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Materials Science and Engineering A369 (2004) 144150

Investigation of an as-sprayed NiCoCrAlY overlay coating.A thermoanalytical approachK. Fritscher, C. Leyens, U. Schulz

DLR German Aerospace Center, Institute of Materials Research, D-51170 Kln, GermanyReceived 2 September 2003; received in revised form 27 October 2003

Abstract

A low pressure plasma sprayed (LPPS) NiCoCrAlY aircraft turbine blade overlay coating was investigated by differential thermal analysis(DTA). A pronounced irreversible endothermic reaction peak was observed at 975C for the as-sprayed coating. No equivalent reaction isobserved for the LPPS coating in the annealed state and in NiCoCrAl (Ni:Co= 2:1) compositions after slow furnace cool-down as well. Thereaction scheme for the partial ternary AlNiCrNi system was taken as a reference standard for understanding the irreversible reaction, andwas reexamined where necessary. The four-phase solid state reaction in the ternary system at 995 C, according to -Ni+ -NiAl -Ni3Al+ -Cr, is compared with the occurrence of the reaction observed at 975 C on the as-sprayed NiCoCrAlY coating material. It runs most likelyaccording to -Ni + -Ni3Al -Cr + -NiAl. The reaction at 975 C is discussed in terms of metastable phase formation sequences. 2003 Elsevier B.V. All rights reserved.

Keywords: NiCoCrAlY overlay coating; Low pressure plasma spraying; AlCrNi; AlCoCrNi

1. Introduction

Superalloy components in the hottest sections of gasturbines need the application of overlay coatings likeNiCoCrAlY compositions in order to resist effectivelythe environmental attack by hot gases and deposits. Thecoatings are commonly deposited by low-pressure plasmaspray (LPPS) or electron-beam physical vapor deposition(EB-PVD) techniques. To an increasing extent the coatingsare also used as bond coats for ceramic thermal barrier coat-ings (TBC) to make the parts even more durable in service.

The mechanisms by which these coatings become ef-fective rely on the generation and maintenance of stableand adherent oxide layers on their surface that form dur-ing surface engineering procedures and during service aswell. Service temperatures at and beyond 1000 C neces-sitate the application of alumina-forming coatings. Themost-suited stable -Al2O3 phase serves to sustain envi-ronmental attacks in at least two respects: (i) it protectsMCrAlY-coated substrates against hot gas corrosion due tothe high chemical stability and low diffusivity, (ii) it can

Corresponding author. Tel.: +49-2203-601-3570;fax: +49-2203-68936.

E-mail address: [email protected] (C. Leyens).

chemically bond to ZrO2-based TBCs by solving Al3+ inZrO2 [1].

There are multiple approaches to make overlay coatingsmore effective. They refer to two fundamental lines: toimprove the coatings by chemical modification (e.g. alloy-ing with optimal trace concentrations of reactive elementsfor better spallation resistance, introduction of diffusion-retarding elements like Re for less substrate/coating in-terdiffusion as well as better creep resistance, addition of-Al2O3 stabilising elements [2], etc.), and by microstruc-tural modification (e.g. microcrystallization for enhancedoxide plasticity and adhesion and for homogenization ofoxide growth via preferential -Al2O3 formation [3,4]).The sputter-deposition of an ordered metastable Ni(Cr, Al,Ti, Si) phase coating isomorphous to CsCl-type -NiAl hasturned out to be most effective in calming down oxidationkinetics on SX superalloys, and it was believed that the finegrain size of the coating promoted formation of a highlyprotective oxide scale [5].

It has been shown that the diffusive fluxes in two-phasealloys depend on the size and the form of second phasedistributed in the matrix phase (e.g. the -phase) whichcan fundamentally alter the oxidation behavior [6,7]. Ac-cordingly early -Al2O3 formation could be successfullyaccelerated even at 900 C, if a narrow-spaced lamellar

