玻璃外文文献加手工翻译.doc
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玻璃外文文献加手工翻译.doc
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Microstructureandsolidparticleerosionofcarbonbasedmaterialsusedfortheprotectionofhighlyporouscarbon-carboncompositethermal
insulation
R.I.BAXTER,R.D.RAWLINGS
DepartmentofMaterials,ImperialCollegeofScience,TechnologyandMedicine,
LondonSW72BP,UK
Multiparticleerosiontestswereperformedoncandidatecoating(colloidalgraphitepaints)andcladding(densecarbonc—arboncompositesandgraphitefoil)materialsemployedtoprotectporouscarbon—carboncompositethermalinsulationinvacuumandinert-gasfurnacesthatutilizeinertgasquenching.ThedependenceoftheerosionrateontheangleofincidenceoftheerodentwasexaminedandrelatedtothemicrostructureandthemechanismsofmaterialremovalasobservedbySEM.Inaddition,theeffectofathinchemicalvapourdeposited(CVD)carbonlayerontopofacolloidalgraphitepaintcoatingandagraphitefoilcladwasinvestigated.Thecoatingandcladdingmaterialsdisplayedagreatererosionresistanceatallanglesofincidencecomparedtotheporouscarbon—carboncomposite.Ingeneral,thegreatesterosionratewasfoundatanangleofincidenceof90°,wheretheerodentstreamisperpendiculartotheerosionsurface,andbrittlefracturewasthepredominantmechanismofmaterialremoval.Theexceptionwasthegraphitefoilmaterialwhichdisplayedmaximumerosionatanangleofincidenceof60°.Forthismaterial,twomechanismswereeffective:
disruptionofthegraphiteflakes,whichare
mainlyheldtogetherbymechanicallocking,andaploughing-likemechanism.TheadditionofathinCVDcarbonlayertocolloidalgraphitepaintimprovedperformance,whereastheerosionresistanceofthegraphitefoilwasslightlydegradedasthe
CVDlayerwastoothintopreventtheploughing-likemechanism.
1.Introduction
Aclassofhighlyporouscarbon—carbon(C—C)composites,withlowdensitiesintherange0.1—0.4Mgm\3,areutilizedasthermalinsulationinvacuumandinert-gasfurnacesattemperaturesupto2800°C.Aconsequenceofthevacuum-mouldingprocessusedintheproductionofthecompositeisthatthediscontinuousfibresareorientatedintolayerstoformatwo-dimensionalplanarrandomstructure.Thevastmajorityofthevolumeofthecompositeconsistsofinterconnectedporesandthefibrenetworkisbondedattheintersectionsoffibresbydiscreteregionsofthecarbonmatrixasopposedtoacontinuousmatrix.Forthisreasonthesecompositesarealsoknownascarbonbondedcarbonfibre(CBCF).Asaresultofthehighporosityandthefibreorientation,thethermalconductivityperpendiculartothefibrelayerplanesislow,atypicalvalueforamaterialwithanominaldensityof0.20Mgm\3is0.24Wm\1K\1at2000°Cinvacuum.Investigationsintothemicrostructure[3,4],mechanicalproperties[2,5—9]andthermalproperties[10,11]ofthesematerialshavebeenreported.(1997Chapman&HallCBCFisusedinfurnacesemployedinhightechnologyapplicationssuchassingle-crystalgrowing(forexample,siliconorgalliumarsenide)ormetalheattreatment.Theheattreatmentofmetals,suchastoolsteels,isincreasinglycarriedoutinfurnacesthatutilizegasquenching(typicallynitrogenisused)[12,13].Thegasquenchmaybeusedtoreducetheturnaroundtimeofbatchprocessesorasanintegralpartoftheheat-treatmentregime.Theadvantageofgasquenchingduringheattreatment,asopposedtoanoilquench,isthatthecoolingratecanbecontrolled;therefore,itispossibletoreducewarpingandcrackinginthecomponent.Duringgasquenching,parti-culatemattermaybecomeentrainedinthegasflows,andimpingementwiththeinsulationmayresultinmaterialremoval.Inthechallengingenvironmentofgasquenching,thereisarequirementforerosionprotectionoftheCBCFbytheuseofhigherdensitycarbon-basedcoatingandcladdingmaterials.Generally,ductileandbrittlematerialsexhibitdifferenterosioncharacteristics;ofparticularinterestistheirrelationshipbetweentheerosionrateandtheangleofincidence[15].Ductilematerialstendtodisplaymaximumerosionatglancinganglesofimpact,approximately30°formetals,andmaterialremovalisthoughttooccurbyamicromachiningmechanismwithacontributionofdeformationwearathigherangles.Ontheotherhand,forbrittlematerials,maximumerosionisfoundwheretheerodentstreamisperpendiculartotheerosionsurface,andmaterialremovaltypicallyresultsfromtheformationofHertzianorlateralcracks.Althoughitisaconvenientapproachtoidealizematerialserosionbehaviourinthismanner,itisanoversimplification,becauseerosionisfoundtodependonotherfactors,includingtheerosionconditions,suchaserodentpar-ticlesizeandshape,aswellasthedetailsofthemicrostructureofthetargetmaterial.Thispaperisconcernedwiththeexaminationofthemicrostructureandtheefectivenessinimprovingtheerosionresistanceofseveralcandidatecoatingsandcladdings.Theresultspresentedinvolvethesteadystateerosionrateasafunctionofimpingementangleunderdefinedconditions.Theoverallaimofthisworkistorelatethemicrostructuretotheerosiondatabymeansofamechanisticapproach.materialsincludedtheFiberMaterialsInc.C3composite],whichisresinimpregnated,andtheToyoTansoG3470.Inaddition,ahigh-densitycarbon—carboncompositewasproducedbyemployingCVDoveraperiodof800htoinfiltratea5mmthicksectionoftheCBCFsubstratetoadensityof1Mgm\3.TheCVDprocessusednaturalgasasthecarbonprecursorandnitrogenasthecarriergas.Thedensificationwascarriedoutatapproximately1100°Cunderareduced.
