Stress distributions in plasma-sprayed thermal barrier coati(7)

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

M.Ahrensetal./SurfaceandCoatingsTechnology161(2002)

26–3533

Fig.9.Radialstressatthepeakasafunctionofoxidethicknessfordifferentamplitudes,A,andperiodlengths,L.Left:fixedL,amplitudes:0.5–20mm;right:fixedA,periodlengths:65–200mm.

increasingoxidethicknessthevalueofRatwhichstressconversiontakesplaceshiftstowardstheinterface.Inaddition,thelocaldependenceofsrceasestobemonotonic.AsalreadymentionedinSection3.1.2,paredwiththepeaklocations,stressconversiondirectlyattheinterfaceisdelayedtolargervaluesofoxidethickness.Inthecaseofhighamplitudestheoxidethicknessobviouslyhastoreachunrealisticallylargevaluesuntilstressconversionisachieveddirectlyattheinterface.

Onefactissimilartothepeaklocation:inbothcasesstressconversioninthevicinityoftheinterfacetakesplacefasterwithgrowingoxidescaleforsmallerampli-tudes.Additionally,Fig.11showstheradialstressattheinterfaceasafunctionofoxidescalethicknessforvariousamplitudesandvariousperiodlengths.AscanbeseenintheleftdiagramofFig.11,theeffectofstressconversionisstrongestfortheintermediaterangeofA(andvanishesforA 0).Apartfromthedepend-enceshownforAs5mm,thereisnostressconversionobservablewithintherangeofoxidethicknessconsidered.

Thedependenceofsr(attheinterface)ontheoxidethicknessfordifferentperiodlengthsLoftheroughnessprofile(Afixed)ispresentedintherighthandsideofFig.11.Incontrasttothepeaklocation(seeFig.11),alargervalueofLleadstoafasterconversionofstresswithgrowingoxidescale.3.3.Summaryoftheresults

TheinfluenceofamplitudeAandperiodlengthLofasinusoidal-shapedBC–TBCinterfaceontheradialstresslevelsandtheirlocaldependenceswithinthetopcoatwasinvestigated.Severalfeatureswereobserved:●WithoutTGOtheradialstressesatthepeakandatthevalleyoftheroughnessareafunctionofthe

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●

●

●

quotientLyAasagoodapproximation.Theyarenotafunctionoftheradiusofcurvature(;L2yA).

Thequitehighstresslevelsofverysharproughnesspeakshaveashortattenuationlengthandhenceonlyinfluencethestressstateinthetopcoatwithinadistanceofsomemicrometresfromtheinterface.Thelocaldependencesoftheradialstressabovethebottomofthevalleyaredifferentcomparedtothoseatthepeakposition.ThedifferenceismostdistinctinthecaseoflargeAandsmallL.InthiscasethevalleysformnarrowtroughsinwhichthematerialoftheTBCisborderedbybondcoatmaterial.Incontrasttothis,thetopcoatmaterialatthepeaklocationsisnearlyfree-standingforallamplitudesandperiodlengths.

AtthepeakpositionwithgrowingTGOthicknessthetensilestresswillatfirstbeconvertedintocompressivestressdirectlyattheinterface.Incon-trast,atthevalleylocationstheconversionfromcompressivetotensilewillfirsttakeplacesomedistanceawayfromtheinterface.

Forbothlocations,peakandvalley,stressconversionwithgrowingoxidescaletakesplacefasterforrough-nessprofileswithintermediateamplitudes.

4.Discussion

Thelastaspectmentionedintheprevioussectionisanimportantfactforourmicromechanicallybasedlifetimepredictionmodelw7x:inthismodelweproposethatinthefirststagesofthermalcyclingcrackslocatedwithinthetopcoatnearpeaksoftheTBC–BCinterfaceareopenedduetothetensileradialstresses(whichoccurduringcoolingtoroomtemperature).Compressivestresslevelsatvalleypositionspreventfurthercrackpropagation.Duringthermalcyclingoxidationofthebondcoatproceeds.Aftertheoxidescalehasreachedacertainthicknessstressconversionoccursandthecrackscanpropagateintovalleylocations.Coalescenceof