HAL Id: hal-01179395
https://hal.archives-ouvertes.fr/hal-01179395
Submitted on 22 Jul 2015
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Optimized sol–gel thermal barrier coatings for long-term
cyclic oxidation life
Lisa Pin, Vanessa Vidal, Fabien Blas, Florence Ansart, Sandrine Duluard,
Jean-Pierre Bonino, Yannick Le Maoult, Philippe Lours
To cite this version:
Lisa Pin, Vanessa Vidal, Fabien Blas, Florence Ansart, Sandrine Duluard, et al.. Optimized sol–gel
thermal barrier coatings for long-term cyclic oxidation life. Journal of the European Ceramic Society,
Elsevier, 2014, vol. 34 (n° 4), pp. 961-974. �10.1016/j.jeurceramsoc.2013.10.013�. �hal-01179395�
O
pen
A
rchive
T
OULOUSE
A
rchive
O
uverte (
OATAO
)
OATAO is an open access repository that collects the work of Toulouse researchers and
makes it freely available over the web where possible.
This is an author-deposited version published in :
http://oatao.univ-toulouse.fr/
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:
staff-oatao@listes-diff.inp-toulouse.fr
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:lisa.pin@mines-albi.fr(L.Pin),
vanessa.vidal@mines-albi.fr(V.Vidal),fabien.blas@mines-albi.fr(F.Blas),
ansart@chimie.ups-tlse.fr(F.Ansart),duluard@chimie.ups-tlse.fr(S.Duluard),
boninojp@chimie.ups-tlse.fr(J.-P.Bonino),
yannick.lemaoult@mines-albi.fr(Y.LeMaoult),
philippe.lours@mines-albi.fr(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.
References
1.PadtureN,GellM,JordanEH.Thermalbarriercoatingsforgas-turbine engineapplications.Science2002;296:280–4.
2.GoswamiB,RayAshokK,SahaySK.Thermalbarriercoatingsystemforgas turbineapplication–areview.HighTempMaterProcesses2004;23:73–92.
3.EvansAG,Mumm DR,HutchinsonJW,MeierGH, Pettit FS. Mecha-nismscontrollingthedurabilityofthermalbarriercoatings.ProgMater Sci2001;46:505–53.
4.WangX,LanWH,XiaoP.Fabricationofyttriastabilizedzirconiacoatings byanovelslurrymethod.ThinSolidFilms2006;494:263–7.
5.RenC,HeYD,WangDR.Cyclicoxidationbehaviorandthermalbarrier effectofYSZ–(Al2O3/YAG)double-layerTBCspreparedbythecomposite
sol–gelmethod.SurfCoatTechnol2011;206:1461–8.
6.ViazziC,BoninoJP,AnsartF.Synthesisbysol–gelrouteand characteri-zationofyttriastabilizedzirconiacoatingsforthermalbarrierapplications. SurfCoatTechnol2006;201:3889–93.
7.SniezewskiJ,LeMaoultY,LoursP,PinL,MinvieBekaleV,MonceauD, OquabD,FenechJ,AnsartF,BoninoJ-P.Sol–gelthermalbarriercoatings: optimizationofthemanufacturingrouteanddurabilityundercyclic oxida-tion.SurfCoatTechnol2010;205:1256–61.
8.PinL,AnsartF,BoninoJ-P,MaoultYL,VidalV,LoursP.Processing, repairingandcyclicoxidationbehaviourofsol–gelthermalbarriercoatings. SurfCoatTechnol2011;206:1609–14.
9.PinL,AnsartF,BoninoJ-P,LeMaoultY,VidalV,LoursP.Reinforced sol–gelthermalbarriercoatingsandtheircyclicoxidationlife.JEurCeram Soc2013;33:269–76.
10.Garriga-MajoDP,ShollockBA,McPahilDS,ChaterRJ,WalkerJF.Novel strategiesforevaluatingthedegradationofprotectivecoatingson super-alloys.IntJInorgMater1999;1:325–36.
11.T.E.Strangman,Columnargrainceramicthermalbarriercoatings,Brevet
USpatent4321311(1982).
12.TolpygoVK,ClarkeDR.Theeffectofoxidationpre-treatmentonthecyclic lifeofEB-PVDthermalbarriercoatingswithplatinum–aluminidebond coats.SurfCoatTechnol2005;200:1276.
13.StraussD,M¨ullerG,SchumacherG,EngelkoV,StammW,ClemensD, QWJ.OxidescalegrowthonMCrAlYbondcoatingsafterpulsed elec-tronbeamtreatmentanddepositionofEBPVD-TBC.SurfCoatTechnol 2001;135:196.
