Local stress evolution in thermal barrier coating system during isothermal growth of irregular oxide layer

Abstract

The local stress associated with the presence and growth of irregular thermally grown oxide (TGO) layer between bond coat (BC) and top coat (TC) may induce micro-cracks and morphological instability in thermal barrier coating system (TBCs). This work is aimed to investigate the local stress evolution during isothermal growth of an irregular TGO layer. TGO convex portion and its vicinity are focused due to commonly observed cracks there. An analytical model based on spherical symmetry is proposed first, which incorporates TGO inward thickening, time-dependent oxide growth strains in both through-thickness and lateral directions, and creep behaviour of TGO and TC layers. Then, a modified finite difference method is formulated to numerically solve the problem. The roles of different growth strains in stress evolution are identified, and the effects of TGO thickening and coatings creep are ascertained. The implication of TC and TGO stress state in crack initiation is consistent with experimental observation. In general, the local stress keeps increasing before creep relaxation overwhelms the consequences of growth strains, and there are two distinctive regimes separated by an extreme point corresponding to the maximum stress value and transition time in the stress variation curve. According to the prediction, TC layer is always subjected to radial compression and hoop tension. However, near TGO/BC and TGO/TC interfaces, TGO stress is affected by the competition between two kinds of growth strains as well as creep in different coating layers, leading to the sensitivity of stress evolution to the related parameters. The parameter sensitivity of maximum stress and transition time of TC hoop stress is also evaluated quantitatively.