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

Abstract

A method to measure the refractive index of the ceramic layer of thermal barrier coating (TBC), based on the intensity ratio of the reflections of terahertz waves from the ceramic surface and a reference metal plate, is proposed. The method uses a variable aperture to change the relative proportions of specular reflection and scattering from the ceramic surface. The refractive index is calculated from 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 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 14µm was obtained, which was in agreement with the microscopic observation result of a TBC specimen whose coating was applied in the same lot as the specimen used in the measurement.