Modeling of interface failure in a thermal barrier coating system on Ni-based superalloys

In the present work, microstructural analysis and finite element modelling are employed to study interface cracking behaviour in a thermal barrier coating (TBC) on a single-crystal Ni-based superalloy. The cohesive zone elements are implemented in the model to simulate interfacial debonding between the top-coat (TC), thermally grown oxide (TGO) and bond-coat (BC). To evaluate the effect of the interface geometry on the residual stress state and cracking behaviour, two units of the TGO profile are analysed: a regular sinusoidal undulation with constant thickness and an irregular (unevenly thick) TGO layer with symmetrical penetrations into the TC and BC layers. It has been found that the morphology of the TGO layer influences not only the magnitude and distribution of residual stresses but also governs the mechanisms of interfacial failure. For the regular TGO shape, the debonding cracks form at the peak of TGO/BC interface and at the valley of TC/TGO interface. Whereas only the TC/TGO interfacial debonding is observed in case of the irregular TGO profile. The debondings induce the stress redistribution in TBC layers that depends on which interface and to what extent is damaged. The TBC system with the regular TGO layer appears to be a more prone to interface failure than that one with the irregular TGO shape. However, much higher compressive stresses in the TGO layer are observed in the latter case. Possible scenarios of the TBC failure in terms of further cracks propagation are discussed.