The fiber coating morphology in the composite was investigated by
TEM analysis in Fig. 4. The coating had a homogeneous thickness and
was composed of BN with scattered dark particles identified as Si3N4
and Si–C–B–N–O, Fig. 4c–e. High resolution TEM images of the interfaces
between coating and matrix, and between coating and fiber, are
reported in Fig. 5. It can be observed that the interface between coating
and matrix is sharp and clean, Fig. 5a–c, while the boundary between
fiber and coating was not clear and an amorphous phase seems to be
present, Fig. 5d–f. STEM analyses performed at the fiber-coating interface,
Fig. 6, confirmed that Zr had not diffused through the coating or
into the fiber. In contrast, Si and C were detected in the coating and oxygen
enrichmentwas observed in the fiber close to the coating, indicating
that even though new phases did not form, some diffusion of C had
occurred. Fig. 7 shows the microstructure of the fiber. Comparing the
center and the periphery, it is shown that SiC crystallites did not coarsen
notably as no significant differences in the mean grain size was measured.
However, a higher quantity of amorphous phase could be seen
in the center, Fig. 7a, c, as compared with the outer zone, Fig. 7b.