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Eprints ID : 14023
To link to this article :
DOI:10.1016/j.jeurceramsoc.2013.10.013
URL :
http://dx.doi.org/10.1016/j.jeurceramsoc.2013.10.013
To cite this version :
Pin, Lisa and Vidal, Vanessa and Blas, Fabien and Ansart, Florence
and Duluard, Sandrine and Bonino, Jean-Pierre and Le Maoult,
Yannick and Lours, Philippe Optimized sol–gel thermal barrier
coatings for long-term cyclic oxidation life. (2014) Journal of the
European Ceramic Society, vol. 34 (n° 4). pp. 961-974. ISSN
0955-2219
Any correspondence concerning this service should be sent to the repository
administrator: [email protected]
Optimized
sol–gel
thermal
barrier
coatings
for
long-term
cyclic
oxidation
life
Lisa
Pin
a,b,1,
Vanessa
Vidal
a,1,
Fabien
Blas
a,b,2,
Florence
Ansart
b,3,
Sandrine
Duluard
b,4,
Jean-Pierre
Bonino
b,3,
Yannick
Le
Maoult
a,5,
Philippe
Lours
a,∗aUniversitédeToulouseMinesAlbi,InstitutClémentAder,CampusJarlard,81013Albi,France
bUniversitédeToulouse,UPS-INP-CNRS,InstitutCarnotCIRIMAT,118RoutedeNarbonne,31062ToulouseCedex09,France
Abstract
Newpromisingthermalbarriercoatings(TBCs)processedbythesol–gelroutearedepositedontoNiPtAlbondcoatedsuperalloysubstratesusing thedipand/orspraycoatingtechnique.Inthisstudy,theoptimizationoftheprocess,includinganappropriateheattreatmentpronetodensify theyttria-stabilized-zirconia(YSZ)top-coatandleadingtothesinteringandthedevelopmentofaresultingcracknetwork,isinvestigated.In particular,relevantinformationoninternalstrainevolutionduringtheheattreatmentareobtainedusinginsitusynchrotronX-raysdiffractionand confirmastabilizationoftheTBCthroughtheoccurrenceofthemicro-cracksthatbeneficiallyreleasesthein-planesinteringstress.SuchTBCsare subsequentlyreinforcedusingadditionalmaterialbroughtwithinthecracksusingsol–gelspraycoating.Theeffectofvariousprocessparameters, suchasthepre-oxidationofthebond-coat,onthesolgelTBCsconsolidationandtheircyclicoxidationresistanceenhancement,ispresented. Reinforcedsol–gelTBCsaresuccessfullyoxidizeduptomorethanonethousand1h-cyclesat1100◦C,withoutanydetrimentalspallation.
Keywords:Thermal-barrier-coating;Cyclic-oxidation;XRD-synchrotron;Sol–gel-processing;Spallation
1. Introduction
Thermalbarriercoatings(TBCs)arewidelyused for vari-ousapplicationsinturbojetenginegasturbinesandcombustion chambers in relation with their excellent thermal protection propertiesallowing drastic improvement of component dura-bilityandefficiency.1,2
Typically, the overall thermal protection system includes: (i) the TBC itself, a ceramic top coat (TC) made of yttria-stabilized-zirconia (YSZ) acting as thermalinsulator, (ii) the superalloysubstratethatsupportsmechanicalloading,and(iii)
∗Correspondingauthor.Tel.:+33563493078;fax:+33563493242. E-mailaddresses:[email protected](L.Pin),
[email protected](V.Vidal),[email protected](F.Blas),
[email protected](F.Ansart),[email protected](S.Duluard),
[email protected](J.-P.Bonino),
[email protected](Y.LeMaoult),
[email protected](P.Lours).
an aluminium rich Bond Coat which enhances the cohesion betweenthesubstrateandtheTBCanddevelopsbyoxidation a fine alumina filmreferred to as the thermally grown oxide (TGO).Withinthismulti-materialsstructure,eachlayer, charac-terizedbyspecificphysical,thermalandmechanicalproperties, shows–uponprocessingand“in-service”thermalexposures– distinct thermomechanicalbehaviour,thereby resultingin the establishment ofinternalthermalstresses. In addition,during hightemperatureexposure,thealumina(Al2O3)TGO,actingas
adiffusionbarrier,continuouslygrowsattheinterfaceBC/TBC whichlikelyinduceslocalincreasesofthemismatchbetween theBCandTBClayers.
As a consequence, in highly complex TBC systems, fail-ure mechanisms upon temperature cycling are very intricate andlotsoftheoreticalaswellasexperimentalresearch investi-gatingmechanisticbehavioursandmicrostructuralmechanisms are dedicatedtounderstandingthe variousprocessesofcrack initiationandpropagation,delaminationandspallation.2,3
Uptonow,twomaincoatingprocessesareusedtodeposit TBCs for industrial applications, namely the electron beam physicalvapordeposition(EB-PVD)andtheairplasmaspray (APS), each generating specific layer morphology, deposit microstructureandthermo-physicalproperties.
EB-PVD results in a columnar structure with grain boundaries roughly normal to the substrate surface allowing satisfactory accommodation of in-service lateral thermome-chanical stresses and related strains. However, the thermal conductivityofEB-PVDcoatingsisnotfullyoptimizedasthe perpendicular growth of the columns favours extensive heat transfer between the outer surface of the multi-material sys-temandthesuperalloysubstrate.Asacomparison,thetypical microstructureforAPScoatingishighlylamellar,resultingin bothlowerthermalconductivityastheporosityisdistributed per-pendiculartotheheatflux,andlowercapabilitytosatisfactorily sustainlateral constraints.Variousalternativewaysto synthe-sizeTBCshavebeen proposedintheliterature,basedonsoft chemicalprocessing.4–6
In thispaper, anewpromising methodfor depositing and reinforcingTBCs,isinvestigated.Indeed,recentlythe synthe-sisanddepositionofTBCsusinganewattractivesol–gelroute has been successfullydeveloped.6–9 This versatile technique, promoting, on contraryto EBPVD andAPS, non-directional deposition,allowstoproduce eitherthin orthick coatingsby usingdiporspraytechniqueorcombinedmethodofboth tech-niquesdepending on therequired result.Sol–gelTBCsshow isotropic microstructure withrandomly distributedporosities, whichstraightforwardly resultsin an interesting compromise betweenthermalconductivityandmechanicalstrength.In pre-viouspapers,8,9theoptimizationofthemanufacturingprocessof sol–gelTBCs,themeanstoenhancetheircyclicoxidation resis-tanceusingstructuralreinforcementaswellasthemechanisms responsiblefortheirpossibledamagesduetolongtermcyclic exposureathightemperaturehavebeendiscussedindetail.
