Aspects of Fatigue Life in Thermal Barrier Coatings

54F5EOI073ULinkoping Studies in Science and Technology, Thesis No. 898

Aspects of Fatigue Life inThermal Barrier Coatings

Hiikan Brodin

INSTITUTE OF TECHNOLOGYLINK OPINGS UN IV ERSITET

Division of Engineering MaterialsDepartment of Mechanical Engineering

Linkopings universitetSE-58 183 Linkoping, Sweden

Linkoping, August 2001

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Linkoping Studiesin Science andTechnology, ThesisNo. 898

Aspects of Fatigue Life inThermal Barrier Coatings

H&m Brodin

INSTITUTEOF TECHNOLOGYLIHKOPINGS UN IV ERSITET

LiU-Tek-Lic-2001:35Division of EngineeringMaterials

Departmentof MechanicalEngineeringLinkopingsuniversitet

SE-581 83 Linkoping,Sweden

Liukoping,August 2001

LiU-Tek-Lic-2001:35ISBN 91-7373-085-8

ISSN 0280-7971

Printedin SwedenbyUniTryck,Link@ing 2001

Abstract

Thermalbarriercoatings(TBC) areappliedon hot componentsin airborneandland-basedgas turbineswhen higherturbineinlettemperature,meaningbetterthermalefficiency, is desired.The TBC is mainly appliedto protectunderlyingmaterialfromhigh temperatures,but also servesas aprotectionfrom the aggressivecorrosiveenvironment.

Plasmasprayedcoatingsareoften duplexTBC’Swith an outerceramictop coat (TC)made from partiallystabilisedzirconia - Zr02 + 6-8% Y203. Below thetop coat thereis ametallicbond coat (BC). The BC ii normallya MCrAIX coating(M=Ni, Co, Fe... andX=Y, Hf, Si...). In gas turbinecomponentsexposed to elevatedtemperaturesnickel-basedsuperalloy arecommonly adoptedas load carryingcomponents.In theinvestigationsperformedhere a commercialwroughtNi-base alloy Haynes230 hasbeen used assubstratefor the TBC. As BC a NiCoCrAIY servesas a referencematerialandin allcases 7°/0 yttriaPS zirconiahasbeen used.PhasedevelopmentandfailuremechanismsinAl% TBC duringservice-likeconditionshave been evaluatedinthe presentstudy.This isdone by combinationsof thermalcycling andlow cycle fatiguetests.The aim is toachievebetterknowledge regardinghow, when andwhy thermalbarriercoatingsfail. Asa final outcome of theproject a model capableof predictingfatiguelife of a givencomponentwill help engineersand designersof landbasedgas turbinesfor powergenerationto betteroptimiseTBC’S.

In the investigationsit is seenthatTBC life is stronglyinfluencedby oxidationof theBC andinterdiffusionbetweenBC andthe substrate.The bond coat is known to oxidisewithtime athigh temperature.The initialoxide found duringtestingis alumina.Withincreasedtime athigh temperatureAl is depletedfrom thebond coat dueto interdiffusionandoxidation. Oxides othersthanaluminastartto form when theAl contentis reducedbelow a criticallimit.It is herebelieved thatspinelappearswhen theAl contentislowered below 2w/o in the bond coat. Here itwas shownthata fastergrowing oxide, richin Ni, Cr and Co forms atthe interface.Al depletionis also linkedto BC phases.Initiallythe bond coat is ay/ &materialpossibly withvery fme dispersedy’. SimultaneouslywithAl-depletion the F-phaseis found to disappear.This occurs simultaneouslywiththeformationof spinel.However, oxidation is not only a disadvantage.Low cycle fatiguetestsreveal thatoxide streakswithinthe bond coat will slow down crackgrowthdue tocrack deflection and crackbranching.Thereforebenefit of or damagefrom oxide growthon crack initiationandpropagationis dependenton crackmode, spallingof the ceramicTC or growth of “classic” cracksperpendicularto the surface.

Fromthe observationsconclusions aredrawnregardingfatiguebehaviourof TBCsystems.The basic idea is thatall cracks leadingto failureinitiatein thethermallygrownoxide (TGO). Following the initiation,they can,however, grow to form eitherdelaminationcracksleadingto top coat spallationor crackstransverseto the surfaceleadingto componentfbilure. -

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Acknowledgement

The work presentedhere has been performed for a Iicentiatedegree atthe departmentofMechanical Engineering, division of Engineering Materials,Linkoping university.Thework has been carriedout in co-operation with ALSTOM Power Sweden and has beenfundedby meansfrom The SwedishBoard for IndustrialandTechnological Development(NUTEK). This work is a part of the project “Fatiguelife in ThermalBarrierCoatings”,KME 706. Financialsupporthas also been provided via the Brinell Centreat the RoyalInstituteof Technology (KTH) in Stockhohn.

Severalpersons have contributedto the completion of this thesis.I am very gratefulforall help andideas andI would like to thank:

Associate Professor Sten Johansson, my academic supervisor at the division ofEngineering Materials,Linkoping university, for his guidance and support during thework.

Professor Torsten Ericsson, my assistant academic supervisor at the division ofEngineeringMaterials,Link6ping university,for guidance in the field of oxidation- andcorrosion-relatedphenomena.

VisitingProfessor Soren Sjostrom atALSTOM Power Sweden andthe division of SolidMechanics, Linktiping university, project leader, unofficial mentor with one foot inrealityandalways a minuteto spare.

Dr Xin-Hai Li at ALSTOM Power Sweden, for great collaborationwithin the field ofthermalbarriercoatings.

My colleague Tekn. Lie. Magnus Jinnestrandat the division of Solid Mechanics,Link6ping university, for useful discussions and for showing me how nice the worldwould look if no irregularitiesexisted or no practical experimentswere needed to bedone.

ProfessorRolf Sandstrom,M.SC.MatsLennartsson,Tekn Lic Uhich 130hnenkamp,M.SC. Peter Szakalos and all other Ph.D. studentsat the Brinell Centre (The RoyalInstituteof Tecnology - KTH, Stockhohn) for useful discussionsand collaborationwithinthe field of coating-relatedproblems.

Mr Bo Skoog andMr Ronnie Fingalfor help, ideas and supportin ourmechanicaltestinglab atLink6pinguniversity.

Mrs Annette Billenius and Mr Nils Larsson at the division of Engineering Materials,Link6ping university,for help, ideas and inspirationin mattersregardingequipmentforevaluationof materials.May LINK be with you.

...m

All otherPh.D. studentsandthe staff atthe divisions of EngineeringMaterialsand SolidMechanics, Linkoping university,for theirfriendship,encouragementand support.

Finally I would like to dedicate this thesis to my beloved PetraHohnberg for beingpatientwith bundles of articles,draftsandresultssheetspreadall over our apartmentandmy latenightsatthe computer.Whatwould I do withoutyou?

H&kanBrodin Linkoping August2001

iv

Content of the thesis

Abstract................................................................................................................................ iAcknowledgements

...............................................................................................................111

Contentof thethesis............................................................................................................v

1 INTRODUCTION ....................................................................................................... 11.1 Land based gas turbines........................................................................................... 21.2 Nickel-base superalloy ........................................................................................... 3

1.2.1 Alloying elements............................................................................................. 41.2.2 Phasesin Ni-base superalloy andNiCoCrAl.X.coatings..............................=.5

1.3 Thermalbarriercoatings.......................................................................................... 61.4 Need for materialmodels....................................................................................... 10

2 AIM OF THE WORK ................................................................................................ 12

3 FATIGUE OF THERMAL BARRIER COATINGS................................................ 133.1 Thermal fatigue......................................................................................................

3.1.1 Thermalshocktesting..................................................................................... ;:3.1.2 Thermalcycling .............................................................................................. 14

3.2 Low andhigh cycle fatigue.................................................................................... 173.3 Thermomechan.icalfatigue..................................................................................... 203.4 Influence of oxidation............................................................................................ 233.5 Otherfactors........................................................................................................... 23

4 RESULTS FROM THE PRESENTWORK .............................................................4.1 Summaryof Appended Papers............................................................................... .

4.1.1 Paper 1: Behaviourof a ThermalBarrierCoating duringHighTemperatureOxidation................................................................................... 25

4.1.2 Paper2: CrackInitiationin APS ThermalBarrierCoatings,Testingand MathematicalModelling of LCF Behaviour .......................................... 27

4.1.3 Paper3: Bond Coat influence on TBC Life ................................................... 284.2 Relatedpapersnot includedin thetiesis ............................................................... 294.3 Relevance of thepresentwork ............................................................................... 29

5 CONCLUSIONS ....................................................................................................... 31

6 REFERENCES.......................................................................................................... 32

v

TBC – Todav’s statusanduocomimzissues

1 INTRODUCTION

Gas turbinesare widely used for aircraftpower propulsion in civil as well as militaryaircraftandhelicopters.Forpower production stationarygas turbinesareavailablefrom afew MW up to 250MW with a thermalefficiency between 35 and 40% in single cycle.The basic principle is shown in Figure 1. A rotating compressor forces air into highpressure(compression ratios20:1 and higher can be applied) increasingthe temperaturefrom room temperatureup to 500”C atthe combustorinlet. In the combustorcompressedair is mixed with fuel (liquid or gaseous) andthe mixtureis ignited.The gas temperaturerises to more than 1400”C in the combustor.When the hot gases are led into the turbinestagesthe pressure drops and energy from the hot gases is utilisedto produce electricenergy in the generatorconnectedto theturbine.

