The ultra-precision -lap grinding U of flat advanced ceramics Journal of Materials Processing Technology

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Journal of Materials Processing Technology

j ou rn a l h o m epa ge :w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

The ultra-precision U _d -lap grinding of flat advanced ceramics

Arthur Alves Fiocchi

, Luiz Eduardo de Angelo Sanchez

, Paulo Noronha Lisboa-Filho

, Carlos Alberto Fortulan

aUniversityofSãoPaulo—USP,DepartmentofMechanicalEngineering,Av.TrabalhadorSaocarlense400,13566-590SãoCarlos,SP,Brazil

bSãoPauloStateUniversity—Unesp,DepartmentofMechanicalEngineering,Av.LuizEdmundoCarrijoCoube,14-01,17033-360,Bauru,SP,Brazil

cSãoPauloStateUniversity—Unesp,DepartmentofPhysics,Av.LuizEdmundoCarrijoCoube14-01,17033-360,Bauru,SP,Brazil

a r t i c l e i n f o

Articlehistory:

Received10April2015

Receivedinrevisedform1October2015 Accepted4October2015

Availableonline23October2015

Keywords:

Ud-lapgrinding Advancedceramics Ductilegrinding Surfacecharacterization Nanotechnology

a b s t r a c t

ThepresentstudyfocusesontheUd-lapgrindingprocessanditsmachine-tooldesignaimingatultra- precision(UP)manufacturingofadvancedceramics.Impactsofthreedifferentoverlappingfactorsona dressing(Ud)andthreeabrasivegritsizesofconventionalSiCgrindingwheelswereanalyzedonflatnano- metricsurfacefinishingofdensediscsof3Y-TZPinaductileregimeofmaterialremoval.Microhardness, contactandopticalprofilometry,SEM-FEG,Ramanspectroscopy,XRD,andconfocalepi-fluorescence wereappliedtocharacterizetheceramicmaterial.Mechanicalandelectric-electronicdesignsofthe machine-toolweredevelopedtowardtheUPceramicgrinding.Thedesignmethodologywassuccessful forsupportingtheachievementofalldesignrequirementsoftheCNCFiocchiLapGrinder.Themachine- toolandtheUd-lapgrindingprocesswerecapableofmanufacturingflat3Y-TZPsurfaceswithnanometric finishingwithoutintroducingcriticaldefects.Thebestfinishing,Ra=60.63nm,camefromthe#300grind- ingwheeldressedwithUd=5.Aflatnessdeviationof0.308␮mwasobtainedthroughthe#800grinding wheelandUd=3.Differencesbetweentheoreticalandrealmacroandmicroeffectsoverthegrinding wheelsaftersingle-pointdiamonddressing,epoxybondstrength,abrasiveprotrusion,abrasivegritsize andabrasivefriabilityplayakeyroleintheUd-lapgrinding.Thereisnoreportofanabrasiveprocess capableofachievingsimilarnanometricfinishingwithoutintroducingcriticaldefectswiththesame micrometricgritsizeandtypeofabrasive.TheUd-lapgrindingcanreplacetheengagementofprocesses suchasgrinding,lapping,andpolishingofadvancedceramics.

©2016PublishedbyElsevierB.V.

1. Introduction

Compared with metals, advanced ceramics are materials of higherhardnessandspecialpropertiesthatpresentperformance, reliability, and safety at elevated temperatures; theseare out- comesoftheirexceptionalchemicalstabilityduetostrongionic and/or covalent atomic bonds (Boch and Nièpce, 2007). How- ever,advancedceramicssufferfromtheirrelativelylowfracture toughness,whichalsosignificantlyaffectstheircostasaresultof post-sinteringprocessingnormallydonebyexpensivesuperabra- sivetoolsandmanufacturingprocesses(Janssenetal.,2008).

Exceptforglass,plasticmoldingandcastingofadvancedceram- icsare exceptionallyhighcost, mainlybecauseofthe chemical bondsthatraisethemeltingpoint.Brittlenessisalsoasignificant

∗Correspondingauthor.

E-mailaddresses:arthuraf@eesc.usp.br

(A.A.Fiocchi),sanchez@feb.unesp.br(L.E.deAngeloSanchez),plisboa@fc.unesp.br (P.N.Lisboa-Filho),cfortula@sc.usp.br(C.A.Fortulan).

characteristicoftheseceramicsbelow1000^◦C.Inadditiontolim- itingtheiruse,thebrittlenessimposesrestrictionsonproduction techniques.Alltheselimitationsofceramicmanufacturingexplain whythebasictechniqueofdensificationisthesinteringofcom- pactedpowder(BochandNièpce,2007).Thisoppositionisquite evidentbetweenmetals(ductile)andceramics(fragile).

Asceramicsaremorebrittle,themainmechanicalfailureusu- allyoccursbycrackpropagation(ArgawalandRao,2008).Initial criticalcracks,precursorsofthefailure,aredefectsusuallyorigi- natinginirregularitiesacquiredintheearlystagesofobtainingthe piece(frompowdertosintering)(Bukvicetal.,2012)and/orduring itspost-sinteringprocessing(hardceramicmachining)(Marinescu etal.,2014),hencethedesignofaproductofadvancedceramic needsspecialcareineachphaseofitsmanufacturing.

Ingenerallines,intheproductionofaceramicartifact,theraw materials (powders) are milledor synthesized, are mixed, and receiveadditivessuchasbindersandplasticizerstoassistthecon- formation,compaction,andsintering(Scheller,1994).Theceramic powderisformedintocompactedpieces,alsocalledgreenpartsdue totheirnotheattreatedstate,whicharesubsequentlysinteredto http://dx.doi.org/10.1016/j.jmatprotec.2015.10.003

0924-0136/©2016PublishedbyElsevierB.V.

E Elasticmodulus(GPa)

EESC SãoCarlosSchoolofEngineering ELID Electrolytic-inprocessdressing FAPESP SãoPauloResearchFoundation FEB BauruCollegeofEngineering HFW Horizontalfieldwidth(mm) HV Hardnessvickers

HV Highvoltage(kV)

IFSC SãoCarlosInstituteofPhysics

k Parameter formonoclinicphasequantificationby Ramanspectroscopy

KIC Fracturetoughness(MPa.m^0.5) LATUS LaboratoryofMachiningTechnology ln Totalmeasuredlength(mm)

LTC LaboratoryofTribologyandComposite l_r Singlemeasuredlength(mm)

m Monoclinicphase

MEMS Micro-electromechanicalsystems m_f Finalmass(g)

m_i Initialmass(g)

MQF Minimumquantityfluid

MRR Materialremovalrate(g/minute) NEMS Nano-electromechanicalsystems ng Grindingwheelrotationalspeed(rpm) nw Workpiecerotationalspeed(rpm) P U_d-lapgrindingpressure(kPa) PABA 4-Aminobenzoicacid

PVB Polyvinyl-butyral PVD Physicalvapordeposition

Ra Arithmeticmeansurfaceroughness(␮m) RMS Rootmeansquare

r_p Dressernoseradiusgeometry(mm) Rt Totalheightoftheroughnessprofile(␮m) r Stylustipradius(␮m)

Sa Arithmeticmeanheightoverthecomplete3Dsur- face(␮m)

S_d Dressingfeed

SE Secondaryelectrondetection

SEM-FEG Scanningelectronmicroscopeequippedwithfield emissiongun

SiC Siliconcarbide

Sku Kurtosisofthe3Dsurfacetexture

Sp Maxpeakheightoverthecomplete3Dsurface(␮m) Sq Rootmeansquareheightevaluatedoverthecom-

plete3Dsurface(␮m)

Ssk Skewnessofthe3Dsurfacetexture

Sv Maximumvalleydepthoverthecomplete3Dsur- face(␮m)

Sz Maximum height over the complete 3D surface (␮m)

T Tetragonalphase TZ-3Y-E Tosohzirconiagrade t→m Martensitictransformation

U_d Overlapfactororoverlapratioondressing UNESP UniversidadeEstadualPaulista

W Realwaviness(␮m)

Y-TZP Tetragonal zirconia polycrystals stabilized with yttria

ZrO₂ Zirconiumoxideorzirconia

3Y-TZP Tetragonalzirconiapolycrystals stabilizedwith 3 mol%ofyttria

␣ Singleabrasivereliefangle(^◦)

␤ Singleabrasivewedgeangle(^◦)

␥ Singleabrasiverakeangle(^◦)

␦ Parameterformonoclinicphasequantification by Ramanspectroscopy

␪ Diffractionangle(^◦)

␭c Cutoff(mm)

␳ Density(g/cm³)

␴ Bendingstrength(MPa) Ø Diameter(mm)

# Meshnumberofthegrindingwheel(abrasivegrit size)

achieve the desired hardness and density (Richerson, 2006).

