Thickness Characterization of multi-layer coated steel by terahertz time-of-flight tomography

International audienceTerahertz time-of-flight tomography is employed to characterize the individual layers in a multilayer coating onsteel substrates. The multilayers, containing up to three layers, consist first of a cataphoretic layer, sealer, andvarnish progressing upward from the steel substrate. The individual layer thicknesses were reconstructed fromthe experimental data using autoregressive spectral extrapolation based on the modified covariance method (AR/MCM), which extrapolates the frequency components of the transfer function into the low signal-to-noise-ratio(SNR) regions from the high SNR regions. Due to the high sensitivity of AR/MCM to the presence of noises,wavelet denoising, in some cases, was needed to precondition the data prior to application of AR/MCM. Themethod is verified numerically and experimentally, showing that it can reconstruct individual layers in the 10-μm range, and good agreement is found between the thicknesses of individual layers extracted by AR/MCM andcharacterized by eddy-current measurements