Figure 7 (a,b) Microscope- and OCT cross-sections of monolayer sample B, respectively (not the same position). (c,d) Microscope- and OCT cross-sections of multilayer sample C, respectively (not the same position). (e) Line scan average of the two samples showing difference in scattering properties of the coating layers. The two traces represent an averaged over ten adjacent A-scans from ten consecutive B-scans, and are aligned to the strong surface reflection (0 μm OPD). Dashed arrows in (b) and (d) indicate the line scan positions.
Figure 8 shows imaging of sample D, which had a thin PU coating deposited directly on top of the composite. The sample was cut from a larger RET blade, which appear to have initiated the formation of cracks near the edge of the cut. From the microscope image of the cut cross-section in Fig. 8(a), a bright white band clearly stands out from the rest of the composite just below the coating (marked with an arrow). Figures 8(b,e) show the surface topography and cross section along the edge of the sample, respectively. On the surface, no defects are visible, but in the cross section, two distinct features can be seen near the edge. It is clear that the features are located in the composite, since the top coating is clearly delineated. The features marked “c” and “d” are also visualized as volume projections in Fig. 8(c) and Fig. 8(d), respectively. The cross section and volume projections together reveal that the features are not only separated in depth, but also in the lateral dimension, where “d” exactly outlines the contours of “c”. This can also be seen from the 90° rotated cross sections in Fig. 8(f,g), which represent a superposition of 10 and 20 B-scans, respectively, at the locations indicated by blue dashed lines in Fig. 8(c,d), respectively. Fig. 8(f) furthermore show that the crack at one point extend all the way to the surface through the top coating, which would likely lead to catastrophic delamination if exposed to harsh environments. A similar feature is seen at the black arrow in Fig. 8(a).