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BLSEVIERWEAR

Wear 203-204 ( 1997) 302-309

Resistance to particle abrasion of selected plasticsKenaeth G Budinski

Eastman odakCornpony , 177Latto Road,Suite 46, Roch rsrer. Y 14612-3092. USA

Ahstnwt

Acceleratedabrasive wear of plastic parts in a piece of productionmachineryprompteda laboratory tud y to find a materialwith betterabrasion esistance. he abrasion ccurred n a machine hat ompacted sand-like pardclesof an inorganiccompound.T h e a b r a s i o n r e s i s ta n c eo f a - w i d e v a r i et y o f p l a s t i c s a n d d i f f e r e n t d u r o me te r p o l y u r e th a n e s (2 1 ma te r i al s ) w a s t e s ted w i t h a mo d i f i c a t i o n o f t h e AST M d r y - s a n drubb er whee l th ree-budy abras io n tes t . On ly one mater ia l , a po lyu re thane, bad be t te r abras ion r s tance tban tbe mater ia l tb a t was cur ren t lyi n u s e . H a rd n e s s , f r i c t i o n a n d s c r a t c h t e s t s w e r e c o n d u c te d o n t h e t e s t ma ter i a l s t o t r y t o u n d e r s ta n d t h e m l e o f ma te r i al p r o p e r t i e s i n t h i s t y p eof abras ion . None of these correlatedwith the wear data.Rcvious investigators f plasticabrasion elatedabrasion esistance o the fractureenergy and friction.The wear datadeveloped in this study did not cormlate with the specific modal pmposed by Ratner. However, it waspossible to obtaina reasonablecorrelationwith a deformation actor h a t i n c l u d e d th e f r i c t i o n o f t h e a b r as i v e o n t b e p l a s t i c a n d a t e rm th a tre la ted to the energy requ i red to de fo rm the mater ia l p las t ic a l l y . A t es t similar to a Brine11 ardness test was used to a r r ive a t tbe fo rmat ne n e r g y o f t h e 2 1 t e s t materials.The more easily the materialdeforms in contactwitha particular brasive, he better he abrasion esistance.0 1997 Elsevier Science S.A. A ll rightsreserved.Keywordr : Abrasionesistaaee; tastia

1. IntYoduction

This study was prompted by an equipment problem thatwas occurring in the production of crystnilites of an inorganiccompound used in the preparation of photographic emulsions.The material was similar to ordinary table salt in size, appear-ance and compressive strength. The problem to be addressedby this study was the accelerated wear of plastic guides thatdirected the powder into a briqueting press. The powder wasdirected into compacting rollers with two parallel plasticplates of ultrahigh molecular weight polyethylene(UHM WPE ) that were about 6 mm thick. The plates wereonly lasting about tw o weeks before they had to be replacedbecause of excessive abrasive wear from the powder rubbingon the plate as it entered a roller nip. The plates wem notparticularly expensive, but replacing these plates was a veryexpensive operation. They were buried well into themachine and their replacement required the loss of up to twodays of production. The plates were made from plasticsb e c a u s e me ta l s ma y i n t r o d u c e c o n ta m i n a t i o n ( w e ar d e b r i s )and because ceramics were too brittle to withstand thelexingthat occurs in the plates. Our assignment was to conductlaboratory tests to determine whether another material wo uldprovide improved service life over the UHM WFE.OW3-1648/97/ t7SlCI t997EtsevkrSeteaeeS.A.PIlSOO43-1648(96)07346-4

All ii Is -ed

Many s tu d i e s h a v e b e e n c o n d u c te d o n t h e w ea r o f p l as t i c sma te d t o me tal s , b u t t h e s tu d i e s o n t h e abrasion resistance ofplastics are fewer and less conclusive. Most reports seem torecommend additional studies [ l-101. There are some stan-dard tests for the abrasion resistance of plastics; one of themost widely used tests is the Taber Abraser test, ASTM D4060 [ 111. This test involves abrasion of a flat sample ofplastic with rotating rubber/abrasive wheels or with sand-paperwheels.Anothertest,ASTMD1242ProcedureA[12],uses loose abrasive distributed on a rotating platen. lhe looseabrasive is pressed into the rotating sample. ASTM D 1242Procedure B [ 131 uses what is essentially a belt sander toabrade specimens on a conveyor that cycles in and out ofcontact with the sandpaper. ASTM G 56 [ 141 has been usedto measure the abrasion resistance of plastic to paper. A bailrider is rubbed on a large paper-covered drum. ASTM G 132[ 151 is an abrasion test in which the ends of vertical pins rubon a large sandpaper-covered drum. Although this test wasdeveloped for metals, the concept has been used by others totest plastics [ 61. All of these tests were considered as cau-didates for a laboratory test to screen materials to address theabove production problem.