Combustor

Fuel r._. 1?

Generator

xCompressorAL

s.*3

z%

Figure 1: Schematic drawing of a gas turbine system.

For gas turbinesthe energy process can be explainedby the Brayton cycle, see Figure 2for the ideal process. Isentropiccompressiontakesplace between 1 and 2 and expansionin the turbinebetsveen3 and 4. During constantpressureprocess partbetween 2 and 3energy is added by the fiel (pressure drop iii the combustor is neglected in thisdiscussion).As a resultthetemperatureandvolume increases.

In gas turbine applicationsa need for increased working temperatureis evident sincehighertemperaturesleadto increasedefficiency according to Equation(l), after [1].

(1)

where II is thermalefficiency, T3 and Tg are temperaturesin Kelvin before and afterpassagethroughthe turbineaccording to the real Joule (13rayton)cycle. ICis an adiabaticexponent.

1

Aspects of FatigueLife in ThermalBarrierCoatings

volume V1 ➤

entropyS

Figure 2: Ideal Brayton cycle pV- and TS-diagrams. Replotted>om [1].

Increasedworking temperatureleadsto higher demandson the load carryingcomponentsin the hot sections. By replacing polycrystalline alloys with directionally solidified orsingle crystal materialone step is taken towards a more durable component, Furtherimprovementscan be done if overlay coatingsareapplied.These coatingsarealuminium-rich metallic overlays and have superioroxidation (formation of alumina)propertiesincomparisonwith the superalloy.By adding a ceramic layer on top of the aluminaformerthe load carrying component is protectednot only from the aggressiveenvironment,butalso from high temperatureif a ceramicwith low thermalconductivityis chosen.

It is clear thatin gas turbineapplicationsa need exists of reliablemodels for fatiguelifein coated hot componentssuch as combustors,vanes andturbineblades.Here focus is seton how cracks initiateand what effect thishas on low cycle fatigue propertiesfor an airplasmasprayedthermalbarriercoating appliedonto a load carryingcomponentmade of anickel-base superalloy. A series of experimentson air plasma sprayed (APS) thermalbarrier coatings (TBC) have been performed Experiments involve static oxidation,thermalcycling andlow cycle fatiguetesting.

1.1 Land based gas turbines

Land-based gas tarbines are in most applicationsused for electric power generation.Insome cases theturbinescan be used for propulsionof ships.The turbinescan of coursebeused in “single cycle”, meaning that energy in the exhaust is wasted. The hot exhaustgases are insteadoften used in a steamprocess in combination with the gas turbine.Inthis way the total systemthermalefficiency can be increased.A land-based gas tarbinefrom ALSTOM Power Sweden (Finspiing)is shown in Figure 3. Air is takenin throughthe air inlet (1) and the pressure is increased by the compressor stages (2). In thecombustors @aced symmetrically around the circumference) the compressed air ismixed with fhel andignited In the turbinestage(4) vanes lead the hot gases towardstheturbineblades. The turbinerotor is via a shaft connected to both the compressor and agenerator(6).

2

TBC – Today’s statusandupcoming issues

6

(1) Air inlet(2) Compressor(3) Combustor(4) Turbinestage(5) Exhaustoutlet(6) Generatorshaft

Figure 3: Land based gas twbinefiom ALSTOMPower, model GTX 100. Power 43 MIT(thermal eficiency 37% (single cycle), 54% @omb cycle). Thermal bam”er coatings arecozwideredfor components in (3) and (4).

As fiel in stationarygas turbinesdifferent types of thin and thick oils are used Fromenvironmentalandmaterialspoint of view it is desirableto lower the contentsof Na, K, Sand ashesas far as possible [2]. Of course othertypes of fuel can come in question, seeTable 1.

Table 1: Different fuel types for stationary gas turbines, after [2].

Liquid fuelsThin oilsThick oil distillatesShaleoilEthanolMethanol

Gaseous fuelsNatural gasPropaneButaneBy-productsfrom chemicalindustries

1.2 Nickel-base superalloy

For use at high temperaturesin a variety of applications nickel-base superalloys areavailable. The alloys are often very advanced but neverthelessthe alloys have gainedmuch attentionandthe behaviour is often well documented.Nickel-base superalloysare .mairdyof threeclasses,namely,wrought, castandpowder metallurgy(PM) alloys.

The PM alloys are less common in gas turbine applications.The strengthis achievedthroughoxide dispersivestrengthening(ODS). Smallparticlesof oxides such as Y203 inlow concentrations(< 1%) are mixed into an alloy powder. After mixing the materialismanufacturedby powder metallurgytechniques.PM alloys arenot consideredhere.

Wrought alloys are often more suitable for machining and welding. To this ~oup ofmaterialcount alloys accordingto Table 2. The wrought alloys are often strengthenedby

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solid solution strengtheningand/or formation of a second coherent phase, called y’(described lateron). The y’-phaseis commonly precipitatedduringheattreatments.

Cast alloys areoften more complicatedto form by turning,milling or similaroperations.In cast alloys y/y’ coexist even if solution heat treatmentsare performed. Alloysmanufactured by casting can be subdivided into polycrystalline and directionallysolidified / single crystalcast alloys. The advantagewith directionallysolidified / singlecrystalsis that fewer / no grain boundaries exist and the creep properties are stronglyimproved. Also the elastic modulus can be tailored with anisotropy giving a three- tofivefold increasein fatigue life [3]. Some typical commercial alloys from each class ofmaterialaregiven in Table 3 andTable 4.

Table 2: Compositions of typical commercial wrought Ni-based superalloys. Taken horn[4].

CompositionAlloy Alloying element[Y.]

trade name Ni co Cr Fe w Iwo Ti Nb c Al B La CuHaynes230 55 <5 22 <3 14 2 0.1 0.35 0.015 0.02HastelloyX 49 <1.5 22 15.8 0.6 9 0.15 2

!.5 3 19 18.5 0.9 5.1 0.08 0.5 <().15~Inconel718 152

Table 3: Cast polycrystalline Ni-based superalloys, typical chemical analyses. Taken from[4].

CompositionAlloy Alloying element[%]trade name Ni co Cr Fe w Mo Ti Nb Al Zr c B

Inconel792 60 9 13 4 2 4.2 2 3.2 0.1 0.2 0.02Inconel738 61.5 8.5 16 2.6 1.75 3.4 2 3.4 0.1 0.17 0.01Waspalloy 57.5 13.5 19.5 1 4.2 3 1.2 0.09 0.07 0.005

Table 4: Compositions of cast single crystal Ni-based superalloy. Taken from [4]..

CompositionAlloy AUoying element [Y.]

trade name Ni co Cr w Mo Ti Nb Al Zr Ta Re c B HfCMSX-2 66.2 4.6 8 8 0.6 1 5.6 6 6ReneN4 62 7.5 9.8 6 1.5 3.5 0.5 4.2 4.8 0.06 0.004 0.15CMSX-4 61.7 9 6.5 6 0.6 1 5.6 6.5 3 0.1

1.2.1 Alloying elements

In high temperaturealloys mechanical strengthis achieved eitherthrough solid solutionstrengthening(as in the alloy Haynes 23O) or by precipitationhardening(for instancein

4


Inconel 718). Precipitatespresentin these alloys are carbides based on various metalsdepending on the alloying elementsin question and will be discussedmore in the nextsection. Several compilationsexist [3, 4, 5] over the use of alloying elementsin nickel-basedsuperalloys.In Table 5 an overview is given over phasespresentin Ni-based alloysfor use atelevatedtemperatures.

1.2.2 Phases in Ni-base superalloys and NiCoCrAIX-matings

In superalloy for use athigh temperaturesFCC materialsuch asNi or Co can be used asan alloy base material. In the present review however only Ni-based alloys areconsidered. These superalloys are typically alloyed with elements as described in theprevious chapter. Some alloying elementsstay in solid solution during the life of a hotcomponent,while othersform dispersionsthat

Table 5: Influence of different alloying elements in Ni-base superalloys.

Elements and concentrations (typical)Classification Large (>10 w/o) Intermediate Small (<2 w/o)

Solidsolutionstrengthener W, Co, Cr Mo Re, Ta, HfPrecipitateformer Al Ti, TaGrainboundaryphases B, C, Zr, HfCarbideformer MC W, Mo,Ti, Nb ~, Ta

M7C3 CrM23C6 Cr W, MoN&c Mo,Nb

Oxidescaleformer Cr Al

will coarsenat high temperatures.Precipitatescan form on dislocationsif hold timesareallowed, during thermal/mechanicalcyclic loading. Especially carbides are known tocoarsen during service. Therefore one could suspectthatalloys where mainly the effectof solid solution strengtheningis utilisedwould be more or less inertto high temperatureexposure regarding change of mechanical properties in comparison to precipitation-strengthenedalloys. One example where this concept has been adopted is the alloyHaynes230 designedwiththesethoughtsin mind.

