On the performance and failure mechanism of thermal barrier coating systems used in gas turbine blade applications: Influence of bond coat/superalloy combination

Surface engineering plays a major role in achieving the performance and design lives of gas turbine components such as the high pressure turbine aerofoils which operate under the most arduous conditions of temperature and stress leading to a wide range of thermal and mechanical loading during service. In this study, emphasis is placed upon the role of composite systems consisting of bond coat and superalloy substrate in determining the performance and useful life of thermal barrier coatings using yttria-stabilized zirconia as top coat processed by electron-beam physical vapor deposition. Three platinum-modified bond coats of the diffusion type and three nickel-based superalloys are included in the study. Thermal exposure tests at 1150 °C in air with a 24-hour cycling period to room temperature have been used to rank the performance of the coating systems. Various electron-optical techniques have been used to characterize the sequence of events leading to coating failure as marked by spallation of the top ceramic coat. It is shown that for a given superalloy substrate, the coating performance is dependent upon the type of bond coat. Conversely, for a given bond coat, the performance becomes a function of the superalloy composition used in the application. However, in both cases, coating failure is found to be predominated by loss of adhesion between the thermally grown oxide and bond coat indicating that the respective interface is the weakest link in the system. The results are interpreted in terms of the phase transformations which occur in the bond coats during exposure at elevated temperatures and the corresponding effects on their oxidation behavior.