The test selected to rank materials was yet another ASTMtest, ASTM G 65, the dry-sand rubber wheel abrasion test[ 171. This test, which is illustrated in Fig. 1, was developed

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to rank the abrasion resistanceof both ha rd and soft metals.The material to be tested is line-contact loaded against arubberwheel and silica sand is metered nto the nip. Wear sassessed by measuring rye volume of material (by masschange) removed from the specimen in a fixed period ofrubbing. This particular est is one of the most used abrasion

2. PrncedureThe dry-sand ubber wheel esteruses a 228 mm diameter

chlorobutyl rubber wheel (6OShoreA) as an abrader. lhc,wheel is 12.7 mm wide and runs at a single speed, 20.9rad s- . Ibe est samplesare from 4 to 12.7mm in thickness,25 mm wide, and 76 m m long. Tbe w ear surfaces are the25 X76 mm* aces.The loading forceof the specimen gain-tthe wheel can be up to 140N. The abrasive s 215 o 300 kmsilica. All test use rspurchase test sand from the same source.The send flow is in the range 30&4O Ct mitt-. There arethreeprocedure s n the ASTA4 est standard hat employ dif-

Tabk IBase polymer wucr w )/wiafollmnYitPolyphmylene sume (PPS) 4o cwbmtiber(cP)PdYrtyrar m NowepOxy (EP) 4o vmwa*PolyphmylenesuRide (PPS) PlpelcboPPcd LLU KG)Phedk (Pm wovmmnGd6berwDM)PolytanauomeIhylaK (Pm?)Polyoxymhykne (PDM) Er+=(cG)Acrylwiuikbtadiadstyme (ABS) NoneEPOXY@PI wova dotb (cot)Phenolic (PF) wovendotb (cot)Poly&rrtherlraonc (PEEK) NonePolylarpRuom(hylcae (Prm) NODI?Polyimide (PI) NOSEPolyethyknc (HDPE) NolvPolyamtde. PA) MePolyurahane (PtJtt) 55A NonePolyamhane (PUR) 9OA NOD?Polyumhaae (PUR) 75D NOtICPolo (Putt) 85D NaUP01yethykm (UHMwpe) oilPolycthykae wHMwm) cotltml NoteReferraametlls:AlSItype316ruinkn~zl(~92HRB).Stdlite6B(hudnss43HRc).typeA2~l~(~6oHRc).

ferent normal forces and test durations. lte test procedureused in this study employeda 45 N force and a test durationof only 200 whe el revolutions (60 s). This procedurewasused successfullyby the ASTM G02.3 Abrasive Wea r Sub-committee n 1990 o conduct n&l&xatq tests on poly-meric coatings. llte test mamriais ncluded the plastics andelastomers isted in Table 1. These materials were selectedbecau se of their successful performanc e in other plantoperations.All test samp lesweremadefrom bulk m aterials.The sam-ples with wovenreinforcementswere estedon the& flat face.Whe re possible, the abrasion tests we re conducted on as-molded surfaces.We ar volumes were calculatedfrom masschangesduring the test.

3. T&resultsThe averagevolume ossesarecomp ared n Fig. 2. Typicalwearseerson the plastic specimens re shown n Fig. 3. Onlyonematerial,a90ShonAdurometerpolyurrthaae.hadbetterwear resistance han the UHM WPEplastic currently n use.An oil-lubricatedUHMWPEwas comparable n wearmsis-tance o the controlmaterialas wereseveralotherhardnessesof polyurethane lastomer.Theglassandcarbonfi~r-ninforcedplssticshadtheworstabrasion resistanceof the reinforcedmaterials. Cotton a ndlinen-reinforcedmaterialshad better abrasion esistancehanthe glass-reinforcedmaterials. llte worst wear on a no n-reinforcedplastic was on polystymne. ltere appeam d o bea preferredhardness or polyurethane-the top of the Shore

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c:

/IIi

3

0 100 200 200 400 so2 coo 700 a00w8w h-2

Fig. 2. Wear voIumes or plastic caadidatles n a modifiedASTM G 65abrssioa test.

