Wear and Wear Mechanisms.pdf

Hhere Werkstofftechnik: Tribologie Wear Mechanisms

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Wear Mechanisms1 www.uni-due.de/wt

Universitt Duisburg-EssenLotharstr 1, 47057 Duisburg, Germany

WerkstofftechnikMaterials Science & Engineering

Wear and Wear Mechanisms





There is no simple relationship between wear and any other materialsproperty

Thus we always have to take into account that wearis not a materials property like e.g. strenghtis a property of a specific tribosystemhas to be evaluated for every tribosystemcannot be generalized in detail

On the basis of known mechanisms oncan understand the charactreistics of a tribosystem andcan take care of well-aimed counter measures

without the knowledge of the mechanisms you can follow trial-and-error


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There is no simple relationship between wear and any other materialsproperty

In order to underastand the mechanism dependant counter measures on can follow the very (!) simple model of Archard (1959)

Archards equation (not law!!) is

W

Ns H

FksWW ==

The wear rate W related to the wear path s is proportional to the normal load FN and the hardness of the worn surface HW and a factor k.

k describes the probability to generate a wear particle during the path s and contains all constants and variables of a tribosystem.




Wear MechanismsAdhesion

Surface Fatigue

Abrasion

Tribochemical Reactions

k depends on theacting wearmechanisms and can be describedonly qualitatively!


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Wear Mechanisms

Verschleikoeffizient k

k might vary between more thannine orders of magnitudedepending on the acting wearmechanism (and submechanisms)

wear coefficient k




Mechanically Dominated Wear Mechanisms

Surface Fatigue

Abrasion


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Surface Fatigue

under rolling wear

under sliding wear

Surface fatigue is brought about by crack initiation and propagation, whichmight take place at the surface or in a certain distance below it. Thatdepends on the contact situation and the microstructure of the contactingmaterials.




Surface Fatigue (rolling wear)

Appearances of surfacefatigue on a wheel of a harbour crane made out of perlitic cast ironGJS-1000 (AISI A538)

Taper section of the worn surface


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Surface Fatigue (sliding wear)Appearances of surfacefatigue on the cylinderwall made out of hardened and temperedsteel 42CrMo4 (AISI4140) under sliding wear of an hydraulic pump.

The counterbody was a chromatized piston ring

Throttle-disc pump




Surface Fatigue (cavitation)

Appearances of surface fatigueunder cavitation on the cylinderwall made out of hardened and tempered steel 42CrMo4 (AISI4140) of an hydraulicpump.

Notice: There is no counterbody under cavitation


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Surface Fatigue (rolling wear)

Appearances of surface fatigue underrolling wear on the cage of a rollerbearing out of hardened and tempered steel 100Cr6 (AISI52100).

Source: VDI5932-5, VDI, Duesseldorf, Germany




Surface Fatigue (3-body abrasive wear)

Appearances of surface fatigueunder 3-body abrasive wear

Groove (abrasion) indentation (surface fatigue)v

de a o

v


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Surface FatigueIf we assume that surface fatigue is similar to fatigue even though the stress filed is much more complictated we can use the general rules of fatigue e.g. Mason&Coffin(1959) adapted to contact mechanics:

mf

fN

= Nf is the number of cycles to fracture, f the

equivalent (plastic) deformation to fracture underone loading cycle, the equivalent deformation per loading cycle, and m an exponent, which is 2 to 3 for metals under a plastic contact

*m

v

vffN

= Nf is the number of cycles to fracture, vf theequivalent stress to fracture under one loading

cycle, v the equivalent stress per loading cycle, and m* an exponent, which is 1.2 to 8 for metals underan elastic contact




Surface FatigueThus the probability for the generation a a wear particle k becomes thereciprocal of Nf:

m

fSFplSF Ck

=

.

*

.

m

vf

vSFelSF Ck

=

giving for an elastic contact situation(after run-in) W

Nm

vf

vSFsSF H

FCW*

.

=


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Surface FatigueCountermeasures

W

Nm

vf

vSFsSF H

FCW*

.

=

In order to gain a small wear rate under surface fatigue with CSF=const. and m*=const. In a given tribosystem we should

1) decrease v (and FN) and 2) increase vf as well as HW

1) decrease by lubrication, introduce compressive residual stresses, avoid internal (non metallic inclusions, hard phases) and external(roughness, pores) notches

2) increase endurance limit by strenght and purity, ductility (f), coldworking capability (HW)

Notice: v depends on FN and




Mechanically Dominated Wear Mechanisms

Surface Fatigue

Abrasion

from kennecott02 (Copyright 2000 by Daniel Ter-Nedden)

Kennecott, UT


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AbrasionAbrasion is brough about by an hard or sharp particle (e.g. mineral particle) or proturberance(e.g. surface asperity) imposedon and moving on a (softer) surface.

