TEM images collected from the matrix confirmed the presence of
ZrO2 and revealed that the SiO2 was crystalline, Fig. 3a, b. The grain
boundaries in these regions generally appeared to be free of any amorphous
phases, Fig. 3b, c. However, amorphous SiO2 was also found
trapped among ZrB2 and ZrO2 grains. In addition, the amorphous SiO2
also contained traces of nitrogen, Fig. 3d–f, probably deriving from
dissolution of some of the fiber coating into the glassy phase.