Materials Selection for Wear Resistance

8/3/2019 Materials Selection for Wear Resistance

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Steve Roberts - Surface Engineering - Surface Engineering 1

Materials selection for wear resistance

Sensible to adopt a systems approach.The materials aspects are partof the whole design problem.

ApplicationFunction

Design

Unalterablefeatures

Alterablefeatures

Wear rate

Time tofailure

BasicMaterials

Economic,etc.aspects

StressState

Surfacetreatments

OperatingEnvironment

LubricationMethod

DesignDetails

(continuum)

Overallshape

more freedom to alter

(very loose categorisation, as most of the abovemutually dependent to some extent)


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Using design to reduce wear

Fretting

Erosion Adhesive wear

Abrasive wear Displacement control

Reduce loadsLubricate

Stress control

Increase loads Use adhesives or fixingsto clamp the components together

Exclude oxygen Use steel rather than Al or Ti

Reduce loads Keep abrasive out Flush abrasive away Filter to keep abrasive particles

small Use as intrinsically hard a materialas possible without sacrificingductility (Could be ceramic if cleansystem)

Reduce gas speeds Keep abrasive out if possible Filter to keep abrasive particles

small Design fluid flow to give low-erosionimpact angle

Use as intrinsically hard a materialas possible without sacrificingductility

HD, EHD or BL lubrication Reduce loads Smooth surfaces Use solid lubes, polymers, soft

metals if HD lubrication impossible Select materials for low adhesion if

lubrication failure. (see next) Design for most wear to be on

replaceable part.


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Materials for adhesive wear

No liquid or solid solubility

Some liquid, v. lowsolid solubility

Limited solid solubility

Extensive solid solubility

Same metals

Better foradhesive wear

applications

N.B. - Pb, Sn, In against Fe and Al


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Using surface engineering to reduce wear

Fretting

Erosion Adhesive wear

Abrasive wear

Displacement controlHarden surfaces ?

try to design out. Stress controlDesign it out !

Coat with intrinsically hardmaterial

Low incidence - ceramic High incidence - v. hard metals ? or

use ceramic and try to keep particlesizes small.

Surface treat one component toharden

Coat one component with v. hardmetal In both cases, use or coat other

component with / of shearable, non-chemically combining material

Depth of treatment can be small

Surface treat one or bothcomponents to harden

Coat one or bothcomponents with v.hard metal (could use ceramic andtry to keep particle sizes small)

Depth of treatment might have to belarge to cope with wear rates


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Surface Engineering Methods

Surface Modification

Surface Coating

Compositionchanged

Compositionunchanged

Plating

Anodising

Weld coatings

High T.spray processing

Electrolytic

Fusion

Vapour phasemethods CVD, PVD

Flame, Induction

Laser, E-beam

MIG weld

Transformation

Melting

Ion Implantation

Thermochemical(solution)

Carburising

Mechanical Shot peening, etc.

Thermochemical(reaction) Nitriding, Metallising


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Mechanical methods

Any Metal: Work harden the surface

Use controlled impingement of:

shot - peeningabrasive - blasting

Must be able to get at all the surface to betreated.

A low - cost, automatible process.

Not much use for wear resistance:

Adhesive wear: spoils surface polish

Abrasive wear workhardening is part of the& erosion: wear process. Intentional w.h.

has little effect.Very good for fatigue resistance.

Examples: valve spring wire, leaf springs, gears



Transformation Hardening (1)

Steels:

Heat surface into austenite range and quench let cold bulk quench

extra surface cooling

Induction Hardening.

Flame Hardening.

Oxy - actetylene or oxy-propane flameDepth - 0.25 - 6mm

R.F. Heating:

f = 3 - 500kHz

Depth: 20/f below 770C (Tcurie)500/f above 770C

Typically 0.5 - 5mm

Both these methods normally use water quenching - spray or bath.

Quench MediumInlet

Induction Coil

Special coils for shaped components- e.g. gear teeth



Transformation Hardening (2)

Laser Hardening.

High - power (0.5 - 15kW) continuous

beam CO2 laser (2-3mm spot size),scanned over surface by mirrors.

Surface is coated with graphite or ironoxide to absorb light.

Heating rate: ~106 K s-1Cooling rate: ~104 K s-1

Surface is quenched by thermalconduction into substrate.

V high cooling rates can give martensiteeven with low C content - avoids distortionand possible surface cracking.

Higher power densities can lead to

melting (laser glasing)

Laser Hardening.