The Ni FCC-matrix found in these alloys is known as y-phase. It contains a variety ofsecond phases discussed in the literature.With increasing aluminiumcontent a secondphase will appear, namely Ni3A1.This phase is coherent with the y -mairix and is anorderedFCC phase called y’. Dependenton composition the y ‘-phasemight be slightlyalteredin composition and Ni3(Al, Ti) will be formed. Except as a second phase in Ni -base alloys Ni3Al is in some applicationsfound as a protective diflision coating forturbine blades. If the aluminiumcontent were farther increased a new phase wouldappear.This phase, called & is not coherentwith the matrix,but forms a second phase.The ~-phase is not normally found in Ni-base superalloy but commonly reportedfrominvestigationsof as-coatedbond coats in TBC-applications[6, 7]. However the ~ -phaseis proneto disappearwithloweredAl contentin thematrix.

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In certaincases a body-centredtetragonalphase(BCT) can be found. Thisphase, denoted1’”>aPPemswhen niobium is addedto the composition [4]. Severalundesirablephasescanexistbut arenot discussedhere in detail.Phasesof thistype arethe Laves phase, ~phas eandthe Vphase.

1.3 Thermal barrier coatings

Nickel-base superalloys have excellent creep- and high-temperaturemecllanic~propertiesup to approximately900°C. Especially single crystalalloys can be adopted ifresistanceto creep is vital. For the temperaturesmentionedearlierfinther actionsneed tobe takenif a designis to achieve a satisfactorylife. Hot partsneed to be internallycooledandin some cases cooling holes lettingout compressedair on for instanceturbinebladesprovide a cooling air fti on the metal surface. The air necessary for cooling is takenfrom the compressorstage.One i%rtherstepis to add a thermalbarriercoating on top ofthe superalloy. The development of materialsystems and the effect of increasing gastemperaturescan be illustratedaccording to Figure 4, where the equivalent metaltemperatureis plotted against the year of introduction for new materialsor materialcombinations.Thermalbarriercoatings areadoptedwhen gas turbinehot partsneed to beshieldedfrom high temperaturesand aggressiveenvironments.Hot partsare componentssuch as combustors, vanes and tarbine blades. Figure 5 shows a typical temperatureprofile over a TBC systeu after [8]. Protectionagainstthe corrosive environmentin thehot gases is desirable,since oxygen, hydrogen sulphuror other aggressive elementscanbe presentin thistype of environment.

Figure 4: Schematic diagram showing the increase in metal temperature in gas turbineapplications as a result of alloy development and adding of thermal barrier coatings. AjlerStiger et al. [9].

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distance from surfsce

Figure S: Temperature gradient over a TBC system on top a superalloy in a ~pica~ combustor

ap~lication. R;plotted a~er [8].

A thermal barrier coating normally consists of two (or more) layers. Facing theenvironmentis the ceramic top coat (TC). Under the top coat is a metallic bond coat(13C).The coating is depositedonto a load-carryingcomponent,in gas turbineapplicationcommonly made from a nickel-base superalloy.A schematicrepresentationof a duplexTBC systemis shown in Figure6.

The top coat is ahnost exclusively manufacturedof zirconia,partiallystabilisedwith forinstance 6-8 w/o yttriain order to produce a tough ceramic. The iiacture toughness ofMgO-PS zirconia is 9-12 MPak, that of Y203-PS zirconia is 6-12 MPa&, iYI

comparison with 3 MPa& for pure zirconia [10]. The TC thicknessrangesfrom 300pmfor a thin coating up to 2mm for a thick coating. Over this thickness a reduction intemperaturebetween 1OO-2OO”Ccan be measured.A condition for even consicking theuse of a thermalbarriercoating is the fact thatthe load-carryingcomponentin questionisinternallycooled.

I 1

I1 TC: ZrOz + 6 – 89’0 Y203 !1

Thermally1 BC: NiCoCrAIY / CoNiCrAIY ~I4

grownI oxideI Substrate: High temperature !I

nickel-basealloy 1I1

L.______ ------ ______ ------_ -_____ -_----l

Figure 6: Schematic description of a duplex thermal bam”er coating. i%e top coat thicknessranges ji-om O,4mm to 2mm. For the bond coat a normal thickness is O,15mm.

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Thermalbarriercoatings can be produced by a variety of processes. Some of the mostimportantare:

- Air PlasmaSpray,APS- Vacuum PlasmaSpray,VPS- ShroudedPlasmaSpray, SPS- High Velocity Oxygen Fuel,HVOF- ElectronBeam PhysicalVapourDeposition,EB-PVD- Low PressurePlasmaSpray,LPPS

In additionto these methods metallicand ceramic coatings can also be applied by othermethods,metalliccoatingsfor instanceby electroplating(33P)or by arc spraying.

All plasma spray processes are applied in similarways, except for the VPS process,which is performed in a vacuum chamber, Here the APS process is briefly describe~since this is the process used for production of the coatings investigated.In some casesthe SPS method has been used for BC production. This method is basically the same asthe APS metho~ but the molten materialis shieldedby argon gas in the samemannerasfor the h41G/MAGweld process.

The APS process is characterisedby gas temperatureof up tovelocities around 400 m/s [11]. A schematic illustrationof aproduction of APS coatingsis shownin Figure7.

(1) (2) (3)

20000°C and particleplasma gun used for

(1) cathode(2) anode(3) powderinlet(4) waterinlet(5) plasmagas inlet(6) wateroutlet

(6) (5) (4)

Fiowre 7: Plama gun, schematic illustration. After [12].

The plasma gun consistsof a tungstencathode and a copper anode. Between anode andcathodean arc is generatedby electricpower. When gases such asAr, He, H or N arefedthroughthe arcthegasesbecome ionised and entera plasmastate.Powder is injected intothe plasma,melts (partlyif coarse) and is acceleratedtowards a substrate.When passing

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throughairparticlesoxidise on the surface.This createsthetypical oxide networkfoundinsideAPS bond coats.The method is usedboth for metallicandceramicmaterials.

APS / SPS and EB-PVD coatings are the processes most widely used today forapplicationof oxidation- or heat-resistamtcoatings. The typical appearancesof APS andEB-PVD coatingsareshown in Figure 8.

Figure 8: T~ical appearance of an APS TC / BC on a nickzl-baed superalloy (le$) and EB-PVD TC IBC (righQ. APS and EB-PVD coatings reproducedfiom [13].

From a historical point of view TBC’S have originally been used for combustor liningsand afterburners in military aircraft. In these early experimentsAPS coatings wereadopted.During the late 1980’s and early 1990’s the firsttrialswith coatings in stationarygas turbines were initiated [14]. In this case the application was fiel nozzles in thecombustors [14, 15]. In the early 1990’s EB-PVD coatings were developed to an extentwhere testing under realistic conditions could be performed. The main reason forintroducing a new coating system for rotatingcomponents is, as mentionedpreviously,increasedefficiency. Why are not APS coatings are satisfactory?Firstly,gas flow over arough surface will give raise to lost power especially at high pressures(see Figure 9).Comparedto EB-PVD coatings, do APS coatings exhibit rough surfaces. Secondly didearly studies show EB-PVD coatings to be superior to the APS coatiugs used at thepresenttime. However, process improvementof APS coatingshave increasedthe life ofsuch a coating systemandthe total life for APS (and VPS) coatingsvaries with alloyiugcomposition%base materialand coefficient of thermalexpansion according to thermalcyclic testson variousMCrAIX coatingsperformedby Hayneset al. [16].

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60

70

60

50

40

30

20

10

00 50 ml 150 200 250 300 360

Surface Finish

Figure 9: E#ect of surface roughness on aerodynamic ej%iency loss over stage 1 and 2 turbinevanes and stage 1 turbine blades.A@ev[14].

1.4 Need for material models

Gas turbinesused for aeroplanepropulsion are designed for quick starts,shutdownsandsudden changes in power output during flights. This type of loading cycle makesmaterialstestmethodslike thermalshock analysesfor materialin the as-coatedstate(andpossibly in a stateafter a shortheat treatment)very importantfor rankingof individualcoating types. Aircrafl are carefullymaintainedwith shortintervalsbetween disassembly/ inspection of the gas turbine engine. In thermal barrier coating applications forstationarygas turbinesloads are applied in anothermanner.Loading up to operation iscarefidly rampedby computercontrol over severalminutes.After start-upof compressorand burner a turbine can be up and running for several weeks before shutdown isperformed However, in some applications for power generation loadings can varydependenton use of a turbine.Some gas turbinesare used as baseloadmachinesand willthereforework underlong periods with only one or a few cycles per year. Othersserve aspeak power machinesused duringshortperiods of high power consumption.The peakerscan be run very intermittentlywith one cycle per day [17]. Thus inspection intemmlsincertaingas turbines(GT) individualswill be quitelong.

With this backgroundit is obvious thatdemandson stationaryGT’s can be very differentborn those of airborne GT’s. For stationaryturbines questions concerning long timematerialbehaviour need to be addresse~ somethingthatoften is neglected during bothtestingand modelling.Examples areoxidatio~ difl%sio~ creepphenomenaandinfluenceof sulphuror hydrogen on materialperformance.As a resultphasechanges and oxidationoccur together with sintering of plasma sprayed porous material. Also inspectionintervalsfor stationarygas turbinesare longer, thus causinghigher demands,not only onmaterials,but alsomodels predictingTBC life.

