On a thermodynamic approach to the chemical phenomena involved in the formation of a BN interphase by CVD/CVI from a BF3-NH3 precursor

Abstract The chemical phenomena involved in the CVD/CVI of BN from BF 3 -NH 3 mixtures are studied through thermodynamic calculations based on a computer minimization of the total Gibbs free energy of the system for 1123 T P =0.1, 1 and 10 kPa, and 0.1 α = [NH 3 ] in /[BF 3 ] in 2 and SiC-SiO 2 thought to simulate the substrates onto the BN interphase is actually deposited in SiC (ex-PCS) fiber preforms, are considered. C and SiO 2 are quasiunreactive whereas SiC-based materials are reactive vs. compared with the gas phase. For the former, the BF 3 /BN conversion yield is low owing to the stability of BF 3 . For the latter, it is almost equal to 100% owing to chemical attack of the substrate, but the deposit is a mixture of BN+C and even of BN+C+Si 3 N 4 (or/and Si 2 N 2 O) for the NH 3 rich precursors. The effects of temperature and pressure on the BN yield and substrate attack are studied as well as that of a contamination of the gas phase by oxygen. It is foreseen that a preform made of SiC (ex-PCS) fibers will not be significantly damaged either chemically or mechanically during the infiltration of a BN interphase from BF 3 -NH 3 if the fiber surface is assumed to be coated uniformly with C, SiO 2 or C + SiO 2 .