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Boron nitride (BN) was synthesized from BCl3/NH3/H2 precursor mixtures via chemical vapor deposition, with a focus on the influence of the total partial pressure of BCl3, NH3, and H2 ( p BCl3+NH3+H2) on the surface deposition rate. The surface deposition rate of BN initially increased and then decreased with increasing p BCl3+NH3+H2, implying that the deposition process transitioned from surface reaction control to mass transfer control. BN deposition from BCl3 and NH3 was mainly attributable to several intermediate gaseous products containing B, N, Cl, and H, such as Cl2BNH2, ClB(NH2)2, and B(NH2)3. The microstructures of BN coatings deposited on SiC fibers were also analyzed. The BN coatings were uniformly and evenly deposited on the SiC fiber surfaces at low p BCl3+NH3+H2 values, whereas excessive p BCl3+NH3+H2 values afforded coatings containing large grains. The as-prepared BN coatings were stoichiometric but amorphous. Heat treatment substantially improved the texture and crystallinity to afford hexagonal BN.
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