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K. Fritscher et al. / Materials Science and Engineering A369 (2004) 144150 145

structure in a NiCoCrAlY coating was provided [8].Since most MCrAlY-based coatings are multiphase alloys,the microstructural arrangement of the phases is of upper-most importance. The two main phases -(Ni, Co, Cr) and-NiAl have the advantageous capability to form aluminiascales on oxidation. The early stages of oxidation between900 and 1100 C, however, are different for both phasesin NiCrAlY alloys with regard to kinetics and generationof transient oxide phases [2]. So the -Al2O3 forming-(Ni, Cr) phase is reported to oxidize slower than thetransient-alumina forming -phase. Hence the oxide growthon NiCoCrAlY coatings will not necessarily proceed co-operatively. Anyway microstructural effects in multiphaseprotective coatings should aim at the provision of a homo-geneous growth process to attain a single-phase oxide layerin order to reduce detrimental interphase stresses withinthe growing oxide [5,6]. Transient microstructures in themetallic sublayer, however, appear to be promising proba-bly due to inherent diffusion properties for the formation offavorable scales. So the potential of any microstructure hasto be considered in terms of its utility. LPPS NiCoCrAlYcoatings are commonly subjected to post-coating abrasiveprocedures for better aerodynamic performance via blast-ing, vibration grinding, or media finishing. Subsequent heattreatments aim at equalizing the microstructures. Hencesome microstructural features will be lost by this way, thatotherwise may exert a beneficial effect on the performanceof superalloy components.

The aim of the thermoanalytical study is to tracemetastable phases and transient microstructures in as-coatedLPPS coatings, that otherwise are mostly devoid in thestate-of-the-art fully processed NiCoCrAlY coatings andhave not been reported so far. By keeping their potentialusability in mind their formation will be discussed.

2. Material and experimental techniques

NiCoCrAlY coating powder (Amdry 365-1, composi-tion stated by the supplier: Ni base, 22.82Co, 17.93Cr,12.47Al, 0.010C, 0.53Y, 0.24 others (in wt.%), 636mparticle diameter size) was LPPS deposited on a moderatelytransferred-arc heated (99,99%) aluminum, nickel, chromium andcobalt (>99.7%) were taken for constitutional investigationsto provide equilibrium reference data in the NiCoCrAlsystem. Solidliquid equilibrium phase transformationswere obtained by DTA, eutectic compositions identified bylight microscopy. Details are given elsewhere [9]. Solid statetransformations on annealed and slowly furnace-cooledVIM cast compositions were followed under high vac-uum by dilatometry (Bhr; 50 mm long sample) and DTA(Setaram TGA 92; typically 0.2 g weight) at 1 K min1.

3. Results

3.1. Characteristic equilibrium phase transformations,their implementation in the constitution of the AlNiCrNisystem, and the effect of Co

The constitution of the ternary system AlCrNi has beencritically reviewed and collated in a compendium [10]. Amore recent thermodynamic assessment giving calculatedtemperatures for invariant reactions differs appreciably [11],particularly for melt equilibria. The accuracy of the data,however, is relevant for this work, which spurred the authorsto reexamine and update the data by DTA, dilatometry andlight microscopy [9,12]. The data of [10,11] and the authorsdata are compared in Table 1. The corresponding partialreaction scheme AlNiCrNi is shown in Fig. 1.

Incorporation of cobalt in the system changes it to aquaternary system with the consequences due to Gibbssphase rule: invariant reactions become monovariant (transi-tion temperatures widen to temperature intervals), and phasecompositions are changed. The invariant four-phase reac-tions E and U2 will be modified through substitution of 1/3nickel in return for 1/3 cobalt and shifted to other temper-ature (and composition) regimes according to Table 2. Thesolid/liquid reaction E goes up by some 34 K according toDTA data [9,11]. DTA peaks of both eutectic AlCrNi andAlCoCrNi alloys establish nearly identical shapes, thelatter being slightly broader; both alloys can be directionally


146 K. Fritscher et al. / Materials Science and Engineering A369 (2004) 144150

Fig. 1. Elongation vs. temperature of AlCrNi and AlCoCrNi compositions of Table 2 obtained at 1 K min1 showing the inflection point at 995 Cfor the AlCrNi alloy that is attributed to the reaction U2 (-NiAl + -Ni = -Cr + -Ni3Al).

solidified to lamellar microstructures the same way indica-tive of similar solidification modes. Hence a minute solid-ification interval in the order of 1 K for the quaternary eu-tectic is supposed). The solid/solid reaction U2 is hypoth-esized to be represented by the turning point on dilatationat 995 C for the AlCrNi eutectic alloy in Fig. 2 whichis mainly attributable to the volume gain of voluminous-NiAl phase forming there at the expense of the moredensely packed -Ni3Al phase, and the temperature for U2is confirmed by the DTA onset point on heating/cooling of996/987 C in Fig. 4 is reduced by some 200 K (first evi-dence of -Ni3Al phase by microscopy below 800 C). Therange of the interval for this reaction downwards, however,is addressed, for e.g. in [13] and may be terminated below700 C.

