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).