Effect of ceramic particulate type on microstructure and properties of copper matrix composites synthesized by friction stir processing

w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original

Article

Effect

of

ceramic

particulate

type

on

microstructure

and

properties

of

copper

matrix

composites

synthesized

by

friction

stir

processing

Issac

Dinaharan

_,

_Ramasamy

_Sathiskumar

_,

_Nadarajan

_Murugan

a_{DepartmentofMechanicalEngineeringScience,UniversityofJohannesburg,AucklandParkKingswayCampus,Johannesburg,} SouthAfrica

b_{DepartmentofMechanicalEngineering,CoimbatoreInstituteofTechnology,Coimbatore,India}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received14November2015 Accepted18January2016 Availableonline14April2016

Keywords:

Coppermatrixcomposites Frictionstirprocessing Microstructure Wear

a

b

s

t

r

a

c

t

Frictionstirprocessing(FSP)hasbeenestablishedasanovelsolidstatetechniqueto pro-ducebulkandsurfacemetalmatrixcomposites.Thepresentworkaimtoproducecopper matrixcomposites(CMCs)usingFSPandanalyzetheeffectofceramicreinforcementtype (SiC,Al2O3,B4CandTiC)ontheevolvingmicrostructure,microhardnessandwear

resis-tancebehavior.Agroovewasmadeon6mmthickcopperplatesandpackedwithvarious ceramicparticles.AsinglepassFSPwascarriedoutusingatoolrotationalspeedof1000rpm, travelspeedof40mm/minandanaxialforceof10kN.Themicrostructureanddistribution oftheceramicparticleswerestudiedusingopticalandfieldemissionscanningelectron microscopy.Theslidingwearbehaviorwasevaluatedusingapin-on-diskapparatus.The resultsindicatethatthevariationinthestirzone,distribution,grainsize,hardnessandwear resistanceofCMCswerewithinashortrange.Nevertheless,Cu/B4CCMCexhibitedsuperior

hardnessandwearresistancecomparedtootherCMCsproducedinthisworkunderthe samesetofexperimentalconditions.

©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevier EditoraLtda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Theadvancementofelectronicandelectricalindustrieshas invoked a demandforhigh strength,high wear resistance and high conductivity connector materials. Pure copper is extensivelyusedinthoseindustriesduetohighelectricaland thermal conductivity, formability and corrosion resistance

∗ _{Correspondingauthor.}

E-mail:[email protected](R.Sathiskumar).

[1–3].However,thelowwearresistanceofpurecopperplaces somelimitations,especiallyforslidingapplications.Copper matrix composites(CMCs)reinforced withceramic particu-latesweredevelopedtoovercomethelimitations[4,5].CMCs arecurrentlyproducedusingpowdermetallurgy[6]and var-iouscastingroutes[7–9].Itisextremelydifficulttoobtaina highperformanceCMCsduetoporosityformation,interface debonding,poordistribution,lowwettabilityandinterfacial

http://dx.doi.org/10.1016/j.jmrt.2016.01.003

2238-7854/©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevierEditoraLtda.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

j mater res technol.2016;5(4):302–316

303

a

b

Cu composite

Cu

Fig.1– (a)Opticalphotomicrographofpurecopperand(b)locationinstirzonetorecordthemicrostructureofCMCs.

reaction.Frictionstirprocessing(FSP)isemergingasa promis-ingtechniquetoproducesoundCMCs[10].

FSPisanovelsolidstatetechniquetofabricatebulkand surfacemetalmatrixcomposites[11].Mishraetal.[12]derived FSP based on the principles offriction stir welding (FSW) tosynthesizemetalmatrixcomposites(MMCs).Agrooveof requiredwidth anddepth[13]orahole ofrequired diame-ter anddepth [14]ismachinedon the metallicplateto be processed. The chosen ceramic particles are subsequently compactedinthe grooveorthehole.Arotatingtoolunder adequate axial force is plunged at one end and traversed along the groove or holes. The material undergoes severe plastic deformation during FSP which is utilized to form MMCs.FSPtechniquehasbeensuccessfullyexploredbymany

investigatorstofabricatealuminum[15],magnesium[16], cop-per[17],steel[18]andtitanium[19]composites.