Eachstephasthepotentialtoproduceundesirablemicrostructural flaws in the piece, which in itself may limit their properties, reliability, and machinability. Scientific investigations are not exclusivelyfocusedonthematerialitself,butalsoinitsprocessing intopowder,granules,andinthesubsequentstagesofobtaininga partaswellasitsmachining.

For less demanding applications regarding form and finish, thesinteringisthelast stageoftheceramicprocessing.Never- theless, several areas of scienceand technological applications requirebetterfinishing,tighterdimensionalandgeometrictoler- ances,andsurfacescompletelyfreeofdamage(Brinksmeieretal., 2010).Suchpartsmust undergo,consequently,furtherprocess- ing stepsaftersintering, especially toconfer onthem complex geometries and finer finishing. Thedemands for bettersurface finishingandhighaccuracyofoptical,electrical,andmechanical componentsaregrowingalongwiththeminiaturizationofhigh performanceproducts, suchaslenses, electroniccomponentsof computersand smartphones,andsensors,as wellas microand nano-electromechanicalsystems(MEMSandNEMS).

Amongtheavailablemethodsofmaterialremoval,theabrasive machiningisthemostwidespreadinindustry.Grindingreconciles stock removal, form correction, and surface quality for large- scaleproductioninaneconomicallyfeasibleway(Malkin,1989).

Although widespread,hard ceramic machiningis an extremely difficulttaskduetotheexceptionalceramicpropertiesinthesin- teredstate,especially:highhardness,highelasticmodulus,high meltingpoint,andfragility(CallisterandRethwisch,2013).These requiresuperabrasivecuttingtools,stiffmachine-toolsandclamp- ingdevices,microandnanometerdepthsofcut,andahighlyskilled workforce(Brinksmeieretal.,2010).

Intraditionalabrasivetechnology,thebrittlemodeisthepri- mary removal mechanism (Marinescu et al., 2004) caused by thepropagationoflateralcracksthatpromotechipping(Xuand Jahanmir,1995).Higherremovalratescanbeachievedinthebrittle modeinoppositiontoductileabrasivecutting(Xuetal.,1995).On

theotherhand,grindingmayalsogeneratelongitudinalcracksthat canremainintheproductaftergrinding(Marinescuetal.,2007) andact,forexample,asstressconcentratorsthatunderspecific conditionsstimulatesubcriticalcrackpropagation(Devilleetal., 2006).

Themain technologicalchallengeofhardceramicmachining is,thus,topromotematerialremovalwithoutintroducingcritical defectsthatwillcompromisetheperformanceofthecomponent (Marinescuetal.,2014).Therefore,thebrittleremovalmodeshould beavoidedinultra-precision(UP)grindingorminimizedinorder toavoidmainlytheformationandpropagationofsurfaceandsub- surfacecracks,microstructuralchanges,andresidualtensilestress (Brinksmeieretal.,2010).

Tominimizetheemergenceandevolutionofcracks,machining shouldbeconductedwithdiscretion,soastoavoidtheintroduction ofresidualtensilestressonsurfacesofmaterialssubjecttostress- inducedphasetransformation(Devilleetal.,2006).Thesearchfor high-techceramicpartswithfunctionalsurfacesrequires,hence, themostsophisticatedmanufacturingprocesses.

Over the last three decades,researchers have respondedto demandsofindustryformachine-toolsandfabricationprocesses thataimtoattainhigh-precisionpartsinananometer(10⁻⁹m)and angstrom(10⁻¹⁰m)forbothdimensionalandfinishrange(Ohmori, 2011).Nevertheless,thesequalitycharacteristicsareverydifficult, oreconomicallyunviable,toachievebyconcatenatingthetradi- tionalabrasiveprocessessuchasgrinding,lapping,andpolishing.

For finishing flat surfaces, researchers have been putting togetherthemainadvantagesofthetraditionalabrasiveprocesses suchas:facegrindingwithconstantpressure,fixedabrasivesfor a two-bodyremoval mechanism,total contactof thepartwith thetool,and lappingkinematicsaswellas somespecificoper- ationstokeep grindingwheelsharpnessandform.In thisway, differentdressingandconditioningtechniqueshavebeenapplied tothetoolbeforeand/orduringgrindingprocesses.Accordingto Sanchezetal.(2011),thesedifferentdressingproceduresyielddis- tinguishedresultsingrindingperformance.Nanogrinding(Gatzen andMaetzig,1997),facegrindingwithlappingkinematicsorgrind- ingonalappingmachine(TomitaandEda,1996),ELID-lapgrinding (Ohmorietal.,2011),flathoning(BeyerandRavenzwaaij,2005), andU_d-lapgrinding(Fiocchietal.,2015)aresomeexamplesof thesehigh-endgrindingprocessesforfinishingflatsurfaces.

TheBrazilianprocess,namedU_d-lapgrinding,wascreatedto meetthedemandsfora gold-standardabrasiveprocessesasan economicalandaccessibletechniqueforindustry,especiallythe Brazilianone,whichuntilnowemployedonlyforeigntechnologies forUPfinishingofflatadvancedmaterials(Fiocchietal.,2015).

U_d-lapgrindinginvestigationsstartedwithanadaptedlapping machine(Sanchezetal.,2011).FiocchiLapGrinderI(Fiocchi,2010 Fiocchi et al., 2015)came next toestablish the first fully CNC machine-tooltogrindflatmetallicsurfacesusingepoxybonded SiCgrindingwheelsdressedaccordingtotheoverlapfactorthe- ory(U_d)proposedbyKönigandMesser(1980).Thesecondversion (Fiocchi,2014),presentedinthispaper,representsacutting-edge CNCmachine-tooldesign,improvedforgrindingmaterialsthatare notonlyductile,butalsohardanddifficulttomachine.

Products made of tetragonal zirconia polycrystals stabilized withyttria(Y-TZP)haveanoticeableinterestinfunctionalsurfaces thatrequiretop-qualitymechanical(Zhuangetal.,2015),electri- cal(Flegleretal.,2014),andtribologicalproperties(Chevalieretal., 2007).

The3Y-TZPusedundersomespecificconditionspresentsacom- plexphenomenonofsubcriticalcrackgrowthdefined,amongother names, asstresscorrosion (Sikalidis,2011)or low temperature degradation(LTD)(Chevalieret al.,2007).Thismechanismpro- motesthechangeofthecrystallinestructurefromtetragonalto monoclinic(t→m)whentheceramicissubjectedtostressand/or

temperature(KellyandRose,2002),especiallyinthepresenceof water(Yoshimuraetal.,1987).Thisstructuralchangeofamarten- sitic nature generates volumetric expansion of the monoclinic phase(KellyandRose,2002)thatgeneratescracksandstimulates theirpropagation(SchmauderandSchubert,1986),thereforeinflu- encingthemechanismofmaterialremoval.

