Effect of Si_3N_4 Addition on Compressive Creep Behavior of Hot-Pressed ZrB_2-SiC Composites

摘要

Compressive creep studies have been carried out on hot-pressed ZrB_2-SiC (ZS) and ZrB_2-SiC-Si_3N_4 (ZSS) composites in air under stress and temperature ranges of 93-140 MPa and 1300℃-1425℃, respectively for time durations of ≈20-40 h. The results of these studies have shown the creep resistance of ZS composite to be greater than that of ZSS. As the temperature is increased from 1300℃ to 1425℃, the stress exponent of ZS decreases from 1.7 to 1.1, whereas that of ZSS drops from 1.6 to 0.6. The activation energies for these composites have been found as ≈95 ± 32 kJ/mol at temperatures ≤1350℃, and as ≈470 ± 20 kJ/mol in the range of 1350℃-1425℃. Studies of the postcreep microstructures using scanning and transmission electron microscopy have shown the presence of glassy film with cracks at both ZrB_2 grain boundaries and ZrB_2-SiC interfaces. These results along with calculated values of activation volumes suggest grain-boundary sliding as the major damage mechanism, which is controlled by O~(2-) diffusion through SiO_2 at ≤1350℃, and by viscoplastic flow of the glassy interfacial film at temperatures ≥1350℃. Studies by transmission electron microscopy have shown formation of crystalline precipitates of Si_2N_2O near ZrB_2-SiC interfaces in ZSS tested at ≥1400℃, which along with stress exponent values ＜1 suggests that grain-boundary sliding involving solution-precipitation-type mechanism is operative at these temperatures.