SomestudiesonCMCsreinforcedwithvariousceramic par-ticlesproducedusingFSPwerereportedinliteratures[20–30]. Barmouzetal.[20–22]producedCu/SiCCMCsandstudiedthe propertiesindetail.Heobservedthatthetraversespeed con-siderablyinfluencedthedistributionofSiCparticleintheCMC [20].Theincreaseinvolumefractionanddecreaseinthesize oftheSiCparticleenhancedthetensileandwearproperties oftheCMC[21].Theincreaseinthenumberofpassesduring FSPimprovedthedistributionofSiCparticlesandreducedthe grainsize[22].Sarmadietal.[23]preparedCu/graphiteCMCs andreportedtheinfluenceoftoolpinprofileon microstruc-ture and wear behavior. Sathiskumar et al. [24] fabricated

Cu/B4CCMCsandinvestigatedtheeffectofB4Cparticleand its volume fraction on microstructure and wear behavior. Sathiskumaretal.[25]developedempiricalrelationshipsto predictthe influenceofprocess parametersonmechanical andwearpropertiesofCMCsreinforcedwithSiC,Al2O3,B4C, TiCandWCparticulates.Akramifardetal.[26]synthesized Cu/SiCCMCsandshowedthattheSiCparticlesimprovedthe mechanical and wear properties. Fenoel et al. [27] formed Cu/Y2O3CMCsanddemonstratedthattheY2O3particleswere effectivetoboostthemechanicalproperties.Khosraviet al. [28]developedCu/WCCMCsandinvestigatedtheinfluenceof numberofpassesonmicrostructure,mechanicalandthermo physicalproperties.ItwasfoundthatWCparticlesimproved themechanicalpropertiesandrefinedthe grainsofcopper. Sabbaghianetal.[29]producedCu/TiCCMCsusingFSPand observedfinegrainswithahomogeneousdistributionofTiC particles.RajuandKumar[30]fabricatedCu/Al2O3CMCsusing FSPandanalyzedtheinfluenceofprocessparametersonthe tensilestrength.

Ithasbeendemonstratedinliteraturesthatitisfeasible toproduceCMCsreinforcedwithvariousceramicparticulates suchasSiC,Al2O3,Y2O3,B4C,TiCandWC.Multiple reinforce-mentswerenotcomparedinasinglestudywhichwouldhelp toevaluatetheperformanceofvarious potential reinforce-mentsunderasetofsimilarexperimentalconditions.Hence, theobjectiveofthisresearchworkistoproduceCMCs rein-forcedwithSiC,Al2O3,B4CandTiC andevaluatetheeffect ofvariousreinforcementsonmicrostructureandslidingwear behavior.Siliconcarbide(SiC)andalumina(Al2O3)are com-monlyusedreinforcements formaking CMCs,whileboron carbide(B4C)andtitaniumcarbide(TiC)areusedforspecific applications[31–33].

2.

Experimental

procedure

Commerciallyavailablepurecopperplatesof100mmlength, 50mmwidthand6mmthicknesswereusedinthisresearch work.Theopticalphotomicrograph ofthe as-received cop-perplate isshowninFig.1a.Agrooveof2.5mmdeepand 0.7mmwidthwasmadeinthemiddleoftheplateusingwire EDMandcompactedwithceramicparticles.Thevolume frac-tionofceramicparticleswas12%.TheSEMmicrographsof asreceivedceramicparticles areshowninFig.2.Apinless toolwasinitiallyemployedtocoverthetopofthegrooveafter fillingwithceramicparticlestopreventtheparticlesfrom scat-teringduringFSP.Atoolmadeofdoubletemperedhotworking steelwas used inthis study [25]. The toolhad a shoulder diameterof20mm,pindiameterof5mmandpinlengthof 3mm.TheFSPwascarriedoutonanindigenouslybuiltFSW machine.Theprocessparametersemployedwere tool rota-tionalspeedof1000rpm,travelspeedof40mm/minandaxial forceof10kN.Thissetofprocessparameterswasselected aftertrailexperiments.Someofthedefectsencountered dur-ingtrialexperimentsareshowninFig.3.Foursuchplateswere frictionstirprocessedbyvaryingtheceramicparticles(SiC, Al2O3,B4CandTiC).AdetailedFSPproceduretoproducethe compositeispresentedelsewhere[20].

