Abstract:

Boron nitride (BN) coatings were prepared using BCl₃-NH₃-H₂-N₂ precursor system by chemical vapor deposition process in a vertically-placed hot wall reactor. The effect of processing parameters on the deposition rates was analyzed. The morphology and microstructure of the BN coating on the surface of the silicon carbide fiber were characterized by scanning electron microscope (SEM), X-ray photoelectron spectroscope (XPS) and X-ray diffraction (XRD) . The dominant gas-surface reactions and crucial gas-phase components during BN deposition process were established. The results show that the deposition rates of BN increase gradually with the elevating deposition temperature from 600℃ to 850℃. And at a defined temperature, the deposition rates of BN gradually decrease along the gases flow direction in the deposition area, indicating that the gas-phase components are gradually consumed along the gases flow direction. The deposition rates of BN increase firstly and then decrease with the increase of system pressures, which suggests that the deposition process change from surface reactions control to mass transfer control. The BN deposition rates increase at 1-3 cm away from gases inlet, but decrease after increase firstly at 4-5 cm away from gases inlet as the residence time getting longer. SEM results show that BN coating on the surface of silicon carbide fiber is relatively smooth and dense. The chemical compositions of coating determined by XPS are BN and B₂O₃. XRD results indicate that the deposited BN at 650℃is amorphous. After heat-treatment at 1200℃ for 2 h, it transformed into the hexagonal BN(h-BN). The deposition of BN is achieved by the intermediate gas-phase species composed of Cl₂BNH₂, ClB(NH₂)₂ and B(NH₂)₃ which were generated by the reaction of BCl₃ and NH₃.

Key words: chemical vapor deposition, boron nitride, deposition rate, microstructures, deposition process