HIGH-TEMPERATURE DEFORMATION OF SIC-WHISKER-REINFORCED MGO-PSZ MULLITE COMPOSITES

The effect of 33.5 vol% SIC whisker loading on high-temperature deform ation of 1 wt% MgO-38.5 wt% zirconia-mullite composites was studied be tween 1300 degrees and 1400 degrees C. At strain rates of 10(-6) to 5 x 10(-4)/s the creep resistance of zirconia-mullite composites without SIC reinforcement was inferior to monolithic mullite of similar grain size, Analysis of the results suggested that the decreased creep resi stance of mullite-zirconia composites compared to pure mullite could b e at least partially explained by mechanical effects of the weaker zir conia phase, increased effective diffusivity of mullite by zirconia ad dition, and to the differences in mullite grain morphology, With SLC w hisker reinforcement, the deformation rate at high stress was nearly t he same as that of the unreinforced material, but at low stress the cr eep rates of the SiC-reinforced material were significantly lowered, T he stress dependence of the creep rate of unreinforced material sugges ted that diffusional creep was the operative mechanism, while the rein forced material behaved as if a threshold stress for creep existed, Th e threshold stress could be rationalized based on a whisker network mo del. This was supported by data on other whisker-containing materials; however, the threshold stress had a temperature dependence that was o rders of magnitude higher than the elastic constants, leaving the phys ical model incomplete, The effects of residual stresses and amorphous phases at whisker/matrix interfaces are invoked to help complete the p hysical model for creep threshold stress.