Specimens were obtained from the center of the fric-tionstirprocessedplatesandwerepolishedasperstandard

Fig.3–Typicaldefectsappearedintrialspecimens:(a) roughsurfaceinducedbyinsufficientplasticflow,(b)tool dragging,(c)incompletebondingand(d)cracks.

metallographic procedure. The polished specimens were etched with a color etchant containing 20g chromic acid, 2gsodiumsulfate,1.7mlHCl(35%)in100mldistilledwater. Thedigitalimageofthemacrostructureoftheetched spec-imens was captured using a digital optical scanner. The microstructure was observed using an optical microscope (OLYMPUS-BX51M) at various locations in the stir zone as indicated in Fig. 1b. Theceramic particle distribution was further viewed using a field emission scanning electron microscope (FESEM,CARLZEISS-SIGMA HV).Energy disper-sivespectroscope(EDS)attached tothe FESEMwasusedto study theelementaldistributionwithinthecomposite.The microhardnesswasmeasuredusingamicrohardnesstester (MITUTOYO-MVK-H1)at500gloadappliedfor15satvarious locationsinthesurfacecomposite.

TheslidingwearbehaviorofCu/X(X=SiC,Al2O3,B4Cand TiC)CMCswasmeasuredusingapin-on-diskwearapparatus (DUCOM TR20-LE) atroom temperatureaccording toASTM G99-04a standard.Specimensofsize3mm×5mm×20mm were preparedfrom the FSP zone bywire EDM. Thewear test was conductedatasliding velocityof1.5m/s,normal force of30N and sliding distance of3000m. The polished surface ofthepin was slidon ahardenedchromium steel disk.Acomputeraideddataacquisitionsystemwasusedto monitor the loss of height. Thevolumetric loss was com-puted by multiplying the cross sectional area of the test pin withits lossofheight. Thewearrate wasobtainedby dividing volumetriclossto sliding distance.Theworn sur-faces of the testspecimens were observed using the SEM. The wear debris, which were scattered on the face of the counterface,werecarefullycollectedandcharacterizedusing SEM.

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Fig.4–CrownappearanceofCMCsreinforcedwith:(a)SiC,(b)Al2O3,(c)B4Cand(d)TiC.

3.

Results

and

discussion

Thecrown appearance is an indication ofthe integrity of thestir zone underneathit. Anydefect on thecrown usu-allyaccompaniesacorrespondingdefectinthestirzone.The crownappearancesoffrictionstirprocessedcopperwith var-iousceramicparticlesarepresentedinFig.4.Thesurfaceof thecrownissmoothwithoutanydepressionsor discontinu-ities.Asimilarcrownsurfacewasobservedirrespectiveofthe typeofceramicparticleused.Semicircularstriations analo-goustothosegeneratedintheconventionalmillingprocess arealsovisibleonthecrown.Theoptimizedprocess parame-terswerecarefullychosentoyieldadefectfreecrownsurface. ThedefectsinthecrownasrevealedinFig.3duringtrailruns canbeattributedtopoormaterialflowbetweenadvancingand retreadingsideandinadequateplasticizationofcopper.

3.1. MacrostructureofCMCs

Fig.5depictsthemacrostructureofCMCsreinforcedwith var-iousceramicparticles.Thestirzoneareawhichcontainsthe CMCisclearlyvisibleinallthefigures.Themarksofagroove madebeforeFSParenotseen.Itindicatesthecomplete for-mationofthecompositeandcontinuousflowofplasticized materialduringFSP.Thefrictionalheatgeneratedbythe rub-bingoftoolshoulderandtheshearingofthepinplasticizes thecoppermatrix aroundand belowthetool.Therotating andtranslationmotionofthetooltransportstheplasticized copperfromadvancingsidetoretreadingside.Thismaterial flowinitiallycausesthegroovetocollapseandmixesthe com-pactedceramicparticleswiththeplasticizedcopper.Therate atwhichthetoolrotatesandtranslatesdeterminesthe inten-sityofmixingleadingtotheformationofthecomposite.The figurerevealsthatallkindsofceramicparticlesmixedwith theplasticizedcopperandformedtheCMC.Itcanleadtoa conclusionthatthetypeofceramicparticlehasnoinfluence ontheformationofcompositeduringtheFSPprocess.The

