EBPVD VS APSTBC system.docx
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EBPVD VS APSTBC system.docx
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EBPVDVSAPSTBCsystem
TBCsystem,whichwasknownbyitstemperaturedecreasefunction,usuallyconsistsof3basiccomponents:
theceramiclayer(Topcoat)depositedbyyttria-stablizedzirconia,followedbymetalliclayer(Bondcoat)andmetallicsubstrate.Therewereseveraltechniquesrecordedtoproducesuchthermal-isolatedsystem.Fortopcoat,twomethodsnamelyelectronbeamphysicalvapordeposition(EB-PVD)aswellasair-plasmaspray(APS)weremainlyusedtoreachthegoalforcomplianceandtolerancestrainconditionswithintopcoatthroughcolumnarandcrack/voidsmicrostructurerespectively.DifferenttypeofbondcoatsweredepositedonthesubstratewhichcouldbediffusionaluminidecoatingscontainingplatinumintoitorMCrAlYoverlayswithcertainpercentageofeachelements.Whenthetemperaturewenthigh,theTBCssufferedfromtheoxidationofbondcoatandformationofoxide(thermalbarriercoating)atinterfacebetweeneachlayers(TC/BC/Substrate),whichwasoneofcriticalfactorstotriggertheTBCsfailure.
However,astwotypeoftopcoatswithdifferentmicrostructuresaswellastwospeciesofbondcoatwithdifferentcomponents,thefailuremechanismvarieswithvariouscombinationoftopcoatandbondcoat.Authorinthispaperinvestigatedandstateddifferentpre-worksandtheoriesproposedfromrelatedpapers.TherewerethreetypeofTBCsystemsinauthor’sstudyincludingEB-PVDTBCwithplatinumaluminidebondcoatandMCrAlYbondcoataswellasAPSTBCwithMCrAlYbondcoat.SeveralfailuremodelswerepresentedandaccordingtoBolcavageetal,assomesortofconclusion,theEB-PVDsystemfailedmainlycausedbytheeffectofbondcoatoxidationwhileinAPS-TBCsystem,thefailurewasgenerallytriggeredbyfactureintopcoat.
Theconclusionhasbeenprovedfromthefailuremechanismquotedbyauthor.ThefailuretakenplacewithplatinumaluminidebondcoatcouldlocatedneartheinterfacebetweentopcoatandTGOwithintopcoat,whichoriginatedbyrumplingandratchetingofTGOitselfbutdegradedbythedisplacementinstabilitycausedbythermomechanicalresponseofbongcoat;thefailurewithsuchtypeofbondcoatalsocouldbelocatedwithintopcoatthatverticalcracksdevelopattheridgesofbondcoatpropagatewhenTGOthickening,resultinginstressformationandwidenthegap.Thethirdtheoryofsuchsystem’sfailurewasexplainedthatcrackinitiationduetotheout-of-planetensilestressatridgesofbondcoat(TGO),followedbytheintergranularoxidationatgrainboundariesthroughthepathwayprovidedbythecrack,thecavityformedwithplasticdeformationofbondcoatandtensilestresscontributingtothestrainenergy,leadingtothefailurewithintopcoat.AstheEB-PVDtopcoatwithMCrAlYbondcoat,thefailurewaspartlyduetothepegswhichindicatestheheterogeneitiesofTGOcontainingyttrium-richoxidemorethanalumina,andthereasonforformationofpegswasduetotheYelementintheBCthatwasmoreeasilytobeoxidizedcomparedtotheotherelements.ThepegsaswellasedgedelaminationandtransientoxidesobservedinthatsystemleadtothefailureofsuchTBCsystem.ForAPS-TBCsystem,onlythe
MCrAlYbondcoatwasappliedandasalreadystatedpreviously,thecrackwasfoundintopcoatinitiallyduetotheresidualstress,atsametimethenormaltensilestressexistingatundulationcrestsatinterfacebetweenTGOandbondcoatleadtocrack,withthelinkingoftwotypeofcracksbyfracture,thetopcoatspallout.
Again,inauthor’sinvestigation,twotypeofthermaltestswereconductedespeciallyfordifferenttypeofTBCsystem,accordingtoBolcavageetal,forthefurnacecycleoxidationtest(FCTtest),itissuitableforsystemwhichofbondcoatoxidationisinvolvedinfailureprocess(EB-PVDsystem)whilejetenginethermalsimulation(JETS)issuitableforsystemwhichfactureinTBCisprimarycauseoffailure(APS-TBC).
Therewerethreeobjectivesforthispaper,firstlytheauthorcomparedfailureofEB-PVDTBCswithAPS-TBCs,whichhasbeenpresentedatpreviousreview.NexttheauthoralsoprovidedsomemodificationmethodstoimprovetheTBCsdurabilityandfinallytheauthorinvestigatedtheeffectsofinfluencesbydifferentexperimentalconditions.
