Effects of bond coat misfit strains on the rumpling of thermally grown oxides

Abstract

One factor governing the durability of thermal barrier systems is the concurrent thickening and elongation of the thermally grown oxide (TGO) upon temperature cycling. The elongation can cause cyclic rumpling of the TGO: influenced by oxide growth, bond coat phase transformations, substrate-bond coat interdiffusion, and constituent strengths. The individual effects of these phenomena cannot be understood by experiment alone. In the current study, simulations are conducted to isolate the effects of the misfit strains between the bond coat and substrate. These strains originate from thermal expansion mismatch, phase transformations, and bond coat swelling. For each calculation, the response of the system throughout an individual thermal cycle is linked to the stresses in the bond coat and TGO. Results obtained for representative misfit strains indicate that all three sources promote rumpling during the early stages, while phase transformations and thermal expansion mismatch are more prevalent upon extended cycling. These misfits also induce tensile stresses in the oxide large enough to cause cracking at high temperature. Further analysis has been used to assess the benefits of developing bond coats having lower phase transformation temperature, higher strength, and a more closely matched coefficient of thermal expansion.