boundariesofthestirzonearemarkedwithblackcolor.Itis interestingthattheboundariesareclearlyvisibleonboththe sidesofthestirzone.Someresearchersreportedtheabsence ofaclearboundarybetweenthestirzoneandthematrix mate-rialontheretreadingsideduringFSW[34,35].Thiscouldbe duetoinadequateforgingatthebackofthetoolorexcessive plasticizationofthematrix.ThemeasuredvaluesoftheFSP area arefurnishedinTable1.ThesizevariationoftheFSP areaacrossthevariousceramicparticlesisnegligible.Under asetofFSPparameters,thevariationintheflowstressofthe materialwillcausevariationinthesizeoftheFSPzone.Itisa wellknownfactthattheadditionofceramicparticleleadsto theincreaseinflowstress[36].Theresultsdemonstratethat thevariationinflowstressofthecopperbythereinforcement ofdifferentceramicparticlesisnegligible.Hence,thereisno significantchangeinstirzonesize.

ItisevidentfromthefigurethatthestirzoneofallCMCsis symmetricaboutthetoolaxis.Asymmetricstirzonewillhave ahomogeneousdistributionofceramicparticles acrossthe zone.Thecombinationoftoolrotationandtranslationtends to inducean asymmetric materialflow inFSP [37]. Higher levelofstirringintheadvancingsidewillalterthe symme-tryofthestirzone.Theformationofthesymmetricstirzone can beattributedto evenstirringand material transporta-tionfrom advancingtoretreadingside.Asamanufacturing process,FSPhasitsowndefectssuchaspinholes,tunnels, voids, cracksandkissingbonds.Defectsreducetheareaof the stir zone and cause the composite susceptibleto slid-ingwearandtensileloading.Noneofthosedefectsappeared inthestirzone.Thestirzoneiscompletelydefectfreeand sound.Defectsariseduetoseveralfactorswhichisnotlimited toinadequateheatgeneration,materialflowand consolida-tion.Theprocessparametersgovernthosefactorstoalarge extend.Absenceofdefectscanberelatedtooptimized pro-cessparametersemployedinthiswork.Itisworthmentioning thatnoonionringscharacterizedbyarclinestypicallyseenin thestirzoneoffrictionstirweldedorprocessedmonolithic alloysarenotobserved.Thisparticularresultagreeswiththe

Fig.5–MacrostructureofCMCsreinforcedwith:(a)SiC,(b)Al2O3,(c)B4Cand(d)TiC.

findingsofsomeearlierworksonCMCsusingFSP[20,21,23]. Theamalgamationofmaterialflowinducedseparatelybythe toolshoulderandtoolpinproducesalternativelayersofhigh andlowvolumefractionofceramicparticlesduetothe tem-peraturegradientalong thedepth ofthewelded plate[37]. Thedepthofpenetrationofthetoolisnotequaltothedepth oftotalplatethickness intheproductionofcompositesby FSP.Theabsenceofonionringspointsoutthatthethermal gradientalongthedepthofthetoolpinpenetrationis negli-gible.Because,thefixturemadeofdifferentmaterialactsas abackupplateinaconventionalFSW.But,thesamematrix materialbeyondthedepthoftoolpenetrationtothefull thick-nessofplateservesasabackplateinFSP.Hence,thethermal gradientisinsufficienttoaidtheformationofonionrings.

3.2. MicrostructureofCMCs

Figs.6–9depicttheopticalmicrographsofCMCsreinforced withvariousceramicparticlesrecordedatlocationsasmarked inFig.1b.Theopticalmicrographsrevealthedistributionof ceramicparticlesalloverthestirzone.Noareainthestirzone isparticlefree.Itisremarkabletoobservethatthedistribution oftheceramicparticleisindependentofthelocationinthestir zone.Thevariationinthedistributionoftheceramicparticles fromtheadvancingsidetotheretreadingsideorfromthetop sidetothebottomsideisinfinitesimal.Butresearchersfound significantvariationinthedistribution ofceramicparticles withinthestirzoneofaluminumandmagnesiumcomposites synthesizedbyFSP[38–40].Thiscanbeattributedtothe mate-rialbehaviorofcopperunderfrictionalheat.Theflowstressof copperisconsiderablehighertothatofaluminumand magne-siumwhichmakesplasticizationofcopperlittledifficult.The plasticizedcopperwillnotfloweasilycomparedtoaluminum

ormagnesium.Thisprovidesmoretimefortheceramic parti-clestobewelldistributedwithintheplasticizedcopperbefore forgingcompletestheformationofstirzone.Hence,the dis-tributionofceramicparticlesissuperiortothatofaluminum ormagnesiumcomposites.

