Abstract
Fracture toughness tests were carried out on HIP-sintered silicon nitride at elevated temperatures. The fracture toughness gradually decreased with an increase in temperature up to 1200°C. In the temperature range from 1200°C to 1275°C, a steel-like brittle-to-ductile transition of fracture toughness was observed: the fracture toughness rapidly increased from 4MPa√m to 8MPa√m. Above these temperatures, stable crack growth was observed. From the detailed observation of the stable crack growth region, β-Si3N4 particles were found to be oriented in the tensile direction. The stable crack growth occurred accompanying with bridging and pulling out of β-Si3N4 particles. The transition behavior of fracture toughness results from the transition of fracture mechanism from the stresscontrolled intergranular fracture to the pulling-out of β-Si3N4 particles with shear deformation of intergranular glass phase. The transition temperature was thought to be consistent with the softening point of the intergranular glass phase. The same transition behavior of fracture mechanism was found in the bending strength test.
