Chemical vapor deposition of yttria-stabilized zirconia as a thermal barrier coating for gas turbine engines

2004·12.4 MB·English

by  Varanasi, Venu Gopal

CHEMICALVAPORDEPOSITIONOFYTTRIA-STABILIZEDZIRCONIAASA
THERMALBARRIERCOATINGFORGASTURBINEENGINES
By
VENUGOPALVARANASI
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHE
UNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHE
REQUIREMENTSFORTHEDEGREEOFDOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2004
ACKNOWLEDMENTS
First,IwouldliketothankDr.TimAnderson,whoismychiefadvisorandthe
AssociateDeanforResearchfortheCollegeofEngineeringattheUniversityofFlorida.
Iappreciatehispatienceandhisknowledgeinthefieldofthermodynamics.Iappreciate
allhiseffortstohelpmeunderstandhowimportantthermodynamicsistothefieldof
chemicalengineering.
Iwouldalsoliketothankmycommittee,whohavebeenveryinstrumentalinmy
developmentasaprofessional.Dr.Crisalleshowedmetheessenceofteamworkandhard
workwhenIparticipatedintheteachingassistantprogramattheUniversityofFlorida.
OthercharacteristicsthatDr.Crisalleshowedmewereorganization,focus,and
thoroughness. Mostimportantly,Dr.Crisallewillbealifelongfriend. Dr.Weaverhas
givenmetremendousinsightintosurfacechemistry. Ialsoconsiderhimagreatfriend
andhopethatheandIcancollaborateinthefuture.
SpecialthanksaregiventoShirleyKelly,Peggy-JoThran,DebbieSandoval,
JaniceHarris,andNancyKrell.Thesewonderfulladiesaremembersofthesupportstaff
intheChemicalEngineeringDepartmentattheUniversityofFlorida. Iwouldliketo
thankDr.GerhardFuchsforhisinsightintothesubjectmatterofthisthesis. Finally,I
wouldliketothankDr.TheodoreBesmannfortheopportunitytoworkatOakRidge
NationalLaboratoryandforhisguidanceinachievingthegoalsofthiswork.
11
1
TABLEOFCONTENTS
Page
ACKNOWLEDMENTS
ii
ABSTRACT vi
CHAPTER
INTRODUCTION
1 1
2 BACKGROUND 4
2.1 Introduction 4
2.2 ThermalBarrierCoating(TBC)System 5
2.3 EvaluationoftheTBCSystem 6
2.4 Zirconia-BasedCeramics 7
2.5 CurrentIssueswithTBCs 10
2.5.1 ThermalBarrierCoatingFabrication 10
2.5.2 ThermalBarrierCoatingLifetime 1
2.6 ChemicalVaporDeposition(CVD):AViableAlternative 13
2.7 ChemicalVaporDepositionofYttria-StabilizedZirconia(YSZ) 14
2.7.1 ChlorideCVDofYSZ 15
2.7.2 ParamtersInfluencingChlorideCVDGrowthRates 17
2.7.3 Metal-OrganicCVD(MOCVD)ofYSZ 24
2.7.4 ParamtersInfluencingMOCVDGrowthRates 29
3 EQUILIBRIUMMODELING 47
34..11 ThermochemiealProperties 47
3.1.1 PureSubstances 47
3.1.2 PhaseDiagramfortheZr-0System 48
3.1.3 PhaseDiagramtheY0i,5-Zr02System 51
3.2 ChemicalVaporDepositionEquilibriumCalculation 54
3.2.1 TheZr-Cl-O-C-H-InertSystem 54
3.2.2 TheZr-Y-Cl-O-C-H-InertSystem 57
3.2.3 TheZr-Y-O-C-H-InertSystem 60
3.3 Discussion 65
4 EXPERIMENTALAPPROACH 110
Materials 110
4.1.1 Reagents 110
4.1.2 SubstrateandSusceptor Ill
iii
81
4.2 Methods Ill
4.2.1 PressureandTemperatureControl 11
4.2.2 PrecursorPreparation 112
4.2.3PrecursorDelivery 113
4.3 ReactionChamber 11
4.3.1EffectofGeometry 120
4.3.2EffectofGasFlowandSubstrate/ChamberHeating 121
4.3.3EffectofChamberPressure 123
4.4WasteandEffluentHandling 124
4.5Post-Processing 124
4.6CoatingAnalysis 125
4.6.1CoatingCrystalStructureAnalysis 125
4.6.2CoatingMorphologicalAnalysis 129
4.6.3CoatingCompositionalAnalysis 130
5 RESULTSANDDISCUSSIONOFCHLORIDECVD 142
5.1 Results 142
5.2 Discussion 146
6 RESULTSANDDISCUSSIONOFMOCVDUSINGBETA-DIKETONATES 155
