Air Plasma Sprayed Thermal Barrier Coatings: Experiments and Finite Element Analysis

The purpose of this research was to examine the primary in-service failure mechanism of an air plasma sprayed thermal barrier coating commonly used in combustor applications, and to use that failure mechanism as a basis in developing a life prediction strategy. The research consisted of an experimental phase, in which the failure mechanism was identified and key features of the coating system measured, and a modeling phase, in which the findings of the experimental phase were used to build a system specific finite element model of the coating in order to extract quantitative data relevant to coating life. Observations were made on multiple thermal barrier coating samples, consisting of a nickel-based super alloy substrate, a MCrAlY bond coat, a thin layer of thermally grown oxide (TGO) which thickens during service, and an air-plasma sprayed yttria-stabilized zirconia (YSZ) coating. Growth of the TGO layer, rumpling, and aluminum depletion from the bond coat were modeled using finite element analysis to ascertain the relative importance of each to the development of stress in the coating. It was determined that TGO growth and rumpling have effects on the major trends of stress development, while aluminum depletion has a more minor role. A previously developed viscoplastic constitutive model, specific to thermal barrier coatings, was employed in the model. From the modeling results, certain inelastic strain output was shown to provide the necessary time and temperature dependent features a damage parameter would need to accurately predict failure live