A Numerical Model for the Cyclic Instability of Thermally Grown Oxides in Thermal Barrier Systems

Morphological instability of the thermally grown oxide (TGO) is a fundamental source of failure in some thermal barrier systems. The instabilities occur when initial non-planarity in the TGO grows in amplitude as the system experiences thermal cycling. By numerical means, this study explores how these instabilities are linked to constituent properties. The associated phenomena involve oxidation of the TGO, plastic flow of the bond coat, thermal expansion misfit between the TGO, bond coat and substrate, and stress relaxation in the TGO at high temperature. A key implication of the simulations is that the incidence of reverse yielding upon reheating differentiates between systems that exhibit a systematic increase in imperfection amplitude upon thermal cycling (ratcheting) and those that exhibit shakedown.