In-situ synthesis and evaluation of Mo5Si3 particle reinforced Si3N4 and Si2N2O composites

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 1850degreesC 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-So(4) 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 712 MPa, 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 6.9 MPa . m(1/2), similar to6% 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.