If the base materialis properly &signed the most probable cause of failure is spallationof the ceramic top coat. In Figure 10 three importantfailure mechanisms on a typical

10

TBC – Today’s statusandupcomingissues

geometry for TBC applications are explaine~ namely spalling due to radial stresses,stresssingularitiesandoxidationinduced delamination.

r Spallingdue tooxidation/

~-,,1Adhesion

j....;

fhilureonconvex surface Spallingdue J

to stresssingularity

Figure 10: Basic TBC failure mechanisms in a gas turbine component. From the present workand [18].

On planar surfaces or surfaces with small curvature,spalling due to growth of smalldelamination can occur. Due to the irregulargeometry of the TC / BC interface smallcracks can initiatein eitherthe TC at the TC / TGO interface or in TGO grown uponmanufactwing or after time at high temperature.Stressesat the interface are introduceddue to (1) TGO growth duringoperationof the turbineor (2) thermalmismatchbetweenthe TC and BC. Since partially stabilised zirconia (PSZ) has low thermal expansioncoefficient in comparison with the MCrAIX, thermallyinduced stressesare introduced.Cracks initiateat interface asperitiesand upon thermalcycling these crack embryos willlink up andform large cracks.Eventuallythese crackswill buckle the TC duringcoolingand spallationis a fact, see Figure 11. This type of behaviour has been discussed byHutchinson et al. [19] and Choi et al. [20]. It was suggested that cracks startat largeasperitiesandpropagatein the TC.

In the case of a large curvature (small radius) a risk exists for severe delaminationbetween the ceramic top coat and the underlying metallic bond coat. Due to thermalmismatch and internalcooling radial stressesarise. Since the adhesive strengthof theinterface is rather limite~ spallation of the ceramic might be initiated. Therefore aminimumradiuscan be determinedfor the TC / BC interface [18]. This fact is importantwhen thermalcyclic testsor TMF testsof tubularspecimensareconsidered.

Near free edges stresssingularitiesarise especially when the TBC (top coat and bondcoat) and substratehave different mechanical and thermalproperties.The risk can bereduced if transitionsfrom coated to uncoated materialare tapered.This phenomenon isrelevantatthe trailingedge of a tarbinevane or blade and also aroundfilm cooling holeson turbineblades.

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TCPTGO

(a) N<< N~ (b) N+N~,T=Tm~~Microcracking in TGO andTC Coalescenceof cracks

(c) N+N~,T+T~oWBuckling, drivenby delamination(due to ct~c< cx~c<a SWIJ

Figure 11: Schematic picture of crack initiation and propagation in a thermal bam”er coatingsystem. Crack initiation at cmpen”ty nudges (a), linking of small cracks (b) and buckling incompression when the crack reaches a cn”ti”calsize (c). The process is dn-ven by oxi&ztz”on-induced stresses and thermal stresses upon heati”ng/ cooling, meaning spalling due to oxkiafi”on/delamination.

2 AIM OF THE WORK

The aim of the presentwork is to establishbasic understandingregarding mechanismsgoverning fatigue in thermalbarriercoatings. These findings will later serve as a basisduringdevelopmentof a reliablemodel for fatiguelife in land-basedgas turbines.Withintheproject focus has so far been set on thinAPS TBC with gas turbinecombustorliningsas a primaryapplication.In other cases such as theturbinestages,more modern coatingslikeEB-PVD’S can be considered.The PVD case is not investigatedfurtherhere.

During the investigationsa main theme has been to achieve resultsfor microstructureapplicableto a real case. This is done by evaluatingnot only the initialmicrostructurebut also heat-treatedandoxidised material.Investigationsof thistype are often neglectedduringplanning stagesof a project concerning fatigueproperties.Furthermoretestingofmaterial under conditions close to load cycles found in a real component must beconsidered. So far many tests consider fatigue under constantloads or thermalcyclingwithoutappliedmechanicalloads. This is not alwayssuitable,but sometimesnecessaryif

12


contributionsfrom differenttypes of loadings are to be investigatedin the developmentof a TBC life model. It mustnot be forgottento perform more advance~ reality-liketestsin conjunction with this type of “simple” tests. Here a suitable type of complex testwould be thermomechanicalfatigue. With this type of test, interactionsbetween thermalfatigue,mechanicalfatigue,creep darnageandoxidation-induceddamagecan be covered.Withinthe frameof the currentproject the following topics have been studied

Initialandheat-treatedrnicrostructuresBond coat oxidationof APS coatingsThermalfatigueof differentAPS TBC’SInfluence of oxidationstateon low cycle fatiguebehaviour

- Crackinitiationandpropagationin TBC’S

When fatigue of thermalbarriercoatings is considere~ top coat, bond coat and oxideinterfacesall play importantroles.However, withinthe presentproject focus is seton thebehaviour of BC and TGO to fatigue. Influence of sinteringof and microcrackingin theTC is not considered.

The work done here is closely related to modelling work done at the DepartmentofMechanical Engineering, Division of Solid Mechanics, Link6ping University byJinnestrandand Sjostrom[21].

3 FATIGUE OF THERMAL BARRIER COATINGS

The topic of fatigue in superalloys and thermal barrier coatings has been heavilydiscussed in the literatureover more thana decade. In early studiesinvestigationswerefocused on thermalshock testsaimingat improving and optimizingcoatings for airbornegas turbines.Standardmechanicaltesttechniquessuch as low cycle fatigue (LCF), highcycle fatigue (HCF) and thermal fatigue tests have also been performed on diHerentcoating systems. For stationarygas turbines loads are applied in another manner andthermal shock tests are replaced or used together with thermal cyclic oxidationexperiments.From all thesedatasourcesconclusions aredrawnregardingthe responseofa given material to loads experienced in hot components. More recentlythermomechanicalfatigue (TMF) testing has been used for simulation with a morerealisticpicture of stressesand strainsin a coating system. The aim here is to provide asurvey over results achieved with the different methods. A comprehensive literaturestadyon the subjectis also provided by Linde [22].

3.1 Thermal fatigue

Thermalfatigue can be divided into two differentbranches,namely thermalcycling andthermalshock testing.The main difference between thd cycling and thermalshocktestingis heatingandcooling rate.In airbornegas turbinesused for propulsionof military

13

Asuects of Fatkue Life in ThermalBarrier Coatims

aircraft rapid thermal cycling is experienced in the turbine hot parts. When thermalfatigue is mentioned sometimes oxidation-induced stresses will cause spallation uponlong time exposure to high temperatures.This is discussedseparatelybelow.

3.1.1 Thermal shock testing

Thermalshock testsare sometimesconsidered in evaluationsof coating systemsfor usein airborne gas turbines. The method is quite rough since rapid heating / cooling isappliedto the tested coating. The method gives qualitativevalues in terms of coatingsystem durabilityand it is possible to rank individual coating / substratecombinations[11, 22]. Oxidationand diffusion phenomenaare in some senseneglected duringthermalcycling since these phenomena are time dependentand thermalshock testingmight notgive a representativematerialbehaviour. Especially for stationarybase-load machinesthis is importantto bear in mind. The issue of shock testingwill not be discussedfurtherin thepresentstady.

3.1.2 Thermal cycling

During thermalcycling the coating systemis subjectedto thermalcycles, which are longin comparison with those of themuoshocktests. The basic idea is, however, the sameexcept for heating and cooling rates that are slower. Due to the differences in thermalexpansion for top coat, bond coat and substrate,temperaturechanges lead to residualstressesupon heating or cooling. Thermal cycling tests of various coating systemsarecommonly reported in the literature.Related articles for plasma sprayed coatings areamong others[23 - 29] andfor EB-PVD coatings [30-35].

On APS coatings Miller [27] performed thermal cyclic tests with various time at hightemperature.Cycling was performed withtimesathigh temperatureaccording to Table 6.

Table 6: Temperatures for the thermal cyclic fatigue study by Miller [27.

Max temperature Time at high temperature [s]1000 301175 1201250 3600

It was found thatthe test coupons failed with white fractures,i.e. the delamination werefound in thetop coat, not in the TOO. One reasonmightbe thatthe TGO hasnot reacheda criticalthicknessor thatno formation of spinelhas been initiated.from otidation induced stressescan influencethe fatiguebehaviour.

However, an effect

14


10000E 7 “!L

i;i, 1

\!, \

:

1000\\\

E ‘\ ‘,=mQ

“\\\‘hUY ‘\

‘\ ‘.: 100 ‘0, “..,al

‘\ ~.\\~.75 s.. . .

G -.. ...-... -.,.-... .>10

-. +

0,1 1 10 100

Heating cycle length (minutes)

Figure 12: Thermal fatigue lfe versus time at high temperature. Replotted ajler [27].

Tamuraet al. [25] performedthermalcycling on cylindricalspecimenwith duplex (TC +BC) and triplex (TC + aluminisedlayer + BC) TBC. Cycling was performed between150”C and 1100”C. They found that an addition of an aluminisedlayer increases thefatigue life. However when the TGO reaches a thicknessof approximately11-13 pm allcoatingsfail dueto spallingof thetop coat. Failureoccurs atthe TC / BC interface,eitherin the TOO or in the boundarylayer facing the TC. Fatigue lives were found to be 260cycles for duplex and 720 * 20 cycles for the triplex TBC. A positive effect regardingfatigue life of an aluminised layer was thus found. The reason would be improvedoxidationproperties.