2.Experimentalprocedure
2.1.Materials
TheCBCFusedasthesubstratewasastandardcommercialmaterial(density0.18Mgm\3)manufacturedbyCalcarbLtd.Thecoatingandcladdingmaterials
wereappliedtothexyplaneoftheCBCFsubstrate(seetheschematicdiagramofCBCFstructureinFig.1);thexyplaneisperpendiculartothedirectionofminimumthermalconductivityandhenceismostlikelytobetheexposedsurfaceoftheinsulationinafurnace.Thecoatingandcladdingmaterialsexam-inedinthispaperwereallcarbonbasedandtheyarelistedinTableI.ThecoatingmaterialsaredefinedasthosethatbondindependentlytotheCBCFsubstrate,whereasthecladdingsarebondedbymeansofacar-bonizingcement.CalcoatandCalcoatMarecolloidal
graphitepaintcoatingsthatwereappliedtotheCBCFsubstratebybrushing.Thematerialwassubsequentlyheattreatedat900°Cinnitrogentocarbonizetheresin
constituentofthecolloid.Higherdensitycarbon—carboncomposites('1.3Mgm\3)usedascladdingpressureof5kPa.(NotethattheCVDofcarbonintheinteriorofaporousmediumissometimestermedchemicalvapourinfiltration,CVI.)AnothercladdingmaterialwasgraphitefoilwhichwasproducedbyToyoTansobycompressingexfoliatedgraphiteflakesinarollingoperation[23].Thefoilisflexibleinnatureandispredominantlyheldtogetherbymechanicallocking,asnobinderisused.FurthersampleswereproducedbysubjectingtheCalcoatcoatingandthegraphitefoiltoaCVDtreatment(samplesdesig-nated#CVDinTableI)foraperiodof75hundertheconditionsdescribedabove.Amoreextensivedescrip-tionofthematerialswillbeforthcominginthedis-cussiononthemicrostructures.
2.2.Erosiontesting
Multiparticleerosiontestswereperformedonagas-blasttyperig,asdescribedbyCarteretal.[24].Inthisapparatustheerodentparticlesentertherigviaanapertureinthebaseofanopenhopper.Aventurifittedinthesystemallowstheparticlestobeentrainedinthecompressedairflow.Afterpassingthroughanozzlewithan8mminternaldiameter,theparticlesstrikethetargetatastand-ofdistanceof40mm.Thetargetspecimenshadnominaldimensions25mm;12.5mm;5mm.
TheerodentusedwasangularequiaxedsilicasandobtainedfromHepworthMineralsandChemicalsLtd,Redhill,UK.Theerodentwassievedtoparticlesizesbetween150and300lm,themeansize(byweight)was230lmwhichwasfoundbyalaserdifrac-tionmethod(Mastersizer1005,MalvernInstrumentsLtd,Malvern,UK).Thevelocityoftheparticleswas6ms\1,foundbythestreakingcameratechniqueatthepositionofthetarget.Thismethodinvolvedexpo-singthefilmforaknownlengthoftimeandmeasuringthelengthofthelinethattheparticleproducesonthe
film.Erosiontestswerecarriedoutatanglesof30°,45°,60°,75°and90°.
Generally,thesampleswereimpactedbyafixedmassoferodent,thencleanedandreweighed.Thisprocesswasrepeatedandtheaccumulatedmasslossplottedagainsttheaccumulatedmassoferodent.The
erosionrate,expressedintermsofmassremovedperunitmassoferodent,wascalculatedfromthegradientoftheseplots.However,inthecaseofthelow-densityCBCFsubstratematerial,whichwasinvestigatedforcomparisonpurposes,asignificantmassoferodentpenetratedandwasretainedwithintheporousstructureofthecomposite.Whencalculatingtheerosionrate,th
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