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

• Published in: Surface & coatings technology 2020-07, Vol.393, p.125765, Article 125765
• Main Authors: Zhai, Min, Locquet, Alexandre, Roquelet, Cyrielle, Alexandre, Patrice, Daheron, Laurence, Citrin, D.S.
• Format: Article
• Language: English
• Subjects: Coated metal; Deconvolution; Electromagnetic pulses; Engineering Sciences; Hematite; Nondestructive evaluation; Nondestructive testing; Oxide coatings; Signal processing; Substrates; Terahertz frequencies; Terahertz imaging; Terahertz reflectometry; Thickness; Thin films; Tomography; Wustite
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2020.125765

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. •Terahertz reflectometry is used to measure mill-scale thickness on steel substrates.•THz reflectometry is nondestructive and contactless.•Axial superresolution achieved by advanced signal processing.•Thinnest film thickness measured is 5 μm.