Brindley [24] has evaluatedthermalcycling from burnerrig testswith heatingto 1150”C.The thermalcycle consisted of 6 min hold time at the high temperature,with constantmaximumtemperaturemaintainedin the BC duringthe last4 minutes.Cooling to roomtemperaturewas achievedby forced air cooling for 4 minutes.The resultsshow crackingsimilarto the analysisby Tamur4 thuscracking in TOO or the TGO/TC interface.Here itwas noticed thatspinel was formed during the fatigue test. Formationof Ni(Cr, Al)204was reportedaftermeasurementsby XRD reflection of the {311} peak. In the samestudyLPPS coatings were investigated It was found that the same spallation mechanismsfound for APS coatingsarevalid even for LPPS-coatings.It shouldbe mentionedthattheoxide thicknessat failurewas thinnerthanwhat was reportedby Tamuraet al. However,spinel formation has been initiatedin the TOO, meaning that the Al supply initiallypresentin the BC is consumed. It was not reportedwhetherextensiveAl d.ifi%sionintothe Wsapalloy substratehas occurred or not. The initialAl concentrationin Waspalloy islow. Therefore it can be argued that the substratehas used a large portion of the Alinitiallyavailable.

15


Rabiei and Evans [36] have investigatedfailure mechanismsin plasma sprayed TBC’Swith a growing TGO interfacebetween top and bond coat. It was concluded thatfailureof TBC’S are dependenton large-scale interface defects (abnormal asperities)and TGOgrowth atthe TC / BC interface.Crackswill continueto ~ow atthe TC / TOO interfaceor propagateout into the TC.

A common opinion of thd fatigue in APS (and similar) coatings seems to beinitiation of interracial cracks at the thermally grown oxide between TC and BC. If theTGO is kept thinthe fatigue life can be increased. Of course, other parts of the TBC willthen be the weak link. It appearsas if the top coat then fails due to thermal mismatch andstresses inducedby TOO growth Also the BC asperity affects residual stresses as shownby others, for instane by Pindera et al. [37]. It was found that an increased surfaceroughnessincreasesthe stressestransverseto the interface.

In the case of EB-PVD coatings it seems as if the case is similar: cracks initiateat theTGO duringthermalcycling. Gell et al. [35] evaluatedthe thermalfatigu; propertiesofYSZ TC + PtAl BC on Rene N5. Here the test specimenswere heat treatedin order toproduce a dense aluminalayer on top of the PtAl bond coat. After this the coating wascycled betweenroom temperatureand 1135°C (60 minutescycle time with 50 minutesathigh temperature).It was shown by fractographicinvestigationsthatthe coating systemfails at the BC/TGO interface due to delamination followed by spallation. Thephenomenon is explained by plastic deformationin the BC during cool-down from hightemperature.After heatingin the next cycle the TOO will be loaded in tensiondue to theprevious plastic deformation in the BC. This eventually causes crack initiationin theTGO. Around microcracks the BC will oxidise more readily due to increased inwardoxygen diffasion in the TGO cracks. Eventually cavities are formed at the initiallycracked TGO. Fromtheseareasdehuninationscan propagate.

Bi et al. [31] have investigatedthe fliilurebehaviour of YSZ TC + NiCoCrAIY BC on In100 (Ni-base superalloy) after cycling. It was concluded that ftiure occurs at the TC /BC interface.Microcracks were seen to form at columnar grains in the YSZ TC. Thecracks grow throughoutthe entireTC thickness.Cracksparallelto the surface initiateinthe TGO and cause spallationupon propagation. It was seen that a pre-treatmentat1273K in vacuumimproves fatiguelife.

In work done by Leyens et al. [32] the samefracturebehaviour is detected Delaminationcracks form between TGO and the BC. In this study two substrateswith YSZ TC andNiCoCrAIY BC TBC were examined.As substratematerialsCMSX-4 andRen6 N5 wereadopted. Thermal cycling was performed between (a) 1100°C and (b) 1250”C. It issuggestedthatcooling from high temperaturecausesthe failure.For fatiguewith thermalcycle according to (a) no differences between substratescan be seen. Fatigue lives arefound to be 80-100 cycles independent on material. At the higher hold temperaturehowever, TBC on Rene N5 substrateseemsto be superiorto CMSX-4. It is believed thatTi and Ta readily difhse from the substratematerialout to the BC / TC interface.Hereadhesion of the TGO layer is believed to be reduced. In the Rene 5 Ti and Ta are notpresent,resultingin betterTGO adherence.

16


Another work done on coatings manufacturedby GeneralElectric (7Y0YSZ + PtAl +Rene N5) by Walteret al. [34] indicatesthe sameTOO behaviourasthatfound by Gell etal. [35]. Studies of the TBC system after isothermaloxidation reveals a planar TGOlayer. After thermalcycling the initially planar interfacehas changed and undulationswith a wavelength of approximately300 pm are apparent.Along the TGO the cycledmaterialis found to exhibitextensivedelamination.

Mumm and Evans [33] have proposed a mo&l for the detachmentof a top coat from aPVD bond coat. If intrusionsinto the BC from TGO are presentthe stressstatearoundthe heterogeneitiesupon cooling will cause nucleationof interracialcracks. The cracksgrow, but do not progressover theheterogeneitiesuntilthe entiresurfacehas cracked.Upto thispoint theheterogeneitieswill serveas ligamentsbetween TGO andBC. As a resultof the internalTGO delamination%the fracturesurfacewill appearblack.

On comparing AI% and EB-PVD coatings some comments can be made. APS coatingstend to fail due to poor oxide attachment.Spinel, or Cr and Ni-rich oxides, can be thereason for the poor behaviour. Fracture surfaces reveal mixtures of black and whitefractures.Oxidation induced stressesand the wavy surface contributesto the problem.EB-PVD coatings do not exhibit the same behaviour, however oxidation is importanteven here.During thermalcycling it has been suggestedthatthe BC will flow plasticallyupon cooling due to large thermalstresses.During the next cycle the TGO will crack.Along the newly createdfree surfacesoxidation is facilitatedby oxygen transportto themetal.Heterogeneitieswill form atthe BC / TGO interfiwe.Around theseheterogeneitiesa similarcase will be fount where cracksinitiatearoundthe oxide intrusions.

3.2 Low and high cycle fatigue

Low- and high-cycle fatigue (LCF and HCF, respectively) testing are both methods forevaluation of materialresponse to mechanical fatigue. The testing can take place atambient or elevated temperature.Figure 13 schematicallyillustratesthe test types. Inmost casesLCF testingis done understraincontrol andHCF testingunderstresscontrol.During LCF tests applied strainsare large enough to give plastic yielding, while HCFfatigue testsare performed more or less in the elasticrange. These testmethods can beused to achieve basic informationon how a coating influencesthe fatigue life of a givensuperalloy.Togetherwith informationfrom thermalfatiguethesetestscan indicatehow acoating systemwill respondto appliedmechanicalandthermalloads.

LCF andHCF behaviourof coated and uncoatedTBC on HastelloyX has been evaluatedin a number of studies,among others by Li et al. [7] who performed tests on an APSTBC, Figure 14. It was found that TBC spallationdoes not occur during HCF or LCFfatigue, Insteadtransversecracks are initiatedand grow into the substrateuntil fdureoccurs. No influence on LCF life of an applied TBC coating is found but HCF life isreduced by 12’Yo.The reasonfor the decreasein fatiguelife in the HCF case is supposedto be therough BC surfacewherefatigue cracksareinitiated

17


A

I

time

/!A!!time

Figure 13: Typical cycle dun”ng standard LCF and HCF test without hold times at maximumstrm”nlstress.

1

0,9

0,8

0,7g

: 0,6.SQ05E,.f 0,4

260,3

0,2

0,1

01W3 10CCI lGOX KmLw

CFk6 to failure(N,)

Figure 14: LCF Strain-&e curves for TBC coated d uncoated Hastelloy X Afier Li et al. [7].

Smith [38] and Schneideret al. [39] have found otherresultsregardingHCF life. Here anincreasein fatiguelife was foun~ somethingthatcould be relatedto smoothBC surfaces.Regarding LCF life a study was performed by Shiozawa et al. [40] on an austeniticstainlesssteel with TBC. Tests were performed both with and without hold times. Theresults show that the fatigue life is reduced during continuous cycling at hightemperature.If the cycling is performed with a hold time of 15 minutes at maximumstraiq the results for coated and uncoated materialdo not differ, see Figure 15. Oneshould however bear in mind thatthe test temperatureis high for this material(973K),somethingthatmightintluencethe creep behaviourin the substratematerial.

18


10.0 i973K, t++=0, in air

# o No-coating 7

i’-” -<-.-!’0

G

ol~“102 ,03 ,.4

105

Number of cycles to failure Nf

5.0 973K, t~=l 5min, in air

4

Tens. COmp.

# No-coating o ●

~Coating(1) n ●

:C0ating{2) A —

!% “

\ —.—

/ =%%%:‘-y:+. + COating(z}.k=c

m&

.g 1.0 ~2siii:+ -%m

‘-%

---+.., -% -Q,,’.

~-,>.0.5 . . ...-. —

100 1000 2000Number of cycles to failure Nf

Figure 15: Strain-fatigue plots for X5CrNi 18-10 austenitic stainless steel with and without APSTBC. In the Iej@gure continuous cycling is perfomed while the righi represents cycling withhold time at maximum strain. A~e~ Shiozawa et al. [40].