Essentially,thedegradationofsol–gelTBCsisinitiatedbythe formationofaregularcracknetworkoccurringeitherduringthe post-depositionthermaltreatmentrequiredtosinterthedeposit orduringtheveryfirstcyclesofoxidation.Itisworthtonotice that,inbothcases,thisregularsurfacecracknetworkisaresult ofthein-planestressreleaseduetothesinter-inducedshrinkage ofthezirconiascale.Subsequently,undercumulativeoxidation cycles,enlargementandcoalescenceofthecracksoccur, pro-motingthedetachmentofindividualTBCcellsandfurtherthe completespallationoftheTBC.
ToimprovethecyclicoxidationresistanceoftheTBCs,two refinementswereproposedbothrelatedtotheoverallprocessing, namely(i)toenhancetheefficiencyofthesinteringthermal treat-mentcarriedoutrightafterTBCdepositionand(ii)tostabilize thecracknetworkbyfillingcrackgroovesusingsupplementary diporspraycoatingpasses.Itwasshownthattheheattreatment parameterssuch as the heating/cooling rates andthe holding timeat dwelltemperature, dramaticallyimpact the geometri-calcharacteristicsofthecracknetworkandconsequentlytheir responsetocyclic oxidation.Afteradjustments,the“optimal” thermaltreatmentparameters,resultinginasignificantextentof theTBClife,correspondtoanexposureat1100◦Cduringtwo hourswithheatingandcoolingratesof50◦C/h.8
In addition, the feasibility of consolidating sol–gel TBCs byadditionalfillingsofzirconiaintothesinter-inducedcracks wasinvestigatedbyadjustingdifferentprocessparameterssuch as the choice of either dip-coating or spray-coating and the
modificationoftheslurryviscosity.9Itsubsequentlyturnedout thatspray-coatingtechniqueleadstoamoreefficientandamore homogeneousfillingofcrackaswellastheselectionofaspecific slurryviscosityforeachindividualpassdependingonthedepth andwidthofcracktofill(bymodifyingtheweightpercentof powder).Thisfillingoptimizationallowssalientimprovement of thecyclic oxidationbehaviour ofthespray-coat reinforced TBCs.9
Note that the failure mechanisms of the optimized and reinforced sol–gel TBCs are more complex than that of non reinforced TBCs. Thisresults from the moreconnected microstructure–thoughmoreuniforminthickness–ofthe rein-forcedTBCshowingacomposite-likemorphologyincludinga skeletonorframe,corresponding tothepartiallyfilledcracks, andamatrix,namelytheinitialsinteredYSZ.
ThedegradationofsuchTBCsresultsfromtheinitiationand propagationofcracks,mainlylocatedattheinterfacebetween the TBCandthe TGO. Asamatterof fact,as reinforcement of crack prevails, crackscan extendmuchmorethan in non-reinforced TBCbefore generating spallation, greatly limiting thedetachmentofindividualspalls.Spallationdevelops follow-ingthecompletepropagationofthecrackthroughoutthewhole specimen,producing–whenoccurringsubsequentlytoahigh number of cumulatedoxidation cycles – large-scale degrada-tionasobservedinEBPVDTBC.Basically,theoptimization ofboththesinteringheattreatmentandtheprocedureforfilling the initialcrack network,allowsasignificantimprovement of thesol–gelTBCdurabilityduringcyclicoxidationat1100◦C. Typically,sol–gelTBCproperlysinteredandadequately rein-forcedcanbecycledfor1hat1100◦Conethousandandfive hundredtimeswithoutspallingwhichisroughlyequivalentto theperformanceofEB-PVDTBCs.
Thepresentpaperproposestoinvestigatethoroughly(i)the crack networkformationduetotheinitialsinteringaswellas the effect of the very first oxidation cycles in sol–gel TBCs using synchrotronradiationtomonitor“in-situ” theevolution of thethermalstrainthroughouttheTBCand(ii)the1100◦C cyclicoxidationdurabilityofsolgelTBCsforwhichoptimized processingandfillingaswellasapreliminaryoxidationofthe bondcoatisapplied.Theoverallperformancesofreinforcedsol gelTBCsarecomparedtopreviousresultsandEBPVDTBCs.
2. Materialsandexperimentaltechniques
2.1. Processingofthesol–gelTBC
Thevariousoperationsconductedtosynthesizethermal bar-rier coatings by the so-called sol–gel route, are presented in detailsinpreviousworksdevelopedinthelaboratory.6,8,9Main stepscanbesummarizedasfollows:
i) First step consists in the production of YSZ powders by hydrolysis/condensation (toobtain a gel),supercritic dry-ing andheat treatment at700◦C of a precursor YSZ sol (9.7%molYO1.5).YSZaerogelpowders crystallizeinthe
Fig.1.Opticalmicrographsofthesol–gelTBCbeforeandafterthesinteringheattreatment.
powdersexhibitsaspecificsurfaceareaSwof26m2/g.This
highSw is correlated tothe small crystallite size (26nm)
andthe alveolarmorphologyofaerogelYSZpowders. So withsuchcharacteristicsofthepowders,supercritical dry-ingappearsasapromisingwaytopreparestableslurriesor loadedsolsfromfineYSZparticlesforTBCapplications. ii) Afterpreparation,nanometricpowders,aredispersedintoa
slurry(ratio of 40wt% powder) before shaping on super-alloys substrates. Thermal barrier coatings (TBCs) are deposited ontoNiPtAl bondcoated AM1 superalloy sub-strates using the dip-coating technique consisting in the immersionofthesubstrateinto theslurrypluswithdrawal ata thoroughly controlledrate (250mm per min) to uni-formlyshapethecoatings.Followingeachdip,theproduced layerisdriedfor5minat50◦C.Thiselementaryoperationis repeateduntiltherequiredTBCthicknessisreachedandthe depositoftheentireTBCiscompleted(Fig.1a).Typically, thethicknessoftheobtainedcoatingsisintherange[50m; 150m].
Finally,processedspecimensaresintered2hat1100◦Cusing the appropriate heating and cooling rates discussed above (50◦C/h)to promote the development of a controlledinitial cracknetwork(Fig.1b).TheTBCsconsistoftetragonalphase t’yttria-stabilizedzirconia(YSZ).