14.BruneseauxF,Aeby-GautierE,GeandierG,DaCostaTeixeiraJ,Appolaire B,WeisbeckerP,MauroA.Insitucharacterizationsofphasetransformations
kineticsintheTi17titaniumalloybyelectricalresistivityandhigh temper-aturesynchrotronX-raydiffraction.MaterSciEngA2008;476:60–8.
15.MuzziL,CoratoV,dellaCorteA,DeMarziG,SpinaT,DanielsJ,Di MichielM,ButaF,MondonicoG,SeeberB,FlükigerR,SenatoreC.Direct observationofNb3Snlatticedeformationbyhigh-energyX-raydiffraction
ininternal-tinwiressubjecttomechanicalloadsat4.2K.SupercondSci Technol2012;25(5):054006.
16.Steuwer A, DanielsJE. In-situ stressand strainmeasurements around cracks using synchrotron X-ray diffraction. J Strain Anal Eng Des 2011;46(7):593–606.
17.PyzallaA,ReetzB,JacquesA,FeiereisenJP,FerryO,BuslapsT. Syn-chrotron radiation in-situ analyses of AA 6061+Al2O3 during tensile
deformationatambientandelevatedtemperature.In:Recentadvancesin experimentalmechanics.Netherlands:Springer;2004.p.527–34.
18.MittemeijerEJ,WelzelU.The“stateoftheart”ofthediffraction analy-sisofcrystallitesizeandlatticestrain.ZKristall–CrystallineMaterials 2008;223:552–60.
19.FenechJ.Nouvelles compositionsde revêtements de zirconesubstituée (Y,La,Sm,Er)élaborésparla voiesol–gel:applicationauxbarrières thermiques multicouches. Ph.D. thesis, University of Toulouse; 2010. p.105–7.
20.Viazzi C.Elaborationparleprocédésol–gelderevêtementsdezircone yttriéesur substrats métalliquespourl’applicationbarrière thermique. Ph.D.thesis,UniversityofToulouse;2007.
21.Popma RLW.Sinteringcharacteristicsofnano-ceramic coatings.Ph.D. Thesis,UniversityofGroningen;2002.
22.SpechtED,TortorelliPF,ZschackP.Insitumeasurementofgrowthstress inaluminascale.PowderDiffr2004;19:69–73.
23.HouPY,PaulikasAP,VealBW.Growthstrainsandstressrelaxationin aluminascalesduringhightemperatureoxidation.In:6thSymposiumon HighTemp.Corr.andProtectionofMaterials.2004.
24.Schumann E, Sarioglu C, Blachere JR, Pettit FS, Meier GH. High-temperaturestressmeasurementsduringtheoxidationofNiAl.OxidMet 2000;53:259–72.
25.CalataJN.Densificationbehaviorofceramicandcrystallizableglass mate-rialsconstrainedonarigidsubstrate.Ph.D.Thesis,VirginiaPolytechnic InstituteandStateUniversity;2005.
26.PanD,ChenMW,WrightPK,HemkerKJ.Evolutionofadiffusion alu-minidebondcoatforthermalbarriercoatingsduringthermalcycling.Acta Mater2003;51:2205–17.
27.RöslerJ,BakerM,VolgmannM.Stressstateandfailuremechanismsof thermalbarriercoatings:roleofcreepinthermallygrownoxide.ActaMater 2001;49:3659–70.
28.CavalettiE.Etudeetdéveloppementdebarrièredediffusionpourles sous-couchesdesystèmebarrièrethermique.Ph.D.thesis,UniversityofToulouse; 2009.
29.VialiasN.Etudedeladétériorationparoxydationhautetempératureet interdiffusiondesystèmesrevêtement/superalliageàbasedeNickel– Prévi-siondeduréedevie.Ph.D.thesis,UniversityofToulouse;2004.
30.TolpygoVK,ClarkeDR.Ontherumplingmechanisminnickel-aluminide coatingsPartI:anexperimentalassessment.ActaMater2004;52:5115–27.
31.Tolpygo VK, Clarke DR. Morphological evolution of thermal barrier coatings induced by cyclic oxidation. Surf Coat Technol 2003;163– 164:81–6.
32.TolpygoVK,ClarkeDR.Surfacerumplingofa(Ni,Pt)Albondcoatinduced bycyclicoxidation.ActaMater2000;48:3283–93.