More recent work by Brodin and Johansson[41] shows a negative influence on fatiguelife of APS coated Haynes 230. At 850”C the fatigue life of the coated material islowered. If the materialsystemis given an initialheat treatmentso thatTOO is formedthe fatigue life decreaseseven farther.If the testing insteadis performed at 500”C thefatigue life is initially lowered. After heat treatmentsthat produce TGO and internaloxides the fatiguelife is againincreasing.The reasonfor the behaviouris believed to beinfluence on crackpropagationof the TOO and internalBC oxide network.At the lowertemperaturethe material behaves more brittle and is more affected by oxide streaksintersectingthe crack path.Furthermoreoxide growth increasesthevolume and since theoxidation is internala compressive residualstresscan be built up. One example of theinfluence of residualstresseson a brittlecoating is a studyby McGrann et al. [42]. Theydiscovered thatan applicationof a WC coating on an aluminiumalloy (6061) decreasedfatigue life for a specimen coated on one side and fatigue tested in bending. Withincreasing compressive residualstressthe fatigue life of coated materialincreasesandfinally approachesthe fatigue life found for uncoated 6061, see Figure 16. However, itshouldbe rememberedthatcompressiveresidualstressesarenot alwaysbeneficial, sincethey can induce spallationif the coating in com–~essionshows poor adherence to theunderlyingmaterial.

19

As~ects of FatigueLife in ThermalBarrierCoatimzs

1 111111

1.E@4 1.E+05 !. E+@

Cvcles to Failure, N,

1.E+07

—Costing Compressive Residual Streaa

+ Bare 6061 Aluminum ● 80 MPa A 500 MPa n 760 MPa

Figure 16: S-N-curves for WC-coated6061-T6511. A@er [42].

3.3 Thermomechanical fatigue

If reliableandrealisticdatafrom thermomechanicalfatiguetestsareto be produced somegeneral considerationsmust be considered. Load cycles representativefor loading in areal component must be achieve~ especially if the real component is subjected to highloads/strainsduringthehigh temperaturepartof the cycle where creep can be considere~T >0,4 Tmas a rule of thumb.(T. = the meltingtemperatureof the materialin question.)Thermalloads must be applied in a correct manner,meaning energy transportfrom theTC surfaceinto the underlyingmaterialduringthe heatingpartof the cycle. Therefore itis dangerousto apply the temperatureby inductionheating unless special measuresaretaken. In an unfortunatecase the highest temperaturewill not be experienced by theceramic but by the underlying bond coat due to that only the metal will be heated byinduction heating. Consequently a totally misleading result could be achievedFurthermoreit is vital that a satisfactorytemperaturegradientis maintainedacross thematerialsystem duringthe complete testingperiod Internallycooled specimens shouldbe used whenever temperaturegradients are needed in a real case. Also cooling ofspecimensmustbe considered If cooling is performed in au incorrectmannerone mightend up with a tensile stress state in components that should experience compressivethermalstressesduringthe cooling period of a TMF cycle. Specimengeometries shouldalso be considered so that effects of radial stressesor stress singularities,desired orundesired,arenot neglected.

TMF testsare performed in eitherin-phase TMF (IP-TMF) or out-of-phase TMF (OP-TMF). The two types of testing are schematicallyplotted in Figure 17. Here no holdtimesatmaximumtemperature/strainhave been plotte~ somethingthatmight have large

20


impact on the TMF fatigue life if oxidatio% diffusionsinteringand creep of coating orsubstrateare considered.A typical TMF uniaxial test rig is shownin Figure 18.

4

time

Figure 17: TMF 1P and OP c.vcles without hold times.

extensometer

fhrnace(open)

internallycooledgrips

Figure 18: T~ test rig to be used in the present project. The system is capable of achievingheating and cooling rates applicable to a real gas turbine hot component.

Some investigationsof wated nickel-based superalloy are presentedin the literature.Zhang et al. [45] have performed some OP-TMF on uncoatecl NiAl overlay coatingCoNiCrAIY overlay coating and PSZ/CoNiCrAIY TBC between 300 and 105O”Cwith

21


phase angle @135° between applied load and temperaturecycles. The results arepresentedin Figure 19. Here the fatiguelife of aluminisedmaterialis superiorto all othercombinations including uncoated CMSX-4. However the resultsfor uncoated CMSX-4arenot representativesince the surfaceroughnesson the inner surfacewas unsatisfactoryfor these specimens. The failurebehaviourreportedhere is growth of transverse(radial)cracks even in the case of the TBC materialsystem. No delaminationbetween top andbond coat is reported.One problem with these resultsis thatthermalloads were appliedby induction heating; therefore the temperaturegradient will have a peak within themetallicbond coat. A directconsequence is a misleadingstressstatein the testspecimensfor theTBC case.

Studies on the response of a TBC system to TMF loads have also been performed byWright [43]. Here the cause of failure was spalling due to extensive TC / BCdelamination. During the tests only small thermal gradients were applied with thehighesttemperatureon the TC outersurface.

A more extensive study has been performed by Tz,imaset al [44]. They performed lP-TMF tests on a single crystal nickel-base alloy. In these investigations a susceptortechnique was adopted for heating, enabling realisticthermalloads with heating of theceramicTC. APS andEB-PVD NiCoCrAIY bond coats were used togetherwith PSZ topcoats. Here the failure mechanism was found to be crack initiationat the TGO / BCinterfacefor high appliedstrains.For both coating types the crackspropagatethroughthebond coat and into the substratematerial where failure eventually occurs. Underconditionsof low strainsthe fracturemode changes and TC / BC delamination followedby spallingare dominant.

0,8

0,7

0,6

g 0,5

j 0,4

q 0,3

0,2

0,1

0

. . . x .

A

,~

o uncoated, h@h T

■ aluminide

A oveiiay

● TBC

x uncoated, low T

o 2000 4000 8000 8030 10000 12000

TMFfatigue life [NJ

Fi~re 19: Reszdts@om TMF tests on dz~erent coatings on the single ciystal CM3X4. Ajier[45].

Anew testmethod for TMF testinghas been proposed by Hasselqvist[5]. No mechanicalloads are applied, instead heating and cooling are locally applied and a “hot-spot” iscreated.The test type is said to be usefid for long-termtestsunderTMF-like conditions.Advantageswiththe “hot-spot” TMF testaccording to Hasselqvistare:

22


Many testscanbe run simultaneouslyLong testtimesarepossible sincethe equipmentis cheapRealisticthermalgradientscan be applied

However, thetestmethodrequiressimultaneousFE simulationsif the stresskimin-stateisto be known in thematerialsystem.

3.4 Influence of oxidation

It was previously mentionedthat oxidation could eitherhave a detrimentalor beneficialeffect on transversecracks during uniaxial loading depending on test temperature.Thenegative influence an oxide interface between TC and BC can have during thermalcycling was also discussed. Since much work on TBC with LCF or HCF testingis doneon as-coated material,literaturedataare sparse.Papertwo in the presentthesispresentsobservations on crack initiation in oxidised APS TBC and a conclusion is that TGOgrowth atthe TC/BC interfacelowers theresistanceto crack initiationbut simultaneouslycrack growth rates are reduced due to crack tip blunting, crack deflection and crackbranching. It is clear that if fatigue properties are to be improved in some way theoxidationpropertiesmustbe improved as far as possible. Especially in APS coatingstheinterdiffusion of aluminiuruneeds to be reduced. It is detrimentalif aluminhun isdepletedfrom thebond coat.

The crack initiationstage preceding delaminationinvolves microcrack formation in theTGO. Pintet al. [46] provide an evaluationof thermalfatigue of TBC on novel substrates(FeCrAl, NiCr, ~-NiAl). The interfaceis relativelyflat and similarto the surfacesfoundafterEB-PVD coatingof bond coats. In thesecases cracking occurs in the TGO interface,seeFigure20. In comparisona typicalwhite fractureis shown in Figure21.

3.5 Other factors

The present discussion concerns the basic features governing the behaviour of TBC’S.Other phenomena also contribute to the performance of a coating system. Creepphenomenahave not been addressedhere, and neitherthe influence from adding of rareearthmetals @EM) for changed adherence of the TGO. Sinteringof the top coat willinfluence stiflhess and consequently thermal residual stresses. By performingoptimisation of the alloy composition Al interdiiTusioncan be hindered or severelyreduced.

Modelling aspects,which are vital for the presentproject, have not been covered. Herethe aim has insteadbeen to give an overview of basic mechanismsresponsible for TBCfailure.The presentedmethods give a fair representationof materialresponseto specificload cases, but if a more complete picture of the degradationprocess is to be achieved,modified TMF testswill be needed.

23


Figure 20: Typical spallation afier thermal cycling on novel substrates. (J)NiAl, (I4 FeCrAl,(11~NiCr. A@er Piti et al. [46].

