Failure mechanism of plasma-sprayed thermal barrier coatings under high-temperature isothermal aging conditions

Abstract

The failure of thermal barrier coatings (TBCs) is caused by the stress generated during service owing to the different physical properties and high-temperature behavior of the ceramic top coat made of Y2O3–ZrO2, thermally grown oxide (TGO) primarily composed of β-Al2O3 that forms and grows at high temperature, and metallic bond coat and substrate materials. The operating environment of TBCs varies depending on the objective of use and the operation stage of the gas turbine to which it is applied; therefore, it is important to understand the stress state by considering the operation environment. In this study, we analyzed the failure of TBCs in an operation environment when they were subjected to high-temperature conditions for a long time. For this, the effects of thermal expansion misfit, creep, and TGO growth on the high-temperature stress states were evaluated and analyzed systematically through finite element analysis(FEA.) The failure mechanism under a high-temperature isothermal condition was presented, considering degradation phenomena such as TGO growth rate decrease and creep rate reduction. It reveals that in the early stage of degradation at high temperatures, a valley-to-valley crack propagation was induced; however, at relatively low temperatures, changes in behavior during long-term exposure caused a peak-to-peak crack propagation. Additionally, the proposed mechanism was verified through analysis of the residual top coat area of the surface of the TBC specimen that was delaminated during the isothermal aging tests performed at different temperatures; this is important because the results of the analysis could not be explained by the mechanisms of previous studies in which cooling stresses under thermal cyclic conditions were focused.