Effect of raw-Si particle size on the properties of sintered reaction-bonded silicon nitride

The effect of raw-Si particle size on the properties of sintered reaction-bonded silicon nitride (sintered RBSN) was investigated by the use of Si powders with different particle sizes containing various native SiO2 oxide contents. Different secondary phases were formed in each specimen reaction-sintered with different particle sizes due to the content difference in native oxide on the surface layer of particles. The specimens prepared by using coarse powders did not show high density at high-temperature gas-pressure sintering, because of the abnormal growth of elongated β-Si3N4 grains owing to the insufficiency and the inhomogeneous distribution of the liquid phase. The specimens made with fine powders shows high density because of the melting of the secondary oxynitride phase. As a result, the content and distribution of the liquid phase became suitable for complete densification, which resulted in density increase. Higher values of fracture strength were obtained for the specimens made by using fine powders, however, higher values of fracture toughness were obtained when large elongated grains were developed in a fine grained matrix. The effects of sintering additives on the densification behavior of RBSN prepared by using coarse powders were also investigated. Densification near theoretical density was attained by using sintering additives, such as 6 wt.% Y2O3+3 wt.% Al2O3+2 wt.% SiO2 (6Y3A2S) and 9 wt.% Y2O3+1.5 wt.% Al2O3+3 wt.% SiO2 (9Y1.5A3S). In the case of 6Y3A2S addition, high fracture strength of 960 MPa and fracture toughness of 6.5 MPa m1/2 were obtained.