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SUMMARY 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. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(12): 37–45, 2013; Published online in Wiley Online Library (http://wileyonlinelibrary.com). DOI 10.1002/ecj.11551

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The characteristics of photoluminescence of Cr 3+ in the thermally grown oxide (TGO) layer in thermal barrier coating (TBC) for gas turbine blades was evaluated. A pulsed Nd:YAG laser of wavelength 532 nm was used to excite Cr 3+ in the TGO layer in TBC specimens, and the luminescence was detected at 694 nm. Photoluminescence of Cr 3+ was observed for TBC specimens heated for 100 hrs and 1000 hrs at 1100°C, but was not observed for the specimen without heating. The intensity of photoluminescence of Cr 3+ for the heated specimens showed two radiative lifetimes, which are thought to be the radiative lifetimes of Cr 3+ in different metal oxides, possibly Al 2 O 3 and AlYO 3 . Effects of self-absorption were not observed. In addition, the photoluminescence intensity was proportional to the excitation intensity, showing that the effects of saturation were absent.