Thereisconcernthat3Y-TZPproductssufferunwantedphase destabilizationunderundersizedconditions,evenwithoutapply- ingstress(Lietal.,2001).Undercircumstancesofexternalloading duringeitherthemanufactureoruseoftheceramicproduct,this destabilizationcanhappen.Inthiscontext,thetransformationcan betriggeredbytemperaturesandstressesfromthemachiningpro- cess(Devilleetal.,2006).Thepresenceofaqueous-basedcutting fluidcanaccentuatethisphenomenonofphasedestabilization.

Themicrostructuralcontrolcapabilityofthiseventmakesthe 3Y-TZP a key material in the evaluation of UP machiningpro- cesses,allowingitsscientificstudybyqualifyingandquantifying themartensitictransformationinducedbychipremoval.

The present work hasas its hypothesis the possibility that the achievement of flat surfaces with nanometric finishing (Ra<100nm) in advanced ceramics without critical defects is possiblebycombiningappropriatemachine-toolkinematicsand grinding pressure, proper cutting tool sharpening, and specific lubricationandcooling.

Toprovethishypothesis,theFiocchiLapGrinder’sdesignwas evolved,aimingatUP facegrindingwithlappingkinematicsof advancedceramics,meetingtheconditionsofremovalandfinish- inginasingleautomatedprocessandequipment.

The 3Y-TZP was chosen as theworkpieces’ material due to its phase transformation under severe removal conditions, for instance,excessivespecificcuttingpressureandhightemperatures inahumidatmosphere.Grindingwheelsmadeofsiliconcarbide (SiC)gritsandepoxybondwereappliedunderdifferentU_ddressing parametersandwaterasadressingfluid.

2. Experimentalset-up 2.1. Machine-tool

AsecondversionoftheFiocchiLapGrinderwasusedinthis work,namedFiocchiCNC LapGrinderII.Themachine-toolwas conceptualizedanddesignedinapartnershipbetweentheLabo- ratoryofTribologyandComposite(LTC)atEscoladeEngenharia deSãoCarlos(EESC)ofUniversidadedeSãoPaulo(USP),SãoCar- losCampus,andtheLaboratoryofMachiningTechnology(LATUS) atFaculdadedeEngenhariadeBauru(FEB)ofUniversidadeEstad- ualPaulista(UNESP),Bauru campus.Themachine-tool wasput togetherandvalidatedatLATUS.Aswellasbeingappliedtobuild thefirstFiocchi’smachine-toolduringhisMaster’scourse(Fiocchi, 2010),designmethodology(Pahletal.,2007)wasusedtoevolve LapGrinder I,aimingtheultra-precision (UP)manufacturingof advanced ceramicsbyimprovingcriticalaspects pointedout in earlystudiesofSanchezetal.(2011),Fiocchi(2010),andFiocchi etal.(2015).

Fig. 1 compares some of the modifications implemented in FiocchiLapGrinders.ThemainimprovementsfromLapGrinder I(Fig.1a) (Fiocchi, 2010; Fiocchiet al.,2011)toLap GrinderII (Fig.1b)(Fiocchi,2014)were:(1)substitutionoftheflexibleshaft byaservomotortocontrolaworkpiece’sangularvelocity;(2)anew spindlehavingtwotaperedrollerbearingsmountedinaback-to- backconfiguration,50Npre-load,andahollowshaftforsupporting avacuumchuck,which hasa universaljointwithtwo degrees- of-freedomtokeepworkpiece’s surfaceparallel tothegrinding wheel;(3)newsteppingmotorsanddrivestopreventlinearaxis resonating;(4)newphysicalconfigurationofthenovelelectrical

Fig.1.(a)FiocchiLapGrinderI,(b)FiocchiLapGrinderII.

componentsinsidethemainelectricalpanel;and(5)updatedsoft- wareandgrindingstrategytoimprovelinearaxisdynamics.

2.2. Workpiecematerial

Zirconiapowderwith3mol%ofyttria(Y₂O₃)fromTosoh(TZ- 3Y-E)waschosenastherawceramicmaterialduetoitsattractive surfaceandmechanical(strengthandtoughness)propertiesafter sinteringinthepresentstudy.Moreover,tetragonalzirconiapoli- cristalstabilizedwith3mol%ofyttria(3Y-TZP)isakeymaterial for medical and dental implants as well as the electronic and mechanicalindustries.AccordingtoTosoh,aftersintering,3Y-TZP canachievehardnessof1250HV,elasticmodulus(E)of210GPa, bending strength (␴) of 1200MPa, fracture toughness (K_IC) of 5.0MPa.m^0.5,and microstructureendowed byfinegrainsunder 0.3␮maveragediameter.

Thematerial alsopresentsinteresting surfacefinishing apti- tudeaswellasthepossibilitythatphasetransformation canbe investigated in outside layers induced by grinding conditions.

Theoccurrenceandmagnitudeofthemartensitictransformation (tetragonal→monoclinic)can,therefore,beusedasascientifictool forevaluatingthequalityofthematerialremovalprocess.

InUPmanufacturingofadvancedmaterials,allproductionsteps areimportantandhaveinfluencenotonlyonthefinalmechanical

andelectricalpropertiesforexample,butalsoonsurfacefinish- ing.Consideringthatfiredadvancedceramicsarehardmaterials that aredifficulttomachine, andtheirmachiningis expensive, an optimized manufacturing routeshould consider a near-net- shapeproduction,inwhichmaterialremovalprocessesareused forimprovingmainlysurfacefinishing.

For thebest machining results, the material properties and geometry must be well-defined and controlled.In the present research,hence,itwaspre-definedthatworkpieces’manufactur- ingrouteshouldstartwithTZ-3Y-Epowderandaimtoaccomplish sinteredflatceramicdiscswith:(1)quasi-zerodefects,(2)homoge- nous grainsize up to 0.4␮m averagediameter, and (3) dense bodies,sothatU_d-lapgrindingcouldfocuson(4)achievingsurface roughness(Ra)under100nm,removinglittlematerialusingsili- concarbide(SiC)grindingwheels.Fig.2illustratesthemainsteps andtheirparametersformanufacturingandcharacterizationofthe ceramicmaterialfrompowdertoU_d-lapgrounddiscs.

2.2.1. Mixing

The powderwasprepared in a ballmillwitha nylonjar of 100ml internal volume(Carvalhoand Fortulan,2006 Carvalho, 2007)rotating at104rpm,corresponding to65% of thecritical velocity(V_crit=1338.2/D^0.5)for a70-mminnerdiameterjar(D).

30vol%oftheusefulvolumeofthejarwerefilledupwithTZ-3Y-

Fig.2. MainstepsandcharacteristicsofTZ-3Y-Emanufacturingandcharacterization.

E(=6.05g/cm³),with0.5kgofmillingmedia(TosohYTZ10mm diameter),and70vol%oftheusefulvolumefilledupwithisopropyl alcohol(=0.786g/cm³)asaslurrysolvent.4-Aminobenzoicacid (PABA; =1.37g/cm³) was added in as a deflocculant in the ratioof 0.5%of theTZ-3Y-E mass. Then, theball milloperated for12h.Afterdeflocculatingcompleted,Polyvinyl-butyral(PVB- butvarB98;=1.1g/cm³)wasaddedtotheslurryasabinderin theratioof2.0%oftheTZ-3Y-Emass,andthemixingcontinuedfor 2morehoursforhomogenizingthecolloidalsuspension.

2.2.2. Drying

Afterthedeagglomerationandmixingsteps,dryingwasmanu- allydonebyagitatingtheslurryinahotaircurrentinsideaclean chamber.Gloves withouttalcwereusedand extremecarewas takeninordertoavoidanycontamination.

2.2.3. Granulationandselection

Manualgranulationandselectionappliedan80-mesh(180-␮m aperture)stainlesssteelsievewiththepurposeofstandardizingthe agglomeratesizeinpreparationforpressing.Granulationandselec- tionwerealsoperformedinsidethecleanchamberwhilewearing gloves.