Fig. 3. Typial appmma ofwaarscmoaplnticrpedmenr.A scale. Harder (Shore D) and softer materials did not wearas well. All of dte plastics tested, except UHM WPE andpolyurethane, had lower abrasion resistance than the scftstainless steel reference material. A2 tool steel at 60 HRC

was more abrasion esistanthan llof the estmaterials, utit was not ignifican tly ore brasionesistant han a9OShoreA polyurethane.hese. estssuggest hatunder ertain on-ditions, omeplastics ndelastomers a nhaveparticle bra-sion resistance comparable with that of hard steels.

4. Dlscuselon

4.1. Role o f h a rdnessWhat controls the abrasion resistance of plastics?Theclas-

sic relationship for abrasive wear of metals is that wear isinversely proportional to the hardness of the metal [ 181. heharder the metal, the lower the abrasion rate. Unfortunatelythere is no single plastic hardness scale that is suitable for thewide range of plasticsieiastomers inch&d in this study.Nonetheless, Shore D and recoil hardness tests were con-ducted on the test plastics to explore the plausibility of ahardness-abrasion relationship. As shown in Fig. 4, thehardest plastic, glass-reinforced epoxy, was harder thanU?M WFE by a factor of about 1.4. but their abrasion ratesvaried by a factor of about 60.

It was thought that the resilience cf the plastics may playsome role in resisting indentation and scratching by hardparticles. here s a comm erciallyvailable ardness esterthatuses therebound elocityof a spherical-endedabot omeasure the hardness of metals. The harder the metal, thegreater the rebound velocity. This device was used on the testmaterials. As shown in Fig. 5, dtese rabound hardnesses didnot show an apparent correlation with the abrasion volumelosses.

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PBPPSGEPvlu~

PPS*PTFEPF NDU

ABBPOM.PTFEEP*OtitlMl

PEEKP7FE-x PI

HDPEPAMCB2

P7FEPUR-MD

0 10 20 20 40 BD BO 70 80 96 100Hv r~ Bbom D

Fig. 4 Shorehsrdsess f testmaterials

PsPPSGF

EP*gluSPPti*PTFE

PF*NDMABB

POMrP7FEEP co1tott

PEEKPTFE-CO

PIHDPE

PAYOSZPTFE

PUR-SSD31B BB

PUR-76DPUS-SSA

SIetIIU SBUHktWPE*o

U+iMWPEPUMOA

A2 tool l 10 100200220400SO0BCO700

(ubnwy unn. 1 to loo01Fie. 5. Reboundhardness f testmaterials basedon the recoilvelocitvofa s h.4ended abot).4.2. The role of scratch resistance

For malAy years the Taber Abraser has been used to rankthe abrasion &stance of plastics [ I ,6.16,19]. As mentionedpreviously, this device can be used for two-body or three-body abrasion. This type of test is not unlike a scratch testwhere the abrader is & gbrasive-filled rubber or a sandpaper-covered wheel. Fixed sharp particles are imposed on the test. .surface. In an attempt to simulate this type of material__removal, scratch tests were conducted on the test plasticsusing a 60 included angle diamond cone stylus with a tipradius of 200 Frn. Yamaguchi [ 161 and Briscoe et al. [20]suggested that scratch hardness is a factor in abrasive wear

of plastics. The scratch hardness is measured by the wid*b ofthe furrow produced by the scratching stylus:H Plbwhere His the scratch hardness, P is the stylus load, and b isthe furrow width.

The tes: o astics w ere scratched for a distance of about 50stylus. Thefo~rerjuiredtoproduccthcscratch was&~~dedcontinuously and this force was converted into static andkinetic f&ion coefficients by dividing by the normal load.Typical scratches arc showo in Fig. 6.

-ks shown in Fig. 7, the mat with he highest scratchhardness. such as the eooxies and ohenolics. had the ooorestabrasion resistance. T&o of the eiastomers; PUR -55 A andPUR -90 A, did not scratch at all. The va rious plasticsappeared ;o scratch by different m echanisms (Fig. 8). The.reinforced plastics displayed ragged edges on the scratchscratch furrows; some produced furrows that varied in widthsuggesling what appeared to be a stick-slip type motion dur-ing surface deformation. Four metals and a cemented carbidewere scratched with the plastics/elastomers to see whetherthey responded properiy to hardness differences. The

Fig.dSentchaintcst~m~~~~S~~~NC ma@ifiatiw: (a) mthm- forced phmolic. (b) polyamide+molybdenum disttlfidc.

