Tensile Properties and Microstructural Characterization of Hi-Nicalon Sic/Rbsn Composites

The room temperature physical and mechanical properties of silicon carbide fiber-reinforced reaction-bonded silicon nitride matrix composites (SiC/RBSN) were measured, and the composite microstructure was analyzed. The composites consist of nearly 24 vol% of aligned Hi-Nicalon SiC fiber yarns in a approx. 30 vo1% porous silicon nitride matrix. The fiber yarns were coated by chemical vapor deposition with a 0.8 micron layer of boron nitride (BN) followed by a 0.2 micron layer of SiC. In the as-fabricated condition, both 1-D and 2-D composites exhibited high strength and graceful failure, and showed improved properties when compared with unreinforced matrix of comparable density. No indication of reaction between the SiC fiber and BN coating was noticed, but the outer SiC layer reacted locally with the nitridation enhancing additive in the RBSN matrix. A comparison is made between the predicted and measured values of matrix cracking strength.