Mo5Si3‐particle‐reinforced Si3N4‐matrix composites were fabricated by sintering molybdenum‐solution‐infiltrated porous Si3N4. Fine Mo5Si3 particles, with an average diameter of ∼0.13–0.17 μm, grew in situ from the reaction between MoO3 and Si3N4 in the grain boundary of the Si3N4. The Mo5Si3 particles resided in the grain‐boundary glassy phase and reinforced the grain boundaries. The four‐point flexural strength and fracture toughness of a 2.8 wt% Mo5Si3‐Si3N4 composite were 1060 MPa and 7.7 MPa·m1/2, respectively. This was higher than those for normally sintered Si3N4 by ∼17% and ∼18%, respectively. The fracture toughness of the Mo5Si3‐Si3N4 composite increased as the content of Mo5Si3 particles increased, but the flexural strength decreased. Improvement in fracture toughness was attributed to a thermal expansion mismatch among the Mo5Si3, the Si3N4, and the grain‐boundary amorphous phases in the Mo5Si3‐Si3N4 composites. Another reason for the improved fracture toughness was the pullout of elongated Si2N2O grains that formed as a result of oxygen gas released from the reaction between the molybdenum‐solution‐obtained MoO3 and the Si3N4. The infiltration method for incorporating desired elements or compounds into a ceramic matrix holds promise as a process for fabricating submicrometer‐ or nanometer‐sized composites with high strength and high fracture toughness.

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