ItcanbeinferredfromFigs.6–9thatthetypeofceramic particle does notplayamajorrole togovernthenatureof distributioninthecomposite.Alltheceramicparticles consid-eredinthisstudycombinedwellwiththeplasticizedcopper andproducedthecomposite.Thiscanberelatedtothenature oftheFSPprocess,whichformsthecompositeinsolidstate withoutmeltingthecopper.Neitherthedensitygradientnor thewettabilitybetweenthetypeofceramicparticleandthe coppercausesunevenmicrostructure.Itisveryhardto pro-duce CMCs reinforcedwithvarious ceramicparticles using liquidmetallurgyroutes.Thedensitygradientwillcausethe ceramicparticle eithertofloat orsinkwhilepoor wettabil-ity willresult inthe rejectionofparticles from the copper melt.FSPprocessissuitableandcapableofproducingCMCs reinforcedwithdifferentceramicparticles.

The optical micrographs in Figs. 6–9 furthershows the presenceoffinergrainscomparedtothegrainsizeofcopper inFig.1a.Theaveragegrainsizeisquantitativelypresented inTable1.Thegrainboundariesare notvisiblecompletely, butconstructedusinganimageanalyzingsoftwarefor mea-surement [24]. Thereinforcement ofceramicparticles and the severeplasticdeformation refinedthe grainsofcopper matrix.But thegrain sizeacross differentCMCs is negligi-ble. Thefollowing factors predominantly contribute to the grain refinement. FSP imposes severe plastic deformation, which leads to high dynamic stress as well as nucleation andincreaseindislocationdensity.Themovementofgrain boundariesandsubsequentlythegraingrowthisprevented.

Table1–Propertiesofcoppermatrixcomposites.

Material FSParea(mm2_{) Averagegrain}

size(␮m) Microhardness (VHN) Wearrate (mm3_/m) Cu – 35 75 385 Cu/SiCCMC 39 6 116 245 Cu/Al2O3CMC 40 3 119 231 Cu/B4CCMC 37 5 135 213 Cu/TiCCMC 42 4 126 225

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Fig.6–OpticalphotomicrographofCu/SiCCMCobservedatvariouslocations.

Secondly,thepresenceofceramicparticleplaysanotherrole. Thepinningeffectoftheceramicparticleonthecoppermatrix impedes the grain growth by suppressing grain boundary sliding.

Fig.10presentstheSEMmicrographsofCMCsreinforced withvarious ceramicparticles whichclearlyrevealthe dis-tribution of ceramic particles in the copper matrix. Fairly homogeneousdistributionwasobserved.Noclusterof parti-clesisseen.Further,thereisnosegregationofparticlesalong thegrainboundaries.Thedistributionisalmostintragranular. ThemechanicalandtribologicalpropertiesofCMCsare dic-tatedbythenatureofdistribution.Homogeneousandintra granulardistributionispreferredtoobtainsuperior proper-ties.TheFSPprocesshasresultedinthedesirabledistribution. Liquidmetallurgyroutesoftenproduceinhomogeneousand intergranulardistributionduetosolidificationphenomena.

Themovementofceramicparticleswithinthecoppermatrix duringprocessingduetodensitygradientisabsolutelyabsent. This resultsin properdistribution. However, occurrenceof ceramicparticleclustersinFSPwas reportedinsome liter-atures[14,15,40,41].TheFSPparameterstoolrotationalspeed andtraversespeedaffectthedistribution.Alowertool rota-tionalspeedandahighertraversespeedwillleadtoclustering andpoordistribution.Thefine,homogeneousdistributionin thisworkconfirmsthatthechosensetofprocessparameters isadequatetoproducethedesirabledistribution.

