BORON-NITRIDE INTERPHASE IN CERAMIC-MATRIX COMPOSITES

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF3-NH3, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000-degrees-C. In a second step, the BN-treated preform was densified with SiC deposited from CH3SiCl3-H2 at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF3-NH3 gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as annealing treatment: (i) the BN inter-phase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO2/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO2 interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.