Composite Materials Applications to Meet Property Requirements of Automotive Bearings2.docx
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CompositeMaterialsApplicationstoMeetPropertyRequirementsofAutomotiveBearings2
CompositeMaterialsApplicationstoMeetPropertyRequirementsofAutomotiveBearings
ABSTRACT
Metal-MatrixComposites(MMCs)haveanirreplaceableshareintheautomotivemarket;theyhavebeenveryattractivetotheautomotivebearingproducersandendusersbecauseoftheiroutstandingmechanicalandphysicalproperties.
CastAl/Gr(p)compositesrepresentoneoftheleastexpensiveofaluminumcomposites.Theyareeasiertomachinethanmonolithicaluminumalloysandhaveimproveddampingcapacity,whichisimportantforautomotiveapplications.
Polyphenylenesulphide(PPS)compositesarewidelyusedinvariousengineeringapplicationsaswell.Andwhenusedinbearings,PPSwouldshowimprovedpropertiesdeliveringacostadvantageandmanytechnicalbenefits.
INTRODUCTION
ModernMetal-MatrixComposites(MMCs)differfromthoseoldermaterialssuchastraditionalaluminumalloysandstainlesssteelinthatanyselectedvolume,shapeandsizeofreinforcementcanbeintroducedintothematrix;inaddition,manufacturingprocesseslendthemselvestoselectivereinforcementofthematrix,therebyallowingthecreationofspatiallyvaryingpropertieswithinamaterialinresponsetospatiallyvaryingperformancerequirementswithinacomponent(Rohatgi,1991).
Forthedevelopmentofautomotiveindustry,theriseofplasticplainbearingshasbeenwitnessedduringthelastseveraldecades.Plainbearingmaterialsrequireuniquecombinationsofpropertiesincludinghighspecificstrength,specificstiffness,dampingproperties,machinability,friction,wearandseizureresistance,andalowcoefficientofthermalexpansion.
Wheelbearingsperformtwoveryimportantjobsinavehicle’ssuspension.Thewheelbearingallowsthewheelstorotatewithminimalfunction,andtheyalsosupportthevehicle’sweight.
Tobeabletodobothofthesejobs,thebearingsmustbeinnearperfectcondition.Thesealsmustalsobeleakfreetokeepthelubricantinsidethebearingsandcontaminantsout.
Plainbearingsmustbemadefromamaterialthatisdurable,lowfriction,lowweartothebearingandshaft,resistanttoelevatedtemperatures,andcorrosionresistant.Oftenthebearingismadeupofatleasttwoconstituents,whereoneissoftandtheotherishard.Thehardconstituentsupportstheloadwhilethesoftconstituentsupportsthehardconstituent.Ingeneral,theharderthesurfacesincontactthelowerthecoefficientoffrictionandthegreaterthepressurerequiredforthetwotoseize(JeffreyA,etal.,2003).
Inanaveragesedanthatweighsaround3,400–lb,eachpairoffrontwheelbearings,aswellastherearwheeloraxlebearings,eachsupport850pounds.Thisalldependsonthefronttorearweightdistributionofthevehicle.It’salotofweightthatneedstobesupportedovertensofthousandsofmiles.
MMCSINAUTOMOTIVEINDUSTRY
AvarietyofmethodsforproducingMMCs,includingdiffusionbonding,powdermetallurgy,casting,pressureinfiltrationandsprayco-deposition,haverecentlybecomeavailable(Rohatgi,1991).1-9
Stir-castMMCsaremadebyintroducingfibersorparticlesintomoltenorpartiallysolidifiedmetals,followedbycastingoftheseslurriesinmolds.Inpressure-infiltratedMMCs,apreformoffibersoraprepackedbedofparticlesismadeandinfiltratedbymoltenalloys,whichthenfreezeintheinter-fiberorinter-particlespacestoformthecomposite.Inboththeseprocesses,mixingandwettingbetweenmoltenalloysandreinforcementsaredesirableintermsofeaseoffabricationandultimatedistributionofparticulatesandfibers.
High-strength,high-stiffnesspoly-crystallineα-alumina(Al2O3)/Alcompositeshavebeenpreparedbyapressure-infiltrationprocess.Fornonwettingmetals,theα-Al2O3iscoatedwithametalbyvapordepositionorbyelectrolessplatingbeforeinfiltration.Titanium-boroncoatingshavealsobeenusedforgraphite(Gr)/AlandAl2O3/Alcomposites.However,intermsfabricabilityandcost,modificationofthematrixbyaddingsmallamountsofreactiveelements(e.g.,Mg,Ca,LiorNa)ispreferred.Alumina-reinforcedaluminumcomposites,aswellasseveralparticle-filledMMCs,havebeensynthesizedbyaddingreactiveagentstothemelts.
Carefulcontrolofreactionsatthefiber/metalinterfaceisrequiredtoimprovewettabilityandbondingwithoutcausingexcessivedegradationofthefibersurface.Theformationofbrittleintermetalliccompoundsattheinterface(e.g.,carbidesinGr/AIandSiC/AIcomposites)duringprocessingcancausefiberdegradationandlossofstrengthinthelongitudinaldirection.Wettabilityandbondingattheinterfacecansometimesbeimprovedbyachemicalreactionthatformsspinelsoroxidesisostructuralwithspinels(MgAI2O4).Spinelpromotesbondingatinterfacesbecauseitformsstrongbondswithmetalsandceramics.Continuousadherentmetalliccoatings(e.g.copperandnickel)havebeenusedonseveralnonwettingparticlessuchasgraphitetoimprovethemelt-particlewettability,permittinghighpercentagesoftheseparticlestoberecoveredinthesolidifiedcastings.