Abrasion has foursubmechanisms, which dependon the structure of thetribosystem and the properties of the materials in contact.




Abrasion (under sliding wear)

Appearance of abrasion under sliding wear of a WC cemented Co hardmetal shaft of a measuring device, which slides against a Ruby (corundum, Al2O3) bearing under water cooling


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Abrasion (under solid particle erosion)

Appearance of abrasionunder solid particle erosionof shovels of a Peltonturbine




Abrasion (under 3-body rolling abrasive wear)

Appearance of abrasion underrolling abrasive wearof a cement millroller

microcutting microcracking


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Abrasion (under sliding wear)

Appearance of abrasion under slidingwear of a cup of a retrieved McKee-Farrar metal-on-metal prosthesis




Abrasion

Microploughinga certain fraction of thegrooved material is just shifted to the edges of the rim. Thus, it is notlost!

MicrocuttingThe groovedmaterial generatesa chip, which islost.

The worn amount of material is smallerthan the volume of thegroove!

The worn amount of material is equal to thevolume of the groove!

MicrocrackingThe groovedmaterial generatescracks underneathand beneath of the groove. The worn amount of material is bigger thanthe volume of thegroove!


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Abrasion

MicrofatigueThe material beneaththe grooves duringmicroploughing ormicrocutting is fatiguedby multiple passes of the indenter

During a single contact the deformationbeneath a groove depends on thedeformation of the equivalent deformation at the first pass v and the work hardeningcapability HW/H according to

3HH

v

W

e=

In analogy to the surface fatigue model onecan write that the number of cycles to generate a wear particle is

m

v

HH

vff

W

eN

=

321




Abrasion1/WAB.s vs. H

precipitationhardening

hardening by straininduced phasetransformationduring wear


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Abrasion

The ratio between the grooved volume and that which is shifted to theedges of the rim can be described by the fab-value, which depends on thelocal equivalent surface deformation during scratching v and theequivalent deformation to fracture vf for ductile materials under e.g. rolling or drawing. In addition the work hardening capability has to beincorporated by Hw/H.

vf

v

WHH

ab efln2

3

1

=

fab - per defintion - lies between 0 forv


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Abrasion

Thus the wear by abrasion under microploughing and microcutting under a multiple contact resulting in microfatigue can be written as

sfatiguescuttingploughingsAB WWW ... += +

( )

+

+=

2tan1

tan101

5

21

22

.3

RHF

e

NffW

defW

N

m

HH

vf

vababsAB

W

where N Nf, R = tip radius of the indenter and 2 the apex angle of the indenter




AbrasionBecause of the action and interaction of these submechanisms no simple relation exists between the hardness of the materials and their wearresistance under abrasion!

In order to increase the wear resistance one should increasethe hardness H, the hardness ratio (HW/H) and the ductility expressed by vf.

Notice: By increasing H usually vf decreases. Thus there is an optimumcombination of both for every single tribosystem.


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Abrasion (by a single hard indenter) = 85, 2 = 115, v = 1.8 m/s, FN = 1 N

def=0.17 def=0.09NiCr20AlTi CoCr27Mo3C0.4HW/H small HW/H large + second hard phase




Abrasion (influence of the tribosystem e.g. hardness of abrasive particle)

The wear rate under abrasionstrongly depends on thehardness ratio between thematerials of body and indenter.

There is a steep increase in the wear rate, if the hardnessratio becomes bigger than 0.7.


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Abrasion (influence of the tribosystem e.g. shape of the abrasive particle)

A blunt or soft indenteror a shallow attackingangle produces lesswear, becausemicroploughing isprevailing;

with a sharp indentermicrocutting does and the wear rate increases.

Thus the geometry of the indenter is as important as the ductilityof the grooved materials!

