such as Ni3Al, NiAl and Ni2Al3
Co-base alloys: CoAl, and Fe-base alloys : FeAl
Inwards coatings :
- produced when Al activity in the pack high, there is apreferential diffusion of the pack through the aluminide layerbeing formed
- Al diffuses inward faster than Ni diffuses outward through thenickel-aluminide intermetallics that initially form on thesurface reaction temp : 700 – 800 C
7-Mar-09 11111 77
PACK CEMENTATION
Modified Aluminide coatings:
Major developments in diffusion coatings:
Modification of aluminide diffusion coatings with Cr, Pt, and
to lesser extent of Si
Thickness of aluminide coatings : 75-100
Major Problems :
- protection of thin sections of cooled blades (cooling passages )
7-Mar-09 11111 78
OVERLAY COATINGS
Coatings of this type are generally called
MCrAlY overlay coatings
where M represents Ni, Co, Fe or somecombination of these metals
- these coatings comprise a monoaluminide(MAl)component contained in a more ductlie matrix of ,in the case of CoCrAly.NiCrAly coatings: a mixture of and ’ Ni3Al
Matrix is Ni or Co
7-Mar-09 11111 79
MCrAlY COATINGS
• Overlay coatings provide design flexibility
• M - Ni, Co or Fe
• Cr- provides hot corrosion resistance
• Al – (10-12 wt%) provides oxidation
resistance
• Y- (1wt%) enhances the adherence of the
oxide layer by combining with S.
7-Mar-09 11111 80
CHARACTERISTICS
General feature of MCrAlY coatings:
(i) An oxide scale on the outer surface
(ii) Material immediately beneath the scale– modified composition and
(iii) an interdiffusion zone in compact withthe substrate
7-Mar-09 11111 81
PERFORMANCE
7-Mar-09 11111 82
THERMAL-BARRIER COATINGS
Top coatCeramic material likestabilized Zirconia
Generally stabilized Zirconia(6 -8.7%yttria)
Why Stabilized Zirconia?
Strain tolerantResistance to Na2So4 and V2O5
Low thermal conductivityIdeal thermal expansion matchHigh thermal stability
7-Mar-09 11111 83
THERMAL-BARRIER COATINGS
Advantages :
• Relatively inert• High melting point• Low thermal conductivity• Resistance to thermal cycling
• Hot corrosion and oxidation resistance ofcoated blade can be further improved
7-Mar-09 11111 84
ROLE OF BOND COAT
• Provided to improve adhesion of TBC
• Forms TGO for oxidation resistance
• Intermetallic material
• Influence on structure and morphology of TGO
7-Mar-09 11111 85
COATING PROCESSES
Air-plasma sprayed coatingcontain porosity & micro-cracks :- help to redistribute thermal stresses but provide
corrosion 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
7-Mar-09 11111 86
THERMAL-BARRIER COATINGSEB-PVD Coatings have a columnar grain morphology:individual grains are strongly bonded at their base
Major advantage :
Columnar outer structure lies in thefact that that it reduces stress buildupwithin the body of the coating
7-Mar-09 11111 87
COATING PROCESSES
1. By vapor deposition onpreheated samples
2. For more mechanically loadedparts
3. High cost
4. Difficult to controlcomposition,
5. Much superior thermal fatigueproperties
6. Columnar microstructure withmulti scale porosity
7. Small parts
1. Either vacuum or air plasma2. For hot components of
combustion chamber3. Low cost, Low thermal
conductivity,4. Compositional flexibility
5. Immediate spallation onthermal fatigue
6. Laminar structure + splat ofcracks
7. Large parts
EB PVDPlasma sprayed
7-Mar-09 11111 88Typical coating thickness/depth of penetration for various coati ngand surface hardening processes
COATING THICKNESS/DEPTH OF PENETRATION
7-Mar-09 11111 89
7-Mar-09 11111 90
COATING METHODSElectron-beam physical vapor deposition
7-Mar-09 11111 91
THERMAL-BARRIER COATINGSTypical coatings for high -temperature applications involve anoxidation resistant coating and a thermal barrier coating (TBC). Theoxidation resistant coating is also called bond coat because itprovides a layer on which the ceramic TBC can adhere.
Thermal Barrier coating designSEM Micrograph of EB-PVD. The turbine blade contains internal channelsfor air-cooling