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?Ps+eFEPyllvrPPS*TPEPF-NOM

HWEPAwoSzPTPEPun-am3mss._ _PU WWPun-s3A

awlI* .aUHSSWPM

UHMWPEPua-wAA2tooIti.ut , I0 10 20 30 40

Fig. 7. Scratchhaninw of testmaterhIs.

scratch hardnesses of all of the plastics were lower than thehardnesses of the metals (Table 2).

Tbe metals disuiayed scratch hardnesses that geaerallycor-related with their hardness, but the relationship was not suc-

cinct. These results suggest a friction effect. Stellite 6B didnot scratcheven though it is much softer than the tool steeland cemented carbide. Overall, the scratch tests did not leadto a relationship that clearly related the scratch hardness tothe observed volume losses in the abrasion tests. A secondand thiid series of scratch tests were conducted with carbidestyli with larger radii (2 and IO mm diameterballs). Only afew of the test materials scratchedwith the 2 nun stylus andnone scratched with the IO mm stylus. The scratch tests wereconcluded and the scratch force data were evaluated forpossible correlation with abrasion rates.4.3. Fiction consdera ions

The force measurements obtained in the scratch tests sug-gest that when a scratch stylus produces plastic deforma tion/fracture of the surface, the force is probably a reflection ofthe energy required to produce de formation and removal ofmaterial. The f r i c t i o n c o e f f i c i en t s p me a s u r e d i n s c r a t c htests on the test plastics are given in Table 3.

These results suggest that me friction coefficient dependsto a significant degree on the degree. f plastic deformationproduced in the scratching operation. The wear test resuhadid not correlate with any of these scratch test results. In the

Fg. 8. Appearance f 600diamond cratchesn various estmaterials.

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Table 2Scratchhe err of four metats edcemeeted arbideMated Scratchhardness

tkgmm-)Vickers uh~ss

6061 T6 aluminum3 6 stainless teelStellie 6BA2 tool steelCemented arbide C2)

25 47loo 143No scratch 435400 62510000 1854

Table 3Ceeffic~ent f friction fi measuredn scratch estson plasticswith variousscratch tyliStyluslloedingmass Avdnge p I rmge60diameedlKJOOg2 mm diameter arbide/1000 g10 mmdiametercubidc/1OOOg

0.58 0.14 0.2-0.9G.21 0.2 0.2-I0.14 0.13 0.05-0.7

Standard eviatiw.

abrasion test, however, the scratching material is 50-70 meshsilica sand. In an effort to determine whether scratching withsilica makes a difference in scratch results. a scratch styluswith silica grams on the rubbing surface was fabricated. Fric-tion coefficients were calculated from the force measure-ments made with a silica sand stylus. The steal stylus was 6mm in diameterand a monolayerof sand was adhered o thehemispherical nd of the stylus with a cellulosic lacquer. Asshown in Fig. 9, the friction coefficient of the plastic/sandcouple did not correlate with the wear test results. The averagefriction coafficient for the sand/plastic couples was 0.32u=O.22. r=O.l to 1.1). Most test samples were perma-

nently scratched by the sand stylus w ith a normal force pro-duced by a 500 g mass o n the stylus. The scratches and thefriction coefficients were smaller than for the diamond stylus.These data suggest that the friction coefficient of this tribos-ystem includes a measure ofsurfaceeformation as proposedmany years ago by Bikennan [ 211.Table 4Some of the models pmposedor the abrasiveWWOf plastics

Fig. 9. Fricuoecc icienu (kinetic) of siticeseadstidiag011estms iats.

4.4. brasion modelsUp to this point, the traditional tribological proper of

tiictionand hardnesshavefailedtocomlatewiththeabrasionresults. Some additional plastic abrasion models from thelitemtttre were reviewed (Table 4) for dition. One modelthat seemed to be quite reasonable was that proposed byRanter et al. [22], where tlte rate of m aterial removal wassaid to he inversely propcmtonal to the product of stress andstrain at rupture. It was n ot clear ho w stress and strain atrupture could he ohtaincd from the G 65 abrasion blocks thatwere available as test materials. It was decided that the load/deflection curve for plastic deformation of the surface by ahemispherical indenter may be a predictor of at least theability of a material to deform plastically as in scratclSng.With this reasoning, the abrasion test coupons were ittdcntedto a fixed depth of I .25 mm with a 6 mm diameter htdenterin a universal tension/compression tester. The area under theload/deflection curve was integrated and it was considered

a, yield strength; load;E elasticmcdulus; c home roughness: H hardness; W wearw-@VPdp ebrasivewear actor;N scmtching fficiency actor;P normal load; d slidingdistance

