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INTERNATIONALJOURNALOFPRECISIONENGINEERINGANDMANUFACTURINGVol.12,No.6,pp.941947DECEMBER2011/941DOI10.1007/s1254101101264NOMENCLATUREFzVerticalforceonagritFzgForceonasinglegritFzsTotalverticalforceactiononsampleσyYieldstressRGritradiushCutdepthforasingleparticleVxWirespeedVzFeedspeedTWiretensionApProjectedareaofthecuttrenchLoCutlengthofsampleLgDistancebetweencuttingparticlesDWidthofcuttrenchSSlidingdistancecMediancracklengthψHalfoftheincludedangleofthegritsEModulusofelasticityofingotHHardnessoftheingotPIndentationforceKcFracturetoughnessoftheingotwDistributedwireloadonthesampleNNumberofcuttingparticlesinthecutlengthαWirebowangle1.IntroductionSiliconwafersusedinthesolarcellandmicroelectronicsindustriescanbecutfromsiliconcrystalsusinginnerdiameterIDsaworwiresaw.WiresawhasadvantagesoverIDsaw.Theseadvantagesarehigherproductivity,lesswafersurfacedamage,andlowerkerfloss.1Moreover,thediameterofwaferthatcanbeslicedbyawiresawishigherthanthatobtainablebyanIDsaw.Wiresawsareusedtocutsapphire,siliconcarbide,lithiumniobate,wood,rock,andalmostallkindsofceramics,includingfoamceramics.13Moller4statedthatthewiresawprocessisresponsiblefor30RoughnessDamageEvolutionDuetoWireSawProcessEgemenTeomete1,1Dept.CivilEngineering,DokuzEylulUniversity,KaynaklarCampusBuca,Izmir,Turkey,35160CorrespondingAuthor/Emaileteometegmail.com,TEL902324127060,FAX902324531192KEYWORDSCeramic,Damagemodel,Ductileregimemachining,Roughness,WiresawThewiresawprocessiswidelyusedforsiliconwaferproductionwithhighyieldandlowsurfacedamageinsolarcellandmicroelectronicsindustries.Thewiresawprocessisusedtomachinebrittlematerialsintheductileregimewherehighyieldandlowsurfacedamagearedesired.Thewiresawprocessisalsousedtocutconcreteandrocksincivilengineering.Inthisstudy,anexperimentalparametricstudywasconductedbyvaryingprocessparameterstodeterminesurfaceroughnessdamage.Ductileregimematerialremovalbytransgranularfailureandbrittlefracturebyintergranularfailureareobservedinelectronmicrographsofthecutsurfaces.Adamagemodelthatrelatestheroughnessdamagetoprocessparameterswasderived.Thedamagemodelpredictstheroughnessdamagesatisfactorily.Themodelshowsthattheroughnessdamageisproportionaltotheratiooffeedspeedtowirespeed.Improvementintheefficiencyoftheprocesswithoutincreasingtheroughnessdamagecanbeattainedbyincreasingthefeedspeedproportionallytowirespeed.Wiretensiondoesnotaffectroughnessdamage.Roughnessdamage,however,isaffectedbypropertiesofthewire.Wireshavingsmallergritradiusandsmallgritspacingcauselessroughnessdamage.ManuscriptreceivedMay4,2010/AcceptedMay15,2011©KSPEandSpringer2011942/DECEMBER2011INTERNATIONALJOURNALOFPRECISIONENGINEERINGANDMANUFACTURINGVol.12,No.6ofthetotalsiliconwaferproductioncost,whichdirectlyaffectsindustry.Thereisaneedtooptimizetheprocessbydevelopingmodelsrelatingprocessparameterstoproductqualityandprocessefficiencymeasures.4Earlywiresawprocessesforwaferproductiondevelopedinthe1990sconsistedofabaresteelwireandabrasivecarryingslurry,resultinginfreeabrasivemachiningusingelastohydrodynamicforces.5,6TheabrasiveparticlescanbeSiCordiamond.Themeangritsizeofabrasiveparticlescanbe5to30µmwitha30to60volumefractionintheslurry.Averagewirediameteris180µm,leadingtoakerflossof200to250µm.Theslurrycanbewaterbasedoroilbased.Oilbasedslurrycausesthewaferstosticktoeachother,anditishardtoseparatethem,whileremovaloftheoilfromthewafersurfaceisanotherproblem.Disposaloftheoilbasedslurryafteruseisalsoaproblem.Hydrogengasproducedfromtheinteractionofwaterbasedslurryandsiliconmaycauseexplosions.However,fromanenvironmentalpointofview,consideringthehighamountofslurrydisposedofduringtheprocess,waterbasedslurriesaregenerallypreferred.4Clarketal.5statedthatinordertoincreasetheproductivityandtobeabletocutharderceramics,diamondimpregnatedwire,whichleadstofixedabrasivemachining,wasdeveloped.Inwiresawingwithfreeabrasives,wirespeedisbetween5to15m/sandwiretensionis20to30N.Thefeedintotheingotresultsinawirebowsothatthewiremakes2oto6owiththehorizontal.6Inthefixedabrasivemachiningwiresawprocess,thewirespeedislowerasmaterialremovalisnotoccurringbyhydrodynamicaction.Inmultiwiretechnology,asinglewireiswindedtoatensioncontrolunitandseveralguidepulleys,whicharegroovedwithconstantpitch.Fivetosevenhundredparallelwiresruntogetherandarecollectedatatakeupspool.Theingotisslicedintohundredsofwafersasitisfedintothewireweb.Thewafersinsolarcellindustryarecutbyrunningthewireinonlyonedirectionatahighspeedbetween5to20m/s,whilethewafersinthemicroelectronicsindustryarecutbyrunningthewireinbothdirectionswithalowerspeedoscillatingthewirefromonespooltoanother.4Researchonthewiresawprocesshasbeenongoinginthreemainareasmaterialremovalmechanisms,kinematicsofwires,andparametricstudiesbetweentheprocessinputsandoutputs.Lietal.7presentedthestressesunderanabrasiveparticle,whichisrollingandindentinginawiresawprocess.MaterialremovalmechanismsforfreeabrasivemachiningweredevelopedusingfracturemechanicsandhydrodynamicbehaviorofslurrybyMoller.4ThematerialremovalrateisdefinedasafunctionofpowersuppliedtotheabrasivebyhydrodynamiceffectandthehydrodynamicfilmpropertiesarecalculatedusingthefiniteelementmethodwhichcouplesReynoldsequationofhydrodynamicswiththeelasticityequationofwire.6Liuetal.8statedthatthematerialremovalmechanismofbeadimpregnatedwiresawcuttingofrockisaHertziantypefractureinwhichthefractureoccursduetothetensilefieldbehindtheslidingbead.WeiandKao9workedonstiffnessanalysesofstraightandbowedwiresundertension.Vibrationcharacteristicsofwirewithrespecttowirespeed,tension,andslurryviscositywasinvestigated.Theincreaseofwiretensionandslurryviscositydecreasesvibrationamplitudeandkerfloss,whilethewirespeedhasalmostnoaffectwhenitisbelow25m/s.1,10Processmonitoringofthewiresawforforces,wirespeed,feedrate,wirebow,andwiretensionwasdevelopedbyClarketal..5Parametricstudiesrelatingprocessparameterstoforces,andsurfaceroughnessandwirewearforcuttingfoamceramicsandwoodwereconductedbyClarketal..2Hardinetal.11conductedaparametricstudyforslicingsinglecrystalSiCwithafixedabrasivediamondwire,relatingwirespeed,rockingfrequency,anddownfeedratewithsurfaceandsubsurfacedamage.ClosedloopdiamondimpregnatedwiresawcuttingofAl2O3andTiCceramicswasstudiedbyMengetal.12HardnessanisotropyofLithiumNiobatewafershasbeeninvestigatedusingnanoindentation.13BhagavatandKao14determinedthedirectionofapproachforthreemostcommonlyslicedorientationsofsiliconconsideringcrystalanisotropy.Damageevolutionduetowiresawingofsiliconwafersisofsignificantinterestasthephotovoltaicandsemiconductorindustrieshavestricttolerancesforsurfacequality.Theprocessinduceddamageonbrittlematerialscanbemodeledstartingwithexistingdamagemodelsofindentationofbrittlematerials.Thereexistseveralmodelsforthefailuremechanismsinbrittlematerialsduetoindentation.1520Ryuetal.studiedindentationonsiliconwafer,glassandsiliconcarbide.21Zhaoetal.observedtheindentationdamagemodesongroundsurfaceofopticalglass.22Ductileregimegrindingofbrittlematerialshasbeeninvestigatedexperimentallybydifferentresearchers.2328Bifanoetal.24statedthatwhenthefeedisdecreasedbelowacertainamountingrinding,atransitionofwearmechanismfrombrittletoductilemodecanbeachieved.Inthisstudy,adamagemodelforwiresawprocessinducedroughnessdamageisdeveloped.Thedamagemodelisbasedonductilemodematerialremovalandbrittlemodedamage,asobservedinSEMimagesofcutsurfaces.Thedamagemodelpredictstheexperimentallymeasureddamagesuccessfully.Theexperimentalworkispresentedinsection2