Residual Stress Measurement in a Pt-Aluminide Bond Coat
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Residual Stress Measurement in a Pt-Aluminide Bond Coat Makoto Watanabe, Daniel R. Mumm, Stefanie Chiras, and Anthony G. Evans Princeton Materials Institute, Princeton University, Princeton, NJ 08540-5211, U.S.A. ABSTRACT The residual stress induced in a Pt-aluminide bond coat formed on a single-crystal superalloy has been measured. The stresses arise because of the thermal expansion misfit with the substrate on cooling from the manufacturing temperature. Since the lattice parameters are unknown, the interpretation of diffraction measurements is problematic, and the "wafer" curvature method has been applied. This method required that the substrate thickness be systematically reduced by mechanical thinning. It was also required that curvature measurements be made with the bond coat present as well as after it had been removed by thinning. This approach revealed that the bond coat is in residual tension, 140MPa, consistent with its thermal expansion coefficient, relative to that for the substrate.
INTRODUCTION The performance of thermal barrier systems is dictated by thermo-mechanical interactions between the four constituent materials: substrate, bond coat, thermally grown oxide (TGO) and thermal barrier coating [1,2]. One of the more important phenomena governing system performance is the yielding of the bond coat in the vicinity of imperfections, upon thermal cycling [3-7]. This response contributes to displacement instabilities that cause failure in some commercial systems. The residual stress in the bond coat caused by cooling from the manufacturing temperature facilitates yielding. It does so by interacting with the stress in the bond coat caused by thermal expansion misfit with the TGO and by TGO growth. The consequence is an acceleration of the instability growth rate and diminished durability. To address the effect, measurement of the residual stress is necessary. The objective of this study is to measure the stress in a commercial thermal barrier system. The system of interest consists of ( β -phase) Pt-aluminide bond coat created by interdiffusion of a Pt/Al layer with the Ni-based superalloy substrate. This bond coat resides in the β − NiAl phase field, but has a sufficient concentration of elements in solution (particularly Pt), that the lattice parameter is unknown. Accordingly, methods based on lattice parameter measurements, such as X-rays, are difficult to use. A viable approach is to use a “wafer” curvature method [8]. On a relatively thin substrate, the force contributed by the residual stress present in a surface layer causes the substrate to bend. The induced curvature depends on the force, as well as the elastic properties of the substrate and its thickness. By systematically thinning the substrate and measuring the change in curvature, the residual stress can be determined. For bond coats, this stress is primarily associated with a thermal expansion misfit with the substrate, being stress-free at the manufacturing temperature, because of creep relaxation [9]. The measurements
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