4 RESULTS FROM THE PRESENT WORK

4.1 Summary of Appended Papers

The work done here focuses on a betterunderstandingof why andwhen a TBC aggregatefails. Earlierstudieshave discussedthistopic and over the pastten to fifteen yearsmanyarticleson the subjecthave been written.However, manyresearchersseemto focus on

the initial microstructure present in a coating, and factors such as oxidation andinterdiffasion can therefore be neglected especially if fatigue by mechanical loads isconsidered. The goal here has been in some sense to get an explanationof how time athigh temperaturewill affect the mechanicalpropertiesof a given coating system.Factorssuch as bond coat initialcomposition, phase development,internaloxidation, oxide phasedevelopment, aluruinium depletion and substrate initial composition need to beconsidered.The work done here is closely connected to FE-modellingwork andthe

24

TBC – Today’s statusanduDcominqissues

Figure 21: White fiactuve in APS TBC, ajler [16J.

goal is to achieve a reliablemodel for fatiguelife. Thereforetheresultsdiscussedin theappendedpapersshouldbe consideredas aninputto thismodel. Since thetopic is verycomplex certainassumptionsneed to be done and decisions mustbe made concerningwhatdegradingfactorsto considerwhen creatinga TBC life model. Below thethreeappendedpapersaresummariseal.An overalldiscussionis also made.

4.1.1 Paper 1: Behaviour of a Thermal Barrier Coating during HighTemperature Oxidation

An air plasma sprayed thermalbarriercoating was investigatedin order to clarifi theconnectionbetweenheattreatment,oxidationand diffhsion behaviour.In the studya thinzirconia (PSZ) layer was used as top coat together with a NiCoCrAIY bond coat. Theinvestigationwas focused on differences for three geometries. Thermalbarriercoatingson flat, concave and convex surfaceswere, therefore, stadied.Isothermaloxidation wasperformedup to 1000 hrs at 1000”C in orderto simulatetrueworking conditions for theinterfacebetween ceramictop coat andmetallicbond coat.

The investigationsshow a presence of AI-rich oxides for shortertimes.When the coatingsystemis heat-treatedfor 1000 hrs a change of oxide composition is obvious and besidesAl the oxides contain Ni, Cr and Co. The oxides tend to grow with different ratesdependingon the macroscopic surfacegeometry. In the study convex surfacesreveal thehighestoxide ~owth ratesand concave the lowest growth rates.At 1000 hrs and 1000”Cthe differencebetweenthefastestandthe slowestgrowing oxide layer is lpm, Figure22.This can be explained by the coating geometry. In the case of a coating on a convexsurfacethe cross-section is reduced upon passage through the TBC. The interfacehas a

25

Asnects of Fatime Life in ThermalBarrierCoatimzs

smaller area than the TC outer surface. Due to this the oxygen concentration willincrease.The opposite is truefor a concave coating.

Some interdifkion is obvious. Between the superalloy substrate and the bond coatoutwarddiffusionof Ni, W and Cr is presenttogetherwith inwarddiffusion of Co andtosome extentAl.

i

0 I , ,

0 200 400 600 800 1000 1200

time [hrs]

Figure 22: Oxide growth on concave, Jut and convex szofiaces. The growth rate is highest on theconvex surface, something that can be explained by increaed oxygen content due to reducedcross secti”on through the TBC.

0 2 4 6di8tance [pm]

8

25

o

o 2 4 6 8distance [~m]

25c)

n

+500h

& 20+ 1000 h

~

gl5-~~clo-g!

: 5-

0

0 2 4 6distance [pm]

8

Figure 23: Formation of spinel is indicated with increasing content of a) Ni, b) Cv and c) Co inthe TGO. “O” indicates TC / TGO interface, “6” represents TGO / BC interface.

26

TBC – Todav’s statusanduucornirwissues

4.1.2 Paper 2: Crack initiation in APS Thermal Barrier Coatings, Testing andMathematical Modelling of LCF Behaviour

In the present paper failure mechanisms in air plasma sprayed (APS) thermalbarriercoatings(TBC) for landbasedgas turbineshave been studied.This has been done by FEsimulationsand fractographicinvestigationsof low cycle fatigue (LCF) testedmaterial,here chosen as an APS 350 pm thick partiallystabilisedzirconia (PSZ) top coat (TC)together with a 150 pm thick NiCoCrAIY bond coat (BC) on a nickel-base substrate(Haynes230).

Both LCF-testing,modelling results and fractographic investigationspoint in the samedirection.& increasedthicknessof the thermallygrown oxide (TGO) does decreasetheLCF life of a coated structuralalloy.

Severalpoints of crack initiationwere foun~ in the TGO at the TC/BC interface, at theoxide network within the BC and at oxide inclusions between BC and substrate,seeFigure 24. During LCF tests the initiated cracks will grow radially into the substratematerial.The behaviour is explained by increased TC/BC delamination stresses andchangedoxidationbehaviourwith increasedoxidationtimes.

Figure 24: Crack formation in thermal bam”er coatings. Le$picture shows cvack formation atthe BC/S inte~ace, n“ghtformation at TGO. Both crack initiation types lead to radial cracks intothe substrate material. Arrows indicate the positions for crack propagah”on.

It was observedthatanAPS bond coat could be beneficial from slow crack growth pointof view. The oxide networkdeflects cracksthatgrow throughthe bond coat. Furthermoreare compressiveresidualstressesintroducedin the bond coat during fatigue testing,seeFigure 25. The compressive stressesare probably due to crack bridging / crack closureeffects of deflectedradialcracks.

27


0 0,2 0,4 0>6 0,8 1 1,2 1,4

total strain range [%]

Figure 25: Resadtsji-om calculations of residual stresses, q, in-the bond coat ajiev mechanicalcycling at 850”C with varying strain range. All specimens have been cycled with R = -1 adunloadedfiom compression. Strain range “O”corresponds to the heat-treated condifi”onbeforeLCF tests are performed.

4.1.3 Paper 3: Bond Coat influence on TBC Life

In the present study the influence of bond coat composition and coating process onthermalbarriercoating (TBC) life has been evaluated Six different coatings have beensubjected to thermalcycling between 100 and 1100°C. After this the diflerent systemshave been characterised by light microscopy and SEM-EDS. Testing of variouscommercial bond coats has been performed on one substrate(Haynes alloy 230). Thetotalfatigue life of the clif3?erentTBC systemsvaries with 30% which is here believed tobe dueto influencedby diffhsion andoxidationphenomenain the bond coat.

It is found thatwhen the ahminium concentrationis decreasedphases otherthanaluminaform at the top coat / bond coat interface. Oxides formed during later stages of thethermal cyclic test are rich in nickel, cobalt and chromium and the results can beinterpreted as formation of nickel-, chromium- or spinel oxi&s. The reason foraluminiumdepletionis due to inwarddiffirsionand formation of thermallygrown oxides(TGO) atthe ceramic top coat (TC) / metallicbond coat (BC) interfaceas well as growthof internaloxides in the bond coat. Thermally grown oxides are known to constitute

tion cracks.This has been mentionedin severalinvestigationsinitiationsitesfor delaminaof fkiled TBC systemsas well as simulationsof stressstatesin thermalbarriercoatings.Here focus is set on the link between oxidation, inter-ion and fatigue life. Ifaluminium diffhsion into the substrate can be hindered or radically decrease~ asignificantlylonger fatiguelife can be achieved. Otherfactors previously suggestedsuchas interface roughness and bond coat composition do not seem to be as importantasoxidationrelatedphenomena.

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Co-base APS Ni-base APS Ni-base EP

Figuve 26: Cornpa@on of TCF lz~efor Co-based APS, Ni-based APS and Ni-based EP-coatings.Here it is obviozu that for a given nickel-bare superalloy the APS’ NiCoCrAIX coatings areinfen”or to CoNiCrAIX coatz”ngs.

4.2 Related papers not included in the thesis

Two papers covering the same topic (APS PSZ / NICoCrAIY coated superalloys)are tobe presented during the autumn/winter.For the reader who wants to follow thecontinuationof thework, paperswill appearin conkrence proceedings according to:

H. Brodin, S. Johanss@ “In#’uence on Low Cycle Fatigue Properties of Bond CoatOxidation for a Thermal Barrier Coating”, to be presentedat ICF 10, 2 – 6 December,2001, Honolul% Hawaii

M. Lennartsson, H. Brodin and R. Sandstrow “Fracture Behaviow of an Air-Plasma-

Sprayed Ni23Col 7Cr12A10.5Y Bondcoat”, to be presentedat Materials Solution, 5-8November 2001, Indianapolis

4.3 Relevance of the present work

It is clear from previous studies that oxidation phenomenamight play an importantroleregarding mechanical performance of a TBC system since a transition from aluminatoNi-, Cr- and Co-rich oxides takesplace with long oxidationtimes. It was suggestedthatoxides formed are of spinel type with an FCC lattice common for a combination ofoxides, here of (Ni, Co)(Cr, Al)204 -type.

Oxidation is to some extentdependenton the macroswpic geometry, since the previousstudiesshow thatoxidation rate4s fasteston concave surfaces,intermediateon flat andslowest on convex exposed surfaces. This is of interest in the case of gas turbinecomponents, since for instance turbine blades have combinations of all three possiblegeometries.

