Influence of Heat-treatment Temperature on Microstructure and Properties of BN Interfacial Coatings

BN interfacial coatings were prepared on the surface of SiC fibers by chemical vapor infiltration (CVI). Then the coatings were heat-treated at several temperatures higher than the deposition temperature. The microstructure and chemical compositions of BN interfacial coatings were analyzed by scanning electron microscope, transmission electron microscope, Raman spectra, and X-ray photoelectron spectroscopy, and the phase stability of the coatings were studied. The results show that when BN interfacial coatings deposited at 850 ℃ were heat-treated at 1050, 1150 and 1250 ℃, respectively, BN was ordered and amorphous BN was transformed into hexagonal crystallographic phase. Moreover, the crystallinity of BN increased with the increase of heat-treatment temperature. The amorphous BN coatings without high temperature heat-treatment were partly reacted with air to form B₂O₃. This kind of BN coatings had poor phase stability in the range of room temperature to 1200 ℃. The coatings with high temperature heat-treatment were almost composed of BN, and there was no phase transformation from room temperature to 1200 ℃.