Failure process of thermal barrier coatings under thermal and mechanical loading

High temperature behaviors of thermal barrier coating (TBC) prepared by electron beam physical vapor deposition were investigated under mechanical loading. The change in microstructure and the formation of microcrack were studied by means of impedance spectroscopy (IS) and acoustic emission (AE) methods. The impedance value is very sensitive to the microcrack in the thermally grown oxidation (TGO) layer in the low frequency range. The formation and propagation of microcracks can be monitored by comparing the change of impedance value combined with acoustic emission. It has been found that the failure mode was quite different for TBC samples tested with or without mechanical loading. When TBCs were tested during high temperature creep, the microcracks formed in the equiaxial crystalline area between TGO layer and columnar structure of YSZ top coat, instead of in TGO layer during thermal cyclic testing without mechanical loading. However, cracks initialized in the diffusion layer between substrate and bond coat when TBCs were tested with low cycle fatigue at high temperature.