Effect of polyethylene glycol on the surface exfoliation of SiC green bodies prepared by gelcasting

MaterialsScienceandEngineeringA368(2004)

255–259

EffectofpolyethyleneglycolonthesurfaceexfoliationofSiC

greenbodiespreparedbygelcasting

FeiLi ,Hai-yanChen,Rui-zhiWu,Bao-deSun

StateKeyLaboratoryofMetalMatrixComposites,ShanghaiJiaotongUniversity,HuashanRoad1954,Shanghai200030,PRChina

Received17July2003;receivedinrevisedform31October2003

Abstract

Thesurfaceexfoliationofsiliconcarbidegreenbodiespreparedbygelcastinginairwasdramaticallyeliminatedbyaddingpolyethyleneglycol(PEG)intheceramicsuspension.Thus,thesizeofsiliconcarbidegreenbodiescouldbepreciselycontrolled.Inthispaper,thezetapotentialsofsiliconcarbidepowdersdispersedinaqueoussolutionandtherheologicalpropertyofthesuspensionwereinvestigatedtodeterminetheeffectoftheadditionofPEG.ItwasfoundthattheadditionofPEGdidnotaffectthecolloidalbehaviorofsiliconcarbidesuspensionmarkedly,butincreasedtheviscosityofsiliconcarbidesuspensionbecausethemacromolecularchainsofPEGinterferedwiththe uidityofsiliconcarbideparticlesinthesuspension.AstheamountofPEGincreased,thesurfacesofsiliconcarbidegreenbodiesbecamemoreandmorerigidandthesurfaceexfoliationwascompletelyeliminatedwhenthePEGwasover2.5wt.%(basedonsiliconcarbideweight)insilicongreenbody.ThemechanismbywhichPEGinhibitedthesurfaceexfoliationofsiliconcarbidegreenbodiesisprobablybecausethatthispolymerpreventsthereactionbetweenoxygenandmonomerfreeradical, R–CH2 ,duringthegellingprocedure.©2003ElsevierB.V.Allrightsreserved.

Keywords:Siliconcarbide;Gelcasting;Greenbody;Surfaceexfoliation;Polyethyleneglycol

1.Introduction

Siliconcarbide(SiC)possessesavarietyofdesirableproperties,suchashighmechanicalstrength,highchemicalstability,goodthermalconductivity,lowcoef cientofex-pansion,andoutstandingerosionresistance.Thismaterialhasbeenrecognizedasapotentialcandidateforhigh-tem-peraturestructuralapplications,e.g.,inadvancedheaten-gineparts,inheatexchangers,infrictionandwear-resistantcomponents,etc.[1].Slipcastinghasbeenemployedinformingthegreenbodiesofcomplex-shapedSiCparts.However,slipcastingtakesalongtimeandproducesweakgreenbodies.Inaddition,densitygradientoccursingreenbodiesformedbyslipcasting.Thus,gelcastingtechniquehasbeenwidelyusedtoformSiCgreenbody.

Thegelcastingprocesswas rstdevelopedintheMet-alsandCeramicsDivision-CeramicProcessingGroupatOakRidgeNationalLaboratory(ORNL),OakRidge,USAbyJanneyandOmatete[2,3].Itisanovelnear-net-shape

Correspondingauthor.Tel.:+86-21-62933751;fax:+86-21-62932914.

E-mailaddress:lifei74@(F.Li).

0921-5093/$–seefrontmatter©2003ElsevierB.V.Allrightsreserved.doi:10.1016/j.msea.2003.11.013

formingprocessforvariousceramicmaterials[4–9].Intheprocess,ceramicpowdersaredispersedinthepre-mixedorganicmonomersolutiontopreparestablehighsolidload-ingslurrywithlowviscosity.Subsequently,theslurrycanbegelledinthemouldswithvariousshapes.Theresultisahomogeneouswetcastbodywithuniformchemistryanddensitythatcontainsonlyafewpercentorganicbinders.Afterdrying,binderremovalandsinteringtakeplaceasinotherceramicprocesses.Thematerialpropertiesaftersinteringareconstantthroughoutthebodies.

