Effect of Particle Size on the Wear Resistance of

CSocietyofTribologistsandLubricationEngineersCopyright󰀆

ISSN:1040-2004print/1547-357XonlineDOI:10.1080/10402000701730494

EffectofParticleSizeontheWearResistanceofAlumina-FilledPTFEMicro-andNanocomposites

STEVENE.McELWAIN,THIERRYA.BLANCHETandLINDAS.SCHADLER

RensselaerPolytechnicInstitute

Troy,NY12180,USA

and

W.GREGORYSAWYERUniversityofFloridaGainesville,FL32611,USA

ItwaslongsupposedthattheabilityofhardparticlefillerstoreducethewearrateofunfilledPTFE(typically∼10−3mm3/Nm)byanorderofmagnitudeormorewaslimitedtofillersofmicroscaleorgreater,asnano-fillerswouldlikelybeencapsulatedwithinthelargemicroscalePTFEweardebrisratherthandisruptingthewearmechanism.Recentstudieshavedemonstratedthatnano-fillerscanbemoreeffectivethanmi-croscalefillersinreducingwearratewhilemaintainingalowcoefficientoffriction.Thisstudyattemptstofurtherelucidatethemechanismsleadingtoimprovedwearresistanceviaathoroughstudyoftheeffectsofparticlesize.Whenfilledtoa5%massfrac-tion,40-and80-nmaluminaparticlesreducedthePTFEwearratetoa∼10−7mm3/Nmlevel,twoordersofmagnitudebetterthanthe∼10−5mm3/Nmlevelwithaluminamicro-fillersatsizesrangingfrom0.5to20μm.Compositeswithaluminafillerintheformofnanoparticleswerelessabrasivetothematingsteel(stainless304)countersurfacesthanthosewithmicroparticles,despitethefillerbeingofthesamematerial.InPTFEcontain-ingamixtureofbothnano-andmicro-fillers,thehigherwearratemicrocompositebehaviorpredominated,likelytheresultofthecontinuedpresenceofmicro-fillersandtheirabrasionofthecountersurfaceaswellasanyoverlyingbeneficialtrans-ferfilms.Despitedemonstratingsuchalargeeffectonthewearrate,thevariationofaluminafillersizedidnotdemonstrateanysignificanteffectonthefrictioncoefficient,withvaluesforallcompositestestedadditionallyfallingneartheμ=0.18mea-suredforunfilledPTFEatthisstudy’s0.01m/sslidingspeed.

KEYWORDS

PTFE;Nano-Composites;Wear;Friction;SelfLubrication;Nanotribology;Self-LubricatingComposites

INTRODUCTION

Polytetrafluoroethylene(PTFE)polymeriswellknownforthelowfrictionitcanprovideindrysliding.Thehypothesisforthelowfrictioncoefficientisthepresenceofathin,highlyorientedtransferfilmaswellasorientationinthewearingbody.Unfortu-nately,ifnotreinforced,PTFEcanadditionallyformverylargelump-orplate-likeweardebris(MakinsonandTabor(1);BahadurandTabor(2)),viaadelaminationprocesswiththicknessesontheorderof10μmanddimensionsintheplaneoftheplateofsev-eralhundredsofmicrometers(BlanchetandKennedy(3)).TheresultingwearratesforunfilledPTFEaresevere,approaching10−3mm3/Nm.AbroadvarietyofhardparticulatefillershavebeenshowncapableofreducingthewearrateofPTFEgreatly,insomecasesbythreeordersofmagnitudedownto10−6mm3/Nmorlower(Lancaster(4)),andthusPTFEcompositeshavebecomecommonlyemployedinmanydryslidingbearingsurfaces.

