Microstructure Evolution and Impedance Analysis of 7YSZ Thermal Barrier Coating during Gas Thermal-Shock

A gas thermal-shock experiment of thermal barrier coating was carried out in air up to 1,250°C in order to simulate the thermal cycling process of the engine blades during the start heating and shut cooling. The growth of thermal growth oxide (TGO) layer and microstructure evolution of YSZ layer during thermal cycling process were investigated systematically by electrochemical impedance spectroscopy testing and scanning electron microscopy. The results show that the thickness of TGO layer increases when increasing the frequency of thermal cycling, and the impedance response of middle frequencies is more and more remarkable. Meanwhile, initiation and growth of microcracks occur in YSZ layer during the gas thermal-shock experiment. The corresponding impedance characterization of YSZ layer after 100 cycles is similar to the as-sprayed sample, indicating that the YSZ microcracks could heal in a short time when sintered at high temperature. But after 300 cycles, the impedance spectroscopy of YSZ layer is quite different from the as-sprayed sample, with the corresponding impedance of YSZ grain boundary becoming more and more remarkable when prolonging the thermal-shock times, indicating that unhealing microcracks form in the YSZ layer, which may be the main reason to induce the failure of YSZ layer.