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.