FSP resulted a change in the size and morphology of ceramicparticlesexceptforAl2O3particlescomparingFig.10 andFig.3.Thesevereplasticdeformationcoupledwiththe rotatingactionofthetoolisabletoshattertheceramic par-ticles.Thevigorousstirringactionofthetoolknocksoffthe sharpcornersoftheceramicparticle.Largesizevariationof

Fig.7– OpticalphotomicrographofCu/Al2O3CMCobservedatvariouslocations.

ceramicparticles(SiC, B4Cand TiC)inFig. 10confirmsthe fragmentation. Similar observations were reported by oth-ers[16,34,36].Thefragmentationdependsuponthesizeand shapeofthe particles.Largesize particles andirregular or polygonalshapeparticleshavethetendencytobreakoff dur-ing FSP.Theretention ofshapeand sizeofAl2O3 particles inFig. 10bafter FSP,which didnotundergo fragmentation confirmsthisstatement.ItisnotableinFig.10thatthelarge ceramicparticlesarenotsurroundedbydebrisgenerateddue tofragmentation.Thereisnoclusteringofsmalldebriseither. Thissuggeststhatthedebrisalsomixedwellwiththe plasti-cizedcopperanddistributedhomogeneouslyintheCMC.The sizeofdebrisisconsiderablylowintheorderofnanometer comparedtothesizeofinitiallypackedceramicparticles.The largesizevariationleadstofunctionallygradedlocalareas withintheCMC.

Fig. 11depicts theSEMmicrographsofCMCs reinforced withvariousceramicparticlesathighermagnification.The etchantusedinthisworkrevealsthegrainboundaries.The interfacebetweenthecoppermatrixandtheceramic parti-cleisdetailedinthisfigure.Theinterfaceisclearwithoutthe presenceofporesorreactionproducts.Eachtypeofceramic particle appearstobebondedwellwiththecoppermatrix. Barmouz et al. [21] encountered a large number of pores aroundSiCparticlesintheCu/SiCCMCfabricatedusingFSP technique.Nosuchporesareobservednearanyceramic par-ticleinFig.11.Thiscanberelatedtosufficientmaterialflow and plasticization ofcopper under the chosen experimen-tal conditions. Theinterface plays acrucial role in tensile loadingand sliding wear totransferthe loadeffectively to the ceramicparticle.Good interfacial bonding isa prereq-uisiteinspiteofhomogeneousdistributiontoenhance the

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Fig.8–OpticalphotomicrographofCu/B4CCMCobservedatvariouslocations.

properties.Thetemperatureoftheprocessingmethod influ-encestheinterfacialstrengthsignificantly.Higherprocessing temperaturetendstoinitiateinterfacialreactionsbetweenthe coppermatrixand theceramicparticle.Thereaction prod-uctsusuallysurroundtheceramicparticleand weakenthe interfacialstrength.Frageetal.[33]detectedreaction prod-uctsaroundB4CparticlesintheCu/B4Ccompositefabricated usingtheliquidmetallurgyroute.Absenceofreaction prod-uctsshowsthatthetemperatureriseduringFSPisinsufficient totriggeranyinterfacialreaction.

Fig.12showstheEDSmapsofCMCsreinforcedwithvarious ceramicparticles.Theelementdistributionofcoppermatrix andreinforcementsareclearlyvisible.Thereinforcement ele-mentsaredispersedalloverthecoppermatrix.Thisconfirms propermixing of ceramicreinforcementparticles withthe copper.Further,thereinforcementelementsareevenly dis-tributedandthereisnopresenceofelementrichzones.The

variationinelementdistributioninthefigureisminimum. Thisvalidateshomogeneousdistributionandtheabsenceof anyinterfacialreaction.

3.3. MicrohardnessofCMCs

Table1displaysthemirohardnessofCMCs reinforcedwith various ceramicparticles. Themicrohardessofas received copper was 70Hv. The reinforcement of ceramic parti-clesincreases the microhardessabove110Hv. Cu/B4CCMC recorded a maximummicrohardnessof135Hv. Therise in themicrohardessofCMCs indicatesthattheceramic parti-clescontributedremarkablytothestrengtheningofthecopper matrix.Thehindrancetodislocationmovementbyhigher dis-locationdensityenhanceshardness.Thestrengtheningcanbe attributedtothefollowingfactors.Thehardnessofceramic particles isextremely highertothat ofcoppermatrix.The

Fig.9–OpticalphotomicrographofCu/TiCCMCobservedatvariouslocations.

