Degradation of boron nitride interfacial coatings fabricated by chemical vapor infiltration on SiC fibers under ambient air/room temperature conditions

Hexagonal boron nitride (h-BN) interfacial coatings were deposited on SiC fibers by chemical vapor infiltration (CVI) and their degradation behavior under ambient air/room temperature conditions was studied with time. Degradation of the interfacial coatings with time was investigated by characterizing the morphology and microstructure of these materials with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Thermogravimetry coupled with differential thermal analysis (TG-DTA) and X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical reactions and phase transitions taking place in the BN coatings. The results showed that the as-deposited BN interfacial coatings fabricated by CVI were compact and well bonded to the SiC fibers. BN coatings remained relatively stable under ambient air/room temperature conditions for 50 h, while severe degradation was observed after 500 h of exposure. The degradation of BN interfacial coatings was mainly caused by two factors, namely, reaction with atmospheric air to produce boric oxide and amorphization of the hexagonal structure. The degradation observed under ambient air/room temperature might be due to incomplete crystallinity of BN interfacial coatings. Presence of water vapor may accelerate degradation of the coatings. The results of this degradation test can be used as a reference for the storage of BN coatings fabricated by CVI.