Subsequently, specimens with controlled cracks are rein-forced using additional filling of zirconia brought up within crack grooves using spray-coating technique. As detailed elsewhere,9 theslurry usedfor thisadditional fillingprocess, iscomposedofthestartingsol(9.7mol%YO1.5)loadedwith 10or 20wt.% of asuspension of well-dispersed commercial YSZpowders(TOSOH8Y)in1-propanolsolvent.
Notethat thespray-coatingtechniqueallowsthecontrolof boththepressureandthedisplacementrateofthespraynozzle. Betweeneachpass,thecoatingisdried5minat50◦Candfinally thespecimensareheattreated2hat1100◦C,usingheatingand coolingratesof50◦C/h.
Themost efficientTBCreinforcement isobtained using 2 succesivepassesusingasolloadedwith20wt%(high viscos-ity)followedby4passesusingasolloadedwith10wt%(low viscosity).9Fig.2showsacrosssectionmicrographofsuchan optimizedreinforcedsol–gelTBC.
Inaddition,theeffectofpre-oxidizingthebondcoatonthe cyclicoxidationbehaviourofthesol–gelTBCsisinvestigated.
Fig.2.Cross-sectionSEMmicrographofasol–gelTBC.
Asamatteroffact,duringpost-processingheattreatmentsuchas cyclicoxidation,aTGO–consistinginanaluminalayeracting asadiffusionbarrier–formsandgrowsattheBC/TBC inter-face. Itis assumed thatprior tothestable ␣-Al2O3,transient
metastable alumina phase (such as ␥-Al2O3, ␦-Al2O3 or
-Al2O3)conferringpoorpropertiestothesystem(weakadhesion,
highgrowthrate,etc.)whentheytransformintostable␣-Al2O3,
mayformduringhightemperatureexposure.10Thereforethese transientaluminaphasesareavoidedinthermalbarrierssystem tothe benefitof the stable␣-Al2O3 whichisadense,slowly
growing,adherentandprotectiveTGOwellknownfor signifi-cantlycontributingtoextentTBClife.11Awaytocontroland favourthe␣-Al2O3formationisthepre-oxidationoftheBC.12,13
Inthispaper,thepre-oxidationoftheinitialAM1superalloys substratecoatedwithNiPtAlbondcoatiscarriedoutat950◦C for2hinasecondaryvacuumofoxygen(5×10−4mbar). Graz-ingincidentXRD,adaptedforsurfacesandthinfilmsstudyis usedtocheckthecrystalstructureoftheresultingaluminaTGO. Data concerning themain processingsteps (pre-oxidation, sinteringheattreatment,reinforcementbyspraycoating)to pro-ducevariousgradesofsol-gelTBCsaresummarizedinTable1. NotethatspecimenS5correspondstothemostadvancedgrade processedusingthefullyoptimizedparameters.
2.2. Strainevolutionmonitoringbyinsitusynchrotron X-raydiffraction
Aspreviouslymentioned,processingsol–gelTBCsincludes a specificheat treatment at1100◦C topromotethe sintering
Table1
Specimensreference,processdata,ageingconditions(fifthfirstcolumns).Investigatedproperties,mechanismsandeffectsandexperimentalmeanforinvestigation (sixthcolumn).
Specimen Processdata Cyclicoxidationageing Effectinvestigatedandmean
ofinvestigation
Pre-oxidation
Sinteringat1100◦C (heating/coolingrate-dwell timeandtemperature)
Reinforcementby spray-coating Numberof1h-oxidation cyclesat1100◦C S1 No InsituinsynchrotronXRD furnace(100◦C/h–1hat 600◦C) No 0 Sinteringmechanismsby synchrotronXRD S2 No InsitusynchrotronXRD furnace(100◦C/h–1hat 600◦C)
No 5cyclesinsituinsynchrotron XRD
Impactoffirst/earlycyclic oxidationbysynchrotron XRD
S3 No Exsituinfurnace50◦/h No 640 Effectofpre-oxidationonthe
cyclicoxidationlife/cyclic oxidationrig
S4 Yes Exsituinfurnace50◦/h No 1025 Effectofpre-oxidationonthe
cyclicoxidationlife/cyclic oxidationrig
S5 Yes Exsituinfurnace50◦/h Yes 1480 Performanceofoptimized
sol–gelTBCversusEB-PVD TBC/cyclicoxidationrig
of theceramic top-coatandtheinduced formationofacrack networkthroughoutthecoating.
Toinvestigatethemechanismsinvolvedduringsintering,the dynamic evolution of the internal elastic strain/stress as the TBCsystemisheated,thenheldat1100◦Candfinallycooled downtoroomtemperatureismonitoredusingsynchrotron radi-ation. Indeed, as the formation of the crack network results from thetime dependent stressrelease occurring as sintering progresses,thedetailedmechanismsinvolved inthe initiation andthepropagationofcracks,mightbeappreciatedby analyz-ingthetimerelatedevolutionofstrainwithintheTBC.Insitu time-resolvedtechniquessuch asreal-timesynchrotron X-ray diffraction,allowingtomonitorboththeshapeevolutionandthe shiftofdiffractionpeaksversustimeastemperaturechanges,is particularlyadaptedtoanalyzestrainvariationaswellasphase transformation during high temperature exposure (heat treat-ment, isothermal oxidation, cyclic oxidation).14–17 The tech-niqueallowsthereal-timecontinuousmonitoringofdiffraction peakshiftsunderspecificconditionssuchasmechanical load-ing,hightemperatureholding,heatingandcoolingatdifferent rates,etc.,Theanalysisofthepeakposition,shapeandwidthcan providerelevantinformationontheevolutionofboththe inter-nal“uniform”strainandthemicrostructure.18Indeed,auniform
“macro-strain”causestheisotropicexpansion(orcontraction) ofthecrystalunitcell,thusleadingtoauniformchangeinthe latticeparametersthatresultsinashiftofthediffractionpeaks. Inaddition,nanostructuraldeviationsfromaperfectcrystal, i.e.whenafewatomsmovelocallyapartfromtheirequilibrium positions,mayresultinabroadeningofthepeakdiffraction.As aconsequence,smallcrystallitesizeassociatedtoahighdensity ofgrainboundaries,defectsattheatomicscalesuchasstacking faults, vacancies, dislocations or “micro-strain”, as well as a poorcrystallinitysystematicallyresultinapeakbroadening.
The use of the high energy X-rays of ID15B at ESRF withveryhighmeasurementspeeds,allowstosuccessfullyand accurately monitor in situ both the continuous shift and the broadeningofBraggpeaks.Shiftisrelatedtothelattice spac-ingevolution,i.e.totheelasticmacro-strainatthescaleofthe phaseandbroadeningisrelatedtolatticedefect,grainsize,i.e. themicro-strainandthesintering.