InaninvestigationbyTanakaandKawakami(5)ofvariousfillerparticlesincludingchoppedglassfiber,bronze,ZrO2,andTiO2,amongothers,theTiO2fillerwasfoundtobeleasteffectiveatreducingPTFEwear.InthatstudytheparticlesizewasnotheldconstantfromonefillertypetoanotherandtheTiO2wasalsothesmallestfilleratlessthan0.3μm,whiletheotherfillershadlargerparticlesizesofseveralmicrometersormore.Itwasthushypothe-sizedthatsuchsmallerfillerparticlesweretransportedwithinthewearingPTFEinitsprocessoftransferringtothecountersurface,incapableofpreventinglarge-scalereorganizationofthePTFEstructureatitsfrictionalsurfaceandlimitingthistransferwearprocess,andwouldthusprovideonlyaweakerwear-reducingac-tion.Assuch,ithasbeenconcludedthattheeffectivenessoffillersinprovidingPTFEwearresistancedependsonhavingareason-ableparticlesizeintherangeofseveralmicrometersupto30μm.Thisconclusion,thatfillerparticlesofinsufficientsizewouldlackeffectivenessinprovidingPTFEwithwearresistance,appearstohavebeencounteredbythestudyof50-nmZnOfillerparticlesbyLi,etal.(6),whoreportedthatthewearexperiencedbyun-filledPTFEcouldinsomecasesbereducednearly100-foldbysuchnanoparticles.However,inthisstudy,largerZnOparticleswerenotsimultaneouslytested,thusitcannotbeassessedwhetherthenano-fillerswereaseffective,orpossiblyevenmoreeffective,than

PresentedattheSTLEAnnualMeetinginCalgary,Alberta,

Canada,May7-112006

ManuscriptreceivedJune9,2007

FinalManuscriptapprovedOctober9,2007

ReviewledbyYeau-RenJeng

247

248S.E.McELWAINETAL.

conventionalmicro-fillers.Actually,inTanakaandKawakami’sstudy(5),theTiO2fillersimilarlyprovidedPTFEwith100-foldwearratereductionsinseveralinstances;still,thesewearratereductionswerenotasgreatasthoseprovidedbythemoreeffec-tivefillershavingparticlesizesinexcessofamicrometer.

Sawyer,etal.(7)showedthattheadditionof38-nmAl2O3nanoparticlesat20%byweightreducedthewearrateofPTFEfurtherdownto1.2∗10−6mm3/Nm,anduponfurthermodificationsubsequentlyattained1.3∗10−7mm3/Nmwithan80-nmAl2O3fillerparticleatanoptimum5wt%concentration(BurrisandSawyer(8)).InBurrisandSawyer(8),a0.5-μmparticlewasalsostudied,towhichthewear-reducingperformanceofthe80-nmAl2O3fillerparticlewasshowntobesuperior.Whatismissingisastudythatteststheeffectofparticlesizefromthenanoscaletothemoreconventional∼10-μmscale,loadedintothePTFEatconstantmassfractionusingparticlesfromthesamesourceinordertoisolatetheimpactofnano-fillers.Thisstudyfillsthatgapbystudyingparticlesfrom40nmto20μm.Additionally,givendifferingextentsofPTFEwearreductionprovidedbythesenano-andmicroscalefillers,compositeswithmixturesofparticlesfromeitherextremeofthisfillersizerangewillbetestedtoinvestigatetherelativepredominanceofthesewearreductionmechanisms.

EXPERIMENTALDETAILSMaterials

AllcompositestestedweremanufacturedthroughtheblendingandsinteringofcommerciallyavailablealuminaandPTFEpow-ders,withaluminafillerdispersedwithinthePTFEmatrix.Alu-minafillerswereinvestigatedatsixdifferentparticlesizesvaryingfrom40nmto20μm,allprovidedbythesamemanufacturerandofthesamealphaphase.Insomecasesthemanufacturer-specifiedparticlesizewasasinglequantitywhileforotherparticlesarangeofsizewasprovided,asindicatedparenthetically.Thisinvestiga-tionstudiedtwoaluminanano-fillersofsize40nm(27-43nm)and80nm,aswellasfouraluminamicro-fillersofsize0.5μm(0.35-0.49μm),1μm,2μm(0.9-2.2μm),and20μm.ThePTFEpowderhadatypicalparticlesizeof30μm.Unlessstatedotherwise,com-positeswereblendedata5%aluminafillerweightfractionintoPTFEofcommercialgradeG580.Inlatertests,somecompositesproducedwithPTFEofanalternate7Ccommercialgradebutofsimilarparticlesizewerealsoinvestigated.

