Abstract-Improvement of Measurement Resolution of Depth Distribution in Cross-Sectional Imaging Using Terahertz Wave

1. TETSUO FUKUCHI¹, 
2. NORIKAZU FUSE¹,
3. MAYA MIZUNO² and
4. KAORI FUKUNAGA²

Version of Record online: 12 MAY 2016

DOI: 10.1002/ecj.11819

© 2016 Wiley Periodicals, Inc.

http://onlinelibrary.wiley.com/doi/10.1002/ecj.11819/abstract

Time-resolved reflection measurement using broadband, short terahertz pulses can provide cross-sectional images of dielectric materials at different depths. Low-frequency components which have responses over a wide time range may result in inclusion of spurious signals which arise from strong reflections at other depths. Model calculations showed that such spurious signals can be suppressed by application of a frequency filter of an appropriate center frequency. The effect of frequency filtering was investigated in cross-sectional imaging of an IC card. Cross-sectional images were obtained at depth intervals of 0.18 mm, which provided images at five different depth intervals. As a result of frequency filtering, spurious signals resulting from strong reflections at different depths could be reduced in intensity, which resulted in improvement of the measurement resolution of the depth distribution.

Abstract-Topcoat Thickness Measurement of Thermal Barrier Coating of Gas Turbine Blade Using Terahertz Wave

1. Tetsuo Fukuchi¹, 
2. Norikazu Fuse¹,
3. Mitsutoshi Okada², 
4. Takayuki Ozeki²,
5. Tomoharu Fujii², 
6. Maya Mizuno³ and
7. Kaori Fukunaga³

Article first published online: 19 JUN 2014

DOI: 10.1002/eej.22624

© 2014 Wiley Periodicals, Inc.

http://onlinelibrary.wiley.com/doi/10.1002/eej.22624/abstract

The topcoat thickness of thermal barrier coating (TBC) applied to a gas turbine blade was measured using terahertz waves. The refractive index of the topcoat was obtained by frequency analysis of the waves reflected from the topcoat surface and the interface between the topcoat and bondcoat. The surface roughness of the topcoat surface was considered for improving the accuracy of the refractive index calculation. The topcoat thickness was obtained from the refractive index and the time separation between the reflected waves. The validity of the method was confirmed using a TBC sample with variable topcoat thickness, and the obtained topcoat thickness was in agreement with measurement results using a contact thickness gauge. Error analysis showed that the measurement error of the topcoat thickness was about 3%. The method was applied to a gas turbine blade with TBC, and the measured topcoat thickness agreed with microscope observation results of the cross section to within 6%, which was within the range of measurement error. The effect of curvature of the gas turbine blade surface did not result in significant measurement error. The results showed that terahertz waves are effective for nondestructive measurement of the topcoat thickness of TBC applied to gas turbine blades.