Effects of in-situ BN coating on thermostructural properties of SiCf/SiC composites prepared via PIP process

SiCf/SiC composites were fabricated by a novel two-stage precursor infiltration and pyrolysis (PIP) process. The two types of SiC fibers (C3-BN and C3-BN-1800) were used as reinforcements. The C3-BN was the third-generation SiC fibers coated with an in-situ boron nitride (BN) layer, C3-BN-1800 was obtained by a 1 min continuous pyrolysis of C3-BN at 1800 degrees C. In stage I, the SiCf/SiC composites (CMC-Is) achieved initial densification by low-temperature PIP process, while the matrix consisted of low crystallinity SiC and amorphous SiOxCy. At 1600 degrees C-1 h, the crystallization of SiC matrix and decomposition of SiOxCy led to a significant reduction in the strength of the CMC-Is (30% strength retention). Through several cycles of high-temperature PIP process (stage II), the SiCf/SiC composites (CMC-IIs) had higher density than CMC-Is, and the matrix had high crystallinity SiC and significantly decreased oxygen content. Thus the CMC-IIs have better thermal resistance and the strength retention is near 50% at 1600 degrees C-10 h.