Oxidation resistance of SiC/SiC micro and minicomposites with a highly crystallised BN interphase

Abstract

A three dimensionally ordered hex-BN is deposited by LPCVD from the BF3–NH3 system at relatively low temperature. This coating was studied in term of crystallisation by X-ray diffraction, and transmission electron microscopy. SiC/SiC microcomposites with such a BN interphase were produced in a first step. Their mechanical behaviour was determined under tensile loading at room temperature. They exhibited a wide non-linear stress–strain domain similar to that commonly observed during the damage of a ceramic matrix composite. Further, static fatigue tests at 700°C in air were carried out on a few microcomposites. The long lifetimes provide evidence that such a BN-based interphase brings a real improvement in the oxidation resistance. In a second step, from these latter results, minicomposites were prepared with a complex interphase consisting of a few layers of BN, which included a highly crystallised layer. Similar mechanical tests at room temperature and static fatigue tests at high temperature in air were performed. Despite the change of geometry (single fibre to a tow), damage capability and improved oxidation resistance are observed in comparison with minicomposites with a carbon interphase.