Sintering-Induced Failure Mechanism of Thermal Barrier Coatings and Sintering-Resistant Design

Abstract

Thermal barrier coatings (TBCs) have been developed to protect superalloys against high-temperature heat fluxes, which are required for the development of high-performance gas turbines. TBCs have porous structures, which are densified by sintering. The resulting stiffening is a major cause of TBC failure in service. Therefore, there is a need to reduce the negative sintering effect on the life span of TBCs. In this study, the sintering mechanism and the dominant factors causing changes in stiffening and mechanical properties were revealed experimentally. The experimental results show that the multiscale undulation of the originally smooth two-dimensional (2D) pore inner surface triggers multipoint contact between the upper and lower inner surfaces, resulting in pore healing during thermal exposure. The healing of 2D pores is the main structural characteristic change in TBCs after thermal exposure and the main reason for the stiffening and changes in mechanical properties. Then, the sintering effect on TBCs with vertically cracked structures was designed and simulated. We found that implanting vertical cracks in the topcoat can reduce the sintering effect and driving force for cracking by 87.9% and 79.9%, respectively. The degree of reduction depends on the space between vertical cracks. Finally, the mechanism responsible for the sintering-resistant TBCs was analyzed and discussed. Vertically cracked structures exhibited scale-sensitive stiffening, indicating that macroscopic stiffening is much lower than microscopic stiffening. In other words, the macroscopic sintering effect was lowered, and the TBCs remained highly resistant to global strain during thermal exposure. The resulting strain energy release rates are much lower than those of conventional TBCs. The results of this study contribute to the long-life thermal protection of superalloy-based components used in advanced gas turbines.