3.2. Differential thermal analysis (DTA) of as-coated PSNiCoCrAlY deposits

The occurrence of metastable phases was traced by meansof differential thermal analysis. A noticeable reaction peak ofas-coated NiCoCrAlY samples occurred at 975 C on mod-erate heating at 5 K min1, signaling phase transformation(see Fig. 3). This reaction is an irreversible one: it vanishedon repeated heating of the same sample.

Table 2Invariant phase equilibria in the AlNiCrNi system with transition points (second column from the left, reexamined data) and corresponding four-phasereactions in the AlCoCrNi (Ni : Co = 2 : 1) system with transition intervals (right)Ternary reactions inAlNiCrNi system

T (C); composition(at.%), (wt.%)

Analogous quarternary reaction T (C); composition(at.%), (wt.%) below

L = -Cr + -NiAl + -Ni 1302; 13.47Al, 37.45Cr,49.08Ni; 7.0Al, 37.5Cr,55.5Ni

L -Cr + -NiAl + -Ni 13051306; 13.47Al,16.36Co, 37.45Cr, 32.72Ni;6.75Al, 19.25Co, 35.5Cr,38.5Ni

-NiAl + -Ni = -Cr + -Ni3Al 995 -NiAl + -Ni -Cr + -Ni3Al 800


Fig. 2. Reaction scheme in the stable ternary system AlNiCrNi (left side), and sector of the scheme transferred to a distinct compositional case inthe quaternary system Al(Co, Ni)Cr(Co, Ni) (column at the right side). In this particular case, the effect of the substitution of 1/3 Ni in return of 1/3Co is shown (arrows), above all the gain in temperature of the four-phase solid/liquid reaction E by some 34 K and the decrease of the four-phasesolid/solid reaction U2 by some 200 K for starting a broad transition interval via a monovariant reaction mode.

quaternary and ternary DTA plots taken on DTA samplesof equal mass. It may probably start at approximately700 C on heating. On cooling now more effects can beseen.

Fig. 3. DTA peak of as-sprayed NiCoCrAlY coating on heating at5 K min1 attributed to a potential solid-state reaction (2) according to-Ni + -Ni3Al -Cr + -NiAl.

4. Discussion

4.1. Differential thermal analysis data and deducedreaction scheme for the as-sprayed coating

Thermoanalytical methods in metallurgy are commonlydevoted to constitutional issues. They are uncommon orrarely used to investigate microstructural subjects in alloys.If, however, a distinctive change in the microstructure yieldsan outstanding thermal effect the occurrence of a dramaticvariation of the thermodynamic potential, e.g. an unexpectedphase transition can be assumed. This is obviously true forthe as-sprayed coating.

The irreversible endothermic reaction observed at 975 Cwill be called into question whether it is related to thefour-phase solid state reaction U2 in the equilibrium ternaryNiCrAl system, according to

-Ni+ -NiAl -Ni3Al+ -Cr (1)

at 995 C. Three arguments at leasta constitutional andtwo kinetic argumentsnegate the validity of this reactionfor the as-sprayed material. The evidence of mainly fcc



Fig. 4. DTA plot of cast slowly furnace-cooled quaternary AlCoCrNi alloy material on heating and cooling at 1 K min1 compared to same-sizedand processed ternary AlCrNi showing absence of essential peaks at the temperature interval under discussion below 1000 C for the quaternaryalloy. Meanwhile the ternary alloy shows distinctive signals on heating at 996 C and on cooling at 987 C indicating an invariant solid-state reactionaccording to -Ni + -NiAl -Cr + -Ni3Al.

phases of the PS quenched-in structure like the / eutecticobtained by XRF and TEM [9,16] plead rather for a modi-fication of the reaction (1) according to-Ni+ -Ni3Al -Cr + -NiAl (2)and correspondingly suggest a very different reactionscheme for as-sprayed material. The disappearance of-Ni3Al on heating to 975 C is in accord with the consti-tutional expectations of this quaternary system [13] where-Ni3Al is not stable at this temperature, but rather below800 C.