Forthegelcastingprocess,acrylamide(AM)isusuallyusedasthemonomerandN,N -methylenebisacrylamide(MBAM)asthecouplingagent.AmmoniumpersulphateaqueoussolutionandN,N,N ,N -tetramethylethylenediamine(TEMED)areusedasinitiatorandcatalyst,respectively.Fortheceramicgelcastinginacrylamidesystem,theinhibitionofoxygeninfreeradicalpolymerizationinairisaseriousproblemthatcausesthesurfaceexfoliationphenomenonofgreenbodies[10,11].Inordertoavoidtheinhibitionofoxygen,gellingoftheceramicslurryisnormallycar-riedoutinN2atmosphere.Thisleadstoahighercostforgelcastingprocess.Recently,somewater-solublepolymershavebeenusedasadditivesinthegelcastingslurrytoavoid

256F.Lietal./MaterialsScienceandEngineeringA368(2004)255–259

theinhibitioncausedbyoxygenwhengelcastingiscarriedoutinair[12,13].Thepolyethyleneglycolisthesimpleststructureofthenon-inoizedwater-solublepolymers.Itpos-sessesgoodstability,adhesionalwetting,lowtoxicityandthecharacteristicthatitburnsoutcompletely,resultinginwideapplicationinthecastingofceramic.ThepurposeofthispaperistoinvestigateanddiscussthepolyethyleneglycolongelcastingofSiC,inthepreconditionofthatthesurfaceexfoliationphenomenonofgreenbodiescastinairisinhibited.

2.Experimentalprocedure2.1.Materialsandchemicals

ThestartingSiCpowdersusedinthisstudyhadanaverageparticlesizeof5 m.Forgelcasting,acrylamide(AM)andN,N -methylenebisacrylamidewereusedasmonomerandcouplingagent,respectively.N,N,N ,N -tetra-methylethylene-diamineasacatalystandammoniumpersulphate(5wt.%watersolution)asaninitiator.Adis-persant,namelytetramethylammoniumhydoxidesolution(TMAH,25wt.%watersolution),wasusedtopreparesta-blesuspensionwithhighsolidloadingandlowviscosity.Polyethyleneglycolhadanaveragemolecularweightof6000.AllthematerialsandchemicalswerepurchasedfromShanghaiChemicalreagentsLtd.,China.2.2.Preparationsofsuspensionandgreenbody

TheschematicformingprocessofgelcastingisdescribedinFig.1.VariousamountofPEGanddispersantwere rstcompletelydissolvedusingmechanicalstirringinapre-mixsolution,whichconsistedof12.3wt.%AM/MBAM(AM:MBAM=20:1,wt.)and87.7wt.%deionizedwater.Thenthesiliconcarbidepowderwasaddedinthesolution.Thesuspensions,withasolidloadingof50vol.%,weremixedmanuallyandthenmilledfor12hinanylonresinjarusingaluminaballsasmillingmediatobreakdowntheagglomeratesandtoachievegoodhomogeneity.Afterde-airing,therheologicalpropertiesofthesuspensionwereevaluated.1vol.%oftheinitiator(basedonthepremixso-lution)and0.1vol.%ofthecatalyst(basedonthepremixsolution)wereaddedtothesuspensionsbeforecastingintopolymethylmethacrylatemould.Aftergellingat60 Cinair,thegreenpartswereremovedfromthemouldanddriedatambientconditions(humidity:90%,temperature:25 C).2.3.Testingmethod

TherheologicalbehaviorsoftheSiCslurriesweremeasuredbyarotationalrheometer(CVO100,BohlinIn-strumentGmbh,Germany) withgapsetupof50 mataconstanttemperatureof25C.ThezetapotentialofSiCpowderwascalculatedfromthemeasured

electrophoretic

Fig.1.Theprocess owchartofsiliconcarbidegelcasting.

mobilityusingelectrophoreticlightscattering(ELS)with5mWlasersource.ThebulkdensitymeasurementsofgreensamplesweredeterminedwiththeHgintrusionmethodbasedonArchimedes’principle.The exuralstrengthofgreenbodywasexaminedbyatri-axialshearinstrument.Three-pointbendingwasemployedforthetest,withaspanof30mm.Thebarsofgreenbodieswerenormally5mm×6mm×40mminsize.Fracturesurfacesofgreensampleswereexaminedwithscanningelectronmicroscope(SEM,PHLIPS515,Holland).

