Figure 4 OCT imaging of transparent coating sample A at 1.3 μm. (a) Close-up en face optical image of the impact area taken with the OCT onboard camera. (b,c) OCT surface- and subsurface en face projection of the impact area, respectively. (d) Single B-scan across the impact area shown by the vertical dashed line in (b). (e) Superposition of 200 B-scans showing the patterns of cracks in the area between the horizontal dashed lines in (b). The scale bars indicate optical depth assuming of n = 1.
While imaging of subsurface cracks in the transparent coating system revealed some interesting structural patterns, it is not representative of the mechanical behavior of actual coatings with pigments and other additives. For this reason, sample B with an opaque coating system was subsequently tested using both the 1.3 μm and 4 μm OCT systems. A camera image of the impact region is shown in Fig. 5(a), while the corresponding OCT surface- and subsurface en face projections are shown in Figure 5(b,c) and Figure 5(e,f), respectively. The limited penetration of 1.3 μm light in such coatings is immediately apparent from the resulting B-scan in Figure 5(g). Apart from a variation in sample orientation, the 1.3 μm OCT subsurface projection is very similar to the camera image, while the corresponding XCT and 4 μm OCT images in Figure 5(d) and 5(f), respectively, show the presence of a cavity near the center of the impact crater marked by an arrow and denoted (1). Due to the difference in sample orientation, the entire 1.3 μm OCT volume was inspected, but no subsurface features could be seen. The XCT cross-section reveal that the void is located very close to the surface (around 113 μm), indicating that the apparent subsurface signal from the 1.3 μm OCT image is most likely from multiple scattering, giving a false impression of the degree of penetration. This is further corroborated by the visible shadowing effect from the cracks in the 1.3 μm OCT B-scan, while the 4 μm OCT B-scan show that the cracks are angled as seen in the XCT image.