High - power (1-10 kW cm-2)electron beam (2-3mm spot size),scanned over surface byelectromagnetic deflection.

No surface coating needed.

Effects like laser treament.

0.7

0.8

0.9

1.0

1.1

0 1 2 3 4 5 6 7 8 910-8 10-6 10-4 10-2 1

108

104

10

6

Laser / e-beamtreatment

Laser melting

Induction / FlameHardening

PowerDensity(Wcm-2)

Interaction time (s)



Surface Melting

Useful for wide range of materials:

Basic effect is grain refinement- though in steelscan also get transformations.

Needs very high input power density, by:

electron beam laser beam T.I.G. welding

Good for:

medium carbon steel (0.4 - 0.9% C)

(low carbon ! soft retained - ferrite)

S.G. cast irons (TIG process)- gives tough core with hard surface

Cast Al alloys:

50m50m

As cast:

hardness80 kg mm-2

wear increased byfracture of large Siplatelets.

17% Al - Si alloy

Laser melted:

hardness:160 kg mm-2

Si platelets now

below critical size forfracture



Thermo - chemical treatments

Interstitial elements(carbon, nitrogen)diffused into surface

Hardening by solutes

Quench and temper to getsurface martensite

Interstitial elements(carbon, nitrogen, boron)

and substitutional elements(e.g. chromium)

diffused into surface.

Hardening by formationofhard reaction products

Either:

hard layer orfine hard precipitates

Principally for ferrous alloys



Carburising (case hardening)

Done in austenite phase field:Typically take surface of medium - low C steel (0.15 - 0.2%) to 0.7 - 0.9%C

Either: Quench immediately and light temper or

Cool slowly, machine and then heat - treat.Will always be

Get: some dimensional change (try to minimise as this is final process) hardening (wear resistance)

surface compression (fatigue resistance)

Gas

Carburising

Vacuum

CarburisingCaseCarburising

PlasmaCarburising

~900C ~900C ~1050C ~1050C

Atmosphere:CO / H2 / N2

orCH3OH / N2

Pack in box with charcoal.(& energiser .. BaCO3)

C + residual O2!CO

Components heated,low P CH3 or C5H12added.

Further diffusionheat

Components heated, inlow P CH3; glowdischarge deposits C on

-vely charged surface.Further diffusion heat



Carburising (2) and Carbonitriding

0

200

400

600

800

1000

0 0.5 1 1.5 2

Hardness(k

gmm-2)

Vacuum

45 min / 1040C

20 mmHg

350mmHg

Gas45 min /

900C

Plasma

52 min / 1050C

Depth (mm)

Comparison of typical carburisingtreatments on 0,18% plain C steel.

Carbonitriding

Carbon and Nitrogen diffused into thesteel:

Quench and temper.

Often can Oil quench, as treated surfacehas higher hardenability; reduces risk ofcracking.

Better wear restance than carburising.

Gas Carbonitriding:

add ammonia to Gas Carburising mixture

Salt Bath Carbonitriding(Liquid Carburising)

immerse in molten salt bath, typically45% NaCN, 40% Na2CO3, 15% NaCl

~880C / 1hr.



Compound - forming surface transformations

Nitriding:Diffuse nitrogen in to form fine nitrides with Al, Ti, V, Mo, etc.

Done at ~400C, i.e. in ferritic regime for C steels.(can also do ne austenitic stainless steels that contain nitride formers)

Typically on ~0.4%C alloy steels that have been heat-treated before nitriding.Ideally on steels which temper at the nitriding temperature.

Hardness produced by nitriding retained up to ~500C(higher than decomposition T of martensite produced in carburising - 200C)

Produces compressive stresses - good for fatigue resistance.Can get brittle white layer (iron nitrides). Careful process control needed to avoid.

Gas Nitriding Plasma / ion Nitriding

Heated in ammonia

400 - 550CMay need 3-4 days to

get layer 500m deep

Cr - Mo steel 650 kgmm-2

Cr - Mo - V 900 kgmm-2

Cr - Mo - Al 1100 kgmm-2

Component in used as cathode at 500 - 1000V

in 10-4 - 10-1 atm H2 / N2.Plasma forms, heats surfaceNitrogen ions diffuse in.~3x faster than gas nitriding,

... can be used down to 350 C,

so useful for temper-sensitivesteels.



Nitrocarburising

Cheap and nasty process - deliberate white - layer like formation.

Usually on low alloy or mild steels.

