The growth and influence of thermally grown oxide in a thermal barrier coating

Surface & Coatings Technology 201 (2006) 1074 – 1079 www.elsevier.com/locate/surfcoat

The growth and influence of thermally grown oxide in a thermal barrier coating W.R. Chen a,⁎, X. Wu a , B.R. Marple b , P.C. Patnaik a a

Institute for Aerospace Research, SMPL, National Research Council Canada, Ottawa, Ontario, Canada, K1A 0R6 b Industrial Materials Institute, National Research Council Canada, Boucherville, Québec, Canada, J4B 6Y4 Received 18 July 2005; accepted in revised form 12 January 2006 Available online 23 February 2006

Abstract The growth of a thermally grown oxide (TGO) layer and its influence on cracking was studied in an air-plasma sprayed (APS) thermal barrier coating (TBC) following thermal cycling. The TGO that formed upon thermal exposure in air was comprised predominantly of layered chromia and spinels, as well as some oxide clusters of chromia, spinel and nickel oxide. The increase in thickness of the TGO exhibited a three-stage growth phenomenon. Cracks formed mostly at oxide clusters as well as at the opening of discontinuities. Crack propagation during cyclic oxidation appeared to be related to TGO growth, with a nearly linear relationship between crack length and TGO thickness. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. Keywords: CoNiCrAlY; Cyclic oxidation; TGO growth; Crack propagation

1. Introduction The metallic bond coat (BC), usually MCrAlY, where M = Ni and/or Co, is an important constituent of TBC systems. It enhances the adhesion of the ceramic thermal barrier layer (the topcoat or TC) to the substrate and also provides oxidation protection to the substrate metal. At elevated temperatures, however, oxidation of the bond coat results in the formation of a TGO layer at the original ceramic/bond coat interface. If the TGO is composed of a continuous scale of Al2O3, it will act as a diffusion barrier to suppress the formation of other detrimental oxides during extended thermal exposure in service, thus helping to protect the substrate from further oxidation and improving the durability of the system under oxidation conditions. Nevertheless, it has been reported that some other oxides such as chromia ((Cr,Al)2O3), spinel (Ni(Cr,Al)2O4) and nickel oxide (NiO) [1–6], may form along with this TGO layer, in APSproduced TBC systems. Oxidation of the bond coat has been recognized as the cause for separation of the ceramic layer from the substrate, leading to TBC failure [2,3,7]. ⁎ Corresponding author. E-mail address: [email protected] (W.R. Chen).

The bond coat oxidation kinetics usually has been quantified by measuring the specific weight change during thermal exposure at elevated temperatures [8–11]. The results generally show a parabolic weight-gain behavior at first, followed by weight losses. This parabolic growth behavior was also

Fig. 1. As sprayed microstructure of ZrO2–8wt.% Y2O3 with Co–32Ni–21Cr– 8Al–0.5Y (wt.%) bond coat.

0257-8972/$ - see front matter. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.01.023

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subjected to thermal cycling in air. The thermal cycle consisted of 8∼12-min ramping, 45-min holding at 1050 °C, and 30∼40-min cooling to the ambient temperature (25 °C). The oxidized samples were sectioned after 10, 50, 100, 200, 300 and 500 cycles, mounted using epoxy, and mechanically polished. Specimens were then examined using a Philips XL30S FEG scanning electron microscope (SEM) with an energydispersive spectrometer (EDS). The area, length, and minimum thickness of TGO were measured, using ImageTool software, based on the micrographs taken from the cross sections of the samples after the predetermined thermal cycles. More than 50 micrographs were taken from each sample for the TGO measurement. 3. Results and discussion Fig. 2. TGO of predominantly a CS layer and CSN clusters formed in as-sprayed APS-CoNiCrAlY after one thermal cycle at 1050 °C.

observed by measuring the thickness of the oxide layer [12,13]. However, the long term TGO growth remained a question, since the weight change could not fully characterize the TGO thickening once oxide started to flake off. In the present investigation, growth of the TGO layer was studied to evaluate the cyclic oxidation behavior of an APSCoNiCrAlY TBC system. Crack propagation in the TBC system was also analyzed to study the relationship between crack length and TGO thickness. 2. Experimental The TBC samples consisted of a Co–32Ni–21Cr–8Al–0.5Y (wt.%) bond coat and a ZrO2–8%Y2O3 topcoat, deposited by the APS method on the flat surface of ϕ16 × 8 mm Inconel 625 disks. The thickness of the ceramic topcoat was 250–280 μm, and that of the bond coat was 140–180 μm. The as-sprayed samples were

