Improved oxidation resistance of thermal barrier coatings

Improved oxidation resistance of thermal barrier coatings

Surface and Coatings Technology 120–121 (1999) 84–88 www.elsevier.nl/locate/surfcoat Improved oxidation resistance of thermal barrier coatings Kh.G. ...

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Surface and Coatings Technology 120–121 (1999) 84–88 www.elsevier.nl/locate/surfcoat

Improved oxidation resistance of thermal barrier coatings Kh.G. Schmitt-Thomas *, M. Hertter Technical University of Munich, Lehrstuhl fu¨r Werkstoffe im Maschinenbau, D-85747 Garching, Germany

Abstract In order to improve the engine output and the efficiency of gas turbines, optimized thermal barrier coatings (TBCs) are required to protect the metallic components at high temperatures. In common TBC-systems, consisting of a Ni-base alloy substrate/MCrAlY-bond coat/ZrO 7 wt.% Y O top coat, an oxide layer grows at the interface bond coat/ceramic under high 2 2 3 temperature service, which limits the life of these coatings. In this paper the oxidation resistance of a new triplex TBC-system, consisting of a CoNiCrAlY-bond coat/Pt-modified aluminide coating/ZrO 7 wt.% Y O top coat is compared with that of a common TBC-system. The as-coated Pt-aluminide coating 2 2 3 consists of an outer region of PtAl +(CoNiPt)Al followed by a single phase layer of (CoNiPt)Al. The results of the oxidation 2 tests at 1000, 1050 and 1100°C in air show excellent oxidation resistance of the triplex TBC-system with the thickest investigated Pt-aluminide coating. In particular, a 28 mm thick Pt-aluminide coating allows the thickness of the oxide layers to be reduced up to 70% compared to the common TBC after 500 h at all examined temperatures. After heat treatment the coating systems were investigated by SEM, EDX and X-ray analysis. Annealing tests with Al O powder indicate which mechanism is probably 2 3 responsible for the improved oxidation resistance of platinum additions. Platinum is evidently capable of decomposing aluminum oxide at temperatures above 900°C. © 1999 Elsevier Science S.A. All rights reserved. Keywords: MCrAlY bond coat; Oxidation resistance; Platinum-modified aluminide; Thermal barrier coatings

1. Introduction One way of improving the efficiency of gas turbines involves increasing the turbine inlet temperature. Despite the development of high temperature alloys, as well as the production of single crystal blades, the high temperature resistance of metallic materials is limited to about 1100°C. To achieve higher process temperatures, gas turbine components are coated with a thermal barrier system. The ceramic outer layer is usually composed of ZrO due to its low thermal conductivity and thermal 2 expansion coefficient, which is in the range of metals. In order to avoid troublesome phase transitions, the ceramic layer is partially stabilized with 6–8 wt.% Y O . Due to the high ionic conductivity of zirconia, 2 3 oxygen can easily diffuse through the ceramic layer to the metallic surface and form an oxide scale. To guarantee sufficiently high temperature and hot gas corrosion resistance, metals should be coated with a protecting * Corresponding author. Tel.: +49-89-289-15290; fax: +49-89-289-15291. E-mail address: [email protected] ( Kh.G. Schmitt-Thomas)

layer of metal before exposure to these conditions. The so called bond coat also helps to adjust the different expansion coefficients of the metal and the ceramic. Typically, bond coats for use in TBCs are either overlay coatings, such as MCrAlYs, or platinum-modified aluminide coatings [1]. Since the early 70s, platinum-aluminide diffusion layers have been successfully deposited as protective layers on high temperature components [2]. However, initially these coatings served exclusively for protecting metals against high temperature oxidation and hot gas corrosion, and therefore did not consist of a ceramic top coat for insulation. The application of platinumaluminide coatings as a bond coat was first mentioned in 1993 in the patent of Duderstadt and Nagaraj [1]. Normally these layers are produced by initially electrodepositing a platinum layer of 8–10 mm thickness followed by a diffusion treatment, either as a separate thermal treatment or combined with the aluminizing step. The aluminisation is performed using a pack cementation or a chemical vapor deposition (CVD) process [3]. The outer layer of the modified aluminides consists of one of the intermetallic phases PtAl , 2 Pt Al , PtAl [4] or platinum in solid solution, depending 2 3

