In-situ Synthesis and Evaluation of Mo₅Si₃ Particle Reinforced Si₃N₄ and Si₂N₂O Composites

Abstract

Mo₅Si₃ particle reinforced silicon nitride and silicon oxynitride composites were fabricated by sintering green compacts of silicon nitride, added with the molybdenum powder or a precursor for molybdenum oxide powder, at 1850°C in nitrogen atmosphere. During the sintering process, molybdenum powders reacted with Si₃N₄ to form the Mo₅Si₃ particles at grain boundary junctions in Si₃N₄, and MoO₃ reacted with Si₃N₄ to generate Mo₅Si₃ particles and Si₂N₂O grains simultaneously. However, because a small amount of oxygen was included in the molybdenum powder, a little amount of Si₂N₂O grain generated in the Mo₅Si₃-Si₃N₄ composite. The Si₂N₂O grains were needlelike crystals in Mo₅Si₃-Si₃N₄ composite, but the shape changed into the tabular in the Mo₅Si₃-Si₂N₂O composite. The flexural strengths of Mo₅Si₃-Si₃N₄ and Mo₅Si₃-Si₂N₂O composites improved slightly by incorporating the Mo₅Si₃ particles, and were 946 and 712MPa, respectively. Because of pulling-out of elongated Si₂N₂O grains during fracture and the effect of fracture toughening by internal stresses due to the Mo₅Si₃ and matrix thermal expansion coefficient mismatch, the fracture toughness of Mo₅Si₃-Si₃N₄ composite was 6.9MPa·m^1/2, -6% higher than that of normal Si₃N₄. However, the fracture toughness of the Mo₅Si₃-Si₂N₂O composite was affected by the tabular crystal Si₂N₂O which has a low value of fracture toughness, and reached the same valu as Si₂N₂O.