Measurement of Refractive Index of Thermal Barrier Coating Using Reflection of Terahertz Waves and Variable Aperture

SUMMARY A method to measure the refractive index of the ceramic layer of a thermal barrier coating (TBC) using the intensity ratio between the reflection of terahertz waves from the ceramic surface and a reference metal plate is proposed. This method uses a variable aperture to change the relative proportions of specular refraction and scattering from the ceramic surface. The refractive index is calculated based on the specular reflectivity, which is obtained from the intensity ratio when the field of view is extrapolated to zero. Measurement of a TBC specimen resulted in a refractive index of 4.7, which was in good agreement with the refractive index of the ceramic material obtained by terahertz time‐domain reflectometry. In addition, the surface roughness of the ceramic surface was obtained based on the frequency characteristics of the reflections from the ceramic surface and a reference metal plate. A surface roughness of 14 μm was obtained, which matched the results of microscopic observation of a TBC specimen whose coating was applied using the same lot as the specimen used in the measurement.