Microplouging

Microcutting




Abrasion

if pc > pcrit

H, HW

vf , KIc

1/Wab.s vs. HW


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Abrasion

pcrit is a function of the size and shape of the abrasive particle and thefracture toughness Kc of the material

Notice: The higher the hardness the lower the fracture toughness!

e.g. for ceramics

tan2sintan

6.2

222

HDKdp

ab

IIcvoidscrit

=

Dvoids = distamve between voidsKIIc = fracture toughness under shear stresses (mode II cracks)H = hardness (Notice: HW=H for ceramics!)Dab = average size of abrasive particles




Abrasion

Microcracking takes place if the local contact pressure pc is higher thanthat for crack initiation pcrit. From these on can derive the probability of microcracking by

crit

cpp

e

= 1

pcrit depends on whether1) there are brittle second phases within the

groove2) There are brittle second phases beneath the

groove3) cracks are generated in and propagate through

the matrix material4) cracks are generated at and propagate along

grain boundaries and5) cracks propagate by combining existing cracks

or voids within the microstructure


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Abrasion




AbrasionThe general wear rate under abrasion can then be expressed in a generalform by

scrackingsfatiguescuttingploughingsAB WWWW .... ++= +( )

++= 222

21. 1c

defWnnabvoids

fabab

W

nsAB

K

HpDf

NNff

HpW

1,2 depend on the shape of abrasive particlesfvoids = volume fraction of significant internal notchesn = 1 for cracks and 3 for grain boundaries


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AbrasionCountermeasures

In order to gain a small wear rate under abrasion with 1,2, Dab = const. In a given tribosystem we should

1) decrease friction def, pN2) increase H, HW, HW/H, vf, KIc,

1) decrease and p, pN by small particles or lower FN2) increase hardness H by martensitic hardening, HW/H by precipitationhardening or choosing materials with a low stacking fault energy, or makebenefit of strain induced phase transformation, embedd second hardphases (volume fraction, size, distribution) if matrix is strong enough to support them, introduce compressive residual stresses3) use 2) without loosing too much of vf and KIc




Chemically-Mechanically Dominated Wear Mechanisms

Adhesion


pictures from www.en.wikipedia.org


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Adhesion

Adhesion is brought about by (friction welded) microjoints. Asperities of body and counterbody are plastically deformed with subsequentdestruction of any intermediate layer. The separation of this joint takesplace within the non-cold worked reagion underneath the contact area. Thus there is materials transfer from body to counterbody and vice-versa.

microjoint

separation




Adhesion (under recipricating sliding wear)Appearances of adhesion followed by abrasion under reciprocating slidingwear of an overheated (and, therefore, unlubricated) cylinder/pistoncontact.

http://knowhow.windstar-club.de/index.php/Kolbenfresser_(BJ.1999-2000)


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Adhesion (under sliding wear)Appearances of adhesion under self-mating sliding wear of a martensitichardfacing alloy (X45CrMnMo6-2, H13 warm-work tool steel).

materials transfer

Pit

grooveDue to the factthat aftermaterialstransfer thesurfacesbecomerougher and harder, adhesion isoftenaccompaniedby abrasion.




Adhesion (under sliding wear) materials transfer

pit

groove

Appearances of adhesion on thecylinder wall madeout of hardened and tempered steel42CrMo4 (AISI4140) undersliding wear of an hydraulic pump.

The counterbodywas a chromatizedpiston ring


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Adhesion

W

Ns H

FksWW ==

Adhesion can only be described roughly and cannot be quantizised; Archards general wear equation is taken

While k is used similar to the k forfriction

fracdefadAD kkkk ++=it contains an adhesive term (for the tendency of materials to adhere to each other), an deformation term (because plastic deformation increasesthe real area of contact in which adhesion takes place) and a fracture term(because for the generation of a wear particle crack initiation and propagation has to take place)

Notice: kAD has a dimension differing from that of friction, because herealso material is dissipated




Adhesion

2

cot21

1

aH

kad

ad kad can be described similar to ad

Thus the tendency to adhere depends on the surface energy ad and on thechemical similarity of surfaces

2,121 +=adif 1 and 2 are dissimilar the interaction term 1,2 might become very large. In addition the solubility within each other play an important role, as well as the lattice orientation (density of atoms)


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Adhesionkad 2,121 +=ad

Nat %nmmJ/mkJ/g atom

1400insoluble0.349500197Pb

500


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AdhesionCounter measures

fracdefadAD kkkk ++=

The three terms of adhesion cannot beseparated

W

Ns H

FksWW ==

1) avoid direct metal-metal contact between body and counterbody2) decrease , ad3) Increase H, HW, HW/H

for 1) lubricate the contactfor 2) decrease FN, use dissimilar materials (e.g. steel-brass (ad), -polymer

(water adsorbtion on surface (, ad)), -ceramic (high bonding energy!) (ad), benefit from inhomogenous microstructure ceramic+metal (perlite) instead of just metal (martensite) or Al-Si casting (Si+AlSi solid solution)

for 3) use ceramics H, HW for a small AC




Reibungszahl f0,4 0,45 0,5 0,55 0,6 0,65

0,01

0,1

1

10MMMM+fHPMM+fHP+gHP

Adhesion under unlubricated self-mating sliding wear

slid

ing

wea

rra

te W

x10

-6

coefficient of friction

adhesion by directmetal-metalcontact

mechanical interlocking byprotruding hard-phases(ceramic-ceramic contact)

pn=0.83 MPav=0.028 m/sRT, air

kad+kdef

Kdef+kfrac

metal matrix

metal matrix + eutectic hard phases

metal matrix + eutectic + primary hard phases

A1010 H13

NiCr20AlTi

D12 hard+temp.