3.Resultsanddiscussion

3.1.EffectofdispersantandPEGonthecolloidalbehaviorsofSiCsuspension

Thestudyoftheelectrokineticbehaviorsthroughmea-surementofzetapotentialisveryimportantforunderstand-ingthedispersabilityofceramicparticlesinaliquidmedium.Thehigherthezetapotential,thehighertherepulsiveen-ergyandthemorestabletheslurries.Fig.2showsthezetapotentialsofSiCpowderwithandwithoutdispersantand/or

F.Lietal./MaterialsScienceandEngineeringA368(2004)255–259

257

20

100

V

m /-10laitn-20teop-30 taZe-40-50-60

2

4

6

8

10

12

pH

Fig.2.Zetapotentialvs.pHvalueofsiliconcarbidesuspensionindeinoizedwater.

PEGindeionizedwater.Itisfoundthattheisoelectricpoint(IEP)ofSiCindeionizedwaterwithoutdispersantwas3.1.AtstronglybasicpHvalues,theabsolutevaluesofzetapo-tentialswererelativelyhigh.Withtheadditionof0.5wt.%TMAHsolution,theIEPofSiCshiftedto2.3;withtheaddi-tionof0.5wt.%TMAHsolutionand2.0wt.%PEG(basedonSiCweight),theIEPofSiCshiftedto2.0.Incompari-sonwiththeabsolutevaluesofthereceivedSiC,thatoftheSiCwithdispersantand/orPEGbecamemuchhigherinthebasicpHrange.

Itiswellknownthatthedispersant,TMAH,isanorganicalkali,whichcanbeionizedinwateraccordingtothefol-lowingreaction:

N(CH3)4OH N(CH3)4++OH

(1)

TheSiCparticlesarecoatedwithanoxygen-richlayeronitssurfaceduringthesynthesisprocess.Theoxygen-richlayercanbehydratedwhenSiCparticleisexposedtoam-bientconditionsorisinwatertoformsilanol(Si-OH)onthesurfaceofSiCparticles.AtlowpHrange,silanolreactswithH+tobecomeSi-OH2+,sothezetapotentialofSiCispositivewhenthepHvalueisbelow3.1.AfterTMAHisaddedtothesuspension,the+IEPofSiCshiftstolowerpHvaluebecausetheN(CH3)4group,whichcomesfromreaction(1),canalsobeabsorbedonthesurfaceofSiCpar-ticlesandtheamountofpositivechargesonSiCparticlesincrease.AthighpHrange,silanolreactswithOH tobe-comeSi-O ,sothezetalpotentialofSiCisnegative.WhenTMAHisaddedtothesuspension,moreOH anionsthatcomefromthedispersantabsorbonthesurfaceofSiCpar-ticles,thusthezetapotentialincreasesandahigherrepul-siveforcebetweentheSiCparticlesisobtained.ItshouldbenotedherethattheadditionofpolyethyleneglycoltoSiCsuspensionhasnoobviouseffectonzetapotentialofSiCparticles.Thismightbebecausethatthehydroxylgroupinthemacromolecularchainofpolyethyleneglycolisnotas

10

10

.s

Pa / tyiosiscV11

Shear Rate / s

-1

Fig.3.Viscosityofsiliconcarbidesuspensionwithdispersantvs.shearrate.

activeasthatinTMAHmoleculeandcannotbeeasilyion-izedinwater.SoitcannotaffectachangeinSiCsurfacecharges.

