The role of cyclic mechanical loading and thermal gradients in damage behavior of thermal barrier coating systems

Abstract

Ceramic thermal barrier coatings (TBC) can exhibit premature in-service failure due to spallation as a consequence of exposure to elevated temperatures, aggressive environments, and cyclic thermal and mechanical loading. In this study, the influence of cyclic mechanical loading and thermal gradients on the damage mechanisms in TBC systems is investigated. Performed tests were thermal fatigue with superimposed mechanical fatigue load and thermal gradient over the cross section as well as isothermal mechanical fatigue at high temperature. The stress states generated by mechanical loading and thermal gradients in the course of the respective test cycle were analyzed by Finite Element calculations. The calculated stress distributions were used to discuss specific damage patterns observed for the different test types. In the case of thermal gradient mechanical fatigue tests, which generate stresses most closely to in-service conditions of rotating gas turbine blades, the observed evolution of cracks parallel to the coating surface can explain accelerated TBC spallation.