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MATERIALSANDMANUFACTURINGPROCESSES,17(2),169–176(2002)

SOLIDIFICATIONMICROSTRUCTUREOFPLASMASPRAYEDANDREMELTEDCARBIDE

COATINGS(Cr3C2/Ni-Al,W2C-WC/Co)

J.Iwaszko*andZ.Nitkiewicz

TechnicalUniversityofCze?stochowa,InstituteofMaterials

Engineering,al.ArmiiKrajowej19,42-201Cze?stochowa,Poland

ABSTRACT

Carbidelayersappliedonthesteelsubstratebytheplasmasprayingmethodwereremeltedusinghigh-energyheatsources.Adiversemethodologyofcoatingtreatmentwasused:rangingfromasingleandmultipletreatmentwithacontinuousmicro-plasmaarc,throughthetreatmentbytheTIG(tungsteninertgas)weldingmethod,toCO2laserremelting.Thequalityoftheremeltedlayerswasevaluatedbasedonmacro-andmicroscopicstructuralexaminations,x-rayphaseanalysis,tribologicaltests,andhardnessmeasurements.Theresultsoftheinvestigationscarriedoutcon?rmedthepossibilityofremeltingsprayedcoatingsandobtaininglayersfreefromfaultsandimperfectionstypicalofplasmaspraying.Themicro-andx-raystructuralexaminationsrevealed,amongotherthings,asigni?cantreductionofporosity(bothopenandclosedones),homogenizationofthechemicalcomposition,smoothingofthesurface,andtheimprovementofcoatingadhesiontothesubstrate.Thepresentstudyisacontinuationoftheinvestigationscarriedoutonthermallysprayedandmicro-plasmaremeltedoxidelayers,thatwerestartedatthebeginningofthe1990sattheMaterialEngineeringInstituteoftheTechnicalUniversityofCze?stochowa.

*Correspondingauthor.Fax(048)(34)3250721;E-mail:iwaszko@mim.pcz.czest.pl

169

Copyrightq2002byMarcelDekker,http://wendang.chazidian.com

外文文献

170IWASZKOANDNITKIEWICZ

INTRODUCTION

Rapidcrystallizationisaprocessthatmoreandmorefrequentlyaccompaniesthestate-of-the-artsurfacetreatmentemployinghigh-energyheatsources(suchaslaser,plasma,orelectronbeam)(1–9).Intensiveheatingofathinupperlayerfollowedbyitsequallyrapidcooling,causesthestructureofthesolidifyingmaterialtobeformedinthespeci?cconditionsofahightemperaturegradientandashortdurationofsolidi?cation.Ononehand,thisprovidesapossibilityofproducingmaterialswithpropertiesunachievablebytheclassicmethodsofheatorheatandchemicaltreatment.Butontheotherhand,however,itisassociatedwithariskofanaccidentalcharacterofthestructure,andthusalsowiththeunpredictabilityof?nalproductproperties.

Underthesecircumstances,theknowledgeofrapidcrystallizationspeci?csandtherelationshipsoccurringbetweenthetreatmentmethodologyandthecharacterofchangestakingplacewithintheremeltedlayergainparticularimportance.

Theaboveproblemsandtheimportanceofrapidcrystallizationinthemodernengineeringencouragedtheauthorstoundertaketheirownstudiesonthepossibilityofthepracticalutilizationofthisphenomenoninthetreatmentofplasmasprayedlayers,andparticularlytheeffectiveeliminationofdefectsandimperfectionsofthesecoatings(10–14).

Inparallelwiththeanalysisofrapidcrystallizationeffectsintheaspectofstructuralandmorphologicalchangesinducedbyremelting,theevaluationwasmadeoftheusefulnessofmicro-plasmaandweldingtechniquesforimprovingthepropertiesofplasmasprayedlayers.

EXPERIMENTALPROCEDURE

Intheexperiment,carbidelayersofW2C-WC/CoandCr3C2/Ni-Altypesappliedbytheplasmasprayingmethodonrectangularprismsteelspecimenswereused.Theexperimentalmaterial,aftereliminatingspecimensthatdidnotmeetthequalityrequirementsdemandedfromsprayedcoatings,wasremeltedusinghigh-energyheatsources.Adiversemethodologyofcoatingtreatmentwasused:rangingfromasingleandmultipletreatmentwithacontinuousmicro-plasmaarc,throughthetreatmentbytheTIGweldingmethod,toCO2laserremelting.DatarelatedtothepowdersusedareshowninTable1.Carbidecoatingsweresubjected

Table1.

PowderType

Tungstencarbides

Tungstencarbides

Chromiumcarbide

ChromiumcarbideCharacteristicsofPowdersUsedChemicalCompositionW2C-WC/Co(16%Co)W2C-WC/Co(12%Co)Cr3C2/Ni-Al(25%Ni-Al)

Cr3C2/Ni-Al(10%Ni-Al)Granularity(mm)0.04–0.1#0.04,0.06,0.06

外文文献

PLASMAANDCARBIDECOATINGS171tocomprehensivemicro-andx-raystructuralexaminations;comparativetestswerealsodoneforwearresistance,aswellashardnessmeasurements.

