Microstructural characterization of a platinum-modified diffusion aluminide bond coat for thermal barrier coatings
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11/9/03
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Microstructural Characterization of a Platinum-Modified Diffusion Aluminide Bond Coat for Thermal Barrier Coatings M.W. CHEN, K.J.T. LIVI, P.K. WRIGHT, and K.J. HEMKER Microstructural and chemical evolution induced by thermal cycling of a platinum-modified diffusion aluminide bond coat was investigated with transmission electron microscopy (TEM), X-ray diffraction, (XRD) and electron microprobe analysis. As-fabricated, the bond coat was confirmed to be an ordered B2 structure, but the underlying microstructure was found to be modulated. Thermal cycling resulted in a primarily outward diffusion of Ni and the formation of a Ni-rich bond coat containing secondary L12 precipitates. Closer inspection of the bond coat revealed a transformation from its original B2 structure to a L10 martensite. In-situ TEM observations indicated that the martensite is stable at lower temperatures and that the parent B2 structure reappears at higher temperatures. These observations can be used to explain the variations in strength that have recently been measured in thermally cycled bond coats. The resulting transformation strain is also argued to play an important role in determining the accumulation of stress and strain in thermally cycled thermal barrier coatings (TBCs).
I. INTRODUCTION
THE use of thermal barrier coatings (TBCs) has resulted in a significant improvement in the life of components for gas turbine engines and detailed descriptions of TBCs can be found in References 1 through 5. Modern TBCs are commonly comprised of a superalloy substrate, an intermetallic bond coat, a thermally grown oxide (TGO), and a ceramic top coat. The useful life of a TBC is determined by its ability to resist spallation, and detailed studies of this failure process have drawn attention to the importance of out-of-plane displacements of the TGO. This failure mode is of particular importance for platinum-modified nickel aluminide bond coat/yttria-stabilized zirconia (YSZ) TBCs, where thermal expansion misfits and TGO growth strains have been related to dramatic roughening of the bond coat/TGO interface.[6,7,8] Numerical simulations of the cyclic instability of the TGO in this TBC system have duplicated many of the experimental observations pertaining to the roughening and failure of platinum aluminide TBCs (for example, References 9 through 15). In reviewing these works, Karlsson et al.[16] have reported that the simulations are only consistent with experimental observations when the bond coat and TGO are both allowed to deform plastically. The realization that this TGO geometric instability is influenced by the mechanical behavior of the bond coat has resulted in efforts to improve TBC life through bond coat development. Bond coat alloys have historically been designed to be good -alumina formers, but issues related to mechanical behavior and microstructural stability of the bond coat are receiving increased attention. M.W. CHEN, Associate Research Scientist, Department of Mechanical Engineering, K.J.T. LIVI, Senior R
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