Analysing the Failure Behaviour of Thermal Barrier Coatings Using the Finite Element Method

The plasma-sprayed zirconia-8-yttria-based Thermal Barrier Coating (TBC) is exposed to thermal cyclic fatigue. Due to thermal expansion mismatch and oxidation of the bond coat, high residual stresses are induced, leading to failure by spalling and delamination. The finite element method has been used in order to obtain detailed stress distributions in TBC systems. Several geometrical configurations of bond coat and oxide layer were modelled to promote micromechanical understanding of the failure behaviour of the two-layer system. The roughness of the interface between bond coat and oxide layer was also taken into account. Thermal stresses due to thermal expansion mismatch and expansion stresses by a high-temperature oxidation of the bond coat were then calculated. The analysis was performed using different material laws considering the elastic, elastic-plastic, and viscoplastic behaviour of the different layers. A blunted crack was implemented in a four-layer TBC structure and the condition of crack propagation was discussed. The fracture mechanical assessment was performed using the J-Integral concept. On the basis of the results gained, a failure model for TBC systems was derived.