Effect of temperature and gaseous medium on the evolved microstructures of carbon fiber reinforced reaction bonded silicon nitride composites

Reaction bonded silicon nitride (RBSN) composites containing 10 vol% carbon fiber were prepared using the direct nitridation method at 1200, 1300 or 1400 °C by purging pure nitrogen or nitrogen-hydrogen (95% N2-5% H2) mixed gas. The effect of nitridation temperature and gaseous medium on the microstructure of RBSN were studied in detail. X-ray diffraction analysis on the resultant RBSN composites revealed a dominant α-Si3N4 phase along with β-Si3N4 and Si2N2O. The growth of silicon nitride whiskers was found to increase with the addition of carbon fibers. The microstructural analysis using TEM and SEM ascertained that there was no formation of beads at the tip of the whiskers. Hence, the growth of whiskers purely depends on gas phase reactions, i.e., it is based on vapor-solid (VS) and not vapor-liquid-solid (VLS) mechanism.