Effects of moisture on tensile stress rupture behavior of a SiC/SiC composite at elevated temperatures

This study characterized the moisture and temperature effects on the stress rupture behavior of ceramic matrix composite (CMC), Syl-iBN/BN/SiC, which was composed of Sylramic (Syl) fibers with an in situ layer of boron nitride (iBN), boron nitride interphase (BN) and silicon carbide (SiC) matrix. For this purpose, stress rupture tests were conducted under 100% steam and laboratory air environments at three temperatures, 400 °C, 750 °C, and 950 °C to develop stress versus time to failure relationships. At a given stress level, time to failure decreased with increase of the test temperature and moisture. However, the stress rupture strength at 750 °C under both laboratory air and steam conditions was relatively low than the expected degradation based on the interpolation from those at 400–950 °C. Similar characteristics were observed in residual strength of the corresponding specimens which survived 100 h and also in the damage mechanisms. The degradation of the BN interphase between the fiber and the matrix was the primary damage which was relatively more in the steam environment than that in air laboratory. This damage was in the form of fracture, recession and loss of BN interphase at 400 °C or 950 °C while there was pesting of fibers due to formation of borosilicate causing relatively more embrittlement of the tested CMC system at 750 °C.