Two simplistic models based on fracture mechanics considerations are used to advance the understanding of instability conditions in TBC systems. First model assumes isostrain behavior at and prior to the onset of crack initiation and is based on elastic energy balance approach. The other model is used for layer buckling and crack propagation behavior. The analysis for crack initiation suggests that the crack tip driving force, KI can reach a high value that is comparable to the fracture resistance of the coating material at and near the TBC/TGO interface even for a small nominal applied stress. The stresses required for the crack driving force (KI or GI) exceeding the fracture resistance of TBC materials are found to be in the range of 0.05 to 0.5 GPa. This appears to be an order of magnitude lower than the reported tangential tensile stress values of 1 to 2 GPa, but matches closely with the simulated transverse stress. High crack driving force resulting from low stress and small size defects (around 2 microns) facilitates early crack initiation in TBC system.

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