Sintering of Si3N4 with Li-exchanged zeolite additive

This paper deals with the densification and phase transformation of Si 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 °C. Upon sintering at 1500 °C the bulk density increases to more than 95% of the theoretical density without phase transformation from α-S to β-Si , i. e. the phase transformation lags behind the densification process. Above 1500 °C the secondary phase is completely converted into a glass and the α-to-β 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.