Effect of Y2O3-Al2O3 additive on the phase formation and densification process of in situ SiC-BN composite

The effect of the addition of Y2O3-Al2O3 additive on the phase formation and densification process of in situ SiC-BN composite has been investigated. The in situ reaction started at 1450 degreesC despite the addition of Y2O3-Al2O3 additive. However, when Y2O3-Al2O3 was added the reaction was much faster and completed after hot pressing at 1700 degreesC. In addition, dense material with well-crystallized SiC and EN grains could be obtained. It was found that denitrification occurred during the reaction process for both specimens with and without Y2O3-Al2O3 additive. However, denitrification occurred continuously at temperatures higher than 1700 degreesC for the specimen without Y2O3-Al2O3 additive and eventually resulted in decrease of nitrogen content. The above results were explained based on the following mechanism, that is, the Y2O3-Al2O3 additive formed a Y-Al-SI-O liquid phase with SiO2 present in Si3N4 raw powder, enhancing diffusion of the elements of the reactants.