3.2.RheologicalpropertiesofSiCsuspensionwithPEGFromthemeasuredvaluesofzetapotentialforSiCsus-pension,itcanbeseenthatTMAHhasanobviouseffectonimprovingtherepulsiveenergybetweenSiCparticlesintheaqueoussystem.However,excessdispersantmaydamagethe uidpropertyofceramicsuspension.Thus,theamountofdispersantinSiCsuspensionwithasolidload-ingof50vol.%wasoptimized.Fig.3givesthelog-linearplotsofapparentviscosityversusappliedshearrateofSiCsuspensionswithoutandwithTMAHaftermilling12h.ItcanbeseenfromFig.3thatallthesuspensionsexhibitedshear-thinningbehavior.Thisisbecauseofaperturbationofthesuspensionstructurebyshear.Atlowshearrates,thesuspensionstructurewasclosetoequilibriumbecausether-malmotiondominatedovertheviscousforces.Athighershearrates,theviscousforcesaffectedthesuspensionstruc-tureandshear-thinningoccurred.Atveryhighshearrates,theviscousforcesdominatedandtheviscosityplateaumeasuredtheresistanceto owofasuspensionwithacom-pletelyhydrodynamicallycontrolledstructure.Fig.3showsthattheSiCsuspensionwithoutTMAHexhibitedhighestviscosityatvariousshearrates.AfteraddingTMAH,theviscosityofSiCsuspensiondecreasedandtheoptimumamountofTMAHinSiCsuspensionwas1.0wt.%(basedonSiCweightinthesuspension).

WiththeadditionofpolymerssuchasPEGintomonomersolution,thesystembecomesamixtureoforganicmonomerandpolymer.TheabsorptionofPEGonthesurfaceofSiCparticleshadanobviouseffectonthesteadyviscosityofSiCsuspension.Fig.4showsthelinear–linearplotsofapparent

258

876

F.Lietal./MaterialsScienceandEngineeringA368(2004)255–259

8

7654321

100

200

300

-1

Viscosity / Pa.s

543210

400

0500

Fig.5.Surfaceofsiliconcarbidegreenbodiespreparedfromthesuspen-sionswithout(a),andwith1.5wt.%(b)and3.0wt.%PEG(c).

Shear Rate / s

Fig.4.Viscosityofsiliconcarbidesuspensionwithpolyethyleneglycolvs.shearrate.

viscosityversusappliedshearrateofSiCsuspensionswithPEG.ItcanbeseenfromFig.4thatallthesuspensionsex-hibitedshear-thinningbehaviorandtheviscosityincreasedwiththeamountofPEGinSiCsuspensions.PEGhashy-droxylgroup,soitiseasilydissolvedinwater.However,PEGcannotbeadsorbedonthesurfaceofSiCinalargeamountandimprovethestabilityofSiCsuspension.ThelargemolecularchainsofPEGdistributeinthesuspension,whichpartlyinhibitsthe uidityandcausestheincreaseofthesuspensionviscosity.

3.3.PreparationandpropertiesofSiCgreenbodiesAccordingtoFig.1,thestablesuspensionsofSiCwith50vol.%solidloadingwerepreparedbydispersingSiCpowdersintheorganicmonomersolutionswithoutandwithPEG.Aftermixingwith1vol.%(basedonthemonomersolution)initiatorsolution(ammoniumpersulphate,5wt.%aqueoussolution)and0.1vol.%(basedonthemonomersolution)catalyst(TEMED),thesuspensionswerecastintopolymermoulds.Theconsolidationtimeofthesuspen-sions,thedegreeofsurfaceexfoliation,bendingstrengthandrelativedensitiesoftheSiCgreenbodiesareshownin

Table1.ItcanbeseenfromTable1that,withtheincreaseofPEGamountinthesuspensions,theconsolidationtimewasextended.Thisisprobablyduetothefactthatthecontactamongmonomers(AM)wasinterferedwithandhinderedbyPEGinthesuspensionduringtheprocedureofin-situpolymerizationofmonomericsystems.Duetotherheologicalbehaviorandconsolidationtime,aminimumPEGconcentrationinSiCsuspensionisrequiredtoinhibitthesurfaceexfoliationphenomenon.Fromtheexperimentsitcanbeconcludedthatatleast2.5wt.%ofPEGinthesuspensionisrequiredtoinhibitsurfaceexfoliation.ThebendingstrengthandrelativedensityofthedriedgreenbodieswithPEGwerelowerthanthatwithoutPEG;butthereducedstrengthishighenoughtoallowinexpensivegreenmachiningofthegreenbodiesintomorecomplexshapes.Fig.5showsthephotosofthegreenbodiesgelcastfromSiCsuspensionswithoutandwith1.5and3.0wt.%PEG.ItcanbeseenthatthegreenbodywithoutPEGhadanobvi-ousexfoliationlayerthatwaseasilybroken.Theboundarybetweenthelayerandrigidpartwasveryclear.Thegreenbodywith1.5wt.%paratively,thegreenbodywith3.0wt.%hadarigidsurfacewithoutexfoliationlayer.ThemechanismofPEGinhibitingthesurfaceexfoliationmightbethatthepolymerpreventsthereactionbetweenoxygenandmonomerfreeradical, R–CH2 duringthegellingpro-cedureinair.Fig.6(a)and(b)showthemicrostructures

