Customized Wavelet Basis Enabled Novel Terahertz Interpretable Thickness Measurement method of Thin Thermal Barrier Coatings

doi:10.3901/JME.2025.24.012

Abstract

Abstract: The thickness of topcoat in thin thermal barrier coating is much smaller than the centre wavelength of the terahertz (THz) system, generating seriously overlapped THz signals. The time of flight and refractive index are difficult to be extracted accurately, which increases the error of thickness measurement. To this end, a novel method is proposed for THz interpretable thickness measurement of thin thermal barrier coatings empowered by wavelet bases, and a customized wavelet de-aliasing layer is innovatively designed to autonomously construct wavelet bases with similar characteristics of the peaks, followed by separating overlapped THz signals and enabling the interpretable network structure to accurately extract time-of-flight and refractive index for thickness measurement. Firstly, an analytical model of THz signals considering the structure of thermal barrier coating is established to explore the shape characteristic of peak as a template for selecting preferred wavelet family for signal de-aliasing. Secondly, a customized Gaussian wavelet de-aliasing layer is developed to autonomously construct a wavelet basis function which is almost identical with the first two peaks to accurately separate the peaks of THz signals. A sparse layer is established to select the key information of peaks and improve the agreement between the simulated training set from analytical model and the experimental test set. Then, a physically interpretable measurement module is constructed to extract time-of-flight and refractive index from the time and frequency domain features, and these two results are connected by a division layer to solve topcoat thickness. Finally, thermal barrier coatings were prepared and THz experiments were carried out. The results show that the correlation coefficient between the proposed customized Gaussian wavelet basis and THz peaks exceeds 0.93, and the proposed method is superior to six existing thickness measurement methods, with the maximum thickness error of less than 5 μm and the time consumed being 8.2 ms.

Key words: thermal barrier coating, terahertz nondestructive testing, customized wavelet basis, analytical model, interpretable neural network

CLC Number:

V263

SUN Fengshan, FAN Mengbao, CAO Binghua, YE Bo. Customized Wavelet Basis Enabled Novel Terahertz Interpretable Thickness Measurement method of Thin Thermal Barrier Coatings[J]. Journal of Mechanical Engineering, 2025, 61(24): 12-27.

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