Sintering of Si3N4 with Li-exchanged zeolite additive

Abstract

This paper deals with the densification and phase transformation of Si3N4 with additives of Li-exchanged zeolite during pressureless sintering at significantly reduced temperatures. Dilatometric shrinkage data show that the first liquid forms as low as 1080 degrees C. Upon sintering at 1500 degrees C the bulk density increases to more than 95% of the theoretical density without phase transformation from alpha-S3N4 to beta-Si3N4, i.e. the phase transformation lags behind the densification process. Above 1500 degrees C the secondary phase is completely converted into a glass and the alpha-to-beta transformation takes place. Under these conditions the grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. The results show that a very effective low-temperature sintering additive for silicon nitride can be obtained from Li-exchanged zeolite.