Abstract-Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location

 

Andrew J. Waddie, Peter J. Schemmel, Christine Chalk, Luis Isern, John R. Nicholls, and Andrew J. Moore

Schematic of experimental setup. M1 – flat mirror; P1 – linear polarizer; BS – Si beam splitter; M2 – parabolic mirror; L1 – PTFE lens; A1 – linear analyzer; PD – photodiode.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-21-31535

We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trials.

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