3.1. Growth of the TGO The as-sprayed samples had some porosity and crack-like discontinuities in the ceramic topcoat, and a partially oxidized bond coat containing segmented Al2O3 flakes (Fig. 1). Upon thermal exposure in air, an oxide layer, which contained 5–16 at.% Ni + Co and 28–43 at.% Al + Cr, with oxygen as the balance, started to form along the interface between the ceramic and bond coat (Fig. 2). The composition of this oxide layer suggested that it was comprised of predominantly chromia and spinel (CS), or (Cr,Al)2O3 + (Co,Ni)(Cr,Al)2O4, as NiAl2O4 contains somewhat 13–16 at.% Ni and 27–30 at.% Al [14]. The Al2O3 portion of this layer was small. At the same time, some oxide clusters of chromia, spinel and nickel oxide, abbreviated as CSN, also formed at the ceramic/bond coat interface. As such, the TGO formed at the ceramic/bond coat interface was eventually composed of a CS layer and CSN clusters (Fig. 2). Since the TGO grows on a rough interface, sectioning of the TBC sample cannot always reveal the true thickness of the

Fig. 3. Minimum TGO thickness, δmin, as a function of the number of thermal cycles at 1050 °C.

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TGO. Therefore, the minimum thickness should be the measurement closest to the TGO thickness. The minimum TGO thickness, δmin, as a function of thermal cycles (Fig. 3) shows a parabolic growth of TGO, which is consistent with previous observations [8–13]. Interestingly, after around 300 thermal cycles, an acceleration of TGO thickening was observed in this study, which will be discussed later. This stage of accelerated oxidation has not been reported for MCrAlY bond coats. A similar behavior was observed in a FeCrAl alloy under oxidation conditions [15]. This behavior would have a significant impact on the durability of the TBC system. Since the TGO is sandwiched between the ceramic topcoat and the metallic bond coat, excessive TGO growth would certainly increase the stress

build-up between the two layers, as a result of volume increase (e.g. an increase of ∼67% in volume can be expected when a Ni unit cell transforms to NiO), and thus lead to TBC delamination. In characterizing TGO growth, however, the minimum TGO thickness only provides some baseline information. It was noticed that the TGO grew heterogeneously along the original ceramic/bond coat interface (Fig. 2). To take into account the entire TGO, an equivalent TGO thickness, δeq, is defined as: deq ¼

X

ðcross sectional area of TGOÞ = ðcross sectional length of TGOÞ X

ð1Þ

Fig. 4 shows the comparison of δeq and δmin in both linear and parabolic plots. Both δeq and δmin curves (Fig. 4a) show an

Fig. 4. TGO growth in APS-CoNiCrAlY bond coat during cyclic oxidation at 1050 °C.

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Fig. 5. Substantial oxide growth underneath the CS layer after 300 thermal cycles at 1050 °C.

Fig. 7. Increased crack nucleation and growth associated with CSN after 200 cycles at 1050 °C.

instantaneous TGO growth at the very beginning, followed by a slow down and then an accelerated growth. The difference between δeq and δmin corresponds to the highly non-uniform TGO morphology, and TGO growth became more heterogeneous as the number of thermal cycles increased. The parabolic 2 plot (Fig. 4b) shows that the δeq curve clearly indicates a three-

stage growth phenomenon: (i) a rapid TGO growth at the onset of oxidation, (ii) a nearly linear growth and (iii) an acceleration stage after 200 cycles, which somewhat resembles the primary, secondary and tertiary stages of creep, respectively. In the last stage, heterogeneous and substantial growth of mixed oxides, mostly (Cr,Al)2O3, (Co,Ni)(Cr,Al)2O4 and NiO, was observed

Fig. 6. Cracks nucleated in the CSN cluster (a) and the CS layer (b), indicated by arrows, and grew into the ceramic topcoat after 50 cycles at 1050 °C.

Fig. 8. Discontinuity opening and propagation in the ceramic (a) after 200 cycles at 1050 °C. (b) is a close-up view of (a).

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underneath the previously formed CS layer (Fig. 5). The discrepancy in time when acceleration of TGO growth begins in Figs. 3 and 4 can be attributed to the heterogeneous formation of CSN underneath the CS layer (Fig. 5). It is speculated that depletion of Al and/or Cr from the bond coat resulted in substantial growth of CSN underneath the previously formed CS layer, leading to the accelerated growth of TGO. 3.2. Crack nucleation and propagation

Fig. 9. Crack propagation within the ceramic topcoat at 1050 °C as a result of discontinuity opening after 300 cycles (a) and crack propagation associated with the CSN after 500 cycles (b).