2.2.4. Forming

Formingtookplaceintwosteps;13.6gofthezirconiagranules weremanuallyinsertedintothecylindricalmoldwiththehelpofa plasticcontainer,leveledwithastainlesssteelspatula,andgently agitatedbyavibratingtableLabortechnikGmbH(THYR2model), seekinghomogenizationofthefilling.Next,single-actinguniaxial presshappenedat83.7MPaduring10sina37-mmdiametermold toshapediscs with4.72±0.03-mmthickness.Subsequentwet- bagisostaticpressingoccurredinanAIPwet-bagpresser(CP360 model)at200MPafor30stohomogenizethegreenceramicbodies, reducingdensitygradientsandincreasingdensification.

2.2.5. Sintering

SinteringoccurredinanambientatmosphereLindbergfurnace (BlueMmodel)with3.89^◦C/minheatingandcoolingratesand2h at1400^◦C,resultinginceramicdiscsof3.77±0.02-mmthickness and29.8±0.15-mmdiameter.

2.3. Ceramiccharacterization

AccordingtoFig.3,severalcharacterizationtechniqueswere applied in this research. Apparent density and porosity were determined by ASTM C73. TZ-3Y-E powder, granules, and greenandsinteredworkpieces’microstructurewereinvestigated by SEM-FEG. Green, sintered, and U_d-lap ground workpieces wereinvestigated by confocal microscopy. Opticalfluorescence microscopywasappliedtothepowder,greenbody, andU_d-lap groundworkpieces.HardnessVickersweremeasuredinsintered pieces.XRDwasmeasuredinpowder,sinteredandU_d-lapground workpieces.Ramanspectroscopyandprofilometrywereusedwith sinteredandU_d-lapgroundpartsaswellasopticalmicroscopyin grindingwheels’activesurfaceafterdressingandU_d-lapgrinding.

SEMappliedanFEIelectronicmicroscopy(InspectF50model) equippedwitha fieldemission gun(FEG), 10kVelectronaccel- erationvoltage,workingdistancebetween6.7and10.3mm,and secondaryelectrondetection(SE).AboveeachSEMimagethereis alegendwiththemaininformationandconditionsapplied,from thelefttorightside:dateandtime,dwelltime,highvoltage(HV), horizontalfieldwidth(HFW),workingdistance(WD),detection mode,andscale.Thespecimensreceiveda thinlayerofgoldby physicalvapordeposition(PVD).

ANikonmicroscope(modelEclipseTS100)andNISElements softwarewereusedtoanalyzeopticalfluorescence.Powdervisu- alizationapplied10×lensesandsinteredpieceswith4×objective and10×ocularlenses.

XRDwasstudiedthroughaPANalyticalDiffractometer(X’Pert ProMRDmodel)ofCoK␣radiation(1.78Å,40kV,and40mA),con- tinuedscanning,0.02angularresolution,6.667×10⁻³degrees,and 2rangingfrom20^◦to65^◦.Quantificationofthevolumepercentage ofthemonoclinicphasewasaccomplishedaccordingtothemodels describedbyTorayaetal.(1984a,b)andSatoetal.(2008).

VickershardnesswasevaluatedbyaMitutoyoMicrohardness tester(HM-200model)withpyramidaldiamondindentatorwith 1kgloadfor10s,andequalloadingandunloadingtimeof4sin sinteredworkpieces.

RamanspectrummeasurementsweredoneinaWitecRaman ConfocalMicroscope(Alpha300A/Rmodel),WitecControlsoft- ware,He-Nelaser(632nm),andUHTSspectrumanalyzer.Different pointsofeachworkpiecesurfacewerefocusedonwitha 1-␮m diameterlaserbeamduring3sofintegratingtimeandtheaverage resultof10integrationswastaken.Quantificationofthevolume

Fig.3. Workpiecefixturingtechnique.

percentageof monoclinicphase wasaccomplishedaccordingto themodelsdescribedbyKatagirietal.(1988),Limetal.(1992), Kimetal.(1997),Casellasetal.(2001),andTabaresandAnglada (2010).

Roughness(Ra,Rt)andflatnessdeviationofsinteredworkpieces weremeasuredbyaForm TalysurfIntrai60contactprofileme- terwitha2-␮mstylustipradius(r_tip),TaylorHobson␮ltraand Talymap Gold software. Dueto green bodies’ fragilityand low strength,anon-contactconfocalLeicamicroscope(DCM3Dmodel) wasusedtomeasuresurfaceandroughnessparameters,controlled byLeicaScan softwareandthedataanalyzed byTalymapGold software.Inputparameterswerea0.25-mmcutoff(␭c),0.25-mm singlemeasuredlength(lr),1.25-mmtotalmeasuredlength(ln), andGaussianfiltertodistinguishroughness,waviness,andshape erroraccordingtoISO25,178andISO4287.

Indirectmaterialremovalrate(MRR)quantificationofthework- pieceswasusedbymeansofananalyticalShimadzuscale(model AUW-D)aftersintering(initialmass,m_i)andU_d-lapgrinding(final mass,m_f).MRRwascalculatedas(m_i−m_f)/time.

2.4. U_d-lapgrinding

For thedressingoperation, brand-newsingle-point diamond dressers,witha60^◦taperand0.5-mmnoseradiusgeometry(rp), wereusedforeachtest.Thedresserwaskeptperpendiculartothe grindingwheel.

ForU_d-lapgrindingexperiments,threedifferentconventional grindingwheelsmadeofdifferentsiliconcarbidegritsizes(#300,

#600,and#800)andhotpressedwithanepoxybondof350-mm diameterand25-mmthicknesswereused.

Filteredwateratan8.33×10⁻⁵m³/sflowratewasappliedby floodduringdressing.A1:40emulsionofsemi-syntheticsoluble oil(RocolUltracut370)wassprayedusingtheminimumquantity fluid(MQF)methodat2.78×10⁻⁷m³/sduringU_d-lapgrinding.The locationofthecuttingfluidnozzleswasthesameasthatusedby Fiocchi(2010,2014),Fiocchietal.(2011,2015),andFiocchiand Sanchez(2011).Ahighercuttingfluidflowratewasnecessarydur- ingdressingtoremovetheslurryformedonthegrindingwheel, thuscleaningitupbeforeU_d-lapgrinding.

U_d-lapgrindingtestswereconductedtoanalyzetheinfluences ofthreeU_dsandthreeSiCabrasivesizesonsurfacefinishingof3Y- TZPspecimens(3×3²experiments).Themainparameterswere:

U_d=1,3,and5;#300,#600,and#800SiC;0.1-mmdepthofdress- ing(a_d);100-kPalapgrindingpressure(P);and100-rpmworkpiece rotationalspeed (nw)andgrindingwheelrotational speed (ng).

Thesefinalparameters,summarizedinTable1,weredefinedaftera longseriesofpretestsaimingtoproducethebestsurfacefinishing.

TheU_d-lapgrindingwasinterruptedat5,10,15,20,25,30,40, and60minand theworkpieces wereremovedfromtheholder, cleanedwithacetoneinanultra-soundbath,dried,weighedfor MRRquantification,andtheirsurfaceroughness(Ra,Rt)andflat- nessdeviationweremeasured.

Bothdressing and U_d-lapgrindingstepsweremonitoredby measuringthemainmotorelectricpower(currentsensor),apply- inga Ciber PowerAnalyzer(CVM NGR96model),described in Fiocchi(2010),whichisintegratedintothemachine-tool.

ANikonandaCarlZeissstereomicroscope(SMZ800andCitoval 2models,respectively),bothlinkedtoaSamtekdigitalcamera(STK 3520model),aswellasPixelViewsoftware,wereusedforgrinding wheelopticalmicroscopy.