Table1

Propertiesandgelationofthesuspensionwith50vol.%siliconcarbideatvariousconditionsNo.12345678

ab

AmountofPEG(wt.%)00.250.51.01.52.02.53.0

Consolidationtime(min)a2030354244505460

SurfaceofsamplesObviousexfoliationObviousexfoliationObviousexfoliationObviousexfoliationPartialexfoliationPartialexfoliationNoexfoliationNoexfoliation

Bendingstrength(MPa)11.798.404.574.344.152.073.374.02

Relativegreendensity(%)b57.9654.4551.2349.8250.2247.8851.4553.78

Consolidationtimereferstothetimeofchange

fromsuspensiontowetgreenbody.Thetheoreticaldensityofsiliconcarbideis3.127g/cm3.

F.Lietal./MaterialsScienceandEngineeringA368(2004)255–259

259

Fig.6.MicrostructuresofsiliconcarbidegreenbodiespreparedfromsuspensionswithoutPEG(a)andwith3.0wt.%PEG(b)(Thearrowsinthepicturesindicatethepolymernetworkingreenbodies).

obtainedbythegelformedbypolymerizationofmonomerwithoutandwithPEGmixedSiCpowder.Maetal.[12]re-portedthattherewasnopolymernetworkinaluminagreenbodypreparedfromaluminasuspensionwithPEG.ButfromFig.6(a)and(b),itcanbeobservedthatboththesampleshadobviouspolymernetwork,whichindicatesthatPEGonlypartlyinhibitedthecontactofmonomersAM,butnotcom-pletelypreventedthecross-linkingofthemonomers.ThusarigidgreenbodycanbeobtainedfromtheSiCsuspensionwithPEG.

Acknowledgements

TheauthorsthanktheNationalKeyBasicResearchDe-velopmentProgramofChina(973Program,GrantNo.G1999064900-4)andPostdoctoralFundofChina(GrantNo.2003033295).TheauthorsalsothankMr.M.J.Dongforthemeasurementofzetapotentialoftheceramicpowder,Dr.Q.WuforthesupporttorheologicaltestsandMr.E.K.Mengfortheassistancetomechanicaltests.References

4.Conclusions

Anewgelcastingsystembasedonaqueoussuspensionofsiliconcarbide/polyethyleneglycol/acrylamidecross-linkedwithacouplingagentN,N -methylenebisacrylamidehasbeendeveloped.Theroleofpolyethyleneglycolinthissystemistoeliminatethesurfaceexfoliationofsiliconcarbidegreenbodiesgelledinairandmakethegel-castingasimpleprocess.Theadditionofpolyethyleneglycolhadnoobviouseffectonthecolloidalbehaviorofsiliconcarbidepowderinaqueoussuspension,butin-creasedtheviscosityoftheslurriesanddecreasedthemechanicalstrengthofthegreenbodies.Astheamountofpolyethyleneglycolincreased,thesurfacesofsiliconcarbidegreenbodiesbecamemoreandmorerigidandthesurfaceexfoliationwascompletelyeliminatedwhenthepolyethyleneglycolinthegreenbodieswasover2.5wt.%(basedonsiliconcarbideweight).Themechanismofpolyethyleneglycolinhibitingthesurfaceexfoliationofsiliconcarbidegreenbodiesisprobablybecausethepoly-merpreventsthereactionbetweenoxygenandmonomerfreeradical, R–CH2 duringthegellingprocedureinair.

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