Uniformization of boron nitride coating thickness by continuous chemical vapor deposition process for interphase of SiC/SiC composites

Boron Nitride (BN) coating on sintered SiC fibers (Tyranno SA) by a continuous chemical vapor deposition (CVD) process was developed with the ultimate goal of controlling the interface in SiC/SiC composites. BN coating by a continuous CVD process was conducted at 1580degrees C under a pressure of approximately 1.3 x 10(2) Pa. Reactant gases were NH3 and BCl3, both diluted by Ar; the yarn speed was 4.5 mm/s. Conventional experiments revealed that the BN coating thickness across the tow varied significantly. Hence, the CVD process was modified not only by adjusting the amounts of reactant gases, but also by reducing the yarn tension and setting a new furnace for removing the sizing agent of the fiber tow. The BN coating thickness obtained by the modified process was significantly more uniform than that achieved by a conventional process. The BN coating had a layered structure; it was nearly stoichiometric and pure, with little O or C. The BN-coated SA fiber retained 90% of the strength of the as-received SA fiber. Moreover, a minicomposite reinforced with a BN-coated SA fiber of 1600 filaments (fiber volume fraction: 10.8%) was fabricated by a polymer infiltration and pyrolysis (PIP) method. This minicomposite exhibited a higher fracture strength (199 MPa) at room temperature and a higher retention ratio (82%) of the fracture strength at 1400 degreesC in air as compared to a minicomposite obtained by a conventional process (173 MPa and 73%).