Stress distributions in plasma-sprayed thermal barrier coati(6)

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

32M.Ahrensetal./SurfaceandCoatingsTechnology161(2002)

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Fig.8.TheradialstressabovethepeakasafunctionofRfordifferentoxidethicknesses(Doxs1,3,6,10mm),differentamplitudesandperiodlengths.

theceramictopcoatandbondcoat.Thusduringcoolingthetopcoattendstocontractmorethantheoxidelayerbetweentopcoatandbondcoat.Foranoxidelayerofacertainthicknessthiswillleadtoacompressiveradialstressstateatthepeaklocationsoftheinterfacerough-nessandtoatensileradialstressstateatthevalleylocationsw7x.SothegradualgrowthofanoxidescaleduringthermalcyclingcanresultinconversionofradialstressesinthevicinityoftheBC–TBCinterface.3.2.1.Peaklocation

Fig.8showstheeffectofoxidethicknessontheradialstressabovethepeaklocationasafunctionofRforvariousamplitudesAandperiodlengthsL.Theoriginoftheradialco-ordinateispositionedexactlyattheTGO–TBCinterface.ItcanbeseenthatinallcaseswithincreasingoxidethicknessstressconversionfirsttakesplacedirectlyattheTGO–TBCinterfacewhilethelocaldependenceofsrisnolongermonotonic(this‘critical’thicknessdependsonAandL).Further,itisobviousthatforthecalculationsshownstressconversiontakesplaceatlowerTGOthicknessforlowerAandL,respectively.Fig.9showstheradialstressattherough-nesspeakasafunctionoftheoxidethicknessfordifferentamplitudesandfordifferentperiodlengths.

Obviously,thechangeofthestressfromtensiletocompressivewithgrowingTGOthicknessisrelativelyfastforintermediatevaluesofA(seeleftdiagramofFig.9).Naturally,theeffectofstressconversionhastovanishinthecaseofA 0(vanishingroughness).Accordingly,thedependenceoftheradialstressontheoxidethicknessbecomesweakforsmallvaluesofA.Theright-handsideofFig.9showsthedependenceofsr(attheinterface)ontheoxidethicknessfordifferentperiodlengthsLoftheroughnessprofile(fixedA).ItisrevealedthatwithgrowingTGOstressconver-siontakesplacefasterforsmallervaluesofL.ForlargevaluesofLthedependenceontheoxidethicknessisgraduallylost,similartothecaseofvanishingA.3.2.2.Valleylocation

Wefinddifferentbehaviourforthevalleylocationsoftheinterfaceroughness.Fig.10showsthecalculatedlocaldependencesoftheradialstressabovethebottomofthevalleyforvariousvaluesofoxidethicknessandamplitudes.Againtheoriginoftheradialco-ordinateispositioneddirectlyattheTGO–TBCinterface.Incon-trasttothepeaklocation,withincreasingoxidethicknessstressconversion(herecompressivetotensile)firsttakesplacesomedistanceawayfromtheinterface.With