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on the deposition parameters such as thickness of the platinum layer, duration and temperature of the thermal treatment or the activity of aluminum during the pack cementation process. According to [4] Pt excludes refractory metal strengthening elements out of the layer. This promotes the selective oxidation of aluminum. Due to the smooth surface of platinum-aluminide, these bond coats are limited to zirconia top coats which are manufactured through the very expensive EB-PVD process. The more economical plasma sprayed coatings, which adhere to the metal by mechanical keying, require a pronounced roughness of the bond coat. Such surfaces could be generated by thermally spraying of MCrAlY bond coats. Practical experience showed that the formation of an oxide scale at the bond coat–ceramic interface and the interdiffusion with the substrate causes the aluminum content to decrease rapidly, which leads to the formation of oxides with a larger volume, such as nickel oxide. In this way the increased residual stresses cause the failure of TBCs [5]. A possibile way of improving the oxidation resistance, and thus the service life of TBCs, lies in the deposition of a platinum-modified aluminide layer on the MCrAlY bond coat, as discussed in this paper. Due to the high price of the initially deposited 8–10 mm platinum layer, a platinum-aluminide coating (with much reduced manufacturing costs but with suitable oxidation resistance) has to be developed first. The optimization of the thin diffusion layer was, moreover, to form a microstructure with a ductility as high as possible.

2. Experimental 2.1. Modification of the coating system For the investigations, Nimonic 90 substrates were coated with a 120 mm Co-32Ni-21Cr-8Al-0.6Y bond coat (LCO-22) by low pressure plasma spraying (LPPS). Afterwards, the bond coat surfaces were electroplated with a 1, 3 or 4 mm platinum layer. Interdiffusion of these different layers was generally performed at 1000°C in inert atmosphere for 30 min. The heat treated specimens were then aluminized by the CVD process in hydrogen. The specimens with the 1 and 3 mm platinum layers were aluminized for 4 h at 800°C. In order to achieve a homogenous distribution of platinum in the bond coat of the samples with a previously 4 mm platinum layer, these were treated by 1000°C for 5.5 h. The effect of plasma coating on the microstructure of platinum-modified aluminide coatings was investigated by coating part of the samples with a 250 mm atmospheric plasma sprayed ZrO 7 wt.% Y O top coat. To simulate 2 2 3 the oxidation attack, the coating systems were treated under isothermal conditions at 1000, 1050 and 1100°C up to 500 h in static air. In this study, specimens without

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an additional platinum-modified aluminide coating were used as a reference system. 2.2. Annealing tests with aluminum oxide Annealing tests were undertaken in order to evaluate the ability of platinum to decompose aluminum oxide. For this investigation c-Al O (Merck) was transformed 2 3 into stable a-Al O by annealing at 1200°C for 50 h. 2 3 After this exposure, a constant weight was reached and no aluminum, other than that in the Al O , was detected 2 3 in the powder using atom absorption spectrometry (AAS ). Afterwards Al O was annealed in a pure plati2 3 num crucible at 900 or 1000°C in argon. To determine the weight change between the initial and final states of Al O and the crucible, the mass was measured with a 2 3 precision of 0.1 mg. Afterwards, the aluminum which originated from the reduction of Al O was detected 2 3 with AAS. To determine the possible influences of platinum on the results, these tests were also carried out with a crucible made of Al O . 2 3 3. Results 3.1. Microstructure of the as-coated bond coat modifications In the as-coated condition, the platinum-aluminide coating with a platinum layer 4 mm thick prior to aluminizing consists, in the outer region, mainly of cubic PtAl (Fig. 1). Beside this phase, the near surface 2 zone also shows the more ductile cubic b-(Co, Ni, Pt)Alphase. Thus mainly aluminum, platinum and small amounts of cobalt and nickel can be detected. The adjacent b-phase intermediate layer contains platinum in solid solution. An accumulation of chrome was detected in the subjacent interdiffusion zone. From an average depth of about 28 mm, the structure of the bond coat, which is unaffected by the oxidation protective layer, consists of the c-phase, Co and Ni in solid solution with c∞-precipitates of composition (Co, Ni) Al and the aluminum rich b-(Co, Ni, Pt)Al-phase. 3 The two platinum-modified aluminide coatings with a platinum layer initially 1 mm and 3 mm thick are indeed composed of the same phases, however, the microstructure differs from that with an original 4 mm platinum layer ( Fig. 2). In the case of the initial 1 and 3 mm platinum layer, a continuous outer zone of brittle PtAl is followed by an intermediate layer of b-(Co, Ni, 2 Pt)Al. This b-matrix also contains the PtAl -phase but 2 its amount decreases continuously with increasing distance from the bond coat surface. In modified bond coats with 1 mm and 3 mm platinum, the entire diffusion layer penetrates to depths of 20 mm and 25 mm respectively. The reference specimen with an LPPS-