A resistive heatershowing a“sandwich geometry” instru-mentedwithtwoopeningsallowingX-raytopassthrough,was mountedatID15Banddiffractionpatternswererecorded simul-taneously intransmissionmodeon thetwo-dimensional (2D) PIXIUMdetectorplacedat746.35mmfromthespecimen.The incidentenergyis87.1keV.
SpecimenS1consistingofasol–gelTBCdepositedon NiP-tAlbondcoatedAM1superalloysubstrateusingdipcoating, wasfirstheattreatedinairasfollowed:(i)heatfromroom tem-peratureupto600◦Cataheatingrateof100◦C/h,(ii)hold1h at 600◦C thenheat upto1100◦C atarate of 100◦C/h, (iii) hold2hat1100◦Cthencontinuouscooltoroomtemperature atarate of100◦C/h.Temperature wasmeasuredbyaS-type thermocouple,previouslycalibrated,andlocatedatthesample surface.Notethatbothheatingandcoolingratesaredifferent fromthe“optimal”heattreatmentparameters(50◦C/h)inorder tolimitthetimeofexperiment.
Duetoveryintensediffractionpeaksfromthesinglecrystal substrate,anappropriatebeamsizeof300mby100mwas chosentoprobeonlythesol–gelTBC.Aschematicdrawingof theexperimentalset-upwiththis“grazingincident”geometry isshowninFig.3.
The raw data consist in 2D diffraction patterns (Fig. 4a) obtained every30minwithanacquisitiontimeof 40seconds, which is long enough to obtain well defined diagrams. The continuousDebye–ScherrerringsinducedbytheYSZcoating
Fig.3.Sketchofthehigh-energyX-raytransmissionset-up(a)anddetailsofthesamplegeometryandX-raybeamsize(b).
Fig.4.Typical2D(a)and1D(b)diffractionpattern(notethatγistheazimuthalangleandqistheradialdirection).
confirmthatsol–gelprocessingresultsinarandomlyoriented TBC microstructure.Integration along the azimuthal angle ␥ allowstoplottheconventional1DdiffractionpatternI(q) dis-playedinFig.4b,withtheradialdirectionqbeingrelatedtothe diffractionangles2θby:q=4/λsinθ.Toavoidanydamageof the2Ddetector, a“beamstop”madeof leadwasinstalledto protectfromthediffuseintensityofthedirectbeam.
Usingthesameexperimentalset-upandbymovingupand downtheresistiveheater,five1-hoxidationcyclesat1100◦C were imposed to a sol–gel TBC specimen (S2 in Table 1), similarlyprocessedandheat-treatedthanspecimenS1.The ele-mentaryoxidationcycleincludes5minheatingupto1100◦C, 55minholdingand20mincoolingdowntoroomtemperature. XRDdatawerecollectedwithfrequenciesandacquisitiontimes thatdependontheconsideredtimeperiodofthecycle.Indeed, duringheating/coolingandholdingstages,2Ddiffraction pat-ternswererespectivelymonitoredevery10s(acquisitiontime 4s)andevery10min(acquisitiontimes40s).
2.3. Cyclicoxidationtests
Cyclicoxidationtestsareperformedinaspecificdedicated automatedriginstrumentedwithaCCDcameratomonitorina realtimebasistheevolutionofthespecimensurfaceuponthe
air-forced5mincoolingfollowingthevariouscumulated5min heatingplus1-hholdingat1100◦C.IndividualCCDimagesare extractedfrom thevideo recording tobetreatedusing image analysis software.From thoseexperimental dataevaluatedin situ,theevolutionofthesurfaceasafunctionoftimefollowing eachcycle,andsotheoverallsurfacedamagekinetics,canbe derived.
First,twounreinforcedspecimensrespectivelypre-oxidized (S4)andnon-pre-oxidized(S3)werecyclicallyoxidizedto eval-uatetheimpactoftheinitiallygrownAl2O3diffusionbarrieron
thelifetime.
In addition,afullyoptimizedsol–gel TBC(S5)processed usingtheappropriatesinteringheattreatment,areinforcement bycrackfillingandapre-oxidationaswellasanEBPVDTBC areconcurrentlyoxidizedtoevaluateandcomparetheiroverall performances.
3. Resultsanddiscussion
3.1. Strainevolutionduringsintering
Apreciseinvestigationoftheelasticstrainevolutionwithin the TBC upon sintering was carried out by studying the shift of the diffraction peak as illustrated in Fig. 5 for the
Fig.5.Seriesof1Ddiffractionpatterns(I=f(Q))forthe(440)peaksoftheYSZonheatingfrom20◦Cto1100◦C(sevenbottomplots)andcoolingfrom1100◦C to20◦C(seventopplots).
Fig.6.(a)Elasticstrainevolutionof(440)planesonheatingandcooling(latticestrainiscomparedwiththeexpectedevolutionresultingfromthermalexpansion only).(b)Enlargementofstrainfluctuationsfrom1100◦Cto600◦Cduringcooling.
tetragonal(440)reflectionoftheyttriastabilizedzirconia(YSZ) constitutingtheTBC.
Thecorresponding Braggpeak positionswereobtained by fittingthe(440)reflectionusingSplitPseudo-Voigtfunctions that wereparticularlywellsuitedformostof theYSZ peaks. Notethat,usingthisfittingprocedure,therelativeerroronthe peak position (d/d) is very low,typically 1×10−3 (0.1%).
Fig.6showsthestrain(ε440T )evolutionversusthetemperatureT
correspondingtothe(440)YSZreflection.Forany(hkl)
reflec-tion,thestrainεhklT isexpressedas:
εhklT = d
hkl T −dRThkl
dRThkl (1)
wheredThklistheinterplanarspacingfor(hkl)planesatagiven temperature TanddRThkl isthereference,stress-freeinterplanar
Fig.7.Elasticstrainevolutionof(440)planesduringearlycycleofoxidation (heatingfor10min,holdingat1100◦Cfor50minandcoolingfor20min).
spacingatroomtemperature.Dottedlinescorrespondtothe the-oreticalelasticthermalstrain(εThermal=αlT)occurringduring
temperaturechanges(T)eitheruponheating(grey)or cool-ing(black)ofthestress-freeYSZ.Itisassumedthatthelinear coefficientofthermalexpansion(CTEorαl)oftheYSZvaries
withtheporosityinthematerial,19thatobviouslychanges dur-ingheating andmost significantlyduring holdingat 1100◦C wheresinteringfullycompletes.Asaconsequence,theCTEof aporousnon-sinteredYSZ,i.e.8.5×10−6◦C−1andthatofa densesinteredYSZ,i.e.11×10−6◦C−1arerespectivelyused duringheatingandcoolingtimeperiods.