dispersionofceramicparticles asahard phaseinthe cop-permatrixresultsinstrengthening.AccordingtoHall–Petch relationship,thegrainsizeinfluencesthemechanical proper-tiesofmetallicmaterials.ThegrainsizeofCMCsissmallerto thatofthecoppermatrixduetograinrefinementofceramic particle.Thefine grainsimprove thehardness.Thirdly,the differenceinthermalcontractionbetweenthecoppermatrix andtheceramicparticlesproducesquenchhardeningeffect. Further,thehomogeneousdistributionofceramicparticlesall overthecoppermatrixinvokesOrowanstrengthening.

ThehardnessofCu/B4CCMCisfoundtobehigheramong theother CMCs produced.But thevariationofhardnessof variousCMCsislessthan20Hv.Themechanicalproperties ofCMCs are generally influenced by the type, size,shape, volumefractionandspatialdistributionofceramicparticles

[21,24,42]. All typesofceramicparticles exhibited homoge-neousdistributioninthecoppermatrixforachosenconstant volume fraction. Absence of clusters and porosity ensures minimum hardness variation. The shape variation can be considerednegligible.Allceramicparticlesarecharacterized by irregular polygonal shape. None ofthe ceramic particle hasasphericalshapetoinducesignificantshapevariation. Al2O3particle issmallerinsizecomparedtoother ceramic particles studiedin thiswork.However,it should betaken into accountthatotherceramicparticles(SiC,B4CandTiC) encountered fragmentation during FSP, which lead to the generationofevenlydistributedfineparticles.Thehardness of B4C is higher compared to SiC, Al2O3 and TiC, which could be the possiblecause forhigher hardnessofCu/B4C CMC.

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Fig.10–FESEMmicrographCMCsreinforcedwith:(a)SiC,(b)Al2O3,(c)B4Cand(d)TiC.

Fig.12–EDSmapsofCMCsreinforcedwith:(a)SiC,(b)Al2O3,(c)B4Cand(d)TiC.

3.4. SlidingwearbehaviorofCMCs

Table1presentsthewearrateofCMCsreinforcedwith var-ious ceramic particles. The wear rate of copper matrix is 385×10−5mm3_{/m. The incorporation of ceramic particles} reducesthewearratebelow250×10−5mm3_/m.Cu/B

4CCMC exhibitedalowestwearrateof213×10−5mm3_{/m.Theresults} indicatethat theadditionofceramicparticlesconsiderably improvedthewearresistanceoftheCMCs.Thereductionin wearrate canbeexplained asfollows.Thevolume lossof materialduetoslidingwearisgivenbythefollowing expres-sion[43]:

Volumeloss=Wearcoefficient×Appliedload×Slidingdistance Hardnessofmaterial (1)

Accordingtothisexpression,volumelossisinversely pro-portional to the hardness of the sliding material. Higher the hardnessof the material, lower will be the wear rate. The enhancement of hardness due to fine distribution of ceramicparticlesandgrainrefinementistheprimarycausefor enhancementofwearresistance.Thecontactareabetween the CMCand thecounterdisk isreducedincomparisonto unreinforced copper due tothe presence ofceramic parti-cleswhichbeartheappliednormalload.Thegoodinterfacial bondingbetweenthecoppermatrixandtheceramicparticle retardsthedetachmentofceramicparticlefromthecopper matrix during sliding. The aforementioned factors lead to higherwearresistanceofCMCs.

Fig.13displaysthewornsurfaceofCMCsreinforcedwith various ceramicparticles.Thewornsurfaceofpurecopper

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Fig.13–FESEMmicrographofwornsurfaceof:(a)purecopperandCMCsreinforcedwith:(b)SiC,(c)Al2O3,(d)B4Cand(e)

TiC.

