Effect of polyethylene glycol on the surface exfoliation of

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

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