Upon heating, the elastic strain ε440T is clearly not linear, revealingthat othercontributionsthan the thermalexpansion havetobeconsidered.
First,fromroomtemperaturetoroughly450◦C,strain fluc-tuationsmayberelatedtothecalcinationofresidualorganics compoundsthat likelydecompose intherange100–400◦C.20 ThisdesorptionmightslightlyincreasethecontactbetweenYSZ particlesurfacesandthuscontributelocallytoalowershrinkage resultinginanadditionalcapillarystrainaround350◦C.21 Fur-therevolutionabove450◦Conlyresultsfromthermalexpansion asexperimentaldataperfectlyfittheplotofthethermalstrain versusthetemperature.
From700◦C,adrasticincreaseoftheelasticstrainand cor-relativelytheelasticstressisobservedastheexperimentalplot stronglydeviatesfromthecalculatedlinearthermalstrain evo-lution.ThisprobablycorrespondstoboththeonsetoftheYSZ sinteringandtheconcomitantgrowthoftheAl2O3TGO
(ther-mally grownoxide) layer.The elastic strain (εel) is then the sumofthethermalexpansionmismatchstrainduetotheCTE difference between the bond coat (BC) and the YSZ (εelTh = (αYSZ−αBC)T)andthestrainduetotheconstrained
sinter-ing(εelS).Indeed,whilesintered,themetallicsubstrate–though
expandingupon heating– restrainsthe shrinkageof the YSZ ceramicleadingtotheoccurrenceofenhancedtensilestrainand stress.Note that assumingan isotropic shrinkage,the dimen-sionalchangesduringsinteringofanunconstrainedYSZcould bestraightforwardlyderivedasL/L0(in-planeshrinkage).In
addition,growthstrains(εelG)duetotheformationandthickening oftheTGOlayercanbecombinedwiththermal(εelTh)and sinter-ingstrain(εelS).Asamatteroffact,hightensilestressintheTGO isreported.22,23Itismainlyattributedtothevolumereduction resultingfromthetransformationofthetransitionaluminainto themoststable␣-Al2O3duringtheearliergrowthstages.This
growth stressmightalso contributetoan increaseof the ten-silestrainintotheYSZatleastuntilthephasetransformation intothestable␣-Al2O3iscompleted.Indeed,asreported,23,24
further growth of stable ␣-Al2O3 only develops low
stress.
Between700◦Cand1000◦C,thermalstressaswellas sin-tering stress andTGO growthstress jointly contribute to the significantincreaseoftheelasticstressandstrainexperimentally measuredwithintheTBC.
From1000◦C,theelasticstrainprogressivelystabilizes,the slopeof thestrain-temperatureplotbeingmuchlower.Atthis temperatureandabove,thenetworkofmicro-cracks–proneto releasethein-planetensilestressandassociatedstrain– initi-atesandcontinuouslydevelopsthroughatwo-dimensionalcrack propagationwithinthethicknessoftheYSZ.Uponcooling, par-ticularly inthe hightemperature range between 1100◦Cand 700◦C,theelasticstrainshowsaswellinstabilities,asindicated by thesuccessive steps inFig.6b.They likelycorrespond to thefurtherdevelopmentofthecracksnetworkleadingtoa sub-stantialreleaseofstressstoredintheYSZduringheatingand holdingat1100◦C.
Duetothemismatchbetweenthermalexpansioncoefficients oftheYSZceramiccoatingandthemetallicsubstrate,themetal tendstocontractoncoolingmorethantheceramic.This pro-vokesthedevelopmentofadditionalstressintotheYSZlayer whose stiffness increased during sintering and thusresponse tomechanicalloadingchanges.Onewaytoreleasethis addi-tionalstressistoformnewordevelopexistingmicro-cracksas detailedintheinsertinFig.6.At600◦Candbelow,thestressin thefullydenseYSZisassumedtobeextensivelyreleasedand theremainingstrainonlyresultsfromthethermalcontraction asperfectlyhighlightedbythefairlygoodagreementbetween theexperimentalandcalculateddataplottedinFig.6.Notethat returningtoroomtemperatureleavessomeresidualtensilestrain –andsoresidualtensilestress–intheTBCsrevealingthatthe completereleasedisnotachieved.ResidualstresswithinTBCs aftertemperatureexposureandcoolingareknowntobemainly compressive,particularlyattheinterfaceTBC/TGO-bondcoat asaconsequenceofthesignificantmismatchinthecoefficients of thermalexpansion between the two materials.This likely resultsinacontinuousandprogressive changeinstress mag-nitude throughout the entirethickness of the TBC,gradually ranging from compressionat the interface withthe substrate to tension at the outer surface incontact with the oxidizing atmosphere.AstheX-raybeamsizeusedinXRDexperiments isidenticaltotheTBCthickness,inputreflectioncollectedto
Fig.8.GrazingincidenceX-raydiffractogramforthepre-oxidizedsample.
Fig.9.Sol–gelTBCsurfaceof(a)apre-oxidizedsampleafter1025one-hourcyclesand(b)anon-preoxidizedsampleafter640one-hourcycles.
calculatestrain,convolutesdataextractedfromthewholeTBC fromitsinnerinterfacetoitsoutersurface.Asaconsequence, thecalculatedstraininthecoatingmustberegardedasamean quote, averaging interfacial, close-to-surface andbulk values (planestrain).
Theevolutionofthepeakshape,clearlyrevealedinFig.6a, canbebeneficiallyanalyzedtoinformonthestructural modifi-cationthatoccursduringsintering.Namely,thesharpeningand the increase inintensity of the Bragg peaks,especially upon heating from 900◦C, can be related to the three successive stagescommonlyreported toaccount for thewholesintering processtocomplete.25Withinthefirststage,particlesrearrange which contributes to enhance contact efficiency and conse-quentlyshrinkageanddensity.Thesecondstagepromotesthe
densificationandthereductionoftheporessize.Thethirdstage correspondstotheeliminationofisolatedporespronetofurther enhancedensificationofthematerialandfavourgraingrowth. ThehighlypronouncedsharpeningoftheBraggpeaks, partic-ularlyfrom900◦C,canbethusrelatedmainlytothisultimate stage of sintering, from900◦Cto1100◦C, wherenoticeable graingrowthgenerallyoccurs.