Thin hard layer of Fe (C,N)3

Substrate hardened by nitrogendiffused in

Low C / low alloy substrate

~20m

500 - 650 kg mm-2

Traditional processes (e.g. Tufftriding ) uses salt bath (~2hrs) - sodium cyanide and cyanate

(typical applications - crankshaft bearings)

More modern processes use less toxic baths or adapted gas or plasma nitriding processes



Boronizing

Iron boride layer on low - C steelLayer is strongly keyed to substrate

Diffuse boron into surface to form iron boride layerOuter layer: FeB

Sub surface layer: Fe2B

Have different T, so careful process control neededto avoid cracking.

Hardness ~ 1500 kg mm-2, (SiO2: 750- 1200), souseful against abrasive wear.

Process:

Like case hardening: surround component with mixture of:B4C, - source of BoronSiC / Al2O3 - inert diluentKBF4 - vapourises, decomposes on steel surfcae and injects boron

K and F reform KBF4 by reaction with B4C

Typically, 900 C, 6 hrs : i.e. in austenite field - can heat treat substrate afterwards.

Also can be done in molten salts baths, or by plasma boronizing

Can do to : WC / Co : borides in both Co and WC

Ti & alloys: TiB (hardness 2500 kg mm-2)



Metallizing / Metalliding

Chromising:

Case - harden steel

Pack, Gas phase or salt bath at ~950C todiffuse Cr into surface

Hard layer of Cr23C6 formed ~40m thick

Hardness ~ 1500 kg mm-2 - abrasive wearresistant.

Hardness kept up to ~700 C.

TD process:

Hard layers of V, Ti, Nb carbides formed,~10m thick

Salt bath Hard layers of Cr23C6 formed at 800-1000C to diffuse metals into surface

Quench direct from bath

Hardness >3000 kg mm-2 - abrasive wearresistant.

Hard layers on non-ferrous alloys:

Deposit alloy on surface electrolytically.

Heat treat to interdiffuse and react.

e.g. Sb / Cd / Sn on brasses:Interdiffuse at 400C

20-30m layer with

hardness 450-600 kg mm-2



Ion Implantation

Process:

Accelarators used to inject ions into surfaces at 50 - 100kV

- gives typical penetrationof 1017 ions / cm2.

! 10-20% solution of implanted species.

Low temperature process~200 - 300C from bombardment heating.

Line of sight- complex shapes may be problematic

Can inject any atomic species into anything.

- gas ions (e.g. nitrogen) easiest to make sources for

Can use neutral ions (e.g. Ar) to knock-on a coating into substrate.

- ion mixing

Displacement damage in substrate gives high diffusion coefficients- can heat to allow chemical reactions after implantation

Extra volume of injected matter gives very high compressive stresses.

Effectively no dimensional change - final process



Ion Implantation - applications

Typical applications:

Nitrogen into steels ! fine surface nitrides (?): hard and wear resistant surfaceTi into steels ! low friction surface: modifies surface oxides (?)Ti, B into steels ! fine TiB2 - hard, wear resistant surface

N into ceramics: ! soft, amorphous surface: abrasive wear resistantN into WC / Co ! increased wear resistanceCr into steels ! increased corrosion resistance

20m

20m

Unimplanted Unimplanted

N- implanted N- implanted

SiC:50g

diamondpoint weartrack

Steel :

abrasive wearon rotating pin



Summary of Methods for Steels

400

500

600

700

800

900

1000

1100

10 100 1000 10000

Nitro-carburising

GasNitriding

PlasmaNitriding

Boronizing

Vac. & Plasmacarburising

Flame &

inductionhardeningCarbonitriding

Laser &electronbeam

Carburising

Tem

perature

(C)

Depth (m)Ion

Implantation



Coatings - Plating & Anodising

Plating Anodising

Typically Cr or Ni on Steel.Hard Chrome / Nickel

Typically 10s m to several mm plated(c.f. ~ m for decoration)Cr: 850 - 1250 kgmm-2:Ni: 400 kgmm-2:In electroplating, must take take not tohydrogen-embrittle the steel.

Can also electroless plateNi:Use bath with Ni ions and a reducingagent catalysed on the substrate.

Reducing agents contain P and B -Ni / P: 500 kgmm-2:Ni / B: 700 kgmm-2:

Can also H - embrittle.

Can adapt both process to include secondphases (e.g. SiC, C, Al2O3) in film.

Develop thick (hydrated) oxide layer onAl.

