Mo
5Si
3 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 Si
3N
4 to form the Mo
5Si
3 particles at grain boundary junctions in Si
3N
4, and MoO
3 reacted with Si
3N
4 to generate Mo
5Si
3 particles and Si
2N
2O grains simultaneously. However, because a small amount of oxygen was included in the molybdenum powder, a little amount of Si
2N
2O grain generated in the Mo
5Si
3-Si
3N
4 composite. The Si
2N
2O grains were needlelike crystals in Mo
5Si
3-Si
3N
4 composite, but the shape changed into the tabular in the Mo
5Si
3-Si
2N
2O composite. The flexural strengths of Mo
5Si
3-Si
3N
4 and Mo
5Si
3-Si
2N
2O composites improved slightly by incorporating the Mo
5Si
3 particles, and were 946 and 712MPa, respectively. Because of pulling-out of elongated Si
2N
2O grains during fracture and the effect of fracture toughening by internal stresses due to the Mo
5Si
3 and matrix thermal expansion coefficient mismatch, the fracture toughness of Mo
5Si
3-Si
3N
4 composite was 6.9MPa·m
1/2, -6% higher than that of normal Si
3N
4. However, the fracture toughness of the Mo
5Si
3-Si
2N
2O composite was affected by the tabular crystal Si
2N
2O which has a low value of fracture toughness, and reached the same valu as Si
2N
2O.
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