Intheexperimental,someofcontrolexperimentswereconductedforbetterexplanations,forEB-PVDTBCs,whichincluding1)Nobondcoatgroup,someoftopcoatwasdirectlydepositedonthesubstrateattwodifferenttimes,namedfirstlotandsecondlot.2)Noaluminizinginproducingplatinumaluminidebondcoat,whichindicatesthetopcoatwasdirectlydepositedontheplatinumonthesubstratewithoutfurtheraluminizingprocess.3)Pre-oxidizedspecimengroup,someofspecimenwerepre-oxidizedat1353Kfor50minutes.ForAPS-TBCs,thelowdensityandhighpuritytopcoatwasdepositedfortwotypeofthickness:
375and1100micrometersrespectively,what’smore,byadjustingsprayingparametersduringdeposition,adenseverticalcracked(DVC)innerlayerswasdepositedbetweentheBCandtraditionalAPS-TBC.ThethermaltestforpreparedTBCssampleswereseparatedinto4types,conductingwith3differentequipment.ForEB-PVDTBCs,thebottomloadingfurnacewasusedtoconductcyclicallyoxidationtestandpseudo-isothermaltests,whichtheholdtimemakesdifferencebetweenthem.The10-minutecycletestwasconductedwithverticaltubefurnace,whichthecyclefrequencymakesdifferencecomparedtotheprevioustests.TheJETStestwasconductedforAPS-TBCs,whichhasshortestholdingtimeathightemperatureandevencausethethermalcycle.Theeffectsofthemwillbediscussedatlater.
TheresultsofsuchexperimentswaspresentedandanalyzedthroughSEMbyobservingthesurfacesexposedbyfailuresandpreparedcross-sections.ForEB-PVDTBCs,thefailuremicrostructuresandfailuremechanismwasfundamentallynodifferencefrompreviousanalysis.However,someofresultsfromcontrolexperimentsthatshowssurprisinglycompetitiveTBClives.Themicrostructuretakenfromstateofas-sprayedaswellasafter-failedbySEMwerecomparedbetweendifferentconditionsincluding“No
Bondcoatgroup”and“Noaluminizinggroup”.Thetypicalcharacteristicsof“Nobondcoatgroup”atcross-sectionwasirregular&oxidesornamedcornkerneldefectsobservedalongtheinterfacebetweentopcoatandbondcoatwhereassimilardefectcouldbefoundat“No
aluminizinggroup”duetotheelectrolyticprocessusedtodepositplatinum.However,bothofthemwerefreefromrumplingorratcheting.ItissuchinterestingphenomenathatthefailuremodewithAPS-TBCsseemstoberelatedwiththethicknessoftopcoat.Thatis,whenthetopcoatinthick,thefailureoccurredwithintopcoatatsplatboundariesandwhenthetopcoatthicknesswasdecreasing,thefailuremechanismturntobethepenetrationmodeloccurredattopcoatandTGOinterfacedescribedabove.What'smore,theAPS-TBCsfailurehadrelationshipwithdifferenttypeofsubstrate.
AuthorinthispaperalsoprovidedsomemethodtoimprovethedurabilityofTBCsystembasedondifferenttypeoftopcoataswellasbondcoat.ForEB-PVDTBCwithMCrAlYbondcoat,twomethodswereproposedwhicheitherdepositingPtoverlayorpolishingthesurfaceofbondcoatbeforedepositingEB-PVDTBC.Someofas-sprayedandthermal-exposedSEMphotoswere
providedtohelpbetterunderstandingthemechanisms.Thepolishedinterfaceshowedobviouslysmoothinterfacebetweentopcoatandbodcoatwhichavoidthedefectgeneratedattheinterface,aftersomethermalexposure,itcouldbeseenthatalthoughTGOthickenduetothereactiveelementoxides,the
interfacewasstillsmoothandfreeofdefects.AsthePt-overlaydepositeduponbongcoat,aratheruniforminterfacebetweentopcoatandbondcoatcanbeobservedandafterthermalexposure,therewerecrackswithintopcoat,butTGOgrowthratewasmuchslowerthanthatofstate-of-theartMCrAlYsystem.
ForEB-PVDwithplatinumaluminidebondcoatdepositedusingheavygrit-blastedtechnique,themodificationcouldbemadebyapplyinglightgrit-blastedbondcoatwhichsmoothinterfacecanbeachieved,andcross-sectionviewafterexposurealsoindicatedthattheTBCsystemdidn’tfailafter8,000cyclesat1373K.
ForAPS-TBCwithMCrAlYbondcoat,theDVCinnerlayersachievedbyadjustingthesprayparameterswouldplayasignificantroleinimprovingthelifetime.ThecrackingusuallywasobservedattheinterfacebetweentheAPS-TBCandDVC.Thelifetimeoftwospecimenwithdifferenttopcoatthicknesswereallimprovedaccordingtothedatafromauthor.
TheauthorinthispaperalsomentionedaboutseveralfactorswhichwouldaffectthelifetimeindifferenttypeofTBCsystems.TheholdingtemperaturewasprovedtohavenegativeeffectonthedurabilityofsystemwithbothMCrAlYandplatinumaluminidebondcoat.Bothofthemshoweddecreaseoflifetime(byafactorof10approximately)whentemperaturewenthighwithintervalof100K.Withthatofdata,anArrheniusplotcouldbeusedtoyieldtheactivationenergyandtheresultwasprettyclosetotheenergywhichα-aluminaofTGOstarttogrowth.
Theeffectofcyclefrequencyshowed“aninterestingdifference”betweenMCrAlYandplatinumaluminidebondcoat.Withthecyclefrequencywenthigher,whichmeansthehightemperatureholdingtimegettingshorter,theperformanceofMCrAlYseemstobeinsensitive,evenshowingtheincreaseoflifetimewhenthecyclefrequencyturntobeveryhigh.However,forplatinumaluminidebondcoat,asignificantdecreaseoflifetimecanbeobservedwhentheholdingtimegettingshorter.Thatdifferencecouldbeexplainedbythedifferentmicrostructuresobservedfromcross-sectionofdiff
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