Oxidation Behavior and Main Causes for Accelerated Oxidation in Plasma Sprayed Thermal Barrier Coatings

Abstract

The growth of thermally grown oxide (TGO) can influence the durability of thermal barrier coatings (TBCs). In order to clarify its main causes and to find a method to restrain it, we examined an influence of surface treatment on a bond coat and its yttrium (Y) content on TOO growth. Shot peening was applied on the bond coat surface of TBC specimens, and high temperature oxidation behavior in air was compared with that of the TBC specimens without shot peening. As a result, in the specimen without shot peening, wart-like oxide was observed on a continuous oxide layer, which was formed between the bond coat and top coat. The wart-like oxide was caused due to unmelted particles of the bond coat. And in the continuous oxide layer, Y-rich granular microstructure was found. On the other hand, by means of shot peening on the bond coat surface, unmelted particles were removed, and the formation of the wart-like oxide was restrained. The growth of the continuous oxide layer, moreover, was also decreased because the Y-rich microstructure in the oxide was reduced. And the TBC specimen with the bond coat eliminating Y-content (Y-free bond coat) was prepared, and its oxidation behavior was examined by the high-temperature oxidation test in air. In the TBC specimen with Y-free bond coat, whether or not shot peening was applied, the growth of the continuous oxide was restrained since Y did not exist in the oxide. The possibility that Y-free bond coat could restrain TGO growth was found.