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Non-destructive evaluation of coatings using terahertz reflection spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) provides a non-contact, non-destructive method for evaluating different materials and their properties. This short review discusses the commonly used numerical models for the non-destructive estimation of thickness, refractive index, surface and interface roughness of paints, thermal barrier coatings, and polymer coatings using THz-TDS in the reflection geometry. To demonstrate the applicability of these models, we used paint layers on metallic substrates and extracted different paraments by fitting the experimental THz-TDS data. We conclude by discussing further steps to improve the efficiency of the fitting procedure used to extract the layer parameters.