Noncontact Thickness Measurement of Multilayer Coatings on Metallic Substrate Using Pulsed Terahertz Technology

In this work, an improved model-based method is proposed to infer the thickness of up to four layers of coatings on metallic substrates using reflected terahertz pulse echoes. Firstly, an analytical model is formulated based on Rouard method. Considering that the surface and the interfaces of coatings are basically rough rather than smooth, the roughness of surface and interfaces is taken into account with Kirchhoff approximation due to scattering when developing the model. The samples with multilayer coatings are prepared by stacking paper sheets on a metallic substrate. Through simulations, it is found that the reflected terahertz signals decrease significantly due to the scattering with an increase of surface roughness, whereas the interface roughness has slight effect on the reflected terahertz signals. Finally, experiments are carried out to validate the formulated model and the proposed improved method. By considering the effect of surface and interfaces roughness, the residuals reduce obviously and the measurement errors decrease to 1% or less.