Non-destructive thickness measurement of thermal barrier coatings using terahertz radiation

Abstract

A non-destructive thickness measurement technique based on terahertz (THz) reflectivity was successfully deployed to interrogate 7 wt.% yttria-stabilised zirconia thermal barrier coatings (TBCs) produced by electron-beam physical vapour deposition (EB-PVD). The THz technique was shown to produce accurate thickness maps for different samples with a resolution of 1 × 1 mm over a surface of 65 × 20 mm that were compared with direct examination of key cross-sections. All thickness measurements on different samples were calculated using a single value of refractive index. Small defects characteristic of EB-PVD, such as “carrot growths” and variations on column inclination, were evaluated and did not produce significant variations in the refractive index of the TBC. Moreover, the thickness maps correctly display thickness variations that are a consequence of the point-source nature of EB-PVD evaporation. In summary, this paper demonstrates the technique can be successfully deployed on large surfaces, and across different coatings of the same material produced under the same deposition conditions. It is shown that a single n value is required to map the thickness distribution for all samples. This combination of qualities indicates the potential of the technique for in-line control of TBC manufacture.

Ultra-Wideband Frequency Modulated Continuous Wave Photonic Radar System for Three-Dimensional Terahertz Synthetic Aperture Radar Imaging

“…A slight thickness change of ~50 μm can be monitored with the proposed system in the 600-GHz band and it shows adequate agreement with the measurement results obtained using the THz pulse system [27]. This system can be applied to inspect coating materials at large scales, such as for ships and planes [35]. Such a long detectable distance with a collimated beam indicates the possibility of introducing the SAR scheme in the THz band to obtain 3D imaging.…”

Characterization of the Internal Stress Evolution of an EB-PVD Thermal Barrier Coating during a Long-Term Thermal Cycling

“…The formula is as follows: where d is the specimen thickness, c is the velocity of light, and α is the angle of refraction. The refractive index of the EB-PVD TBCs was 3.977 [ 61 ]. The refraction angle, α , was 10.242, based on the refraction law and Equation (4), corresponding to cosα being 0.98406.…”

PVD for Decorative Applications: A Review

“…In 2020, MC simulations were also employed with success to make calibration curves for XRF thickness measurements [168]. Another emerging nondestructive methodology was proposed in 2021 by Isern [169]; it is based on terahertz (THz) reflectivity and was successfully employed to map the PVD-deposited yttria-stabilized zirconia thermal barrier. In 2023, Cruz [170] proposed a standardless method, tested on TiN coatings, based on EDS and MC simulations that correlates the acceleration voltage, the type of substrate, and the intensity ratio of peaks of the substrate and the deposit to the coating thickness.…”