RESULTSANDDISCUSSION

Lightandscanningelectronmicroscopyexaminationsrevealedsubstantialchangesinthestructureandbuildoftheremeltedcoatingscomparedtotheinitialmaterial(anopticalmicrographoftheplasmasprayedcoatingisshowninFig.1).Aspeci?cfeatureoflayersremeltedonlypartiallytoacertaindepthwas,inparticular,thepresenceoftwodistinctlyformedandmutuallydiversezones:anupperlayerthatwascharacterizedbyresidualporosityandreducedroughness(Fig.2a—“A”),followedbyazoneretainingthefeaturesandcharacterofthesprayedmaterial(Fig.2a—“B”).Themutualquantitativerelationshipbetweenthelayersdiscussedwasaresultantoftreatmentparametersandtheremeltingtechniqueused.Thus,itwaspossibletomodel?exiblythequantitativerelationshipbetweenthezonesuntilthecompleteeliminationoftheporouspart,theevidenceofwhichisFig.2b.Inthecasewhereremeltingcoverednotonlythecoatingbutalsothesubstrate(which,asarule,tookplacewithmultipleremeltingorwithtreatmentusinglargecurrentsandlowvelocitiesofheatsourcemovement),thepresenceofanintermediarylayerbindingthecoatingwiththesubstratewasseen(Fig.2c).Thislayerexhibitedthefeaturesandcompositionofboththesubstrateandthecoating,andwasaresultofpreviousmeltingofboththemediawiththeirsubsequentmixingintheliquidstateandeitherpartialorcompletedissolutionofthecoatingmaterialinthesubstratematerial(Fig.2d).Thebindinglayerformedsigni?cantlyimprovedadhesionofthecarbidelayertothesubstrate.Asideeffectofremeltingaccordingtotheaboveprocedurewasthepresenceoflargesphericalpores.Theseporesoccurredmainlyinlocationswherethecoating-substrateinterfacehadbeeninexistencepreviously.Itshouldbenotedherethatthiseffectwasonlylocalincharacter,whichisindicatedinFig.2bandcshowinglayersfreefrommaterialdiscontinuitiesofthistype.

Inallspecimensanalyzed,areductionofporositywasnoted.Thesechangestendedtoreducetheopenporosity.However,itisbelievedthatthistype

of

Figure1.Plasmasprayedcoating,opticalmicroscope,Nitaletched.

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172IWASZKOANDNITKIEWICZFigure2.ExamplesofthediverseremeltingdepthofW2C-WC/Cotypecoatings,opticalmicroscope,Nitaletched(a–c);agraphicalmodel

(d).

porositymighthavebeendominatingintheinitialremeltingphaseasaresultoftheintensi?eddisplacementofinternalporestowardsthesurface.Quantitativeporositytestsperformedusingacomputerimageanalyzershowedadropinporositytoalevelrepresentativeoflayersproducedbyothermethods,suchashighvelocityoxy-fuel?amespraying(HVOF)orsmallparticleplasmaspray(SPPS)(14–16).

外文文献

PLASMAANDCARBIDECOATINGS173

Dependingonthematerialcoolingrate:beingafunctionofcurrent–voltageparametersandthetimeofinteractionoftheenergymediumwiththelayersurface,differentmorphologiesofphaseswereobserved.Withincreasingrateofheatinputtothenonheatedpartofthematerial,theintensi?cationofstructurere?nementtookplace,anddisappearanceofdendriticstructurefeaturewasobservedinfavorofstructurewithasimplerbuilt,i.e.,globularandcellular-dendriticones.Globularandcellular-dendriticstructureswerepredominatinginlayerssubjectedtolaserandplasmatreatments.Dendriticstructures,ontheotherhand,prevailedinlayersremeltedbytheTIGweldingmethod.

Thescanningelectronmicroscopyexaminationsofetchedmetallographicsectionsshowedthepresence(inthetungstencarbide-basedlayersremeltedusinglargecurrentsandlowvelocitiesofheatsourcemovement)ofhighly?ne-dispersedeutecticstructuresbelongingtothegroupofregulareutecticswithalamellargeometry.ItwasfurtherdemonstratedbyEDXandx-rayexaminations,amixtureoftheW2Ccarbidephaseandasolidsolutionrichintungstenandcobalt.Thepresenceofeutecticstructurewasalsofoundinchromiumcarbide-basedlayers.Alsointhiscase,lamellargeometryeutecticspredominated(Fig.3).

Thechangesinboththebasematerialandthestructureoftheremeltedlayer,notedduringthemicroscopicexaminations,expectedcausefavorablechangesinitsproperties.Theseexpectationswerecon?rmedbycomparativehardnessandwearresistancetests.TheresultsofwearresistancetestsareshowninFig.4.

X-rayexaminationsperformedwithinthescopeofthisstudycon?rmedthepreviouspresumptionsofsigni?cantchangeslikelytooccurinthephasecompositionsoftheinitialandremeltedmaterials.Thepresenceofnewphaseswasfound,includingnonequilibriumphasestypicalofrapidcrystallization.InthecaseofW2C-WC/Cotypecoatings,decompositionoftheWCcarbide(constitutingoneofthecomponentsofthepowderusedforspraying)wasobservedamongotherthings,andtheformationofcomplexcarbidephasesbeing,asarule,acombinationofcarbon,cobaltandtungsten(Co6W6CandCo7W6).Inthechromiumcarbide-basedlayers,ontheotherhand,thetransformationoftheFigure3.EutecticstructuresinremeltedlayersofW2C-WC-Co(a)andCr3C2/Ni-Al(b)types;metallographicsections;scanningelectronmicroscope,Nital

etched.