The clamping technique was brought from the electronic ceramic industry by melting of Thermoplastic Quartz Cement (model70CfromLakeside)onaglasssubstrateetchedbyhydroflu- oricacidtoenhanceadhesion.Oneworkpieceattime wasfirst gluedontothecenterofthediscusingatemplatetocentralize theworkpieceontheglass.AQUIMIShotplateat100^◦C(Q.261.2 model)wasusedtomelttheglue.Duringcoolingoftheglue,asec- onddiscanda2-kgmasswereplacedovertheworkpieceinorder tohomogeneouslydistributethemoltenglueandkeepthework- piece’ssurfaceparalleltotheglass.Next,thesmoothfaceofthe discwasputincontactwiththevacuumchuck,thevacuumpump turnedon,andthevacuumvalveclosedtoholdthediscinposition onthechuck.Fig.3showstheworkpiecefixturesequence.

A2400-Wvacuumcleanerwasusedtocleanupthegrinding wheel’s surface duringdressingand U_d-lapgrindingoperations inordertoavoidanylooseparticlesonthegrindingwheelthat mayhavedamagedthefinishingand/ortheworkpiece’ssurface integrity.Therefore,onlytwo-bodyabrasionisexpected.Suction nozzles are strategically located before and after grinding and dressingareasaccordingtoFiocchi(2010,2014)andFiocchietal.

(2015),ascanbeseeninFig.4.

3. Resultsanddiscussion 3.1. LapGrinderII

Fig.4showsthegrindingareaofLapGrinderIIanditskeyele- ments,suchascontrolpanel,servomotor,grindingwheel,vacuum chuckandvalve,aswellascuttingfluidsandsuctionnozzles.

AccordingtoPahletal.(2007),theevolutionofthemachine-tool regardingdesignmethodologyisconsideredanadaptivedesign, since it kept toknownand establishedsolution principlesand adaptedtheembodimenttochangedrequirements.Ontheother hand,theoriginalmachine-toolbasedonamodifieddesignthat confersonitnewfeaturesandcapabilitiesisconsideredadevelop- mentaldesignbyKerala(2002).

Thevacuumchuckandglued workpieceshowedthemselves efficientandpractical,withgoodresistancetocuttingfluidsand grindingforcesinadditiontoeasycleaning.

Thedevelopmentaldesignofthespindlewasnecessarytoclamp theworkpiecebyavacuumandtoaddtwodegrees-of-freedom tothejointconnectingthevacuumchuckandthespindle’sshaft.

Thisarticulationdynamicallycompensatesforanymisalignment, keeping theworkpiece’ssurface alwaysparalleltothegrinding

Table1

InputandoutputparametersstudiedthroughUd-lapgrinding.

Inputparameters Outputparameters

Dressing U_d-lapgrinding Workpiece Grindingwheel

ad=0.1mm P=100kPa 3Y-TZPTosoh SiliconCarbide(SiC) Workpieceroughness(Ra,Rt) 60^◦taperand0.5mm

dressernoseradius(rp)

ng=100rpm 1400^◦Csintering temperature

Hotpressedepoxybond Workpieceflatnessdeviation(␮m) nw=100rpm nw=100rpm 29.8±0.15mmdiameter Ø350×25mm DressingandUd-lapgrindingpower(W) 8.33×10⁻⁵m³/s

filteredwater

2.78×10⁻⁷m³/s 1:40

RocolUltracut370

3.77±0.02mmthickness #300 Grindingwheelsurfacemicroscopy

Flood MQF 0.45␮mRa

7.47␮mRt

#600 Workpiece3Dprofilometry

Ud=1,3,and5 60min 29.3±1.7␮mflatness #800 MRR(g/minute)

Fig.4.CNCFiocchiLapGrinderII.

wheel.Thesefeaturesresultedinastifferandmorestablemechan- icalsystem.Theservomotorofferedconstantangularvelocityto thevacuumchuck,andthenewsteppingmotorsanddrivespre- ventedthelinearaxisfromresonating.Thus,alltheirregularities pointedoutwereovercomeandarobustUPmachine-tooldesign wasachievedandvalidated.

3.2. U_d-lapgrinding

3.2.1. Grindingwheeltopography

Figs.5and6showtheopticalmicroscopiesof4×4mmareasof thegrindingwheels’active/functionalsurfacesafterdressingand 60minofU_d-lapgrinding,respectively.U_dvaluesarearrangedin rowsandabrasivegritsizeincolumns.Darkerregionsrepresent deeperareasofthegrindingwheel.

TheimagesshowedinFig.5aresimilartothosefoundinFiocchi (2010)andFiocchietal.(2011,2015).Themacroeffectwasevident forallU_d dressingsappliedonboth#800and #600andonthe

#300grindingwheeldressedwithU_d=1.AfterU_d-lapgrinding, allgrindingwheels(Fig.6)kepttheirstructuresopenedwithout adhesionofmaterialfromtheworkpiece,asopposedtothatseen inthemachiningofAISI420stainlesssteelinFiocchi(2010)and Fiocchietal.(2011,2015).Thelowerductility,higherhardness, andlowerfrictioncoefficientofthe3Y-TZPaswellasthegrinding conditionsmaysustainthesefindings.

3.2.2. RMSelectricpower

Forthosegrindingprocess,i.e.,peripherallongitudinalgrinding (Fig.6a),inwhichtheactivesurfaceisthecylindricalperipheryof

thegrindingwheel,ataconstantdistancefromitsrotationalcenter, thegrindingpower(Pc)canbecalculatedas:

Pc=Ft×Vc (1) whereFtisthetangentialgrindingforceandVcthetangentialveloc- ityofthetool(Tönshoffetal.,2002).Insuchprocess,forinstance, F_tcanbeaccuratelyobtainedbycommercialpiezo-dynamometers and,hence,Pccanbecalculated.

Ontheotherhand,theabrasiveprocessstudieddoesnothavea constantrelativetangentialvelocityduetotherelativekinematics oftheworkpieceandgrindingwheel(Fig.6b).Thus,everysingle point(i)ofthetool/workpieceinterfacehasitsownV_c,i.In this particularfacegrindingwithconstantpressureandlappingkine- maticsprocess,theuseofahalleffectsensorisrecommendedto determinePc,consideringthatadynamometerwouldnotbeable todeterminelocaltangentialgrindingforce(F_t,i).

Commercialpiezo-dynamometers providethemost accurate measurementofcuttingforces.Thesetransducershavehighstiff- ness,butsincetheyusepiezoelectricceramics,themeasurementof staticforcesoveralongperiodoftimemayresultinsignificantdrift (Byrneetal.,1995),andbecauseoftheirdifficultiesforinstallation intheforceloop,theapplicationofpiezo-dynametersisrestricted (Jemielniak,1999).Forcetransducersarealsosubjectedtodynamic influencesfromthemachine/tool/workpiecesystem(Oliveiraand Valente,2004).

AccordingtoOliveiraandValente(2004),currentsensorsfor powermeasurementarethesimplestandlowest-costalternative forprocessmonitoring;inaddition,theirinstallationissimple,with

Fig.5. Opticalmicroscopyofthegrindingwheel’sactivesurfacesafterdressing.(ad=0.1mm,0.5-mmnoseradius,ng=100rpm,andfilteredwaterappliedbyflood).

Fig.6. Opticalmicroscopyoftheworngrindingwheel’sactivesurfacesafter60minofUd-lapgrinding.(P=100kPa,ng=nw=100rpm,andemulsionappliedbyMQF).

noinfluenceonthesystemstiffness.Thepowerisusuallyobtained bymeasurementofthevoltageandelectriccurrentatthegrind- ingwheelspindlemotor(MalkinandKoren,1980).Byrne etal.