AluminafillerandPTFEmatrixpowderswereblendedin10-12gbatchesusingaHauschildmixer.AsseeninFig.1,thenano-filleraluminaparticlesintheas-receivedpowderwereclusteredintoagglomeratesonthemicroscalethatdependonthissubse-quentblendingtobebrokenup.Eachblendedmixturewasthenusedtopreformtwo5-6gpucksofapproximate5mmheight,eachcold-pressedfor15minwithina22-mm-diametercylindricaldieat40MPa.Afterpressing,thecompositepuckswereremovedfromthedieandsinteredbyheatingatarateof100◦C/hto360◦Cwheretheywereheldfor3h.Aftertheholdtimeelapsed,thespecimenswerethencooledatarateof100◦C/hbackto20◦C.Allheatingwasdoneinanitrogen-purgedenvironment.Theprocessingstepsforthenanocompositeswerenodifferentandnomoredifficultthanthosefortheconventionalmicrocomposites.UnfilledPTFEcontrolspecimenswerealsoproducedbythesameprocesses.

Fig.1—Secondaryelectronimagesofas-receivedaluminapowder(a)

nanoparticles(40nm),and(b)microparticles(20μm).

GivenPTFE’sinabilitytobemelt-processedandthepress/sintertechniqueinsteademployed,theresultingdistributionofnano-fillersinthecompositeisheterogeneous,sincesuchfineparticlesmayonlyexistattheinterfacesbetweenthemuchlargermicroscalePTFEmatrixparticlesandalsobecauseofincompletebreakupofnano-filleragglomeratesduringblending.

WearTesting

Wearandfrictionaltestingwasperformedonathree-pin-on-disktribometerinambientairatroomtemperature.Asetofthreecompositepinswith4mm×4mmcrosssectionand12-mmlengthwasmachinedfromthecenterofeachpuck.Allcompositepinsetsweretestedagainststeel(304stainless)countersurfaces.Thecoun-tersurfaceswerepolishedwith0.3-μmaluminaparticlesindistilledwateronafeltwheel,yieldingameanvalueofaverageroughnessofRa=0.048μm,thenultrasonicallycleanedinmethanol.

Foreachtestasteeldiskcountersurfacewasattachedtoarotatingspindle,whilethethreeflat-endedpolymerpinswerese-curedwithinaholderatopanairbearing.Thepinswereloadedagainstthesurfaceoftherotatingsteeldisk,underanominalcon-tactpressureof3.125MPafromapneumaticpistonapplyinga150Nnormalload,withthepinsarrangedsoastobeequallyspacedaboutthecommoncircularweartrackof17-mmmean

AbilityofHardNano-ParticleFillerstoReducetheWearofPTFE249

radiuswithinwhichtheyallslidonthecountersurface.Spindlero-tationprovidedaslidingspeedof0.01m/s.Duringsliding,frictionwasmeasuredbystraingagesmountedonstationarycantileversthatcontactthepinholderandresistitsrotationwhenpinsslideagainsttherotatingdisk.Slidingwasinterruptedperiodicallytoquantifywearviapinmasslossmeasurementsusingananalyticalbalanceof0.1mgprecision.Thesemasslossescouldbesubse-quentlyconvertedtovolumelossesusingthecompositedensities,approximatedfromtheinitialdimensionsandmassesofcompos-itespecimens.

Testsofeachcompositewererunforatleastasufficientdura-tionsuchthataneventualsteady-stateregionwasclearlyidentifi-able,inwhichtheincreaseinwearisroughlylinearwithincreasingslidingdistancewhilefrictioncoefficientfluctuatesaboutamean.Acorrespondingwearrate(mm3/Nm)wasdeterminedfromtheidentificationoftheslopeofthissteady-statewearvolumeversusslidingdistancebehavior,vialinearregression,dividedbythe150Nnormalload.Foreachtest,a95%confidenceintervalwasde-

terminedonthesteady-statewearrateestimatefromthelinearregression.Meanvaluesofthefrictioncoefficientwerealsodeter-minedwithinthissteady-stateregion,aswellas95%confidenceintervalsaboutthismean.