The second argument still referring to reaction (1) willrecall that a reaction peak can be transferred on rapid heat-ing to higher temperatures due to sluggish transformationkinetics, but will never occur at a lower temperature thanthat of equilibrium. The transient for the disappearance of for Astraloy, e.g. is raised by 125 K at a heating rate of300 K min1 [16]. So it is not reaction type (1) to occur at975 C which is essentially lower by 20 K on heating instead.

The third argument, admittedly speculative, is the sharp-ness of the DTA peak and its high intensity. A sharp DTApeak is typical for invariant reactions to occur in unary, bi-

nary or ternary metallic systems. It is atypical in quaternarysystems in agreement with Gibbs phase rule (one degreeof freedom left). The apparent contradiction can be solvedlike this: the two phases -Ni + -Ni3Al identified in theas-sprayed structure have compositions which, although ex-tremely supersaturated, do not necessarily represent quater-nary concentrations, as shown in Table 3. The compositionsare essentially binary at least for the -Ni3Al phase. The phase contains additional Y and Ti impurities, whereasthe Ni and Co atoms may be considered to be very similarin their basic aspects and can easily replace each other innearly most deliberate mixtures. At high solidification ratesof the coating a tendency was observed that ternary and qua-ternary elements will be rejected from the / structure, and

Table 3Composition (in at.%) of matrix and -Ni3Al second phase in LPPSeutectic grains analyzed by EDS [9]Phases Ni Co Cr Al Y Ti

-Ni3Al Balance Traces Traces 33.0 2 -Ni, Co 21.61 12.07 64.04 1.94 0.3



instead extended solid solutions will be established [9]. Soin essence binary phases are supplied to the reaction whichnow can be understood to perform analogous to invariantreactions, for e.g. in ternary systems.

The composition of the supersaturated -Ni3Al phasecomes close to that of stable -NiAl in binary -NiAl inequilibrium with (37.4 at.% Al at 1000 C [17]), whereasthe supersaturated -Ni phase is represented by + oreven single -Cr phase under the corresponding equilibriumconditions. So the former fcc / structure will decomposeto bcc phases at reaction temperature and simultaneouslyapproach quaternary compositions. This accumulation of re-arrangements as well of phases and in compositions will re-sult in a pronounced thermal effect as observed. So reaction(2), although fed by metastable phases in a quaternary sys-tem, is essentially of the invariant type due to essentiallybinary phases. Consequently it establishes a distinct partic-ular transition temperature, which is different from that ofthe stable reaction U2. And apart of that the reaction itselfis not related in any way to reaction (1). It is irreversiblesince the metastable phases become transformed into morestable phases. They, of course, will follow the stable reac-tion scheme according to reaction (1) on repeated heatingof the same sample.

The intensity of the DTA peak suggests two items thatrefer to the demand for energy: at first excessive energy isneeded for the transformation of metastable (and highly Alsupersaturated) to -NiAl, which is a compound showinga particularly high energy of formation. Secondly, transportenergy is needed for the spontaneous provision of matter forinstantaneous formation of the stable phases (and a differentmicrostructure and composition) that depends on enhanceddiffusivity of matter.

In superalloys, is commonly stabilized by some moreelements like Ta, Ti and Nb so that the transition U2 occursat somewhat higher temperatures than 995 C [18]. The Tiimpurities in the coating, however, cannot account for thestabilization of the -phase in the coating as Ti, althoughinvolved in reaction (2), has not been identified in this par-ticular -Ni3Al phase but exclusively in the -Ni phase.Nonetheless involvement of Ti would increase the temper-ature of reaction (1). What we observe is reaction (2) at alower temperature instead. It is obviously one more argu-ment that reaction (2) is not related in any way to reaction(1).

