The effect of sintering additive on fracture behavior of carbon-whisker-reinforced silicon carbide composites

Hot-pressed silicon carbide composites reinforced with carbon fiber were prepared. Aluminum and yttrium oxides served as sintering additives and low-cost α phase SiC was used as starting powder, instead of the more expensive β-SiC. In the sintering process, the SiC-matrix grains grew larger via solution reprecipitation. Reaction of Al₂O₃/Y₂O₃ additives with SiO₂ on the surface of SiC or its oxidation products caused formation and distribution of a low-eutectic-point phase around the SiC grains and carbon whiskers. Such amorphous films can be found in triple-junctions and boundaries of SiC grains. Excess sintering additives improve the room-temperature flexural strength, but reduce the fracture toughness. Coupled with a higher sintering temperature, they contribute to the diffusion of yttrium ions into carbon fiber, and make the reaction layer thicker. Non-homogeneous amorphous inclusions between grains and whiskers are harmful for mechanical properties. A combination of grain bridging, crack deflection and whisker debonding can improve fracture toughness.