RESULTSANDDISCUSSION

TherecordsofwearvolumeasafunctionofslidingdistanceforPTFE(G580)filledto5%weightfractionwithaluminafillerofvaryingparticlesizeisdepictedinFig.2.Eachtestadoptsasteady-stateofwearvolumeincreaseproportionaltoincreasingslidingdistance,suchthatforeachcompositeasteady-statewearratecanbequantified.AscomparedtotheunfilledPTFE,whichwearssorapidlyastobeinclinedalongtheverticalaxis,eachofthefillerswithparticlesizeintherange0.5-20μmformedmicro-compositesthatweresimilarlymorewearresistantsoastoallfallwithinthesamediagonalbandacrossthewear-slidingdistancegraph.Itisimmediatelyapparentthatthesmaller40-and80-nm

Fig.2—(a)WearrecordsforunfilledPTFE(G580)aswellasforPTFEmicrocompositesandnanocompositesincorporatingaluminafillerparticlesat5wt%.

(b)Expansionoflowerwearvolumeportionofwearrecordstohighlightwear-resistantbehaviorofnanocomposites.

250S.E.McELWAINETAL.

Fig.3—Steady-state(a)wearrateand(b)frictioncoefficientofalumina-filledPTFEcompositesasafunctionoffillerparticlesize.Compositesformed

witheitherG580or7Cresin.

aluminaparticlesusedtoformnanocompositeswerenotonlyabletoprovidethewearresistanceofthemicrocompositesbutwereabletoimproveuponittosuchanextentthatthewearrecordsarecomparablyflat,fallingalongthehorizontalaxisofthewear-slidingdistancegraph.AninsetwithexpandedwearvolumeaxisisalsoprovidedinFig.2bsothatthenanocompositewearrecordsmaybemoreclearlyseen.

Thesteady-statewearratesof5%alumina-filledPTFEquan-tifiedfromthewearrecordsofFig.2areplottedasafunctionofthefillerparticlesizeinFig.3a.AscomparedtotheunfilledPTFEdatumnear0.7∗10−3mm3/Nm,themicrocompositeseachprovidedwearreductionsofnearlytwoordersofmagnitude,withwearratesfallingmorenearto10−5mm3/Nm.Thetwonanocom-positesprovidedanadditionaltwoordersofmagnitudeofwearresistance,withwearratesnear10−7mm3/Nm.Despitedrasticallyalteringwearbehavior,Fig.3bindicatesthatthealuminafillerpar-ticleshaveverylittleeffectonunfilledPTFE’sfrictioncoefficientunderthese0.01m/sslidingconditions,measuredtobeapproxi-matelyμ=0.18.Thesewearandfrictionbehaviorsasafunctionoffillerparticlesizewerealsoduplicatedusingthealternate(7C)commercialPTFEgradeinaseriesofcompositeswithnano-fillersof40-or80-nmsizeandmicro-fillersof1-and20-μmsize.

AsshowninFig.4,followingaunidirectionalslidingtestofunfilledPTFE,thecountersurfaceiscoveredwithabundantplate-likelargedebris,havingin-planedimensionsofseveralhundredmicrometers.Givensufficientslidingdistanceapproaching50kmtogenerateasimilaramountofwearasthatfromtheunfilledPTFE,thedebrisfromthemicrocompositegenerouslygatheredabouttheedgesoftheweartrackandisfiner,withdimensionsofmorenearly10μm.Despitebeinggivenmorethantwicethisslidingdistance,theweardebrisfromthenanocompositeabouttheweartrackedgesisbothsparseandfine.Secondaryelectronimagestakenwithintheweartracks(Fig.5)indicatethatthemi-crocompositesleaveabrasiongrooveswithinthestainlesssteelcountersurfacealongtheslidingdirectionwithlooseweardebrisalsonoted.Thenanocompositesdonotappeartocausesuchabra-sionbutinsteadleavethintransferfilms,andeventhoughcrack-ingmayappearinthethickestregionsofthistransferitappearstoremaincoherentandwell-adheredwithoutliberatingnumer-oustransferweardebris.BurrisandSawyer(9)havepreviously

Fig.4—DebrisdepositedaboutthecountersurfaceweartrackfortheunfilledPTFEaswellasthe20-μmand40-nmfilledPTFEcompositesfollowingthe

wearrecordsdetailedinFig.2.Thecountersurfacedimensionsare50mm×50mm.

