Vol.25 No.5 TRANSACTIONS OF MATERIALS AND HEAT TREATMENT PROCEEDINGS OF THE IFHTSE CONGRESS October 2004 Influence of Heat Treatment on the Bond Coat Cyclic Oxidation Behaviour in an Air-plasma-sprayed Thermal Barrier Coating System W.R. Chen 1 , X. Wu 1 , B.R. Marple 2 , and P.C. Patnaik 1 1. Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council Canada, Ottawa, Ontario, Canada, K1A OR6 2. Industrial Materials Institute, National Research Council Canada, Boucherville, Quebec, Canada, J4B 6Y4 Abstract: It is generally believed that a thermally grown oxide (TOO) layer of alumina provides enhanced protection to the metallic bond coat in thermal barrier coating (TBC) systems at elevated temperatures. However, in an air-plasma-sprayed (APS) TBC system with Co-32Ni-21Cr-8Al-0.5Y (wt%) bond coat, the TGO layer formed upon thermal exposure in air was predominantly chromia and spinels, which would not effectively protect the bond coat at above 1000°C. In addition, mixed oxides of chromia, spinel and nickel oxide formed heterogeneously between the ceramic coating and CoNiCrAlY bond coat, which would promote crack initiation and lead to premature TBC failure. A heat treatment in a low-pressure condition was applied to the as-sprayed TBC system, with the aim to produce an alumina layer as well as reduce the amount of detrimental oxides. The influence of this low-pressure oxidation treatment (LPOT) on the bond coat cyclic oxidation behaviour of the TBC system was also investigated. Key words: TBC, CoNiCrAlY, TGO, low-pressure oxidation treatment, cyclic oxidation THE METALLIC BOND COAT is an important constituent in a TBC system. It enhances the adhesion of the ceramic thermal barrier layer (the topcoat) to the substrate and also provides oxidation protection to the substrate metal. The composition of the bond coat, generalized as M-Cr-Al-Y, where M represents Ni, Co and/or Fe, generally allows a layer of alumina (A1 2 O 3 ) to form during high temperature exposure. If a continuous scale of A1 2 O3 forms along the interface between the bond coat and the ceramic topcoat, 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. Generally, depending on the composition of the bond coat, the amount and type of other oxides vary significantly in a real TBC system. The oxide products newly formed along the bond coat/topcoat interface are collectively called thermally grown oxides (TGO). For example, spinel (Ni(Cr,Al) 2 O4) and nickel oxide (NiO), have been found, in addition to alumina (A1 2 O 3 ), in NiCrAlY systems [1-6]. It has been suggested that these two oxides are detrimental to the durability of TBC systems, because the rapid local volume increase accompanying their formation [4] may enhance the local stress build-up. This was corroborated in high- temperature experiments by comparing the effect of exposing NiCrAlY in air to that produced when exposed in an argon environment with low-pressure oxygen [6]. The one exposed in air, where the formation of the mixed oxides occurred, failed prematurely as compared to the one exposed in argon where only A1 2 O 3 formed. The latter did not fail until the A1 2 O 3 scale and a significant amount of the bond coat transformed into spinel-like oxides. In an air- plasma-sprayed (APS) CoNiCrAlY system exposed in air, the TGO layer was found not to consist of a continuous layer of A1 2 O3 but rather to be composed predominantly of mixed chromia/alumina, (Cr,Al) 2 O3, and spinels (Co,Ni)(Cr,Al) 2 O 4 [7], which would not effectively protect the bond coat from further oxidation at temperatures >1000°C [8]. In some applications, a continuous A1 2 O3 layer has been artificially introduced, either through physical deposition or pre-oxidation treatment before the deposition of the ceramic topcoat. This has been shown to have a positive influence on the TBC durability [9]. The objective of this study is to modify the TGO to form a thin Al 2 Os layer through a low-pressure oxygen treatment (LPOT). The secondary objective is to minimize formation of the mixed oxides, (Cr,Al) 2 O 3 -Ni(Cr,Al) 2 O 4 -NiO (abbreviated as CSN [10]), in the TBC system. The cyclic oxidation behaviour of both as-sprayed and LPOT-treated TBC with a CoNiCrAlY bond coat will be examined in detail. 1. Experimental The TBC samples used in this study consisted of a Co-32Ni-21Cr-8Al-0.5Y (wt%) bond coat and a ZrO 2 - 8%Y 2 O 3 topcoat, deposited by the APS method on Inconel 625 disks of 12.5 mm in diameter. Some samples were heat-treated in a low-pressure oxygen environment of about 2x10~ 3 torr (Po 2 =0.056 Pa) at 1080°C for 24 hours (LPOT treatment). The as-sprayed