STEADY-STATE CREEP OF HOT-PRESSED SIC WHISKER-REINFORCED SILICON-NITRIDE

Constant compressive stress creep experiments have been conducted in nitrogen on hot-pressed silicon nitride containing, as sintering aids, 6 wt% Y2O3 and 1·5 wt% Al2O3, in addition to 0, 20 and 30 vol.% of SiC whiskers derived from rice hulls. The temperature and stress ranges of these studies were 1470-1720 K and 50-350 MPa, respectively. All materials showed a change in thermally-activated creep behavior above 1570 K. For example, under a constant stress of 300 MPa, the value of the activation energy for the 20 vol.% material increased from 403 kJ/mol to 715 kJ/mol for the temperature ranges 1470-1570 K and 1570-1670 K, respectively. In the higher temperature range both the 20 and 30 vol.% composites deformed at creep rates equal to or lower than that of the unreinforced material. This phenomenon, however, was reversed in the low temperature range where the composites consistently crept at higher than the unreinforced material. Under all conditions, the 30 vol.% material crept at a lower rate than the 20 vol.% material. In addition, the steady-state creep rate versus log stress curves for the composites showed a change in slope at 230 MPa. Below this stress, the composites showed a stress exponent of 0·4-1. Above 230 MPa, the stress exponent increased to 1·5-2·5, values which were approximately equal to those for the unreinforced material over the whole stress range. Kinetic data, coupled with the results of transmission electron microscopy, indicate that grain boundary sliding by viscous flow was the primary mechanism of steady-state creep in these materials. Viscous flow was limited by extensive crystallization of the grain boundary amorphous phase. In the low temperature régime, grain boundary sliding was controlled by diffusion through the glass phase. The higher creep rates in the composite were caused. © 1989.