AbilityofHardNano-ParticleFillerstoReducetheWearofPTFE251

Fig.5—Secondaryelectronimagesfromwithinexampleweartracksformedby(a)microcomposite(20-μmfilled),and(b)nanocomposite(40-nmfilled)

uponthematingsteelcountersurface.

reportedthatPTFEcompositesproducingthinnertransferfilmscorrespondinglyexperiencelowerratesofwear.

ThesecondaryelectronimagesfromthepinspecimensinFig.6alsoshowsuch“mudflat”crackingthroughoutanother-wisesmoothandflatcoherentsurfacelayercoveringthewornnanocomposite.Highermagnificationimagingrevealsthatfibril-latedPTFEspansthesecracks(Fig.7)andappearstostabilizethesurfacelayeragainstbreakdownandweardebrisformation.Incontrast,thewornmicrocompositeshowsanincompleteflow-ingsurfacelayerthatappearstobeflakyandlessadherent,trans-formingtoweardebrisofthe∼10-μmdimensionpreviouslynotedabouttheweartrackinFig.4.EnergydispersiveX-rayspectraalsotakenfromtheseworncompositesurfacesrevealKαpeaksatenergiesof0.85,1.49,and6.40keV,respectively,forFfromthePTFEmatrix,Alfromthealuminafiller,andFefromsteelparticlesabradedfromthecoun-tersurfaceandmixedintothecomposite’swearsurface.AratiooftheheightoftheseAlandFepeaksnormalizedtotheFmatrixpeakcanserveasindicatorsoftherelativeamountsofaluminafillerandsteelweardebrisuponthecompositesurface.Forun-wornmicrocompositesandnanocompositesat5wt%aluminatheAl/Fratioisobservedtobeapproximately0.2,whereasforwornsurfacesthisratioincreasesasthefillerismorewearresistantandthereforetendstoaccumulateinthenear-surfaceregionasthe

Fig.6—Secondaryelectronimagesfromexamplewearsurfacesofa(a)microcomposite(1-μmfilled),and(b)nanocomposite(80-nmfilled).EDXSspectra

fromeacharealsoprovided,indicatingFfromthecompositematrix,Alfromthecompositefiller,andFefromdebrisabradedoffthematingsteelcountersurface.

252S.E.McELWAINETAL.

TABLE1—PEAK(Kα)HEIGHTSOFALFROMTHEALUMINAFILLERANDFEFROMDEBRISABRADEDOFFTHESTAINLESSSTEELCOUNTERSURFACE,RATIOEDTOTHEPEAKHEIGHTOFFFROMTHEPTFEMATRIX,ASMEASUREDUPONENERGYDISPERSIVEX-RAYSPECTROSCOPYOFWORNCOMPOSITESOFVARIOUSALUMINAFILLERPARTICLESIZECompositeSurface

Unworn

PTFE-20μmaluminawearsurfacePTFE-2μmaluminawearsurfacePTFE-1μmaluminawearsurfacePTFE-0.5μmaluminawearsurfacePTFE-80nmaluminawearsurfacePTFE-40nmaluminawearsurface

Al/F0.21.121.981.961.910.350.48

Fe/F00.321.280.420.220.150.17

Fig.7—Secondaryelectronimageoffibrilsspanninga“mudflat”crack

inawornnanocompositesurface.

PTFEmatrixispreferentiallywornaway,aspreviouslyreportedformicrocomposites(HanandBlanchet(10)).AsshowninTa-ble1,nanocompositesexperienceamuchlesserextentoffilleraccumulationattheslidingsurfacethanmicrocomposites.

Withlessfilleraccumulatedattheslidingsurface,theextenttowhichananocompositeabradesthemetalcountersurfaceandcausesdebristobemixedintoitswearsurfaceislessthanthatformicrocompositesasindicatedbyitslowerFe/FratiosinTable1.Thisreducedabrasivityofnano-fillersisfurtheremphasizeduponalsoconsideringfromFig.2thatitwasmaintaineddespitethemuchgreaterslidingdistancesthenanocompositeswereexposedtoinformingthesewearsurfacesandthemuchlesseramountofvolumetheylostduringthissliding.Obviously,agreaterslidingdistanceincreasesopportunityforabrasion.Additionally,alowercompositewearrateoffersincreasedopportunityforcountersur-

faceabrasiontobeobservedwithinthatcompositesurfacesinceabrasiondebrisinputintothecompositesurfaceatearlierstagesofslidingislesslikelytohavebeenexpelledduringsubsequentcompositewear.

