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COATINGS FOR HIGH TEMPERATURE

APPLICATIONS

Department of Metallurgical and Materials EngineeringIndian Institute of Technology Madras

Chennai 600 036

DR. M. KAMARAJ

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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

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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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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

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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

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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

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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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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)

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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

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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

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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:

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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

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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.

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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

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COATING AND WELD PATTERNS

Cross-sections and bead appearances by diode laser and TIGcladding.

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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

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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

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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

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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.

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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

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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 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

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METHODS OF DIFFUSION COATINGS

Main types of coating : 1. Pack cementation

2. Slurry-fusion

3. Gas-phase (out-of-contact) process

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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

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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)

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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

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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 )

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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

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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.

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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

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PERFORMANCE

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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

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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

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ROLE OF BOND COAT

• Provided to improve adhesion of TBC

• Forms TGO for oxidation resistance

• Intermetallic material

• Influence on structure and morphology of TGO

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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

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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

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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

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COATING METHODSElectron-beam physical vapor deposition

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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