Stress distributions in plasma-sprayed thermal barrier coati

Stress distributions in plasma-sprayed thermal barrier coatings as a function of interface roughness and oxide scale thickness

SurfaceandCoatingsTechnology161(2002)26–35

Stressdistributionsinplasma-sprayedthermalbarriercoatingsasa

functionofinterfaceroughnessandoxidescalethickness

¨M.Ahrens*,R.Vaßen,D.Stover

¨¨WerkstoffeundVerfahrenderEnergietechnik1,D-52425Julich,¨ForschungszentrumJulichGmbH,InstitutfurGermany

Received29May2001;acceptedinrevisedform13May2002

Abstract

Duringthermalcyclicloading,plasma-sprayedthermalbarriercoatings(TBCs)oftenshowfailurewithinthetopcoatcloseto

theinterface.Inallcasesthisresultsfromcrackpropagationofpre-existingcracksnearthebondcoat(BC)–topcoatinterface.StressesdevelopingonamicroscopicscaleneartheBC–ingafiniteelement(FE)methodthelocaldependenceofstressesinthevicinityofthisroughinterfacewasinvestigated.Measurementsofrealroughnessprofilesprovidedgeometricalparametersneededforthecalculations.AsignificantdifferenceinthestressdistributionswasfoundforpeakandvalleylocationsoftheBCroughnessprofile.TheeffectofBCoxidationonstressdevelopmentwasmorepronouncedinthecaseoflessroughness.AnalyticalfitsoftheFEresultsrevealedhowtheparametersofroughnessandtheoxidethicknesscorrelatewiththestresslevels.Inthenextstageofresearchthesefitswillserveasinputdataforamicrostructuralbasedlifetimemodel. 2002ElsevierScienceB.V.Allrightsreserved.

Keywords:Burnerrigtest;Elasticproperties;Roughness;Plasmaspraying;Thermalbarriercoatings

1.Introduction

Theuseofthermalbarriercoatings(TBCs)allowstheincreaseoftheturbineinlettemperatureandhenceanincreaseoftheefficiencyofturbineengines.Atypicalplasma-sprayedTBCsystemconsistsofanair-plasma-sprayed7–8wt.%yttria-stabilizedzirconia(YSZ)andavacuum-plasma-sprayedMCrAlYbondcoat(BC).Thelatterprovidesprotectionfromoxidationofthesubstrateandalsogoodmechanicalinterlockingbetweentheceramictoplayerandmetallicsubstrateduetoitsroughsurface.DetailsonthefunctionalityandstructureofTBCscanbefoundinw1–4x.However,thelifetimeofTBCsisatpresentnotpredictableandthisinturndiminishesanefficientuseofTBCsinindustrialapplications.Areliablemodelthatcorrelatesthemicro-structureandmechanicalpropertiesofthecoatingwithitslifetimeisstillnotavailable.Severallifetimemodels

*Correspondingauthor.Tel.:q49-711-689-1765;fax:q49-711-689-1722.

E-mailaddress:m.ahrens@fkf.mpg.de(M.Ahrens).basedonsemi-empiricalapproachesw4–6xdonottakeaccountofthemicrostructure.Incontrast,wefocusonamodelwhichisbasedonmicrostructuralconsidera-tionsw7x.

TBCfailureoftenresultsfromthermo-mechanicalstresseswhichariseduringthermalcyclingduetothermalexpansionmismatchandBCoxidationw5x.Thethermalexpansioncoefficientsarehighforthesubstrateandthebondcoat()15=10y6yK),intermediatefortheYSZtopcoat(10–11=10y6yK)andquitelowfortheoxide(8=10y6yK)w8,9x.DamageisprimarilyinducedclosetotheBC–ceramicinterface.Thepro-nouncedroughnessofthatinterface,necessaryinthecaseofplasma-sprayedTBCs,createslocationsofhighstressconcentration.Thesestresseswillinitiatecrackpropagationparalleltotheinterfacefinallyleadingtocrackslongenoughtocausespallationofthetopcoatw10–12x.Thereforetheunderstandingofthestressdis-tributionneartheinterfaceisfundamentalforlifetimeprediction.AtpresenttherearemanymodelspredictingthestresseswithinaTBCw13–20x.Hereweinvestigate

0257-8972/02/$-seefrontmatter 2002ElsevierScienceB.V.Allrightsreserved.PII:S0257-8972 02.00359-6