Stir-CastingTechniques
Abasicrequirementofthefoundryprocessingl-4ofMMCsistheinitialintimatecontactandbondingbetweentheceramicphaseandthemoltenalloy.Thisisachievedeitherbymixingtheceramicdispersoidsintomoltenalloysorbypressureinfiltrationofmoltenalloysintopreformsoftheceramicphase,followedbystirringofthecompositeinmelts.Becauseofthepoorwettabilityofmostceramics,intimatecontactbetweenfiberandalloycanbepromotedonlybyartificiallyinducingwettabilityorbyusingexternalforcestoovercomethethermodynamicsurfaceenergybarrierandviscousdrag.Mixingtechniquesgenerallyusedforintroducingandhomogeneouslydispersingadiscontinuousphaseinameltare:
•Addingparticlestoavigorouslyagitated,fully/partiallymoltenalloy.
•Injectionofthediscontinuousphaseintothemeltwithaninjectiongun.
•Dispersionofpelletsorbriquettesofadiscontinuousphase.
Withalloftheabovetechniques,externalforceisusedtotransferanonwettableceramicphaseintoameltandtocreateahomogeneoussuspensionoftheceramicinthemelt.Theuniformityofparticledispersioninametalbeforesolidificationiscontrolledbytheinitialclusteringoftheparticlesintheloosepowderstateandthedynamicsofparticlemovementinagitatedliquids.Themelt-particleslurrycanbecasteitherbyconventionalfoundrytechniques,suchasgravity,pressure-dieorcentrifugalcasting,orbynewertechniquessuchassqueezecasting(liquid-forging),spraycodepositionormeltspinning.Thechoiceofcastingtechniqueandmoldconfigurationisofcentralimportancetothequality(i.e.,soundnessandparticledistribution)ofacompositecastingbecausethesuspendedparticlesexperiencebuoyancy-drivenmovementinthesolidifyingmeltuntiltheyareencapsulatedinthesolidifyingstructurebycrystallizingphases.Particlessuchasgraphitearelighterthanmostaluminumalloysandtendtosegregatenearthetopportionofgravitycastings,leavingaparticle-impoverishedregionnearthebottomofthecasting.Similarly,heavierparticlessuchasSiCandAl2O3tendtosettleandsegregatenearthebottomofgravitycastings.2
ThespatialarrangementofthediscontinuousceramicphaseinthecaststructureprincipalIydeterminesthepropertiesofthecastcomposite.Thedistributionofphasesdependsonthequalityofthemelt-particleslurrypriortocastingandonthefollowingadditionalvariables:
•Viscosityofthesolidifyingmelt.
•Shape,sizeandvolumefractionofparticles.
•Specificgravitiesoftheparticlesandmelt.
•Thermalpropertiesoftheparticlesandthematrixalloy.
•Chemistryandmorphologyofcrystallizingphasesandtheirinteractionswithparticles,nucleationofprimaryphasesonceramics,andentrapmentorpushingofparticlesbysolidifyinginterfaces.
•Theflocculationofparticles,
•Thepresenceofanyexternalforcesduringsolidification.Thevarioustechniquesusedtosolidifymelt-particleslurriesarediscussedinthesidebar.
Theprincipalattractionsfortheuseof
MMCsare
•Reductioninmass,especiallyinengineparts.
•Improvedwearresistanceorlubricationcharacteristics.(Itmaybepossibletoreplacepiston-ringgrooves,valves,andvalveseatsbyusingMMCs,althoughthismeansthereplacementnotjustofthecomponentbutthesurfaceitrubsagainst.Thismaynecessitatetotalengineredesignoratleastare-evaluationofthepowerunit.)
•Improvedmaterialproperties,particularlystiffnessandstrength,eitherprovidingincreasedcomponentdurabilityorpermittingmoreextremeserviceconditions.
•Reducedthermal-expansioncoefficient.
Drawbackscenteron
•Reducedductilityandtoughness.
•Extracostofreinforcementoveraluminumalloys.
•Lackofdesigndata.
•Needtoimprovelow-cost,high-volume,secondaryprocessingtechniques.
•Doubtsaboutrecyclability.
CASTAl/Gr(p)COMPOSITES
Beginningin1965,castAI/Gr(p)compositesweredevelopedusingstircastingtechniquesforautomotiveantifrictionapplications.20,21
Suchalloypotentiallyrepresentoneoftheleastexpensive1-10ofaluminumcomposites.Theyareeasiertomachinethanmonolithicaluminumalloysandhaveimproveddampingcapacity,whichisimportantforautomotiveapplications.Thesealloyshavebeencastintoenginecomponents,includingpistons,linersandbushings,usingpermanent-moldcasting,squeezecasting,centrifugalcastingandpressurediecasting;theyhavebeentestedinautomotiveapplications.
CastAI/Cr(p)compositeshaveexhibitedlowerfrictioncoefficientsandwe