Lw-p:;p friction coefficient; load;H hardness; estress nd strainat mpnrrew-tanstee 6 dampingparameter btained romdynamicmechanical nalysis DMA)

puta:Iv-- Yp centact orce;u slidingvelocity; f time; a mughes of surface; y surface energy

Yamsgacni I61

Rmlcrctal. [U]

Blw I241

visweluJuthendBellow [zs]

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2 4 6 e 10 i2Raformmtionactor(r=--aruBwu-1u(D~v.s,

Corralatlon CtfiClMt -0.7273(deformation factor)

as a measure of the energy required to deform plastically thetest material.

Wear data were p lotted vs. the reciprocal of this energyterm as proposed by Ratner. lbere was poor correlation. Aftermanipulating the energy data in a variety of ways it wasdetermined that the best correlation existed with a deforma-tion factor that included the friction coefficirbii f the sandon the test material:w-/&Q)where w is the abrasion rate, /.c is the friction coefficient ofsand on the plastic surface, and Se is the area under the load/deflection curve from tbe bag indent test.

The lower the product of friction and deformation energy,the ower theabrasion. s shown n Fig. IO, the correlationis much less than perfect (correlation of 0.73), but this cor-relation is certainly better than the correlation obtained withthe hardness and scratch parameters. This relationship alsoseems reasonable. The deformation factor for elastomers islow and they have good abrasion resistance. The same canhe said about he UHMWPRs. The contributionf frictioncoefficients thought o be that ow friction anreducewearbecause h e abrasive in thm e-body brasion) s less likelyto dig in and form a plow mark or scratch. However, high-friction material such as PUR has good abrasion resistancebecause. the abrasive grains tend to roll through the wearinterface rather than hecotue fixed on one member and plowa furrow. The correlation seemed plausible and further workwith the other models was decided against.

5. Conclusions1. Polyurethane with a durometer of 90Shore A has moreabrasion esistan ce o AFS 50-70 silica in a three-body

abrasion test than WE.

2. The hard, reinforced and filled engineering plastics hadrelatively poor abrasion resistance to silica sand in thethree-body test used in this study.

3. llte plastics tbat defcrm easily when acted on by looseabrasive particles (e.g. silica) are less likely to producematerial removal by scratching/fracture.

6. SumruuryTwenty-one plastics/elastomers were subjected to a three-

body abrasion test to find an improved material to solve aproduction problem. The tests did not identify material withthe desiredIO-fold ncrease n abrasion ife. Ihe mprov e-mentwas only of the order of two times, much less tbananticipated. However, this study reconfirmed that UHM WPEand high Shore A polyurethanes have better abrasion resis-tance than most other plastics and elastomers.

The explanation for their excellent abrasion resistanceappears to be their bility o deform asilyand heir favorablefriction haracteristicsgainstmost other materials.Themodel uggested y thisstudyneedsmoredevelopment, utit s felt hat t mayhave orrelated etterf this study did notinclude suchdiverse plastic/elastomer systems (glass-nin-forced composites, injection moldable commodity plastics,carbon fiber-reinforcc.$t engineering plastics, and elrsto-mers) .They were not from similar groups or families. Futurestudies need a less diverse roup of test materials. Also, itappears that the abrasionmodel should nclude a fracturetoughness term since the more brittle materials lost materialby brittle fracture in the scratch tests. The elastomers mayneed a term relating to tbeii rcsiliencc (branching) and theneat materials may need a more well defined friction term. In1981, Bartenev ndLavrcntcv 191 concluded he chapteron abrasive wear in their book on the friction and wear ofpolymers with the statement: To the present there is notheory of abrasive wear of polymers. This situation appearsto prevail, and it may not be possible IO improve on tbeUHhIWPE s and polyurctbanes for abrasion esistance n tila bettermodel s deduced.

References[ I J.M. Tborp. Abrasive wearof omecommeminl polymers. Tritd. hr..

I5 ( 1982)59-68.1211. Larsen-Basx and T. Abmad. Slurry abrasion of polymers undersimuktcd submarine conditions. Wear, 122 2) (1988) 131-149.

