Formation and Microstructure of Silicide-Particle-Reinforced Si₃N₄ Composites with Crystallized Grain Boundary Phase of Yb₂Si₂O₇

Abstract

Silicide-particle-reinforced Si₃N₄ composites with a crystallized grain boundary phase of Yb₂Si₂O₇ were synthesized in-situ by hot-pressing Si₃N₄ with the metal oxides M_pO_q (silicide-forming oxides), which can react with Si₃N₄ to form silicide, and Yb₂O₃ as sintering additives. The reaction between Si₃N₄, silicide-forming oxide (Ta₂O₅ or MoO₃) and Yb₂O₃ at high temperatures generated silicide (Ta₃Si or Mo₅Si₃) particles and a grain boundary phase, Yb₂Si₂O₇, simultaneously. The silicide particles mainly existed at the grain boundaries, but a small amount of Ta₃Si particles were detected from Si₃N₄ grains. Ta₃Si particle grew up to a polyhedron shape, but Mo₅Si₃ particle to a spherical shape. To obtain the crystallized grain boundary phase of RE₂Si₂O₇, the molar ratio of Yb₂O₃ to M_pO_q should be adjusted to q/4 (q: the number of oxygen atoms in M_pO_q). However, because a small amount of oxygen was included in Si₃N₄ powder and existed on the surface of Si₃N₄ as SiO₂, the excess SiO₂ reacted with Si₃N₄ to generate a trace of Si₂N₂O grain. In the silicide-Yb₂Si₂O₇-Si₃N₄ composites, the grain boundary phases were crystallized, but thin amorphous films with a thickness of 1 nm were detected from the interfaces between the silicide particle, Si₃N₄ grain, and the grain boundary phase of Yb₂Si₂O₇. The dense silicide particles reinforced Si₃N₄ matrix composites can be obtained by using this in-situ synthesis method, and the flexural strength and fracture toughness of Ta₃Si-Yb₂Si₂O₇-Si₃N₄ composite were 1209 MPa, and 6.0 MPa•m^1/2, respectively.