inFig.13ashows largeamountofplastic deformationand deepcraters.Theseareprominentcharacteristicsofadhesive wear.Intheabsenceofceramicparticles,thecopperpin sur-faceisindirectcontactwiththecounterface.Theasperities ofthe copper pin and the counter disk are deformed dur-ingslidingwearandsolidstatejointiscreated.Thiscauses slidingdifficultincreasingwearloss.Thecyclicdeformation hardens the area nearthe adhesion region leading to the generationofmicrocracksandcavities.Thesemicrocracks coalescenceandeventuallydetachthesurfacelayer intensi-fyingtheadhesivewear.Hence,adhesivewearisthereason

forhighwearlossofpurecopper.Theworn surfaceofthe CMCs inFig. 13b–epresentsroughlyauniformflatsurface. ThevariationinwornsurfacemorphologyofvariousCMCsis insignificant.Littleplasticdeformationisevidentontheworn surface.Theceramicparticlesrestrictthefreeflowofsoftened copperduetofrictionduringslidingwear.Thewornsurfaces are coveredwithnumerousweardebris.Thewearmodeis changedtoabrasivewear.

Fig.14displaystheweardebrisofCMCsreinforcedwith variousceramicparticles.Theweardebrisofpurecopperin Fig.14ashowslargesizedebrisaswellassmalldebris.The

Fig.14–FESEMmicrographofweardebrisof:(a)purecopperandCMCsreinforcedwith:(b)SiC,(c)Al2O3,(d)B4Cand(e)TiC.

sizeandmorphologyofthedebrisaredirectlyinfluencedby thewearmechanism.Largesizedebrisconfirmsthatthewear mechanismofpurecopperisprominentlyadhesion.Theworn surfaceoftheCMCs inFig. 13b–epresents finesize spheri-calshapeparticles.Thissuggeststhattheadditionofceramic particlesinfluencedthewearmechanism.Thedifferencein weardebrissizeandmorphologyofvariousCMCsistrivial. Thegenerationoffinedebrisduringthewearprocesscanbe attributedtothe followingcauses;(a)the increasein hard-nessoftheCMCs comparedtocopper and (b)thereduced

probability ofthe slidingpin and thecounterfacedisk due tothe incorporationofceramicparticles.Theceramic par-ticles aredetachedfrom the coppermatrix asslidingwear progresses.Thesedetachedparticlesaltertwobodyabrasion wearintothreebodyabrasionwear.Thelatterproducesfine sizeweardebris.Thedebrisparticleslyingontheweartrack arerepeatedlysubjectedtocrushingactionbetweenthe slid-ingpinandthecountersurfaceduetotheappliedload.The wearprocessbecomesanalogoustohighenergyballmilling resultinginfinerdebris.

j mater res technol.2016;5(4):302–316

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4.

Conclusion

Cu/X(X=SiC,Al2O3,B4CandTiC)CMCsweresuccessfully syn-thesizedusingFSP.Themicrostructure,microhardnessand slidingwearbehaviorwereevaluated.Thefollowing conclu-sionsarederivedfromthepresentwork.

• Thevariationinthestirzone,grainsize,microhardnessand wearratewaswithinashortrange.Nevertheless,Cu/B4C CMCshowedsuperiorhardnessandwearresistance com-paredtootherCMCsproducedinthisworkunderthesame setofexperimentalconditions.

• ThedistributionoftheceramicparticlesintheCMCswas independentofthelocationinthestirzone.Thedistribution washomogeneousandunaffectedbythetypeofceramic particleused.

• SiC, B4C and TiC particles encountered fragmentation during FSP due to severe plastic deformation and inter-actionwiththerotatingtool.Thefragmenteddebrisalso mixed well with the plasticized copper and distributed homogeneously inthe CMC.Al2O3particle didnotsuffer fragmentationduetoitssmallersize.

• Therewasnointerfacialreactionbetweencopperandany typeofceramicparticleandgoodinterfacialbondingwas observed.

• Alltypesofceramicparticlesenhancedthewearresistance ofpurecopperandaffectedthewearmechanism.Thewear modechangedfromadhesiontoabrasion.Theplastic defor-mationonthewornsurfacedisappearedwiththeaddition of ceramic particles. The wear debris became finer in size.

• FSPisasuitableprocessingmethodtoproduceCMCs rein-forcedwithanykindofceramicparticleswithacceptable properties.

Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

Acknowledgement

TheauthorsaregratefultotheManagementandDepartment ofMechanicalEngineering,CoimbatoreInstituteof Technol-ogy,Coimbatore,Indiaforextendingthefacilitiestocarryout thisinvestigation.TheauthorsarealsothankfultoDr.S.J.Vijay andMr.I.Devamanoharanfortheirassistance.

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