.Themodelispresentedinsection3.Theresultsanddiscussionofthestudyarepresentedinsection4.Theconclusionsarepresentedinsection5.2.ExperimentalProcessWiresawexperimentswereconductedonaluminaceramic.Thewirebowangle,wireaxialspeed,Vxandfeedrate,Vzweremeasuredduringthewiresawcuttingtests.Thesurfaceroughnessofcutsurfaceswasalsomeasured.TheSEMimagingofcutsurfaceswasobtained.Theequipmentusedinthesemeasurementsandtheprocessparametersarepresentedinthissection.2.1WireSawCuttingandWireBowAngleMeasurementAwiresawmachineMillenniummodelproducedbyDiamondWireTechnologyinColorado,Springswasusedintheexperiments.Thisspooltospoolwiresawmachinewithrockingmotionofthewirecanbecontrolledbythewirespeed,Vx,downINTERNATIONALJOURNALOFPRECISIONENGINEERINGANDMANUFACTURINGVol.12,No.6DECEMBER2011/943feedspeed,Vz,andwiretension,T.Thetensionwascontrolledbywiretensionpulleyspoweredbyairpressure,whiletherockingmotionwascontrolledbywireguidepulleysascanbeseeninFig.1.Thecutlengthofthewirewas300ft91.4m.Thus,ateverydirectionreversal,300ftofwirewastransferredfromonespooltotheother.AcoolantconsistsofwatertolubricantSawzitProductofSyntheticLubricants,Inc.ratioof50/1wasusedduringcuttingtests.Fourdifferentdiamondgritcoatedsteelwireswereusedinthewiresawexperiments.TheaveragehalfincludedangleofthegritsonDWS2wasψ71o.ThediamondgritcoatedsteelwireDWS3wasaproductofWellDiamondWireSawsInc.DiamondgritcoatedsteelwiresDWS4andDWS5wereproductsofSaintGobainAbrasivesInc.TheDWS4andDWS5weremanufacturedbynickelelectroplatingonsteel.Thegritswereaffixedintotheelectroplatednickellayer,whilethecoreremainsintact.Aluminaceramicsampleshavingtensilestrengthofσfr300MPa,fracturetoughnessKIC4MPam1/2,YoungsmodulusofE370GPa,29andhardnessofH22GPa20wereusedinthecuttingtests.ThecutlengthofthesampleswasbetweenLo1520mmandtheheightwasHs7.1mm.AgroupoftestsweredonewithDWS2withthewirespeedvariedoverVx1.3,1.8,2.95,3.5m/s,thewiretensionvariedoverT13.3,17.8,22.4,26.7N,andthedownfeedvariedoverVz5,6.35,10.16µm/sec.Inordertoexploretheeffectofdifferentwirescharacteristicsonsurfacequality,twelvetestsweredonewithprocessparametersVx1.35,2,3,4m/s,Vz6.35µm/sec,andT13.3NusingthewiresDWS3,DWS4,andDWS5fourtestswereconductedwitheachwire.AmegapixeldigitalcameraKodakEasyShareDX7630of28562142pixelswasusedtomeasurethewirebowangleseeninFig.2.TheimagesofthewireandsamplewerecollectedduringthetestandanalyzedusingMatlabMathworkstoobtaintheangleαbetweenthewireandthehorizontal.Theaverageofthesteadystatewirebowangles,α,wasattainedtothetestasthesteadystatewirebowangleofthattest.2.2SurfaceRoughnessMeasurementsandSEMImagingThesurfaceroughnessofthecutsurfacesweremeasuredbyusinganopticalnoncontactprofilometer,ZygoNewView6000,manufacturedbyZygoCorporation.A10xlenswasusedforthemeasurements.Theprofilometerhadaverticalresolutionontheorderof3nanometertheresolutioninthehorizontalplanewas1.1µm,whilethefieldofviewusedwas0.70.53mm.Inastitchmeasurement,theprofilometertakescontinuousmeasurementseach0.70.53mmandstitchesthemtogetherintoonedataset.Threestitchmeasurements,eachof0.73mmdimensions,wereappliedinthedirectionofcuttingforeachsampleontheleftmiddlerightofthecutsurface.Afterthemeasurementsweretaken,thedatawasprocessedusingthesoftwareMetroProVersion8.1.5developedbyZygoCo.Ahighpassfilteringwasappliedtoremovethesurfacewaviness.Arithmeticaveragedeviationfromthecenterlinebestfitplanewasobtained.TheaverageofthreemeasurementswastakenassurfaceroughnessRaofthetest.AScanningElectronMicroscopeSEM,JEOLJSM606LV,Fig.1Singlewire,spooltospoolwiresawmachine.Thewiretrackismarkedbythedashedline.DWTInc.,MillenniumModel,Colorado,Springs,USAFig.2WirebowangleinwiresawtestsFig.3SEMimageofawiresawcutsurfaceofaluminaceramicVx1.3m/sec,Vz5µm/sec,T13N944/DECEMBER2011INTERNATIONALJOURNALOFPRECISIONENGINEERINGANDMANUFACTURINGVol.12,No.6wasusedtoimagethecutsurfacetopology.TheSEMimagesweretakenfromthelowerhalfofthesample,onthecenterlineofthecutsurface.Itisseenfromtheimagesthatthematerialremovalmechanismisthetransgranularfailure.Intergranularfailure,inwhichgrainboundaryfailureresultsingraindislodgementinabrittlemode,isalsoobserved.BothmechanismscanbeseeninFig.3.3.RoughnessModelDerivationDuctilematerialremovalandbrittlefractureisobservedinSEMimages.TheproposedmodelisshowninFig.4.ThematerialremovaloccursinaductilemodeasseeninSEMimages,whilethedamageoccursduetomediancrackingasinFig.4.AsdiscussedbyEvansandMarshall,15removalofplasticallydeformedmaterialinthecuttingzonereducesresidualstress.Thisreducesthetendencyoflateralcrackformationinbrittlematerials.Fuetal.30derivedtheforceonasinglegritinductilemodematerialremovalaspresentedinEq.1,whereσyisyieldstress,Riscuttingparticleradius,andhiscutdepthforasingleparticle.zzgyFFRhπσ1ThemasscontinuityofthecuttingprocessgivesusEq.2.ogzxpoggLDhsLdVolddShhVVdtAdtLDdtLL2Volumeisthetotalamountofmaterialremoved,Apistheprojectedareaofthecuttrench,Loisthecutlengthofsample,Lgisthedistancebetweencuttingparticles,Diswidthofcuttrenchthatcanbetakenasdiameterofwire,Sisslidingdistance,Vxistheaxialspeedofwire,andVzisthefeedofwire.Theforceonasinglegrit,Fzg,canbeobtainedintermsofprocessparametersbyusingEq.1andEq.2.zzgygxVFRLVπσ3Thedamageresultingfromwiresawcuttingiscorrelatedwithmediancrackdepth.Lawnetal.16derivedthemediancracklengthusingfracturemechanicsprinciples.ThemediancracklengthispresentedinEq.4.Lawnetal.16calibratedtheindentationcoefficients0.032and0.017inEq.4usingindentationdataofsodalimeglassandnotedthattheyareapplicabletoallbrittlematerials.2132230.0320.017cotcEPcHKψ4InsertingEq.3inplaceofPFzginEq.4givesusEq.5.221323230.0320.017cotygzcxRLEVcHKVπσψ5Fig.4WiresawroughnessdamagemodelductilematerialremovalandbrittlefractureThedamageduetothewiresawprocessispresentedintermsoftheprocessparametersinEq.5.Thedamageisafunctionofthehalfoftheincludedangleofthegrits,ψthemodulusofelasticityofingot,Ethehardnessoftheingot,Hthefracturetoughnessoftheingot,Kcandwireproperties,feedspeed,andwirespeed.4.ResultsandDiscussionDecreasingfeedrateingrindingbelowathresholdyieldsductileregimegrindingofbrittlematerials.2328Inductileregimemachiningofbrittlematerials,thematerialremovaltakesplacewithplasticdeformationofthegrains.23,24,2628,31Whilethematerialremovalisinductilemode,brittlefractureisstillobservedinductileregimegrinding.24,28ThematerialremovalanddamageformationinthewiresawprocessisanalogoustoductileregimegrindingasseenfromSEMimagesofwiresawprocessedsurfaces.Adamagemodelisderivedforroughnessdamageinducedbywiresawprocess.ThemodeliscomparedtoexperimentaldatainFig.5.Themodelhasagoodperformanceinpredictingroughnessdamageduetothewiresawprocess.ThedamagemodelstatesthatifthefeedspeedtowirespeedratioVz/Vxisincreased,theroughnessdamagewillincrease,whileifthisratioiskeptconstant,roughnessdamagewillbeconstant.ThetwoexperimentsmarkedinFig.5havedifferentfeedspeedsandwirespeedsbutaverycloseVz/Vxratio,andtheirroughnessesarealsoveryclosetoeachother.Inawiresawprocess,ifefficiencyshouldbeincreasedbyincreasingthefeedspeed,inordertokeepthelevelofdamageconstant,thewirespeedshouldbeincreasedproportionallytothefeedspeed.Inordertoexplaintheeffectofwiretensiononroughnessdamage,thechangeofforceswithwiretensionshouldbeconsidered.ThetotalforceanddistributedforceactingonthesamplebythewireduetowirebowandtensionispresentedinEq.6andEq.7,respectively.Thetotalforce,Fzs,isdistributedonthecuttinggritsascuttingforcespergrit,Fzg,byEq.8.TheNLo/Lgisthenumberofcuttingparticlesinthecutlength,Lo,and,
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