Crack nucleation was usually associated with formation of CSN clusters and the CS layer (Figs. 6 and 7). As thermal cycling proceeded, some of the pre-existing crack-like discontinuities in the ceramic developed into fully open cracks. Most of them remained in the ceramic, but some had penetrated into the CS layer (Fig. 8). In the meantime, small cracks also nucleated between the TGO and the unoxidized bond coat. After 300 cycles, the opening and coalescence of crack-like discontinuities in the ceramic were so extensive that large cracks in the order of 100 μm started to form (Fig. 9a). Moreover, extensive crack propagation associated with CSN also occurred after 500 cycles (Fig. 9b), resulting in a total crack length of about 180 μm located near the ceramic/bond coat interface. On the other hand, newly formed cracks associated with the CS layer remained small, usually below 50 μm. During thermal cycling, the opening of the pre-existing discontinuities appeared to be the result of thermal fatigue, and CS and CSN nucleated cracks appeared to be stress buildups/ strain mismatch between oxide constituents. It is likely that CS and CSN nucleated cracks would assist the crack coalescence process, leading to the formation of large cracks that could become life threatening. Sintering of the ceramic topcoat may also assist the opening and propagation of long pre-existing

Fig. 10. Maximum crack length as a function of the number of thermal cycles at 1050 °C.

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Fig. 11. Correlation between maximum crack length and equivalent TGO thickness.

crack-like discontinuities during thermal cycling; however, such a phenomenon was not observed up to 500 cycles at this temperature. Most of the perceptible crack propagation occurred in the TGO and ceramic topcoat within 150 μm from the TGO. Fig. 10 shows the maximum length of cracks in the ceramic and/or TGO, as well as the equivalent TGO thickness δeq plotted against the number of thermal cycles. It is clearly shown that crack length increased with increased cycling in proportion to the TGO thickness (Fig. 11). Below 50 cycles, opening of the pre-existing discontinuities was not observed and the cracks nucleated within the original ceramic/bond coat interface region were all smaller than 20 μm. 4. Summary The growth of a TGO layer and crack propagation in an APS-TBC system with Co–32Ni–21Cr–8Al–0.5Y (wt.%) bond coat under thermal cycling to 1050 °C in air were investigated. The TGO was found to contain predominantly layered chromia and spinels, with some clusters of mixed oxides composed of chromia, spinel and nickel oxide. These mixed oxides formed heterogeneously at the original ceramic/ bond coat interface. Thickening of the TGO exhibited a three-stage growth phenomenon. Crack propagation during thermal cycling, was linearly proportional to TGO growth, which occurred as a result of opening of pre-existing discontinuities, and growth and coalescence of oxide induced cracking.

Acknowledgments The authors would like to thank Mr. Sylvain Bélanger of the Industrial Materials Institute of National Research Council Canada for thermal spraying the TBC samples. The support from the SURFTEC consortium is also appreciated. References [1] J.A. Haynes, E.D. Rigney, M.K. Ferber, W.D. Porter, Surf. Coat. Technol. 86–87 (1996) 102. [2] E.A.G. Shillington, D.R. Clarke, Acta Mater. 47 (1999) 1297. [3] A. Rabiei, A.G. Evans, Acta Mater. 48 (2000) 3963. [4] C.H. Lee, H.K. Kim, H.S. Choi, H.S. Ahn, Surf. Coat. Technol. 124 (2000) 1. [5] L. Ajdelsztajn, J.A. Picas, G.E. Kim, F.L. Bastian, J. Schoenung, V. Provenzano, Mater. Sci. Eng. A338 (2002) 33. [6] W.R. Chen, X. Wu, P.C. Patnaik, J.-P. Immarigeon, Proceedings from the 1st International Surface Engineering Congress and the 13th IFHTSE Congress, Columbus, Ohio, 2002, p. 535. [7] R.A. Miller, C.E. Lowell, Thin Solid Films 95 (1982) 265. [8] M.A. Gedwill, NASA TM-81567, 1980. [9] R.A. Miller, High Temperature Protective Coatings, 1982, p. 293. [10] K.S. Chan, N.S. Cheruvu, GT2004-53383, Proceedings of ASME Turbo Expo 2004, Power for Land, Sea, and Air, 2004. [11] B.S. Sidhu, S. Prakash, Oxid. Met. 63 (2005) 241. [12] S.M. Meier, D.M. Nissley, K.D. Sheffler, Proceedings of the 1990 Coatings for Advanced Heat Engines Workshop, 1990, p. II57. [13] W.R. Chen, X. Wu, B.R. Marple, P.C. Patnaik, Surf. Coat. Technol. 197 (2005) 109. [14] V. Kuznetsov, in: G. Petzow, G. Effenberg (Eds.), Ternary alloys, vol. 7, VCH Publishers, New York, NY, 1993, p. 434. [15] G.H. Meier, F.S. Pettit, J.L. Smialek, Mater. Corros. 46 (1995) 232.