Stress Rupture in Tension and Flexure of a SiC-Whisker-Reinforced Silicon Nitride Composite at Elevated Temperatures

Stress rupture of a 20 vol% SiC whisker-reinforced Si₃N₄ composite processed by gas pressure sintering was investigated by both tension and flexure methods. The stress exponents for the stress rupture decrease with increasing temperature. The fracture surfaces of both tensile and flexural stress rupture at 1000°C consist of mirror, mist, and hackle regions. The size of the mirror region increases with decreasing stress. Crack propagation is a mixture of intergranular and transgranular modes at 1000°C. Both tensile and flexural fracture surfaces under constant stress at 1200°C were characterized by a rough zone and a mirror zone; the size of the rough zone increased with decreasing stress. Creep crack growth occurred at 1200°C, which is a process of cavity nucleation, growth, and interlinkage in front of a crack. The transition of fracture mechanisms with temperature is discussed.