Assessment of Cyclic Lifetime of NiCoCrAlY/ZrO 2 -Based EB-PVD TBC Systems via Reactive Element Enrichment in the Mixed
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INTRODUCTION
THERMAL barrier coating systems (TBCs) are widely used in gas turbine applications to improve their overall efficiency and prolong their lifetime. Typical TBCs consist of four layers: ceramic topcoat, thermally grown oxide (TGO), metallic bond coat, and superalloy substrate. The final deposition of the ceramic topcoat is favorably conducted by plasma spraying (PS) or by reactive electron beam physical vapor deposition (EB-PVD). Apart from improved durability, the latter one imparts a smooth surface finish to the component. The bond coats—NiAl(Pt) diffusion coatings or overlay coatings—act as TGO formers during the final coating processing and successive exposure to service temperature. The overlay coatings mostly stem from the principal elements Ni, Co, Cr, Al, and Y and are habitually termed NiCoCrAlY coatings. Overlay bond coatings are commonly applied by a low pressure PS process or by EB-PVD. In either case, the bond coats need a post-treatment to establish appropriate surface conditions. In EB-PVD TBCs, the fineness of the bond coat surface and the steady growth of a smooth TGO/ ceramic interface are considered to be essential requirements for attaining good adhesion and inhibiting premature failures.[1] For APS TBC systems, however, superior adhesion and lifetimes are attained via shot KLAUS FRITSCHER, Retired, WOLFGANG BRAUE, Senior Scientist, and UWE SCHULZ, Senior Scientist and Department Head Coatings, are with the Institute of Materials Research, DLR German Aerospace Center, 51170 Cologne, Germany. Contact e-mail: [email protected] Manuscript submitted February 15, 2012. Article published online December 1, 2012 2070—VOLUME 44A, MAY 2013
peening production routes.[2] The reliability of TBCs (lifetime concepts) in terms of predictive times of adhesion to their substrates at temperature is—except for defect-induced constrains—critically influenced by diffusion and oxidation phenomena in the TGO. The growth kinetics of the CAZ demonstrate protective diffusion-controlled parabolic[3–5] or sub-parabolic[6,7] growth rates as is typical for alumina-forming alloys.[8] The MZ is unprotective[7] and retains a constant thickness over long periods.[6] The TGO scale of TBC systems having formed on a genuine NiCoCrAlY overlay bond coat may exhibit a typical double-layer scale structure. The inner portion is a columnar alumina zone (CAZ) and the outer one the mixed zone (MZ).[3,9–14] In the early stages of TBC deposition, the MZ is typically formed via uptake of zirconia by a spinel-derivative and/or transition-alumina scale. It results in a fine-grained globular microstructure of a-alumina and zirconia dispersals. The MZ develops a variable microstructure throughout the TBC lifetime.[3,13] Local minor Cr2O3 nests are observed in the upper MZ.[6,11,15–17] They may be related to later formation of some (Al,Cr)2O3-modified matrix at the MZ/TBC interface[12] that establishes a reaction front growing into the TBC bottom section of the top coat. This situation is to be seen in Figure 1.[18] A transition
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