InvestigatingtheelasticstrainevolutionintheTBCinareal timebasisclearlyrevealstheimportanceofcontrollingtheheat treatmentwheresinteringoftheTBCassociatedtothe forma-tionofthecracknetworkoccurs.Indeed,theestablishmentof aperfectlystabilizedcracknetworkresultingfromacomplete sinteringoftheTBCshouldbeachievedbeforeusingTBCsin realindustrialconditions.Thisisindeedofprimaryimportance
toavoidanysupplementarystrainandstressincreaseassociated toapossiblein-servicecompletionofthesol–gelTBCsintering process.
Furthermore,aspreviouslymentioned,growthstresswithin theTGOmayalsocontribute toalargeextent totheincrease of the residual stress into the sol–gel TBC, particularly dur-ing further oxidation cycles generally performed with high heating and cooling rates. The possibility to limit growth stress using an initial thoroughlycontrolled pre-oxidation of thesubstrate beforesol–gel depositionandthe impactonthe durabilityupon cyclic oxidation isdiscussed inthe next sec-tion.
3.2. Cyclicoxidationbehaviourofsol–gelTBC
3.2.1. Evolutionoftheelasticstrainduringearlystagesof thermalcyclingoxidation
As to investigate any possible change in strain provoked by cyclic oxidation, the tetragonal (440) YSZ reflection of a non-reinforced sol gel TBC (specimen S2) upon the first five 1h-cycles at 1100◦C was monitored in situ. The impactof theseearlyoxidation cycles isappreciatedbothby analysing data obtained right before and after each individ-ual cycle and comparing of elastic strain at the beginning and end of the exposure at dwell temperature (1100◦C). Shown inFig.7 are the evolutions of the elastic strainupon first, third, fourth and fifth cycles, calculated using the ref-erence room temperature inter-planar distance right before the first cycle, as well as the theoretical thermal strain esti-mated for a fully dense stress-free YSZ (εThermal=αlT) at
1100◦C.
The first thermalcycle does not generatenoticeable over-allvariationoftheelasticstrain.However,duringexposureat 1100◦C,theelastic strainintheTBC,essentiallyconstant,is slightly smaller than the expected elastic thermal expansion. Thiscanresultfromtheintrinsicnatureofthecycleincluding fastheatingandquiteshortexposure,whichlikelyestablishesa significanttemperaturegradientthroughoutthethicknessofthe TBC.Asaresultthestraincanbehighlydifferentdependingon thelocationwithintheTBC.Theelasticstraincalculatedfrom X-raydataconvolutedfromthewholethicknessoftheTBCcan consequentlydifferfromthesolethermalstrainεThermal,which
wouldbe homogeneouslydistributedonlyifsteadystate was ensured.
Uponfurthercyclicexposure(3rd,4thand5thcycles)and converselytothefirstcycle,anincreaseofthestrain,very simi-larforeachcycle,ismeasuredwhiletheTBCisheldat1100◦C. ThisstronglysuggeststhattheTBCmechanicalresponseto tem-peratureexposureisimpactedbyvariousadditionaleffectssuch asthemismatchinthermalexpansion coefficientbetweenthe YSZ,theTGOandthebondcoat,specificdeformation mecha-nisms–especiallycreep–occurringathightemperatureinthe BC,TGOandTBCaswellasconstrainttypicallyduetothe con-tinuousslowgrowthoftheTGO.Indeed,literaturereportsthat NiPtAlBCisexpectedtoyieldattemperatureabove600◦C26
anddeformsbycreepat1100◦C,asTGO3,27andTBCcando aswell.Asaconsequence,creepofthebondcoatisassumed
toimposeadditionaldeformationtothesol–gelTBCresulting intheslightelasticstrainincreaseoccurringduringexposureat 1100◦C(Fig.7).
Beyondtheprogressive,timedependentevolutionof strain duringisothermalexposure,itisinterestingtonotethatthestrain beforeandaftereachelementarycycletendstochange,either increasing(3rdand5thcycles)ordecreasing(4thcycle).This unambiguously indicates that in all cases residual strain and stress establish within TBC.However, after cooling,residual stressisheterogeneouslydistributedthroughouttheTBC thick-ness, from essentially compressive at the top of the TBC to tensile atthe BC/TGOinterface.Result from X-rayanalysis, standingforanoverallmeanvalue,partiallyconcealsthestrain heterogeneity.
Neverthelessit can be concluded that as soon as the very firstoxidationcyclesareimposedtotheTBC,various deforma-tionmechanismscanoperatetomodifyandaccumulateresidual strain,includingtypically:
(i) creepassumedtooccurmainlyintheBCduringisothermal exposureat1100◦CthatlikelyelongatestheTBC, (ii) thickeningoftheTGO,whichdevelopsgrowthstress
dur-ingheatingaswellaslargeresidualcompressionstresson coolingtoroomtemperatureasitsCTEislowerthanthat ofboththeBCandtheTBC,
(iii) mismatchinCTEoperatingbothduringheatingand cool-ing.
It is interesting to point out that these observations may be alsorelatedtothepossibleoccurrenceofinterfacialrumpling. Indeed,whereasthisundulationdevelopmentisnotfully under-stood, several works attribute rumpling to combined effects such as coating-substrate thermal expansion coefficient mis-match and cyclic plastic strains and creep inthe bond coat. Rumplingisacommon,frequentlyobservedmaterialresponse tocyclic oxidation, whichdevelopspreferentiallyundersuch conditions andisonlylittle pronouncedfollowingisothermal oxidation.Itcorrespondstotheabilityofthebondcoat, depend-ingonitsintrinsicmechanicalproperties,toaccommodatemore or less the plastic deformation related to stress produced by thermal cycling andTGO growth. Development of rumpling inthebondcoatclosetotheinterfacewithaluminaaffectsthe deformation of the TGOand the top-coat TBC. Asa matter of fact, synchrotron X-ray diffraction, well suitable for esti-mating residual strain andstress, canbeneficially be used to address thisimportantmechanism typicalof cyclic oxidation ofTBCandparticularlydetrimentalregardingdurabilityofthe system.
Finally,inordertoassesstheeffectsofthecyclicoxidation ontheevolutionoftheresidualstrainandstressontheTBClife durationandfailuremechanisms,thermalexpansioncoefficient mismatchaswellastheonsetofcreepdeformationintothe dif-ferentstackedlayers(BC,TGOandTBC)shouldbeconsidered fromtheveryfirstcycles.
Fig.10.SEMmicrographsofthecrosssectionofTBCsystems(a)withoutpre-oxidation(after6401-hcycles)and(b)withpre-oxidation(after10251-hcycles).
