Measurement of Refractive Index and Thickness of Topcoat of Thermal Barrier Coating by Reflection Measurement of Terahertz Waves

A method to determine the refractive index and thickness of the topcoat of a thermal barrier coating (TBC) from the reflected waveforms of terahertz waves was developed. The method was applied to specimens of yttria-stabilized zirconia (YSZ), which is the topcoat material. The obtained refractive index was in agreement with that obtained by time-domain terahertz spectroscopy in a transmission configuration, and the obtained thickness was in agreement with measurement results using a contact thickness gauge. The method was also applied to a TBC specimen, and the planar distribution of the refractive index and thickness of the topcoat were obtained. The obtained thickness was in agreement with microscope observation of the cross section of the specimen. The results show that reflection measurement of terahertz waves is useful for nondestructive testing of TBC applied to gas turbine blades. (c) 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(12): 37-45, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11551