29


It was shown here thatthe TGO interfacebetween top- and bond coat is responsible fortransversecracksduringLCF conditions.The sameis truefor the thermalcyclic teststhatwere performed on dii3!erentTBC systems.In all cases cracks initiatein the TOO. Afterinitiation it is a matter of how loads are applied if the cracks will interlink intodelamination or if they are bound to deviate and form radial cracks. The theory issupportedby the TMF-work presentedby Tsimas et al. [44]. It can be argued that athresholdstrain/ stresswili transfercrack mode from delaminationto transversalwiththermalcyclic testing beiny one extreme and LCF tests the other. Therefore LCF andthermalcyclic testsarenot zwdly misleadingif mechanismsbehind crack initiationandgrowth are to be investigated.If data for fatigue lives under TMF conditions are to bedeterminedreliable test methodsnear real load cases are needed. A model capable ofdeterminingg component fatigue lives must take into account, or at least consider, allfactors mentionedabove.

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5 CONCLUSIONS

In gas turbine applicationsnickel-base alloys are used for burner linings, vanes andturbine blades. These components need to be shielded from the hot, corrosiveenvironmentin the combustorandturbinestages.By applyinga thermalbarriercoating itis well known thatthe performance of gas turbinescan be improved. The presentworkaimsat discussingfactorsthatinfluencefatiguelife for gas turbinehot parts.This is doneby literaturereviews compared with new work done on the same class of material.Theresultsdiscussedhere aresummarisedbriefly below.

In both plasma sprayed and PVD coatings oxidation plays an importantroleconcerning fatiguelife.It is vital thatthermallygrown oxides arenot allowed to form spinel.If spinelhasformed this is anindicationof low Al contentin theBC.The initial bond coat aluminiumresewe is consumed during high temperatureexposure. Two major factors contributeto the ahuniniumconsumption, namelyoxidation (formation / growth of TGO and bond coat internaloxides) and inwarddiffhsion of Al intothe substratematerial.H’ a diffasion barrieragainstAl diffusion can be applie~ the fatigue life can beincreased. Depending on substratecomposition the fatigue life can be radicallychanged if elementsfrom the substratecan contributeto oxide formationor oxideadherence.Thermal cyclic life and formation of spinel are closely related Introductionofspinelin anAPS bond coat initiatesa mixed black / whitetlacture.Crack propagation under thermal cyclic conditions is always related todelamination.Under low cycle fatigue conditionscracksgrow throughthe coating into the basematerial.CracksstillinitiateattheTGO interface.Results fi-om thermomechanicalfatigue show that low strain ranges lead tospallationas for thermalcycling. Large strainrangeswill change crack mode andthe samebehaviour,asunderLCF conditions,will be foundWhen cracks pass through an APS coating the fatigue life is increased if aninternaloxide networkhas been allowedto grow. Crackdeflection, branchingandbluntingW OCCUr.

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M. Tamura,M. Takashaki,J. Ishii,K. Suzuki,M. SatoandK. Shimomur~J Thermal Sprq Tech., vol. 8, no. 1, (1999), pp. 68-72D. Zhu andR.A. Miller, Surf Coat. Tech., vol 94/95, (1997), pp. 94-101R.A. Miller andC.L. Lowell, Thin Solid Films, vol 95, (1982), pp. 265-273J.A. Haynes, M.K. FerberandW.D Porter,J Thermal Spray Tech, vol. 9, no. 1,(2000), pp. 38-48M.F.J.Kooloos and J.M. Houben, J Thermal Spiny Tech, vol. 9, no. 1, (2000),pp. 49-58M.E. Walter,B. Eigenmatm,Mater. Sci. Eng. vol. A282, (2000), pp. 49-58X. Bi, H. X% S. Gong, Surf Coat. Tech., vol. 130, (2000), pp. 122-127C. Leyens, U. Schulz,B.A. Pint,I.G. Wright,Surf Coat. Tech., vol. 120-121,(1999), pp. 68-76D.R. Mumm, A.G. Evans,Acts Mater., vol. 48, (2000), pp. 1815-1827M.E. Walter, B. Onipede,W. Soboyejo, C. Mercer, J Eng. Mater. Tech., vol. 1-22,pp. 333-337M. Gell, K. Vaidyanathan,B. Barber,J. Cheng andE. Jordan,Metallurgical andMaterials Trans., vol. 30A, (1999), pp. 427-435A. Rabiei andA.G.Evans,Acts Mater., VO1.48, (2000), pp. 3963-3976M.J. Pindera,J.Aboudi and S.M. Arnold, Mat. Sci. Eng., vol. A284, (2000),Pp. 158-175

R.W. Smith,Thin Solid Films, vol. 84, (1981), pp. 59-72K. SchneiderandH.W. Gnmling, Thin Solid Films, vol. 107, (1983), pp. 395-416K. Shiozaw~ S. Nishino,N.Yoki andY. Haruyanx+Transactions of the JapanSociety of Mechanical Engineers, PartA, vol. 60, no. 575, July 1994, (1994),pp. 1510-1516H. Brodin and S. Johansson,Proceedingsof ICF 10,2-6 December, (2001),Honolulu, HawaiiR.T.R McGrann,D.J. Greving, J.R..Shadley,E.F. Rybicki, B.E. Bodger andD.A.Somerville,JThermal Sprq Tech., vol. 7, no. 4, (1998), pp. 546-552P.K. Wright,Mater. Sci. 13ng., vol. A245, (1998), pp. 191E. Tzimas,H. Miillerjans,S.D. Peteves,J.Bressersand W. Stamm,Acts Maler.,vol. 48, (2000), Pp. 4699-4707Y.H. Zhang, P.J.Withers,D.M. Knowels, Proceedingson the 7thInternationalFatigueCongress,Beijing, (1999), pp. 1945-1950B.A. Pint,I.G. Wright,W.J. Brindley,J Thermal Spray Tech., vol. 9, no. 2, (2000)

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Aspectsof Fatigae Life ia Thermal Barrier Coatings

F&fattareAuthor III&m Bmdin

SaauaanfatodnsAbstract

Themnal bani= coatings (TBC) me applied on hot anaponrMs in airbinne and land-based EWStdines v.hem bighez turbine iafet tempme,

meaning better tkmnal eflkkacy, is desixed. The TBC is mdnfy applied to protect IUIdtmlyingmatmiaf fiwn high temperate@., bul aka saves as aprotecdon from the a~es5ve ccmmtive eawirommmt.

Ffasnu? sflrayed coatings are otkn duplex TBC’S titb an onteI ~tiC tq COW(TC) ma& flom pd~y 5t2bihSd titia - ~ + d-8’YoY2G. BeIOW the tq COatthere is a metaflic bnndmat (BC). The BC is nmnallys MCrNx coadni?(M=W co, Fe... ~d X=Y, ~ Si..). ~ Wturbine componentsexposedto efsvatedteaqmaturea nickel based sqmralloys are conunmdyadopted as load CaU@S cmfmmts. h theh“~gations peafonnedhere a conunmial WOW@ Ni-base alloy lbynea 230 has b= wed as subshate far the TBC. AS BC .S~GC~Y SW=

as a refemux mataial and ia all cases 7% @a PS zirconia has been used Fhase development and fkilure mechanisms in AFS TBC duringsezvice-like conditions kave b= evalvated in tke presd stady. This is done by combinatias of themnal cycling and low cycle fa@ae tests. Theaim is to achieve bett= knowkdge regarding how, when znd why thwnaf btier coa!ings i%l. As a tinaf outcome of the project a madel capable ofIxddns fadgue life of a siven conym=t T@ hdp e@w=s ad d=@u=s Of ~d ~s~ %S ~~es f~ POW g~~~~ tO b~~ e=-iwk.

In the investigations it is seai thatTBC life is stroagfy inflamed by oxidation of the BC and intdiflikm between BC and the sabstxze.The bcmd coat is knovm to oxidise with b at high temparatme. The initial oxide fomd king testingis akmdaa With increasedtie at hightemq=me Al is dqddsd li.nn thebondcoat dneto intdiffnsh aad axidadan.Qxidesothezs than alumina start to fona when the AI contezMisreduced Mow a tiiical limit. It is hae bdieved that spinel appeammhm the Al contmt islowe%edbdow 2w/0in the bond coat. Hue it was shcwm

that a fimter growing oxide, rich in Ni, Cr and Co fbnns at the interhce. Al dqk+ioa is also linked to BC phases. Initially the bond coat is a yl &

matezia3 possibly with vw fine dispersed Y’. simultaneously with Al-depletion the ~-phase is hmd to disappeax. This occurs sinmftanecmdy wititk fonnadcm of spiad. However, oxidation is not only a diwdvaage. ~W de fitifw t- IW~ th~ ofi& **s ~~ the ~d mat tislow down crack srowth due to crack detlectiun and crack branching, Tfmefore benetit of m damage from oxi& growth on crack initiation andpropagation is dependent on crack mad? qadling of the ceramic TC m SXov.tb of “classic” cracks pqendicuk+r to the suct%e.

Fmm the obsenmtkms conclusions are drawn regarding fitigue bahaviom of TBC systems. The basic idea is that all cracks leading to time

initiate in tie tbatnally gmvm oxide (TfiO). Follov.ing the initiation, tkey cm, howew, grow to h eitk &lamination cracks leading to topcoat sprdltim or cracks tisnsverse to the surface lrading to compcmrat tiikre.

I

Nyckelord-dtkermal barrier coatings, low cycle fatigue, oxidation, delamination% NiCoCrAIY, CoNiCrAY, APS, SPS, tidygown oxide, spinel

Aspects of Fatigue Life in Thermal Barrier Coatings

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