Boron Nitride Interphase in Ceramic‐Matrix Composites

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Abstract
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°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 an annealing treatment: (i) the BN interphase 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.