Detection of Surface Crack on the Substrate under Thermal Barrier Coatings Using Microwave Non-Destructive Evaluation

Thermal barrier coatings (TBCs) have been widely used in gas turbine engines in order to protect the substrate metal alloy against high temperature and to enhance turbine efficiency. Thermal effects in the work process will produce a certain stress to induce the metal substrate surface defects such as cracks, which affect thermal conductivity of TBCs. Therefore, it is necessary to detect the cracks in order to ensure the quality of TBCs. In this paper, an approach for microwave non-destructive evaluation of surface crack on the substrate under thermal barrier coatings will be presented. The relationship between reflection coefficient and defects of TBCs were studied, based on the propagation feature of microwave isonified through multi-layer. A network analyzer operated in the frequency range of 26.5 GHz to 40 GHz was used to measure the reflection coefficient. Scanning images were obtained by using phase difference and amplitude at the optimum detection frequency. The results indicate that the amplitude, and the phase difference of the reflection coefficient, can represent the defects under TBCs using microwave non-destruction evaluation when the stand-off distance is suitable. The experimental setup that validates the microwave non-destructive inspection of TBCs is presented, and the feasibility to evaluate the cracks on the substrate surface using microwave non-destructive evaluation is verified in this paper.