The influence of the grain boundary phase on the mechanical properties of Si3N4–MoSi2 composites

The microstructure and mechanical properties of Si3N4–MoSi2 composites doped with two different sintering additive systems, Y2O3–Al2O3 and Lu2O3, were investigated. It was found that the composite doped with Y2O3–Al2O3 had an amorphous grain boundary phase, while the grain boundary phase of the Lu2O3-doped composites was completely crystallized. The Si3N4–MoSi2 composite containing Lu2O3 had higher elastic modulus and better creep resistance at elevated temperatures (>1000 °C) than the composite doped with Y2O3–Al2O3. This is attributed to the crystallization and higher softening temperature of the Lu2O3-doped grain boundary phase compared with that doped with Y2O3–Al2O3. However, the toughness and strength were not influenced significantly by the grain boundary phase. The inclusion of MoSi2 particles in Si3N4 can improve their fracture toughness through residual stresses induced by the coefficient of thermal expansion mismatch of Si3N4 and MoSi2. The strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi2 phase.