4.2. Phase composition and solubility range extensionissues

The tendency for the rejection of ternary and quater-nary elements from the phases in the / structure duringsolidification of the as-sprayed NiCoCrAlY coating, asdocumented in [9], is worth while to be considered in moredetail. Either it deals with an effect, that is generally validfor higher-order systems, but not detected so far; or it is ittypical for this system only. No references report on this dis-

tinct effect to the authors knowledge but refer to a shiftingof solubility ranges on splat-cooling in binary and ternarysystems [1921]. So, in spite of mental reservations, it isconsidered in the following if this effect is a characteristicof this NiCoCrAlY coating.

In general, melts on splat-cooling experience the low-est temperatures for solidification attainable. For e.g.glass-forming deep eutectic systems can attain a specialsolid state which is still isomorphous with the liquidstate, without a liquidsolid transition during cooling. Be-sides the glassy phases from deep eutectics, there aremany examples for systems that form metastable phaseson high cooling rates. Due to this trick they still havea liquidsolid transition, but at a lower temperature. Themost prominent example for it is the ironcarbon systemand the metastable FeFe3C variant. So in consequence itshould be asked, if there are some more tricks to get lowsolidification temperatures.

Moderate undercooling at the melt front is necessary toinitiate eutectic solidification. The undercooling intervalon quenching of regular eutectic structures forming melts,however, is limited to a few degrees, e.g. to approximately15 K for the NiAlTi system [22]. In cases where the eu-tectic reactiona complex long-range ordering processisthe only one feasible in a system, it may pay off in the end,however, to support the melting-point depressant tendencyof this reaction by introducing a variant of eutectic reactions(equal to preference of binary instead of multi-elementalphases) to lower the solidification temperature for thisparticular case below the regular limit for undercoolingeutectics even more. In this system, the formation even oftwo metastable phases and (instead of one e.g. forthe FeFe3C system) is just involved for this very purpose.More melting point depression is still desirable. As shownin Fig. 1 and Table 2, the lowest melting temperature (reac-tion E) in the (stable) ternary system AlNiCrNi is 34 Klower than in the quaternary Co-containing system. This is again of 20% for a potential lowering of the temperature nec-essary for undercooling, provided that a similar tendency oftemperature depression is valid for the metastable solidifica-tion reaction by following a ternary scheme. In essence, thetrick on splat-cooling the quaternary system is assumedto be the switching over to a ternary reaction mode in orderto benefit from the lower solidification temperature. Thecompositions of the phases need to become pre-adjustedtowards binary compositions to apply to the require-ments in a ternary system, e.g. for performing invariantreactions.

An explanation, different to constitutional issues, may re-fer to kinetic arguments. It may be mentioned that crystalsgrow more easily from multi-component melts on solidifi-cation if they are composed of no more than two elements.Under stringent conditions of rapid solidification they willdo it even more. But examples showing the opposite can becited [21]. Which of the explanations is more likely is be-yond the scope of this work.



5. Conclusions

As-sprayed NiCoCrAlY coatings exhibited an irreversiblethermal reaction peak on DTA heating at 975 C. It isattributed to the reaction of metastable -Ni3Al and super-saturated -(Ni,Cr) phases to stable -Cr + -NiAl. Theformation of the metastable and supersaturated phases isdiscussed with reference to the transformation temperaturesof the reaction schemes in which the respective phases areinvolved.

Acknowledgements

The authors thank Dr. Rudolf Henne, DLR Institute ofApplied Physics, for procuring plasma sprayed samples. Wehighly appreciate Horst Gedanitz for conscientious conduct-ing the thermoanalytical experiments.

References

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Investigation of an as-sprayed NiCoCrAlY overlay coating.A thermoanalytical approachIntroductionMaterial and experimental techniquesResultsCharacteristic equilibrium phase transformations, their implementation in the constitution of the AlNi-Cr-Ni system, and the effect of CoDifferential thermal analysis (DTA) of as-coated PS NiCoCrAlY depositsDTA of slowly solidified bars of the Ni-Co-Cr-Al system

DiscussionDifferential thermal analysis data and deduced reaction scheme for the as-sprayed coatingPhase composition and solubility range extension issues

ConclusionsAcknowledgementsReferences

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