D12 soft

Stellite21

FeCr5B5


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Chemically-Mechanically Dominated Wear Mechanisms

Adhesion






reaction layer

Tribochemical Reactions are brought about by the tribologicallyinduced chemical reaction of the materials of body an counter body withthe interfacial medium and the environment.

Notice: Tribochemical rections bring about reaction products which arenon metallic (e.g. oxide) and, therefore, hinder adhesion!


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Tribochemical Reactions (under fretting)Appearance of Tribochemical Reactions (films, layers, scales)

Brownish or reddish layer within contact area of a bar and a plate generated by small strokesbetween press-fitted parts (fretting) caused bymachine vibrationfrom http://www.gitz-online.de/EWIS-GITZ-Workshop-2003.pdf




Tribochemical Reactions (under sliding wear)Appearance of Tribochemical Reactions (films, layers, scales)

Denatured proteins on thesurface of a metal-on-metal hip ball, which has been run againsta pin in bovine serum

torque sensor

ball

pin

Notice: There are also appeaeances of abrasion (grooves) and surfacefatigue (indentations)!

FN=750 NTE=37Cv~0.0293m/s


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Tribochemical ReactionsTribochemical Reactions are very complex and less understoood. They canbe seen as a combination of

- Tribosorption- Tribodiffusion (accelerated by lattice defects of deformed surface

volume or any other mechanism) and- Tribochemical Reaction e.g. Tribooxidation

The local energy must be very high, so that chemical reactions take place, which normally wouldnt. E.g. oxidation of Cu in CO2 atmosphere accordingto

COCuCOCu ++ 22 24 has a free energy of G=+102 kJ/molCO2. Thus it would never take place. Butunder tribological circumstances in a ball mill with Cu chips it has been observed.




Wear and Wear Mechanisms COCuCOCu ++ 22 24Under thermodynamic equilibrium one can write

)(2OMOMox yGxGGG yx +=

In addition we know that Gox depends on the partial pressure of O2 acc. to

yO

oxp

G2

1ln

This would require a pressure of 5.7x1035 bar ( 83x1035 psi)!!

Thus it is more likely that GM becomes extremely large by plastic deformation!

or anything else is going on


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Tribochemical Reactions under mild sliding wear ( < 0.6)For tribooxidation Quinn(1983) has described the wear rate as function of physical, chemical and mechanical properties of the reaction products. He states that wear is brought about by spalling off an oxide layer, which has reached its critical thickness dc

222.oxox

RTQ

pC

csTCR

g

eAvA

ddW

C

p

=

d = surface separationdc = critical oxide layer thicknessAp = Arrhenuis constant fortribooxidationQp = activation enerty for oxidationgox = weight content of O in oxideox = density of oxide




Tribochemical Reactions under mild sliding wear ( < 0.6)

22.oxox

RTQ

pC

csTCR

g

eAvA

ddW

C

p

=

This means that WTCR.s becomes smallfor a slow growth rate of the oxide, described by the Arrhenius part (smallAp and TC, high Qp)

Notice: For a const. Qp under staticand tribooxidation Ap under wearbecomes 11 orders of magnitudebigger compared to static oxidation!

But because of Ta and Tf the heat conductivity of the oxides as well as the coefficient of thermal expansion of oxides and substrate play an important role as well.


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Tribochemical ReactionsCounter measures

Because tribochemical reactions cause less wear and hinder adhesion onewould not really look for any counter measures unless the electricconductivity is of importance or you are in the field of MEMS or NEMS.

In that case useoxidation resistantalloys (noble metals) or those with a slowgrowth rate of oxides(ceramics). In addition vacuum oran inert gas atmosphere mighthelp as well.

e.g. Polysilicon MEMS fatigue life test setup. Crack initiation within the 10 nm thin oxide reaction layer inside the notch might limit the fatigue life (from R.O.Ritchie, UCB)





Wear path in m

Ni (200 HV) < NiO (400 HV)

Al (200 HV)

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Wear and Wear Mechanisms.pdf