3.2.2. Effectofpre-oxidationoncyclicoxidationbehaviour ofsol–gelTBC
Asmentionedpreviously,theirreversiblegrowthofthe alu-mina Al2O3 layer during the post-processing heat treatment
generatesprejudicialgrowthstressintheTBC,mainlydueto thetransformationoftransientmetastablealuminaphasesinto stable ␣-Al2O3. Preventing or atleast limiting the formation
of thosetransient phasesshouldhavebeneficial effectonthe resistance to cyclic oxidation. Favouring the nucleation and preferentialgrowthofstable␣-Al2O3canbeachievedby
pre-oxidizingthesubstratepriortodeposition.Pre-oxidationmust bethoroughlycontrolledtodeveloppurecoherentandthin alu-minalayer.Fig.8showsthegrazingincidenceX-raydiffraction pattern obtained for a NiPtAl bond coat pre-oxidized 2h at 900◦C under secondary vacuum of oxygen (5×10−4mbar). To limit the depth of penetration of X-rays into the bond coatand investigatemainlythe thin alumina layer,a grazing incidenceanglewasfixedtoα=1.5◦.OnlyBraggpeaks char-acteristicofeitherthebondcoatorthe␣-aluminaareidentified showingclearlythatmetastablephasesdidnotformduring pre-oxidation.
Theeffectof thisbeforehand treatment andtheassociated presenceofathininitialpure␣-aluminalayerpriortosol–gel processing of the TBC on the cyclic oxidation behaviour is
analyzed by comparing behaviour of non reinforced sol–gel TBC either pre-oxidised (specimen S4) or non pre-oxidised (specimenS3).Theaspectoftheoutersurfaceofsuchspecimens following1025(forS4)and640(forS3)1h-cyclesat1100◦C areshowninFig.9aandbrespectively.Bothspecimensshow identicalcharacteristicsintermsofsurfacemorphology show-ingthepresenceofthecracknetworkissuedfromthesintering heattreatmentandpronetoreleaseconstrainedshrinkagestrain asquantitativelyestimatedinSection2.2.WhilethewholeTBC showsperfectadhesiontothepre-oxidisedsubstratebondcoat,it extensivelyspalledofffromthesubstratewithnopre-oxidation. Indeed,inthisformercase,thesurfacefractionofspalledTBC, highlydiscohesive,largelyexceeds50%eventhoughthe num-berofoxidationcyclesis40%lowerthanforthepre-oxidised system.
ComplementarySEManalysisofcrosssectionsfromthetwo specimens(Fig.10)highlightstwomainfeatures:
(i) Withoutpre-oxidation,failure,eitheradhesiveorcohesive, occursoverdifferentzonesoftheTBCsystemsuchasthe interfacesbondcoat/TGOandTGO/TBCaswellaswithin theTBCitself.NotethattheTGO,withthicknessaround 10m,showshighrumpling.
Fig.11.Opticalmicrographsofthesol–gelTBC(lefthandsideimages)andEB-PVDTBC(righthandsideimages)samplesbeforecyclicoxidation(aandb),after the500th1h-cycleat1100◦C(candd),afterthe1050th1h-cycle(eandf)andafterthe1480th1h-cycle(gandh).(Notethatimagesb,d,fandhareobtained usinggrazinglightconditions,whichenhancessurfacedefects.)
(ii) With pre-oxidation,the TGO, much thinner (about 2m thick)presentssmootherrumplingandshowshighadhesion tothebondcoat.
Thiscomparisonrevealedthat thepre-oxidationontheinitial AM1superalloy substratecoated withNiPtAl at 950◦C dur-ing2h,clearlyimprovesthecyclicoxidationbehaviourofthe
sol–gelTBCbycontrollingtheformationofastableand thin-ner ␣-Al2O3 layer adherent to the bond coat. Indeed, initial
hightemperatureoxidationpriortoTBCdepositionresultsin areductionoftheoxidationrateoftheTBC-coatedmaterialand concomitantlyin anextended timetoreach the TGOcritical thickness,standingfortheonsettospallation.Prejudicialissues relatedtotheformationofmetastablephase12 arethuslimited
Fig.12.SEMmicrographsofthecrosssectionof(a)thereinforced/optimizedsol–gelTBCand(b)EB-PVDTBCfollowingthe1480th1h-cycleat1100◦C.
throughasignificantdecreaseofthelevelofTGOgrowthstress aswellasanenhancementofdiffusionbarrierpropertiesbefore in-serviceconditionsareapplied.28,29
Notethattheoccurrenceofrumplingdoesnotdependonthe specificgrowthmechanismoftheTGOasbothpre-oxidisedand non pre-oxidisedsystems exhibit fairly corrugatedinterfaces. ThissuggeststhatTGOgrowthstressandoxidationratedonot impactrumplingsignificantlyasalsoobservedbyTopygoand Clarke.30
Finally,thiscomparisonconfirms,thepossibilityof improv-ingthecyclicoxidationbehaviourbyusingpre-oxidationofthe initial substrateand proves its feasibilityandapplicability to sol–gelTBCsystems.
3.2.3. Cyclicoxidationbehaviouroftheoptimized reinforcedsol–gelTBC:comparisonwithanEB-PVDTBC
Aspreviouslyillustratedintheliterature,thefailure mecha-nismofTBCsiswelldifferentforEB-PVDandsol–gelsystems. Basically,EB-PVDTBCsgenerallyexhibitlongtermresistance tospallationfollowingcyclicoxidationexposurewithvery lit-tle degradation up to, say, oneto two thousands numbers of onehour-cyclesat1100◦C.Once,themechanicalstrainenergy storedinthesystemandthedevelopmentofrumplingarelarge enoughfortheonsettospallation,theEB-PVDTBCgenerally failsupononesinglecoolingsubsequenttoanultimate expo-sureathightemperature. Failureaffectsthe wholeTBCor at leastalargesurfacefractionoftheTBCfollowingtheinitiation andpropagationofcracksatthesubstrate/TGOinterface.Inthe caseof sol–gelTBC,theinitialcrack networkresulting from the sintering heattreatment, concentratesstress duringcyclic exposureandacts as zones of crack formationthat generally tendtopropagateattheTGO/TBCinterface.Individualcells, delineatedbythisnetwork,cansubsequentlyspalloff contin-uouslyandgraduallyasoxidation cyclescumulate.Spallation kinetics,possiblyestablishedwithintheveryfirstcycles,ismuch moreprogressivethanforEB-PVDTBCscharacterizedbysharp andsuddendegradation.Fornopre-oxidisedandnoreinforced
sol–gel TBC,the overalllifeisgenerallyshorter thanfor the EB-PVDcounterpart.