6.1 Results 155
6.2 Discussion 159
7 RESULTSANDDISCUSSIONOFMOCVDUSINGN-BUTOXIDES 189
7.1 Results 189
7.2 Discussion 193
8 SUMMARY,CONCLUSIONS,ANDFUTUREWORK 218
8.1 SummaryandConclusions 218
8.2 RecommendationforFuturework 222
8.2.1 ImprovementsinCoatingDeposition 222
8.2.2 ThermalCycleTestingofCoatings 223
8.2.3DeterminationofaGrowthMechanism 224
APPENDIX
A PDFCARDSFORUSEINX-RAYDIFFRACTIONANALYSIS 232
B CALIBRATIONFORTEMPERATUREMEASUREMENTS 239
IV
REFERENCES 243
BIOGRAPHICALSKETCH 255
V
AbstractofDissertationPresentedtotheGraduateSchooloftheUniversityofFlorida
inPartialFulfillmentoftheRequirementsfortheDegreeofDoctorofPhilosophy
CHEMICALVAPORDEPOSITIONOFYTTRIA-STABILIZEDZIRCONIAASA
THERMALBARRIERCOATINGFORGASTURBINEENGINES
By
VennVaranasi
August2004
Chair;TimAnderson
MajorDepartment;ChemicalEngineering
Thegasturbineengineusesanyttria-stabilizedzirconia(YSZ)coatingtoprovide
thermalinsulationforitsturbineblades.ThisYSZcoatingmustbetetragonalincrystal
sturcutre,columnarinmicrostructure,andbe100-250pmthicktoprovideforadequate
protectionfortheturbinebladesinthesevereengineenvironment. Currently,YSZ
coatingsarefabricatedbyelectron-beamphysicalvapordeposition(EB-PVD),butthis
fabricationmethodiscostintensive.Chemicalvapordeposition(CVD)isamore
commerciallyviableprocessingmethodandapossiblealternativetoEB-PVD.The
depositionoftetragonalYSZfromgaseousmetalandoxidationsourceswerestudied.
Achemicalequilibriumanalysismodeledthefeasibilityofdepositingtetragonal
YSZforbothchlorideCVD(Zr-Y-C-O-Cl-H-Inertsystem)andmetal-organicCVD
(MOCVD)(Zr-Y-C-O-Hsystem).PurethermochemicalpropertiesandtheassessedYSZ
phasediagramwereusedinthisanalysis. Usingthemolarinputofmetals{{ny+«zr)and
{ny/{ny+nzr)=0.08))asbases,equilibriumcalculationsshowedthattetragonalYSZ
VI
formationwasfeasible. TetragonalYSZformationwasfeasiblewithhighoxygen
content{hq/{ny+nzr)>8)andhightemperature(T>100°C)inthecaseofchloride
CVD(Zr-Y-C-O-Cl-H-Inert). TetragonalYSZformationwasfeasiblewithhighoxygen
content(no/{ny+nzr)>5)andhightemperature(T>950°C)inthecaseofMOCVD
(Zr-Y-C-O-H). AlthoughsolidcarbonformationdidnotappearinchlorideCVD,
additionaloxygen{no/(ny+nzr)>32)andlowhydrogencontentrelativetocarbon{uh/
nc<2)wererequiredtoavoidsolidcarbonformationinMOCVD.
Coatingsweredepositedusingasetofbaseconditionsderivedfromthechemical
equilibriumanalysis. InchlorideCVD,YCI3wasnotincludedbecauseofitslowvapor
pressure,thus,ZrCUwasoxidizedwiththeH2-CO2gasmixture. MonoclinicZr02
coatingsweredepositedatthethermochemicallyoptimizedconditions{hq/{ny+nz/)>
8,T>1004°C)withapproximately5.5pmh growthrate.
Inmetal-organicCVD(MOCVD),liquidprecursorsolutionsofY-andZr-(3-
diketonateandY-andZr-n-butoxideprecursorswereusedasthemetalsourcesandO2
gaswasusedastheoxidationsource. UsingtheY-andZr-P-diketonateliquidprecursor
solution,tetragonalYSZwasdepositedwithalayeredmicrostructureapparentanda
maximumgrowthrateofapproximately14pmh’’(ctivationenergy(Ea)of50.9±4.3kJ
mol'*). Thegrowthrate(approximately43pmh withEa=53.8±7.9kJmol"')was
improvedusingY-andZr-n-butoxideliquidprecursorsolutions,andthemicrostructure
wascolumnar. Yet,two-phasedepositionofmonoclinicZr02andtetragonalYSZ
occurred. Resultsofelectron-probemicro-analysisshowedthattheny/(ny+nzrratio
waslessthan45%oftheny/(ny+nzrratiointheliquidprecursorsolution.