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Fig. 1. Back scattered electron (BSE) image and X-ray mappings of the main elements of the 28 mm Pt-aluminide coating (initial platinum layer of 4 mm) on the CoNiCrAlY: b-phase dark regions, c-phase light regions.

CoNiCrAlY-bond coat, however, consists of a homogenous structure of c-phase with c∞-precipitates and a finely divided b-(Co, Ni)Al-phase in a cobalt matrix. 3.2. Oxidation resistance of platinum-modified aluminide coatings Fig. 3 shows the oxidation layer thickness of different Pt-Al coatings after 250 h exposure at 1000°C. Accordingly, a significant increase in the oxidation rate can be seen by the thinnest Pt-Al deposition on bond coats compared to the reference. An improvement in the oxidation behavior of CoNiCrAlY bond coats is

obtained with platinum-aluminide coatings corresponding to an initial platinum thickness of 3 mm. The best results in oxidation resistance were obtained by samples with the thickest Pt-Al layer of about 28 mm, corresponding to a 4 mm platinum layer. At 1100°C only the 28 mm platinum-modified aluminide coating leads to a reduction in the oxidation growth, compared to bond coats without an additional diffusion coating. Figs. 4 and 5 show the oxidation layer thickness vs. time up to 500 h after an isothermal exposure at 1000 and 1100°C of the 28 mm platinum-modified aluminide coating, corresponding to an initial 4 mm platinum layer and of the reference bond coat. During the first 100 h

Kh.G. Schmitt-Thomas, M. Hertter / Surface and Coatings Technology 120–121 (1999) 84–88

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Fig. 4. Oxide layer thickness vs. time at 1000°C for reference bond coats and for an additional 28 mm Pt-aluminide layer on the CoNiCrAlY, standard deviation for three samples. (a)

Fig. 5. Oxide layer thickness vs. time at 1100°C for reference bond coats and for an additional 28 mm Pt-aluminide layer on the CoNiCrAlY, standard deviation for three samples. (b) Fig. 2. Back scattered electron (BSE ) image of the initial (a) 1 mm and (b) 3 mm platinum coated Pt-aluminide coatings on the CoNiCrAlY.

beginning of the heat treatment, higher oxidation rates. However, after 500 h of exposure at all examined temperatures, the oxide layer was about 70% thicker than on bond coats with an additional platinum-aluminide coating, corresponding to a 4 mm platinum layer. As can be seen with X-ray diffraction after exposure at 1000 and 1050°C, selective a-Al O is formed on the 2 3 surfaces of the thickest modified bond coats. Only after 500 h of exposure at 1100°C, small amounts of spinels were detected. In contrast, reference bond coats showed spinels in addition to a-Al O inside the thermally grown 2 3 oxide at all examined temperatures and times. 3.3. Bond coats with a ceramic top coat

Fig. 3. Oxide layer thickness with standard deviation of one sample of the reference bond coat and modified bond coats with an initial 1, 3 or 4 mm platinum layer (oxidation conditions: 1000°C, 250 h, static air).

especially, the Pt-Al deposition on the CoNiCrAlY bond coat leads to a decrease in oxidation growth. Due to the formation of a protective oxide layer, there is also slower oxide growth for a longer exposure time. In contrast, reference bond coats show, particularly at the

Even with the thickest platinum-aluminide coating on top of the bond coats, these can still be coated with zirconia without any delamination. Probably, due to the high interfacial temperature during plasma spraying, some PtAl in the outer zone of the initial 4 mm platinum 2 coated diffusion coating dissolves. As a consequence, the ceramic coated modified bond coats with initial 4 mm thick platinum layer showed only a 50% reduced oxide scale thickness compared to reference bond coats when the treatment is done at 1000°C for 500 h.

