The effect of thermally grown oxide on multiple surface cracking in air plasma sprayed thermal barrier coating system

The objective of this paper is to understand the effect of thermally grown oxide (TGO) on the multiple surface cracking behaviors in an air plasma sprayed (APS) thermal barrier coating system (TBCs). The extended finite element method (XFEM) and periodic boundary conditions are used to investigate the TGO dependences of periodic surface crack driving force and crack propagation path. It is seen that the effect of TGO thickness on the strain energy release rate (SERR) is negligible while ignoring the thickening and elongation strains of the TGO layer. However, the effect of elastic modulus of TGO layer is significant. In particular, for relatively stiff TGO layer the SERR drops to zero as the ratio of surface crack length to coating thickness approximates to one. And vice versa, for a severely damaged TGO layer with relatively compliant modulus, the SERR approaches infinity as surface cracks propagate to the interface. This kind of fracture mechanism is mainly governed by the elastic mismatch across the bimaterial interfaces, which can be used as a guide for the design of good strain-tolerant coating film in APS-TBCs.