Can be done for decorative purposes (dyes

in the oxide layer)

For wear resistance, produce layer 25 -150mm thick:Hardness:350 -600 kgmm-2

(v. soft for an oxide)

Can only use alloys with low alloyingelement content (< 5% Cu, 10% Si).

Can impregnatefilm with PTFE orMoSi2:

High(ish)hardness &

low friction.



Hardfacing - Weld Coats

Generally applied to Steels.

Coating applied by standard welding methods - Oxy-acteylene, Arc, MIG, TIG, etc.

Deposits are typically several mm thick - can be a lot thicker.

Typically: Austenitic (Mn) steels Martensitic steels Cast irons contining carbide formers WC / Co

Generally used to apply sacrificial material where rher is high abrasive wear.



Spraying - Flame Spraying

Air

Oxy - Acet.Mixture

Feed

~100m/s

~200C ~2000C(particles)

~3000C(flame)

Oxyactelyene flame melts the wire & heats the particlesAdditional air is to accelerate the particles.

Feed can be:Metal wireMetal tube containing

alloying elements orrefractory metal powderWC / Co powder

Ceramic rod

Can reheat by flame or R.F. toremove porosity in the coating.

(Also electric - arc spraying -similar)


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Spraying - Plasma Spraying

~250 - 500m/s

~200C ~2000C +(particles)

~15 000C(plasma)

+

-

Ar + He / H

Powder

High plasma temperature makes it possible to spray ceramics and refractories

Inert gas prevents oxidation.

Trapped air gives some (1-10%) porosity.

Needs good surface preparation (grit blasting) to key coating on.

Often usebond-coats

(Ni / Al) as intermediate layer if ceramics (typically ZrO2) sprayed.


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Spraying - Vacuum Plasma Spraying

Can spray Ti and Al basedmaterials - explosive in air.

More as new processing routethan for surface engineering)

Ni - superalloy

VPS aluminised layer

VPS CoNiCrAlY

bond coat(Oxidation resistant -near fully dense)

APS ZrO2 8%Y2O3Thermal barrier layer

(porosity aidsthermal shock)

100m

VPS Unit at Oxford

Avoids trapped air problems:

Low porosity - generally bettertribological properties forceramic coatings.


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Physical Vapour Deposition

Target

Liquid

Heat

vacuum

Target

0.1 - 10Pa Arglow discharge

Solid

+ 0.5-5kV

Target

1 - 10Pa Ar

glow discharge

-

0.3-3

kV

Liquid

Heat

Evaporation SputteringIon Plating

Vapour from sourcecondenses onto target.

Only useful for low m.p.

coatings - optical films etc.

Weak adhesion; can(re)heat target tointerdiffuse.

Ar ions sputter atoms fromsource, hit target with fewkV - good adhesion.

Use of a.c. allows non-conductive source.

Can do in reactive gas todeposit nitrides, carbides,oxides.

Evaporated atomsbecome ionised inplasma, attracted totarget.

Can do in reactive gas todeposit nitrides, carbides,oxides.e.g. Ti in N2/ Ar to depositTiN at ~400C onto steels

(without affecting temper).


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Chemical Vapour Deposition

Pass reactant gas(es) at lowpressure over heated substrate.

Reaction occurs on the substrate.Typically: 0.1 - 1m / hr

500-1000C

1 - 10m thick layers

Columnar microstructures.

TiN /TiC

Peoples Choice for abrasion resistance

TiCl4 + N2 + 2H2 ! TiN + 4HClTiCl4 + CH4 ! TiC + 4HClTiCl4 + C + 2H2 ! TiC + 4HCl

(using C in high C steel or WC / Co)

PACVDUse an RF or microwave plasma - bringsdeposition temperature down to < 500C

SiC

Many possible reactions:Most common is methyl trichlorosilane (MTS)

CH3(SiCl3) ! SiC + 3HCl

MTS is toxic, very inflammablebut not(likeother possibles) explosive on contact with air...

< 1200C: amorphous SiC

> 1200C: - SiC, with increasing grain size asT increases

H2

Diamond

~ 600C: deposit from methane / H2/ O2

Need microwave plasma so the H can etchaway graphite, leave diamond.

Narrow band of CH3 : H2 : O2 to get diamond


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Diamond Films

Top & side views of a polycrystalline CVD diamond film grown on a Si

substrate. Process gas mixture was 1% methane in H2.

The initial stages ofnucleation of diamondon a Ni substrate.

For more information:http://www.tlchm.bris.ac.uk/~paulmay/diamhome.htm


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Materials Selection for Wear Resistance