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

Abstract The microstructure and mechanical properties of Si 3 N 4 –MoSi 2 composites doped with two different sintering additive systems, Y 2 O 3 –Al 2 O 3 and Lu 2 O 3 , were investigated. It was found that the composite doped with Y 2 O 3 –Al 2 O 3 had an amorphous grain boundary phase, while the grain boundary phase of the Lu 2 O 3 -doped composites was completely crystallized. The Si 3 N 4 –MoSi 2 composite containing Lu 2 O 3 had higher elastic modulus and better creep resistance at elevated temperatures (>1000 °C) than the composite doped with Y 2 O 3 –Al 2 O 3 . This is attributed to the crystallization and higher softening temperature of the Lu 2 O 3 -doped grain boundary phase compared with that doped with Y 2 O 3 –Al 2 O 3 . However, the toughness and strength were not influenced significantly by the grain boundary phase. The inclusion of MoSi 2 particles in Si 3 N 4 can improve their fracture toughness through residual stresses induced by the coefficient of thermal expansion mismatch of Si 3 N 4 and MoSi 2 . The strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi 2 phase.