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Si 3 N 4  ceramics have been densified in air atmosphere furnace at low temperatures of 1600∼1650°C with Li 2 O-Al 2 O 3 sintering additives. Molar ratio of 0.4∼1.6:1 of Li 2 O and Al 2 O 3  were used to study the effects of additives on sintering behavior of Si 3 N 4  materials. It is found that the densities of sintered samples increased and then decreased with the increase of molar ratio accordingly. Highest densities in excess of 95%oftheoretical are obtained with the ratio of 0.7:1. Densification process is accompanied by phase transformation. Therefore, the primarily phases in sintered specimens areβ-Si 3 N 4 and α-Si 3 N 4 , with a small amount of Si 2 N 2 O existed because of minor oxidation of the specimens during sintering. However, relatively low α- to βSi 3 N 4  transformation (∼63%β-Si 3 N 4 ) took place in the present densification conditions. The microstructures of the sintered samples exhibited typical β-Si 3 N 4  elongated grain morphologies with small amount of homogeneously distributed residual pores. The sintered Si 3 N 4  materials have the hardness of 14GPa and fracture toughness of 5.08MPa·m 1/2 , indicating a potential method for low-cost processing of Si 3 N 4 -basedceramics.

Sintering Behavior of Si3N4 Ceramics at Low Temperature in Air Atmosphere Furnace