Stress state and failure mechanisms of thermal barrier coatings: role of creep in thermally grown oxide

Abstract

The mechanical loading of the thermal barrier coating (TBC)/thermally grown oxide (TGO)/bond coat interface region is calculated for a TBC coated superalloy specimen using a finite element model. It is shown that the evolving stress state depends crucially on the ratio of the loading rate caused by growth and swelling of the oxide layer and the unloading rate by creep relaxation. For grain sizes below 1 μm, creep of the oxide layer cannot be neglected. From this calculations, possible damage scenarios are inferred and implications for future design of TBC layers are considered. Tailoring the TGO creep properties, for example by reducing its grain size or designing a multilayer TGO scale, is identified as the most relevant aspect.