A numerical assessment of the durability of thermal barrier systems that fail by ratcheting of the thermally grown oxide

Interactions between cracks induced in thermal barrier coatings (TBCs) upon thermal cycling have been calculated. The interactions are motivated by displacement instability in the thermally grown oxide (TGO). The results indicate that the energy release rate G cycles as the temperature changes, with the largest value arising at ambient temperature. It increases on a cycle-by-cycle basis. Cracks that converge from neighboring imperfections exhibit a minimum energy release rate prior to coalescence. Equating this minimum to the toughness of the TBC provides a criterion for coalescence and failure. Imposing this criterion allows the change in crack length upon cycling and the number of cycles to failure to be ascertained. This simulation capability is used to explore various influences on durability. The roles of the heating/cooling rate and the high temperature hold time are assessed, demonstrating substantial variation in durability, especially when the bond coat is relatively soft. The trends from these simulations are compared with experimental results for furnace cycle and burner rig tests. Improvements in the durability upon increasing the high temperature strength of the bond coat and upon decreasing the growth stress in the TGO are established, as well as the influence of the geometric imperfections in the bond coat. Some effects of the thermal expansion misfit between the bond coat and the substrate are explored.