Theseresultsledtothehypothesisthatthenanocompositewearmechanismisoneoftransferwear,whereadditionalre-movalofnanocompositematerialtoreplenishtransferfilmuponthecountersurfacemaynotbeactivateduntiltheprevioustrans-ferhaseventuallybeendetachedanddiscardedasdebris.ThenanoparticlescauseachangeinthePTFEsuchthatathintrans-ferfilmformsonthecounterfacethatiswelladheredandre-mainsstablebecausethenanoparticlesdonotaggressivelyabradeit.Thereasonforthethinnerandbetteradheredtransferfilmisnotclearbutmightberelatedtoincreasedcrystallinityin-ducedbynano-fillers(McElwain(11)).Ifthishypothesisiscor-rect,thentheadditionofthemoreabrasivemicroscalefillerstoanano-filledcompositeshouldleadtoanincreaseinitswearrate.

Inordertotestthehypothesis,anadditionaltestprogrampro-ducedcompositeshavingamixtureofnanoparticles(40nm)and

Fig.8—WearandfrictionbehavioroffivedifferentPTFEcompositematerialsslidingagainst304stainlesssteel,indicatingtheeffectofinclusionof

aluminamicroparticles(20μm)andnanoparticles(40nm),aswellasmixturesofmicro-andnanoparticles.

AbilityofHardNano-ParticleFillerstoReducetheWearofPTFE253

Fig.9—SecondaryelectronimageofthewornsurfaceofaPTFEcompos-itewithamixture(5%each)of40-nmand20-μmaluminafillers,displayingtheincompleteandflakysurfacelayercharacteristicofthemicrocompositewearmechanismaspredominant.

microparticles(20μm).AsshowninFig.8,thesemixed-fillercom-positeswereproducedeitherwith5wt%ofeachfillerorwith2.5%ofeachsothatthetotalfillercontentwouldbethesameasthecontrolcompositesfilledwitheitheronlynanoparticlesormi-croparticles.Ineithercase,themixed-fillercompositesdisplayedwearratesmoreneartothe10−5mm3/Nmmicrocompositevaluethanthe10−7mm3/Nmwearratesofnanocomposites.AsshowninFig.9,thewearsurfaceofthesemixed-fillercompositesalsomorenearlyresemblesthatofthemicrocompositesinFig.6thanthenanocomposites.Materialflowsintoanincompletesurfacelayerthatappearstobeflakyandbreakingupintofinedebris.So,thoughthemicroparticlesinthesemixed-fillercompositesstillin-terferewiththewearmechanismsthatcreatethelargeplate-likedebrisandresultintherapidwearofunfilledPTFE,theysupplantthewearresistanceotherwiseofferedbythenanoparticlesbyap-parentlymakingavailableawearpathwaynototherwiseavailableinthenanocomposite.

Insummary,whileduplicating∼10−7mm3/NmlevelsofwearratefornanocompositesasreportedbyBurrisandSawyer(8)using80-nmaluminaat5%,itisadditionallydemonstratedthatsuchwearratesaregreatlyreducedfromtheintermediate∼10−5mm3/Nmlevelobservedutilizingthesamefillermaterialbutatlarger,conventionalmicroscales.Bothstudiesalsoindicatedsim-ilarlyhighlevelsofwearrateofunfilledPTFEat0.6–0.7∗10−3mm3/Nm.BurrisandSawyer(8)werefurthermoreabletomain-tainthe∼10−7mm3/Nmlevelofcompositewearratewhilere-ducingthemassfractionof80-nmaluminafrom5to1%.Ourcontinuedstudieswillincludeinvestigatingtherelativeabilityofnano-fillerstocontinueprovidingPTFEcompositeswithreducedwearrateevenatreducedmassfractionloadings,comparedtothatofmicro-fillers,andthepossibleeffectsofdifferentpowderblendingtechniquesandtheirresultantfillerparticledispersions.