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3.4. Annealing of aluminum oxide After annealing Al O at 1000°C in a pure platinum 2 3 crucible, a significant mass loss was detected, and aluminum which originated from the reduction of aluminum oxide was identified within the Al O powder. In con2 3 trast, no differences were found after annealing at 900°C compared to the initial state. Also, no weight change and no other evidence of reduction of aluminum oxide was detected after annealing Al O in an Al O crucible. 2 3 2 3 It thus seems reasonable to assume that platinum is probably able to decompose Al O at temperatures 2 3 above 900°C.

4. Discussion As can be seen in Figs. 1 and 2, it is possible to produce a homogenous platinum distribution inside the outer zone of the Pt-aluminide coating even on the rough bond coat surface by electrodeposition followed by different heat treatments. Due to the, still, good roughness after deposition of the diffusion layer, the bond coat surface can be coated with zirconia without any delamination. The microstructure of the 28 mm thick Pt-aluminide layer on the CoNiCrAlY bond coat is similar to that of a conventional platinum-modified aluminide coating on nickel-base alloys, which is usually called RT-22 [6 ]. Due to the homogenous distribution of the brittle PtAl -precipitates within the b-matrix, these coatings 2 are more ductile but less oxidation resistant than layers with a continuous outer zone of PtAl [6 ]. However, 2 the bond coats with a 4 mm thick platinum layer prior to aluminizing show considerably better oxidation resistance than the thinner coatings consisting of a continuous outer zone of PtAl . Apparently, a minimum 2 Pt-aluminide coating thickness is necessary for improved oxidation behavior also at higher temperatures. The dramatically reduced oxide layer thickness of the 28 mm thick Pt-aluminide, compared to the reference bond coat at all oxidation heat treatments examined, may be a result of the protective thermally grown aluminum oxide

layer effected by the selective oxidation of aluminum. However, a-Al O has the lowest diffusion coefficient 2 3 for aluminum and oxygen. Moreover, platinum probably reduces the oxidation rate, as the annealing tests with aluminum oxide show. Due to the dissolving of the PtAl -phase during plasma spraying of the zirconia, 2 the oxidation resistance of the 28 mm Pt-aluminide coated bond coat is somewhat reduced compared to bond coats without a ceramic top coat.

5. Conclusions The new triplex TBC system, consisting of a CoNiCrAlY bond coat, a 28 mm thick platinum-modified aluminide coating and a ZrO -Y O top coat shows 2 2 3 excellent oxidation resistance at high temperature exposure. In further investigations, the effect of the Pt-aluminide on the thermal-mechanical behavior will be examined.

Acknowledgement The authors wish to thank both Daimler-Chrysler, Forschung und Technik, Ottobrunn and DaimlerChrysler, MTU Munich for coating the samples, as well as for the useful discussions.

References [1] E.C. Duderstadt, A.N. Bangalore, U.S. Patent 5 238 752, 24 August, 1993. [2] K. Bungardt, G. Lehnert, H. Meinhardt, German Patent 1 796 175, 1 July 1971. [3] Z. Mutasim, W. Brentnall, J. Am. Soc. Mech. Eng. 96-GT-436, 1996. [4] H.M. Tawancy, N. Sridhar, B.S. Tawabini, N.M. Abbas, T.N. Rhys-Jones, J. Mat. Sci. 27 (1992) 6463–6474. [5] B.C. Wu, E. Chang, C.H. Chao, M.L. Tsai, J. Mat. Sci. 25 (1990) 1112–1119. [6 ] P.C. Patnaik, R. Thamburaj, T.S. Sudarshan, in: T.S. Sudarshan ( Ed.), Superalloys, Surface Modification Technologies III, Minerals, Metals & Materials Society, 1990, pp. 759–776.