Numerical investigation of residual stress fields and crack behavior in TBC systems

Due to thermal expansion mismatch and bond coat (BC) oxidation, high residual stresses are induced in the thermal barrier coating (TBC), leading to failure by spalling and delamination. Using the finite element method (FEM), an analysis of the stress distributions in TBC systems, which is a prerequisite for the understanding of failure mechanisms, was performed. As cracking usually occurs at or near the interfaces between BC/thermally grown oxide (TGO) and TBC/TGO depending on the processing mode of the TBC, cracks in the interface region were considered in the FE models in order to determine the loading conditions for their propagation and, thus, the failure criteria of the TBCs. Due to the mode mixity of these cracks, suitable methods are required for the determination of the fracture mechanics parameters needed for their assessment, such as the strain energy release rate G. The modified crack closure integral method (MCCI) was found to be a very efficient tool which can be combined easily with an FE analysis and leads to highly accurate energy release rate values. Moreover, this method enables the determination of mode-dependent energy release rates. Using this tool and appropriate crack propagation criteria, TBC failure models could be developed and verified.