Effects of intergranular phase chemistry on the microstructure and mechanical properties of silicon carbide ceramics densified with rare-earth oxide and alumina additions

Based on the processing strategy of improving the mechanical properties of liquid-phase-sintered materials by modifying the secondary phase chemistry, four rare-earth oxides (RE2O3, RE = La, Nd, Y, and Yb), in combination with alumina, were used as sintering aids for a submicrometer-size beta -SIC powder. Doped with 5 vol% RE2O3 + Al2O3 additives, all specimens were hot-pressed-to near full-densities at 1800 degreesC, and they exhibited similar microstructures and grain Size distributions. The SiC grains in all specimens revealed a core-rim structure after being plasma-etched, indicating that they were densified via the same solution-reprecipitation mechanism. It was found that a decrease in the cationic radius of the rare-earth oxides was accompanied by an increase in Young's modulus, hardness, and flexural strength of the SiC ceramics, whereas the fracture toughness was improved by incorporating rare-earth oxides of larger cationic radius. The changes in the mechanical properties were attributed to the difference in the chemistry of the intergranular phases in the four ceramics.