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CHAPTER
1
INTRODUCTION
Demandforlowercostpowergenerationhasspurredresearcheffortstoward
lower-costmaterialsfabrication. Akeypowergeneratingdeviceknownasthegas
turbine,hasgarneredmuchattentionbecauseofitsefficientpowergenerating
capabilities. Yet,thegasturbineincurshighcapitalandmaintenancecostsbecauseofits
complexmaterialsdesignandfunction. Thus,improvementsinmaterialsandfabrication
willboostthegasturbine’soveralllifetimeandefficiencyandpotentiallyreducecapital
andmaintenancecosts.
Costefficiency,engineefficiencyandenginelifetimearepartiallylimitedby
turbinebladeperformanceandcyclelife(Haynes1997,Miller1997). Theturbineblades
(whicharethemovingpartoftheturbine)requireprotectionfromexposuretohot
combustiblegasesassociatedwiththeengineenvironment. Athermalbarriercoatingis
usedtoprotecttheseturbinebladesfromhightemperaturesandcontaminantsinthefuel
andintakeair(Haynes1997,Marple2000).
Thisstudyfocusesonthecost-efficientfabricationofthermalbarriercoatings.
Theceramic,yttria-stabilizedzirconia(YSZ)isusedsinceithasadequatehigh-
temperaturepropertiesnecessaryforgasturbineenvironments(Wahletal.2001a).
Currently,YSZisfabricatedusingairplasmaspray(APS)orelectron-beamphysical
vapordeposition(EB-PVD),however,thesetechniquesarecapital-andoperational-cost
intensive(Wahletal.2001b). Aneconomicalalternative,chemicalvapordeposition
2
(CVD)showspromiseasapossiblereplacementforAPSorEB-PVD(Krumdiecketal.
2003,Wahletal.2001a,Wahletal.2001b,PulverandWahl1997).
ThegoalofthisworkisthesuccessfuldepositionofYSZusingtheCVDmethod.
Thisworkusesachemicalequilibriumanalysistoidentify,optimize,anddeterminethe
effectofCVDconditionsongasandsolidphaseformation. TheCVDconditionsused
wereatommoleratios(e.g.,theratioofthetotalnumberofhydrogenatomsinthesystem
tothetotalnumberofcarbonatomsinthesystem,regardlessofwhichmoleculesthey
residein),thesubstratetemperature,andthereactorpressure. TheseoptimizedCVD
conditionswereusedasabasisforselectingexperimentalconditions. Further,the
equilibriumanalysiswasusedtoexplainexperimentalresults.
ExperimentswereattemptedinaCVDreactor. Avertical,cold-wall,stagnation
flowreactorwasimplementedinthiswork. Amongthemyriadofpossibleprecursors,
onlythreetypeswereused. ForchlorideCVD,HClreactedwithZrchipstoprovidea
controllableZrvaporsource. ForMOCVD,Y-andZr-p-diketonatesandY-andZx-n-
butoxideswereused. Sincetheexperimentsusedliquiddelivery,atetrahydrofuran
(THF)solventwasusedtodissolvetheY-andZr-P-diketonatesandtolueneandn-
butanolwereusedtodissolveY-andZr-n-butoxides,respectively.
Toanalyzethecoatingdeposition,severalcharacterizationtoolswereused. For
crystalstructureanalysis,x-raydiffraction(XRD)wasused. Forcoatingmorphological
analysis,scanningelectronmicroscopy(SEM)wasused. Forcoatingcompositional
analysis,electronprobemicro-analysis(EPMA)wasused.
Thenextchapterfollowswithabackgroundintowhyyttria-stabilizedzirconiais
usedasathermalbarriercoatingandwhyCVDcanbeusedtofabricatethismaterial.
3
Thefollowingchapterdelvesintothechemicalequilibriumanalysistooptimizetheatom
ratiosandintensivevariables(temperatureandpressure). Then,thefollowingchapter
describestheexperimentalapproachforCVDandmaterialscharacterization. The
followingthreechapterspresentanddiscusstheresultsofexperimentalefforts. Thelast
chapterthensummarizes,concludes,andrecommendspossibleavenuesforfuturework.