Effect of rare-earth oxides on properties of silicon nitride obtained by normal sintering and sinter-HIP

This paper presented the microstructure, mechanical properties and oxidation behavior of silicon nitride obtained by both conventional sintering and sintering followed by hot isostatic pressing (HIP). Silicon nitride with additives such as 5 wt.% Al2O3 and 5 wt.% Ln2O3 (Ln= La, La concentrate, Gd or La+Gd) were studied. The results revealed that Gd2O3 additions increased the formation of elongated grains of β-Si3N4, the fracture toughness and oxidation resistance. La2O3 additions led to higher densification and hardness values, while addition of La2O3 concentrate promoted the formation of materials with intermediate properties, compared to the other studied compositions. Hot isostatic pressing increased the hardness but decreased the fracture toughness of the material, mainly because it allowed residual pores to close and also reduced the average aspect ratio of β-Si3N4 grains.