CONCLUSIONS

1.AscomparedtounfilledPTFE’shighwearrate(approximately0.7∗10−3mm3/Nm),theadditionof40-or80-nmaluminapar-

ticlesat5%massfractiondrasticallyreducedthewearrateto∼10−7mm3/Nm.Compositesutilizingmoreconventionalmi-croparticlesatthesamemassfractionofaluminafillerwithsizerangingfrom0.5to20μmonlyreducedthePTFEwearrateto∼10−5mm3/Nm.

2.PTFEcompositeswithaluminafillerintheformofnanoparti-cleswerelessabrasivetothematingsteel(304stainless)coun-tersurfacesthanthosewithmicroparticles,despitethefillerbe-ingofthesamematerial.

3.Theresultssuggestthatthenano-filledPTFEdepositsathin-ner,well-adheredtransferfilmthatisstablebecausethenano-fillersdonotabradeit.Compositeswithbothnano-andmi-croparticlesatequalamountsbehavedasamicrocompositewithahigher∼10−5mm3/Nmwearrateprobablyduetotheremovalofthetransferfilmbythemoreabrasivemicroscalefillerparticles.

4.ThefrictioncoefficientofthesePTFEcompositeswasunaffectedbyaluminafillerparticlesizeanddidnotdif-fersignificantlyfromthe0.18valuemeasuredforun-filledPTFEatthe0.01m/sslidingspeedemployedinthisstudy.

ACKNOWLEDGMENTS

ThismaterialisbaseduponworksupportedunderAFOSR-MURIgrantFA9550-04-1-0367.Anyopinions,findingsandcon-clusions,orrecommendationsexpressedinthismaterialarethoseoftheauthorsanddonotnecessarilyreflecttheviewsoftheAirForceOfficeofSponsoredResearch.

REFERENCES

(1)Makinson,K.R.andTabor,D.(19),“TheFrictionandTransferof

Polytetrafluoroethylene,”ProceedingsoftheRoyalSociety,281A,pp49-61.

(2)Bahadur,S.andTabor,D.(1984),“TheWearofFilledPolytetrafluoroethyl-ene,”Wear,98,pp1-13.

(3)Blanchet,T.A.andKennedy,F.E.(1992),“SlidingWearMechanismof

Polytetrafluoroethylene(PTFE)andPTFEComposites,”Wear,153,pp229-143.

(4)Lancaster,J.K.(1968),“TheEffectofCarbonFibreReinforcementonthe

FrictionandWearofPolymers,”BritishJournalofAppliedPhysics,1,pp549-559.

(5)Tanaka,K.andKawakami,S.(1982),“EffectofVariousFillersontheFric-tionandWearofPolytetrafluoroethylene-BasedComposites,”Wear,79,pp221-234.

(6)Li,F.Hu,K.,Li,J.andZhao,B.(2002),“TheFrictionandWearCharac-teristicsofNanometerZnOFilledPolytetrafluoroethylene,”Wear,249,pp877-882.

(7)Sawyer,W.G.,Freudenberg,K.D.,Bhimaraj,P.andSchadler,L.S.“A

StudyontheFrictionandWearBehaviorofPTFEFilledwithAluminaNanoparticles,”Wear,254,pp573-580.

(8)Burris,D.L.andSawyer,W.G.(2006),“ImprovedWearResistancein

Alumina-PTFENanocompositeswithIrregularShapedNanoparticles,”Wear,260,pp915-918.

(9)Burris,D.L.andSawyer,W.G.(2005),“TribologicalSensitivityofPTFE-AluminaNanocompositestoaRangeofTraditionalSurfaceFinishes,”Tri-bologyTransactions,48,pp1-7.

(10)Han,S.W.andBlanchet,T.A.(1997),“ExperimentalEvaluation

ofaSteady-StateModelfortheWearofParticle-FilledPolymerCompositeMaterials,”ASMEJournalofTribology,119,pp694-699.

(11)McElwain,S.(2006),MasterofScienceThesis.RensselaerPolytechnicIn-stitute,Troy,NY.