Nondestructive measurement of mill-scale thickness on steel by terahertz time-of-flight tomography

Zhai, Min, Alexandre Locquet, Cyrielle Roquelet, Patrice Alexandre, Laurence Daheron, and D. S. Citrin. "Nondestructive measurement of mill-scale thickness on steel by terahertz time-of-flight tomography." Surface and Coatings Technology (2020): 125765.

Abstract

We measure in a nondestructive and noncontact fashion the thicknesses of three scale films with thicknesses 28.5 ± 1.4 μm, 13.4± 0.9 μm, and 5.1 ± 0.3 μm on steel substrates employing terahertz time-of-flight tomography combined with advanced signal-processing techniques. Wüstite is the dominant phase in the scale films, though magnetite and hematite are also present. Because wüstite is electrically insulating, the incident terahertz electromagnetic pulses largely penetrate into the scale film; however, the pulses are entirely reflected by the underlying electrically conductive steel substrate. Because the film layers are thin, in some cases optically thin, the distinct pulses reflected at the air/scale and scale/steel interfaces overlap in time and thus are not visually evident in the reflected terahertz signal, necessitating the use of deconvolution techniques to recover the sample structure. We compare the merits of three deconvolution techniques, one unsuccessful (frequency-wavelet domain deconvolution) and two successful (sparse deconvolution and autoregressive extrapolation), to characterize the thicknesses of these scale films.

for full paper see  https://www.sciencedirect.com/science/article/abs/pii/S0257897220304345

ExperimentalThe THz experiments were carried out in an air -conditioned laboratory at 22 o C with humidity < 48 %. Otherwise, the samples were stored in a dry box to retard the transformation of wüstite from transforming into magnetite. The experiments shown here were conducted within the 3 months of sample fabrication to ensure that the scale films were primarily composed of wüstite. The THz reflection measurements are taken on a TeraView TPS Spectra 3000, a pulsed, broadband THz time -domain spectroscopy (TDS) system, shown in Fig. 4.