(1995),Jemielniak(1999),and OliveiraandValente(2004)con- tendthatthismethod(halleffect/currentsensor)ispreciseandit haslowintrusiveness,but itisdampeddue tothesysteminer- tia,i.e.,grindingwheel,andthesignalprocessingcharacteristics needed.Thisleadstoadelayedresponsethatisnotimportantfor

thisresearch,sincetheworkisnotfocusedonshortevents.Thus onlytherelativemagnitudeofU_d-lapgrindingpoweramongthe grindingconditionsisimportantinordertorevealtheinfluenceof U_dandgrindingwheelgritsizesalong60minofU_d-lapgrinding.

3.2.3. Dressingpower

The updated machine-tool permits deactivation of the workpiece-holder rotation during dressing. In the older ver-

Fig.7.(a)Discretizationofforcecomponentsinperipherallongitudinalgrinding(Tönshoffetal.,2002).(b)GeometricalmodelforUd-lapgrinding.

sion this was not possible due to the permanent mechanical connection between the workpiece-holder and the main shaft throughtheflexibleshaft(Fiocchi,2010;Fiocchietal.,2011).In suchcase, LapGrinder IIdemands less energy by stoppingthe servomotorand,consequently,thevacuumchuckspindle,during dressing.

Asforthereductionintheamountofenergyrequiredfordress- ing,somecorrelationwasexpectedbetweenthepresentandearlier results(Fiocchi et al., 2015) regarding dressing power (Fig.8).

Althoughthecurrentresearchhasusedpreciselythesamegrinding wheels,thesamedresser’sgeometry,andexactlythesamedress- ingparametersappliedbyFiocchietal.(2015),therewasachange intheRMSdressing’selectricpower.The#600and#800grind- ingwheelsintegritiesmayhavebeendamagedbytheageofthe toolspurchasedin2004.Asofteningofthesegrindingwheels,by looseningabrasivesparticlesmoreeasily,wasnoticed;thiscaused theirmacrogeometriestodeterioratemorerapidly,especiallyfor the#800grindingwheel.

3.2.4. U_d-lapgrindingpower

Fig. 9 indicates that, on average, the higher the U_d value, thelowertheRMSU_d-lapgrindingpower: 176.71W forU_d=1, 173.91WforU_d=3,and172.34WforU_d=5.Thisgraphicsuggests, therefore,that,onaverage,higherU_dshaveatendencytodemand lesspowerinzirconiaU_d-lapgrindingusingSiCgrindingwheels.

Thedifferencebetweentheoreticalandrealmacroandmicro effectsoverthegrindingwheelandtheweakeningoftheepoxy bondcanexplainthosevariances.Fig.10illustratessomeimportant aspectschangedingrindingwheeltopographybyasingle-point diamonddressing,suchasthenumberofactiveabrasives(cutting edgesinteractingwiththeworkpiece),aswellasthewedgeangle (␤),rakeangle(␥),reliefangle(␣),andmacroporosity.

Suchanalysisisbynomeansatrivialtask.Itdependsonabra- sivesizeandfriabilityaswellasthe“health”ofthebondholding theseabrasiveparticlesandagglomeratesinplace,whichinareal dressingsituationinvolvediversemacroandmicroeffects,besides structuraldamageingrindingwheel,whichcanbeextremelyhard topredict,quantify,andqualify.Thepredominanceofoneormore effects canthusjustify thedispersal ofU_d-lapgrindingpowers depictedinFig.9.

3.2.5. Materialremovalrate

A very important consequence of applying different U_ds is showninFig.11,whichpresentsdifferentconditionsofmaterial removalasafunctionofallU_d-lapgrindingsituations.Therange

oftheresultsusingthesamegrindingwheelreinforcesthecapa- bilityoftheU_ddressingtomodifygrindingwheels’aggressiveness.

ThelowestMRRwas1×10⁻⁶g/minuteforU_d=1and#800.The highest,19timesbigger,happenedwithU_d=5and#300.

3.3. 3Y-TZPcharacterization

Unfortunately,itwasnotpossibletostudy/focusonexactlythe sameregionsduetolackofacommonreferencesystemamong theconfocalmicroscopy,contactprofilometer,SEM-FEG,XRD,and Ramanspectrometer.Allanalysishappenedinthesamevicinity, whichalsorepresentsthecharacteristicsoftheentiresurface.

3.3.1. 3Y-TZPpreparation

TwodifferentmagnificationsofSEM-FEG ofTZ-3Y-Epowder areshowninFig.12aandb.Therawmaterialpresentedthepre- dominanceofaroundedagglomerateof0.06±0.03-mm(Fig.12a) andparticlesizeof0.08±0.02-␮maveragediameter(Fig.12b).

Someagglomerateshapeswereobtainedafterdrying(Fig.12c)and granulation/selection(Fig.12d).Fig.12epresentsthecompacted ceramicat200MPa,representingthenewbiggeragglomeratesand theresidual interagglomerateporosity as described byGerman (1987).Fig.12fdemonstrates zirconia’s 0.35±0.07-␮maverage grainsizeachievedby2hofsinteringat1400^◦C.Table2displays thefinalzirconia’sproperties.

3.3.2. Profilometry

Uniaxialpressinginducteddensitygradientsinthegreenbodies, compactedaswellexplainedinliterature.Thosegradientsgen- eratedplasticdeformationsand,therefore,formerrorinthethin ceramicdiscs.Aftertheisostaticpressingstepthosedensitygradi- entswereconsiderablyreduced,buttheformerrorremainedand producedconcaveandconvexoppositesurfacesinthesamework- piece.Theformerrorincreasedordecreaseddependingontheside oftheworkpieceexposedtothehighertemperatureinsidethefur- naceduringsintering.Anamplificationofformerrorwasnoticed whentheconcavesurfaceswereexposedtohighertemperatures.

Ontheotherhand,betterformcorrectionhappenedwhenconvex surfacesweresubjectedtohighertemperatures.Alsoobservedwas animportantinfluenceofgravityaccelerationduringsinteringas consequenceofdeformationofthinceramicdiscsthathadrested onirregularsurfaces.

IntheuniverseofproducingnearnetshapesinUPceramicman- ufacturing,thefollowingshouldbeconsidered:thetypeofpressing anditsassociations;theadoptionofgreenceramicmachiningto

Fig.8.RMSdressingelectricpowerasfunctionofoverlapfactorandSiCgritsize.(ad=0.1mm,0.5-mmnoseradius,ng=100rpm,andfilteredwaterappliedbyflood).

Fig.9.RMSUd-lapgrindingelectricpowerasfunctionofoverlapfactorandSiCgritsize.(P=100kPa,ng=nw=100rpm,andemulsionappliedbyMQF).

Fig.10.Schematicrepresentationoftherealandtheoreticalmacrogeometry(dashedlines)ofthegrindingwheelsdependingontheabrasivesizeandoverlapfactor(Ud).

Table2

Finalzirconiadiscspropertiesaftersintering.

TZ-3Y-E

Microhardness(HV) Openedporosity(%) Density(g/cm³) Averagegrainsize(m)

1414.10±339.28 0.002±0.001 6.0±0.06 0.35±0.07

Fig.11.MRR(g/minute)asfunctionofoverlapfactorandSiCgritsize.(P=100kPa,ng=nw=100rpm,andemulsionappliedbyMQF).

shapethepartandalsoremovethehighest-densitygradientsin outsidelayers;theflatnessofthebase,andthetemperaturedis- tributioninsidethefurnace.Alltheseconsiderationwillminimize theeffectsofheterogeneouspressingandsinteringshrinkage.The judiciouschoiceoftheseparameters,therefore,shallresultinless machiningtimeforcorrectionofform,whichwillfocusmainlyon finishing.

3.3.3. Surfaceroughness

Foralltestedconditions,therewasasharpdiminishinginthe valuesofsurfaceroughnessinthefirst5minandaslightvariation ofsurfacequalityuntiltheendof60minofU_d-lapgrinding.Ascan beseeninFig.13,allthreegraphicshadthesamescaling.

