Microstructural characterization of a creep-deformed SiC whisker-reinforced Si₃N₄

Constant compressive stress-creep experiments have been conducted in the temperature and stress ranges of 1470-1720 K and 50-350 MPa, respectively, on a 30 vol% SiC whisker-reinforced Si₃N₄ composite containing sintering aids of 1.5 wt% Al₂O₃ and 6.0 wt% Y₂O₃. This work focuses on two independent creep experiments conducted at 1470 and 1620 K, both in the stress range 50-350 MPa. At 1620 K, the stress exponent, n, exhibited a bilinear behavior with n<1 in the low-stress regime and n>1 in the high-stress regime. This behavior was not observed at 1470 K. Various electron microscopy and X-ray diffraction techniques have been used to completely characterize the microstructure of the as-received, annealed, and crept materials. Transmission electron microscopy observations on samples taken before and after the change in n at 1620 K were compared to observations on material crept at 1470 K to clarify the microstructural phenomena controlling the break in the creep curve. Interfaces between Si₃N₄ grains and the whisker reinforcement were examined using high-resolution electron microscopy. The relationships between observed microstructural changes and high-temperature mechanical properties are presented.