Effect of initial alpha-phase content on microstructure and mechanical properties of sintered silicon carbide

By using alpha- and beta-SiC starting powders with similar particle sizes, the effects of initial cu-phase content on the microstructure and the mecha nical properties of the liquid-phase-sintered and subsequently annealed mat erials were investigated. The microstructures developed were analyzed by im age analysis. When beta-SiC powder was used, the grains became elongated. T he average diameter decreased with increasing alpha-SiC content and the asp ect ratio showed a maximum at 10% alpha-SiC and decreased with increasing a lpha-SiC content in the starting powder. Such results suggest that microstr ucture can be controlled by changing alpha-phase content in starting powder s. The strength increased with increasing alpha-SiC content in the starting powder while the fracture toughness decreased with increasing alpha-SiC co ntent. There may be a trade-off in improving both the strength and toughnes s in SiC ceramics sintered with oxide additives.