[31 D.H. Buckky and R.L. Johnson. Friction wear and decomposit ionmechanisms for variouspolymercompcsitesn vacuum o IO+ mmKg.NASA ech. orcD-2373, Dccm ber 1963 NA SA, Washington,DC).f41 G.F. Cok and R. Travksco. Wear by paper on nylon matrixcomposites . In L.H. Lee (cd.), Adwnces in Polymer Friction andWear. Vol. SB. PIcnum New York, 1974, pp. 689-702.IS1 I.M. Hu~cbings. Trih&~.CRCPress.London. 1992. pp. 156-162.

t6108.PowerandJ.H.Dumbleton,Animprovedmethodfordemrminingthewearof polymeric coadngs, Wear. 5 ( 1973) 373-380.

8/12/2019 Resistance to Particle Abrasion of Selected Plastics

8/8

KG. &din& i/ Wear 03-204 1997) 302-309 309

171 B.J. Briscoeand P.D. Evans.Tk influence of asperity defomwionconditions on the abmive wear of imdii PIPE. lo K.C. Ludma(cd.). Wear o Maferialr 1989, Vol. 2. Amc lican sociay ofMcchaniul Enghan. New York, 1989. pp. 449457.

[El R.KamkoaadE.~Microwearpoceuerofpolymcrrurf~.wear, 162-164 ( 1993) 370-377.

I91 D.G.BellowandN.S. Virw~alll.An~ysiroflhcwearofpolymm,Wear,162-164 (1993) 1048-1053.

[IO S Bahadur md D. Don tbmdation of the model for opumalpmpmtiw of Clkr in polymer for weat n&tatce. In K.C. L&ma(cd.), Wear o Mamar 1991 Anhan Sociay of MechanicalEngineers. New York. 1991. pp. 177-187.

[lIlASTMD4060.T~me~forabrasion~sisulneeoforganic~ngsby Taivr Abrascr, Amacan Society for Testing and Materials. W.Conshohocken. PA.

[ 121 ASH D 1242. Test method for resistance of plastic mawi& toabrasion. prowdun A American Soce y for Testingand Materials.W. Conshohockcn, PA.

1131 ASTM D 1242, Test method for raistance of plastic materials toabrasion, pucedue B. AmericanSociety or Testing and Mat&d.s.W. Conhhocken, PA.

( 14) ASTM G 56. Test maid for the abrasiveness of ink-impregnatedfabric prinlerribbons. Amriran Sac&y forTestinS and Mauials. W.Conshohocken. PA.

1151 AsrMGI32.Sundsrdttstmahodforp~abrasiontffting.AmricanSaciity for Testing and Materials. W. Conshohocken, PA.

( I61 Y. Yanquchi. Tribo&y OfPJastic am iaJs, E&via, hmadq1990.9.125.

[l7lASTMG65,PmcticeforconduchSdry-smdrubbu~~W.Is. Americ=m oday for Testing and M*cr*lr. W. coarhohodrto .PA.

t181.abinwin. Friction and Wear ~Mat&7Jr. Wii. Nnv Y&1966. p. 168.f I91 G.M. Bartcnev and V.V. Lawewcv. in KC. Luknu sld LR. h

(edr.). Fricdun and Wearo/PoJymers.Ekvier. Amsmbm, 1981.p. 239.

[ZOI B.J. Briscoe.S.K.BerinamdS.S.Paoesw.ll~scrachhudacrrlodhictionofaMArigidm*Q*1.~~C.LUdCm;LUSdR.G.B8~(~.),wearof MOI.zriaLv991. Amaiaa .9ociuy oft&&&alE,NewYork. 1991.pp.451~56.

(211 J.J. Bikerman.The nahnt of polyma friction. In L.H. Lee (cd.).Polymer cience nd Techwlo8y. Vol. 5 Pkaum, New Yak. 1974.p. 168.

t221 S.B. hmer. 1.1. Pattwmva, O.V. RaJyakaidt and E.G. Lw. Sov.PllsI... M4) 37.

(231~. Hombogen.Tltcmkoffmctuetoughcssinthe~ofnwb,wear. 33 ( 1975) 251-259.

[Xl PJ. Blau. Friction Schce awJ Techmb~. k4me1 Jkkk a. NcvYolk, 1996, . 194.

WI N. Viswanath nd D.G. BcUow.Development f 60 apai~~ fa thewearof polymers.Wear. 181-183 ( 1995) 42-49.