Toevaluatethecombinedeffectofthethreeproposedwaysof optimization,namelythepre-oxidation,theuseofappropriate heattreatmentparametersandthefillingofcracksbyspray coat-ing,anoptimizedsol–gelTBCwascyclicallyoxidizedtogether withanEB-PVDTBCforcomparison.
Fig.11showsseveralphotographsobtainedonboth speci-mensbeforeoxidationandafter500,1050and1480one-hour cyclesat1100◦C.Attheinitialstate,beforeoxidation, differ-encesbetweenthesol–gelandtheEBPVDTBCssurface mor-phologies areclearlyevidenced.Despitetheadditionalfilling broughtbyspraycoating,acracknetworkoutlinedbyadjacent homogeneouscellsstilldelineatesthewholesol–gelTBCouter surface,whiletheEB-PVDTBCshowsasmooth,slightlyrough surface.
Following500ththen1050thone-hourcycle,bothsamples remain essentially undamaged. Note however that a grazing lightingofthesol–gelTBChighlightsasmallcircularblister, besidesinitiallypresentbeforecyclicoxidation.Uponcycling, the blister furtherdevelopsslightly.Damagemechanisms are significantlydifferentforEB-PVDTBCsystems.Indeed,inthe caseofEB-PVDTBCs,fewsmallcracksinitiateontheedgeof thespecimen sampleasshowninFig.10f.Finally,after1480 cycles,failureoftheEB-PVDTBCoccursthroughthe exten-sivepropagationofthisinitialcrack.Forthesol–gelTBC,even thoughtheblisterremainsunchanged,spallationofafew indi-vidual cells isobserved overthick edges indicatingprobably the onsetfor the sol–gel TBC degradation. Nevertheless, the optimizedreinforcedsol–gelTBCshowspromisingbehaviour, exhibiting highdurability very similartothat of anEB-PVD TBC.
SEMmicrographsofthecrosssectionofthetwospecimens are presented in Fig.12. In sol–gel TBC,defects within the YSZ layer–whichisadherenttotheTGO–as wellas rum-plingofthebondcoat/TGOareobserved.Inturn,degradation of theEB-PVD TBC ismainlycharacterized bya delamina-tion at thebond coat/TGOinterface, alsoassociated to some
rumpling.Besides,thehigherTGOthicknessexhibitedbythe EB-PVDTBC,suggeststhattheestablishmentofhighergrowth stress andstrain duringcyclic oxidation might be oneof the reasonsofthevariationintermsoflifetimeandfailure mecha-nismsbetweenoptimizedsol–gelandEB-PVDTBCs.Indeed, althoughtheTGOlayerisgenerallythinascomparedtotheTBC scale,itmaydevelophighstresseswithinthesystemasa conse-quenceofsignificantthermalexpansionmismatchbetweenthe TGOandthebondcoat.So,failureinEB-PVDTBC mainly originatesatthebondcoat/TGOinterface.
Notethatitwouldbeworthtoanalyzefurthertherumpling mechanismstoinvestigatepreciselyinwhichsystemitoccurs mostextensively as it is reportedtobe potentially arelevant mechanismcontributingtofailure.31,32
So,thecomparisonof the sol–gelTBC andEBPVDTBC overallperformancesduringcyclicoxidation,revealed,forboth systemacomparablelifetime,characterizedbyalmostno notice-able damages even after the 1500th oxidation cycle. These encouragingresultspoint outthat pre-oxidationassociated to the use of appropriate heat treatment parameters as well as thefillingofcracksbyspraycoatingsignificantlyimprovethe thermo-mechanicalperformancesofsol–gelTBCs.
4. Conclusions
Extending cyclic oxidation life of sol–gel TBCs, show-inganinterestingcompromisebetweenthermalinsulationand mechanicalstrengthperformance,requirestoconductperfectly thevarioussuccessive stepsforprocessingthe YSZtop coat. Beyondtheinitialdip-coatingandcontrolledwithdrawingofthe NiPtAlcoatedsuperalloysubstratesfromaslurrycomposedof thestartingsolandpreviouslydriedYSZpowder,itisofoutmost concerntocarryoutandcontrol:
i) Apre-oxidationof theNiPtAlbondcoatdepositedonthe substratetoenhancethebeneficialdiffusionbarriereffect. ii) Asinteringheattreatmentthatdevelopsaregularin-plane
cracknetworkwithintheTBCasitgetsdenser.
iii) AreinforcementoftheTBCbypartiallyfillingthosecracks usingsol–gelspray-coating.
UsingbothXRDsynchrotronradiationandcyclicoxidationtest, theperformancesofsol–gelTBCsintermsofmicrostructural evolution,mechanicalresponse tohigh temperature exposure anddurabilityuponsinteringandbothearlyandlong-term oxi-dationcyclesareinvestigated.Theinsitumonitoringofthestrain withinthesol–gelTBC duringpostdepositionheattreatment revealstheoccurrenceofvarioussuccessivemechanisms includ-ing–asthetemperatureisincreasedfromroomtemperatureto 1100◦C–thecalcinationoforganiccompounds,theconstrained thermalexpansion, the volumereduction of the Al2O3 TGO
resultingfromtheto␣transitionandtoalowerextenttothe TGOgrowth,thesintering-induceddensificationandthe initia-tionandgrowthofthecracknetwork.Residualstrainremainsin theTBCaftercompletionofsinteringandtheveryfirstoxidation cyclefurtherenhancethisstrain,throughvariouseffectsuchas creepofthebondcoat,thickeningoftheTGOandmismatchin
coefficientofthermalexpansionbetweenthebondcoatandthe barrier.
Pre-oxidationofsol–gelTBCdrasticallyextendcyclic oxida-tionlifeastheinitialAl2O3TGOlimitsthegrowthkineticsofthe
scalethusreducingthedepletionoftheAlreservoirinthebond coatanddecreasingthestressassociatedwiththeTGO develop-ment.Finally,optimizedsol–gelTBCsincludingpre-oxidation and spray-coating reinforcement are shownto present cyclic oxidationlifeverysimilartostandardstate-of-the-artEB-PVD TBC.
Furtherdevelopmentoftheworkmayfocusonthe investi-gationofthestrainevolutionwithinreinforcedsol–gelTBCsas wellasofthemonitoringofthecrackinitiationandpropagation usingXRDtomography.
Acknowledgment
TheauthorsacknowledgetheEuropeanSynchrotron Radia-tionFacility(ESRF)forbeamtimeonID15Band,particularly, V.HonkimäkiandT.Buslapsfortheirhelpanduseful discuss-ions.
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