Abstract
Near- and mid-infrared optical coherence tomography (OCT) is evaluated as a non-destructive and non-contact reflection imaging modality for inspection of industrial and marine coatings. Near-infrared OCT was used to obtain high-resolution images (~6/2 µm lateral/axial) of hidden subsurface cracks and defects in a resin base coating, which had been exposed to high pressure and high temperature to study coating degradation in hostile environments. Mid-infrared OCT was employed for high-resolution (~15/8.5 µm lateral/axial) subsurface inspection of highly scattering marine coatings, demonstrating monitoring of wet film thickness and particle dispersion during curing of a 210 µm layer of antifouling coating, and detection of substrate corrosion through 369 µm of high-gloss alkyd enamel. Combining high-resolution and fast, non-invasive scanning, OCT is therefore considered a promising tool for studying coating performance and for industrial inspection.
Highlights
Coatings science is in constant development to meet the increasingly demanding requirements of the industry
Figur f are optical coherence tomography (OCT) cross-sectional images (B-scans) of the exposed sample taken along the dire of the white arrowheads across the 3 mm length of the scanned area6. oTfh14e vertical bars show the approximate scale in the axial direction, indicating a film thickness va between ~140 and180 μm, assuming a refractive index of n = 1.5
MIR OCT was employed to monitor the decrease in wet film thickness (WFT) and particle dispersion during the curing of a 210 μm layer of antifouling hull coating, which is not possible using traditional non-contact techniques, such as profilometry
Summary
Coatings science is in constant development to meet the increasingly demanding requirements of the industry. It is essential to implement quality control steps for the inspection and testing of coatings at all stages during development and deployment. A common practice is the holiday testing, in which deviation in electrical conductivity due to coating defects is used to, for example, identify improper coating of the substrate resulting in pinholes. Such methods are simple and effective for locating defects but are time-consuming and provide no information about the type or structure of the defect. In 2009, Alig et al demonstrated detection of subsurface defects, such as cracks, bubbles, and delamination, in 20–130 μm thick polyurethane coatings on steel substrate with a lateral resolution of about 10–20 μm using 100–150 MHz transducer frequency [4]. The contact medium can permeate pores and cracks in the coating, potentially causing structural or even chemical changes
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