Influence of pressure on chemical vapor deposition of boron nitride from BCl3/NH3/H2 gas mixtures.

Boron nitride (BN) was synthesized from BCl 3 /NH 3 /H 2 precursor mixtures via chemical vapor deposition, with a focus on the influence of the total partial pressure of BCl 3 , NH 3 , and H 2 (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 BCl 3 and NH 3 was mainly attributable to several intermediate gaseous products containing B, N, Cl, and H, such as Cl 2 BNH 2 , ClB(NH 2) 2 , and B(NH 2) 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. [ABSTRACT FROM AUTHOR]