Forthe#800dressedwithU_d=1and3,theroughnessdecreas- ingrateispracticallythesameandseveraltimeshigherthanfor U_d=5inthefirst5min.Theworstperformancecanbeexplainedby theprematurewearofthe#800grindingwheeldressedwithU_d=5, whichlostitsmacroeffectsooner,ascanbenoticedcomparingFigs.

5-7and6-7.

The#600grindingwheeldidnotshowavisiblemacroeffect forU_d=3and5(Fig.5-5and5-8).However,themicroeffectmay havepositivelyinfluencedsurfaceroughnessbyofferingnewcut- tingedgeswithdifferentanglesandlowerdepthofcutaswell assufficientgrindingwheelporositytocarrydebrisandcutting fluid,thereforeshowingthattheU_d throughasingle-pointdia- monddressingchangesgrindingwheelaggressiveness.Fig.6-5and 6-8showsimilartopographiesandmayexplainthesimilarityin roughnessproducedbythe#600dressedwithU_d=1and5during thelast20min.

TheinfluenceofU_dwasmoreevidentoverthe#300grinding wheel.Duringthefirst5minallU_dsproducedquitesimilarrough- nessdecreasingrates.ThebestroughnesscamefromU_d=5andthe worstonefromU_d=3.

Figs.14and15presentRa(nm)andRt(␮m)roughnessparam- etersvaluesattheendof60minofU_d-lapgrinding,respectively.

TheideabehindFig.10canjustifytheroughnessdepreciation withthesubstantialincreaseofdeeperrisksproducedbythe#600 and#800grindingwheels;higherRtvaluesareshowninFig.15.

TZ-3Y-EisharderthantheAISI420stainsteelstudiedbyFiocchi etal.(2015)andthusnomaterialisimpregnatedintothegrinding wheel’ssurface,resultinginhigherabrasiveprotrusionthatmay depreciatefinishing,butkeepsthetoolcleanerandwithopened porosity.Ontheotherhand,theconstantabrasiveexposureassoci- atedwiththehigherhardnessofzirconiawearsthegrindingwheel easily.

3.3.4. Topography

Fig.16and17illustrateconfocalmicroscopyofagreenanda firedpiece,respectively.Surfacefinishingimproved36%inarith- metic mean height (Sa) from green (0.71␮m) to the sintered (0.52␮m)stage,aswellasproducingflatterandsmootherareas.

Fig. 18 represents workpieces’ surface obtainedby confocal microscopyafter60minofU_d-lapgrinding.Inthisfigurecanbe seenapredominanceofductilecutting,absenceofcracks,andpat- ternofaleatoricscratches.Atleasttwolevelsofscratchingcanbe visualized:oneseverewithlargerwidthanddepth,andanother gentlerwithshallowmarks.Inbothcasesthemicrocuttingseemsto bethemajorductileremovalmechanisminallconditionsstudied.

Insomeworkpiecestherearestillremnantsofareasfromthe sinteredgenitor surface,suchas thepresenceof openedpores, depressionswithsphericalgeometry,andnon-machinedareas.In thecontextofthiswork,thosezonesareclassifiedasirregularities andmightberemovedbyincreasingtheU_d-lapgrindingtime,since thestudiedcuttingconditionsprovidedlowMRR.

TheconditionofU_d=1andthe#300grindingwheelraiseda suspicionthatbesidesductilecutting,therewasalsomicroplowing andplasticdisplacementofmaterial,asdemonstratedinFig.19.

The white arrows point to microcutting marks. The white dashedcirclesdelimitareasofplasticdeformationattheintersec- tionofthescratches,whichrevealsthechronologicalsequenceof events.Thenewestscratcheshaveovershadowedthepredecessor ones.

Theblackarrowspointoutpossiblemicrocracks.However,SEM provedthatthoseregionscamefromplasticdislocationofmaterial towardthecenterofthescratch.Anotherfactthatsupportsthis thesisisthatthewhiteleftarrowpointstoadeeperscratchwithout cracking.Howcouldalessaggressivecondition,pointedoutbythe blackarrows,generatesuchfailure?

Thepresenceofplasticdeformationputforwardtheideathat strainhardeningmaybetakingplace.Insuchcase,bythetimeits limitisreached,thematerialsuffersbrittlefracture.Thisdiscussion willberesumedbySEManalysis.

3.3.5. Flatnessdeviation

Fig.20displaysawidevarietyofflatnessdeviationandrein- forcestheability thatthesingle-pointdiamond dressinghasto modifygrindingwheels’characteristicsinU_d-lapgrinding.

3.3.6. SEM-FEG

ThequalityoftheimagesacquiredbytheSEM-FEGismuchbet- terthanthoseobtainedbyconfocalmicroscopy;however,theSEM employedwasnotcapableofmeasuringroughness.Figs.18and21

Fig.12.SEM-FEGofTosohTZ-3Y-E:(aandb)rawmaterial;(c)driedand(d)granulated/selectedby#80(180␮m)sieve;(e)isostaticallypressedat200MPa;(f)sinteredby 2hat1400^◦Candaheating/coolingrateof3.89^◦C/minute.

representthesamesurfacesshownbydifferenttechniques.Such comparisonisimportanttodisplaythedifferencesininterpretation ofthesurfacesdependingonthemicroscopytechniqueused.

The confocal and electron microscopy images demonstrate removal mechanismssuch as microcutting,microplowing, and microgrooving. During both microscopy procedures the work- pieceshadalloftheirsurfacesscanned,andnocrackintroduced byU_d-lapgrindingwasdetected.

Themicrocuttingwasmoreactiveatthebeginningofthepro- cess,whentheabrasiveswerestillsharp.Theothermechanisms happenedpracticallyduringtheentiremachiningtime,becoming moreevidentwiththewearofthecuttingedgesthatincreasedthe tip’sradius,exactlyasdescribedbyMarsh(1964).

Fig.22ashowsthemicroplowing,whichplasticallydislocates materialtotheborder ofthefurrow.Themicrogroovingcanbe seen inFig.22b;zirconia’sgrains aresmashed, minimizing the grainboundariesbymeansoflongitudinalplasticdeformationof theceramicmaterial.

Insomecircumstancestheabrasivepenetrationintothework- pieceprovidedacuttingcrosssectionthatpromotedductilecutting withlowresidualplasticdeformation.Thisspecificcuttingcondi- tionsdidnotraisematerialattheborderofthescratchandleft parallelabrasivemarksinthecuttingdirectionatthebottomof thescratch.Fig.22cillustratesthismicrocuttingmechanism.

According to Schinker and Döll (1987), microplowing and microgrooving mechanisms promote not only lateral deforma- tion,butalsothecompressionofthematerialinthedirectionof

Fig.13.Roughness(Ra)versustimefordifferentSiCsizesandUds.(P=100kPa,ng=nw=100rpm,andemulsionappliedbyMQF).

Fig.14. Finalroughness(Ra)after10minofUd-lapgrindingfordifferentSiCgritsizesandUds.(P=100kPa,ng=nw=100rpm,andemulsionappliedbyMQF).

Fig.15.Finalroughness(Rt)after10minofUd-lapgrindingfordifferentSiCgritsizesandUds.(P=100kPa,ng=nw=100rpm,andemulsionappliedbyMQF).

machining.Byincreasingdepthofcutthemicrocuttingbecomes predominantagain.Thiswasoneoftheexplanationsfoundtojus- tifythebestroughnessproducedbythelargestabrasivegrit(#300 SiC),whichalsohashigherfriability.

TheSEMimagesshowthatU_d-lapgrindinghasachievedplas- ticityconditionsof 3Y-TZP,which leadus awaytobelieve that thenanometricsurfacefinishingwasproducedmainlybyductile

microcuttingandpulverizationremovalmechanisms.Inthefirst mechanism,thevolumeofremovedmaterialisequaltothescratch volumeleft.Itisbelievedthatthesecondmechanismhappensdue tosuccessiveplasticdeformationaccumulatedby3Y-TZPindis- tinctdirections(defects)thathasachievedacriticalstrainstate, inwhichmicrocrackswerenotonlynucleated,butalsoformeda recrystallizedlayerofnanometricthicknessaccordingtoTabares

Fig.16.Confocalmicroscopy(LeicaDCM3D)ofagreenpiece.20×objective,darkfield,andmonochromaticlight.

