High-Precision Thickness Detection of Coatings Based on Terahertz Propagation Simulation Model

In this study, to address the problem of inseparability of echoes from the upper and lower surfaces of a thin coating during terahertz nondestructive thickness testing, the optical parameters of the common top coating were measured in the frequency range of 0.2-7 THz. According to spectral characteristics in the range of 0.2-2 THz, constant models of refractive index and extinction coefficient were used to establish the reflecting terahertz wave propagation simulation model. The difference equations between the simulation waveforms and the actual terahertz testing waveforms were taken as the objective function, which were optimized by constructing a least squares thickness optimization method and introducing a distance parameter between the terahertz transmitter and the sample. Samples with different step thicknesses of the top coating were designed for nondestructive thickness testing. The experimental results reveal that the proposed method considerably enhance the accuracy of quantitative nondestructive thickness testing. For the top coating, the accuracy of thickness testing reach 7.9 mu m, which is two orders of magnitude compared existing methods. The introduction of the distance parameters enables the visual analysis of the distance distribution between the terahertz transmitter and the sample during the testing process.