Role of platinum in thermal barrier coatings used in gas turbine blade applications

Current technology of thermal barrier coating systems used in gas turbine blade applications relies on the use of a metallic bond coat, which has a twofold function: (i) it develops a thin layer of aluminum oxide enhancing the adhesion of the ceramic top coat and (ii) it provides an additional resistance to oxidation. It was the objective of this study to develop an understanding of the role of platinum in bond coats of the diffusion-type deposited on a nickel-based superalloy. Two Pt-containing bond coats were included in the study: (i) a platinum-aluminide and (ii) a bond coat formed by interdiffusion between an electroplated layer of platinum and the superalloy substrate. In both cases, the top ceramic coat was yttria-stabilized zirconia. For reference purposes, a simple aluminide bond coat free of Pt was also included in the study. Thermal exposure tests at 1150°C with a 24 h cycling period at room temperature were used to compare the coating performance. Microstructural features were characterized by various electron-optical techniques. Experimental results indicated that Pt acts as a "cleanser" of the oxide-bond coat interface by decelerating the kinetics of interdiffusion between the bond coat and superalloy substrate. This was found to promote selective oxidation of Al resulting in a purer Al₂O₃ scale of a slower growth rate increasing its effectiveness as "glue" holding the ceramic top coat to the underlying metallic substrate. However, the exact effect of Pt was found to be a function of the state of its presence within the outermost coating layer. Of the two bond coats studied, a surface layer of Pt-rich gamma prime phase (L1₂ superlattice) was found to provide longer coating life in comparison with a mixture of PtAl₂ and beta phase. This could be related to the effectiveness of gamma prime phase as a sink for titanium minimizing its detrimental effect on the adheXrence of aluminum oxide.