Fig.17.Confocalmicroscopy(LeicaDCM3D)ofasinteredpiece.100×objective,brightfield,andwhitelight.

etal.(2011).Thisspecificremovalconditioncanbeassociatedwith metalhardeningasaconsequenceofcoldworking(strainharden- ing),whichproducesverysmallfragmentsofmaterial(debris),as wellillustratedinFig.22d.Itisalsobelievedthatthelastremoval mechanism is a consequenceof low-cycle fatigue,producing a smoothandpracticallynotscratchedsurface,asshowninsome areasofFig.22c.

3.3.7. Ramanspectroscopy

Fig.23presentsRamanspectraoftheworkpiecessubmittedto thenineU_d-lapgrindingconditions.Thisfigurealsocontainsthe spectraofaworkpieceassinteredaswellasacompactedmono- clinicphasezirconia.Verticalblackandredlinesaresuperimposed onthegraphicinordertohelpdifferentiatetetragonal(t)andmon- oclinic(m)Ramanintensities.

Thevolumepercentageofthemonoclinicphasewaslowerthan 1%,accordingtoTable3,whichpresentsdifferentapproachesfor determiningthemartensitictransformation.Theresultswerequite similarineachappliedtheory.Consideringthosevalues,themon- oclinicphasemaybenegligible.

3.3.8. XRD

Fig.24presentsx-raydiffractionpatternsfor(1)rawmaterial (TosohTZ-3Y-E)assupplied,(2)sintered3Y-TZP,(3)sintered3Y-

TZPafter60minofU_d-lapgrindingusing#300SiCdressedwith U_d=5,P=100kPa, ng=nw=100rpm, and 1:40emulsion applied byMQF,and (4)compacted monoclinicpowderwithoutbinder pressedat80MPa.

Therawmaterialhastracesofthemonoclinicphase,asindicated bythepurplearrowsinthevicinityof2␪=33^◦and36.5^◦(Fig.24-1).

Sinteringresultedinafullytetragonalmaterial(Fig.24-2).

Fig.24showstheXRDpatternsforallnineU_d-lapgrindingcon- ditionsstudied.

The#600and#800grindingwheelspracticallydidnotalter thespectrainrelationtothesinteredworkpiece,onlytenuous, asymmetric,leftbroadeningaround35^◦.Thesingularitywasmore pronouncedusingthe#600grindingwheel.

Two characteristic phenomena arising from U_d-lap grinding usingthelargestabrasivegritwereobserved.Theseweretheasym- metric broadening of tetragonalpeaks near35^◦ and 59^◦ (black arrows),andreversaloftheintensityofthetetragonalpeaksat40^◦ and41^◦(Figs.24-3and25).VirkarandMatsumoto(1986),Mehta etal.(1990),andTabaresetal.(2011)affirmthatreversaloccursdue toferroelasticdomainswitching,whichcanbeexplainedasareori- entationofthecrystallographicplanesatthelevelofcrystallite.The asymmetricbroadeningmayalsoindicateasuperficialgrainrefine- ment.ThisfindingisinagreementwithTabaresetal.(2011)that

Fig.18. Confocalmicroscopy(LeicaDCM3D)after60minofUd-lapgrindingwith9differentconditions.100×objective,brightfield,andwhitelight.

Fig.19.Confocalmicroscopy(LeicaDCM3D)ofasurfaceproducedbya#300SiCgrindingwheeldressedwithUd=1.100xobjective,brightfield,andwhitelight.

Fig.20.Flatnessdeviationafter60minofUd-lapgrinding,asafunctionoftheoverlapfactorfordifferentSiCgritsizes(P=100kPa,ng=nw=100rpm,andemulsionapplied byMQF).

Fig.21.SEM-FEGafter60minofUd-lapgrindingwith9differentconditions.

Fig.22.SEM-FEGof(a)microplowing,(b)microgrooving,(c)microcutting/scratching,and(d)chipsproducedbyUd-lapgrinding.

Fig.23.RamanintensityversusRamanshift(cm⁻¹)ofthenineUd-lapgrindingconditionsappliedin3Y-TZP,3Y-TZPassintered,andacompactedmonoclinicphasezirconia.

VerticalblackandredlinesaddedtothegraphictoindicateRamanshiftoftetragonalandmonoclinicphases,respectively,accordingtoPezzottiandPorporati(2004).

Table3

Volumepercentageofmonoclinicphase(Vm)measuredbyRamanspectroscopyinUd-lapgroundworkpieces.

Author/equation Parameter Grindingwheelmesh Overlapfactor(Ud) Vm(%) Vmmax.[%] Vmmin.[%]

k ␦

Clarke and Adar (1982)

0.97 1 800 1 0.50

600 1 0.47

300 1 0.50

800 3 0.50

600 3 0.50 – –

300 3 0.50

800 5 0.50

600 5 0.50

300 5 0.50

Katagiri etal.

(1988)

0 2.2±0.2 800 1 0.48 0.50 0.46

600 1 0.49 0.51 0.47

300 1 0.48 0.50 0.46

800 3 0.48 0.50 0.46

600 3 0.48 0.50 0.46

300 3 0.48 0.50 0.46

800 5 0.48 0.50 0.46

600 5 0.48 0.50 0.46

300 5 0.48 0.50 0.46

(1992) 1 0.33±0.33 800 1 0.75 1.0 0.60

600 1 0.73 1.0 0.57

300 1 0.75 1.0 0.60

800 3 0.75 1.0 0.60

600 3 0.75 1.0 0.60

300 3 0.75 1.0 0.60

800 5 0.75 1.0 0.59

600 5 0.75 1.0 0.60

300 5 0.75 1.0 0.59

(1997) – – 800 1 0.20

600 1 0.20

300 1 0.20

800 3 0.20

600 3 0.20 – –

300 3 0.20

800 5 0.20

600 5 0.20

300 5 0.20

Casellas etal.

(2001)

– – 800 1 0.53

600 1 0.52

300 1 0.53

800 3 0.53

600 3 0.53 – –

300 3 0.53

800 5 0.53

600 5 0.53

300 5 0.53

Fig.24. X-raydiffractionpatternsfor(1)TosohTZ-3Y-Epowderassupplied,(2)sintered3Y-TZP(2hat1400^◦C),(3)sintered3Y-TZPafter60minofUd-lapgrindingusing

#300SiCdressedwithUd=5,P=100kPa,ng=nw=100rpm,and1:40emulsionappliedbyMQF,and(4)compactedmonoclinicpowderwithoutbinderpressedat80MPa.

Fig.25.X-raydiffractionpatternsfor(1)compactedmonoclinicpowder,(2)TosohTZ-3Y-Epowderassupplied,(3)sintered3Y-TZP,(4-12)sintered3Y-TZPafter60minof Ud-lapgrindingusingP=100kPa,ng=nw=100rpm,and1:40emulsionappliedbyMQF.

foundarecrystallizedzoneofgrainsof∼20nmdiameteringround 3Y-TZP.

Tabaresetal.(2011)alsoshowedthattheasymmetryobserved inthespectrumofgroundzirconiaisrelatedtotheoverlapping of thetetragonal and rhombohedral phases.The rhombohedral

phase hasbeenassociatedwithdistortion ofthecubic phasein the[111]directionaccordingtomanyauthors,suchasHasegawa (1983),Kitanoetal.(1988),Mitraetal.(1993),andTabaresetal.

(2011).Therefore,theprobabilityoffindingarhombohedralgrainis equaltotheprobabilityoffindingacubicgrain.Ontheotherhand,