ON A THERMODYNAMIC APPROACH TO THE CHEMICAL PHENOMENA INVOLVED IN THE FORMATION OF A BN INTERPHASE BY CVD CVI FROM A BF3-NH3 PRECURSOR

The chemical phenomena involved in the CVD/CVI of BN from BF3-NH3 mixtures are studied through thermodynamic calculations based on a computer minimization of the total Gibbs free energy of the system for 1123 < T < 1723, P = 0.1, 1 and 10 kPa, and 0.1 < alpha = [NH3]in./[BF3]in. < 15. Various substrate materials, i.e. a purely inert substrate, C, SiC, SiO2 and SiC-SiO2 thought to simulate the substrates onto the BN interphase is actually deposited in SiC (ex-PCS) fiber preforms, are considered. C and SiO2 are quasi-unreactive whereas SiC-based materials are reactive vs. compared with the gas phase. For the former, the BF3/BN conversion yield is low owing to the stability of BF3. 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 + Si3N4 (or/and Si2N2O) for the NH3-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 BF3-NH3 if the fiber surface is assumed to be coated uniformly with C, SiO2 or C + SiO2.