Effects of yttrium on the sintering and microstructure of alumina-silicon carbide "nanocomposites"

Alumina and alumina-based "nanocomposites" with 2 and 5 vol% silicon carbide and varying amounts of yttria (0-1.5 wt%) have been prepared by pressureless sintering in the temperature range 1450 degrees-1650 degrees C. The effects of composition and sintering temperature on density and microstructure are reported. Yttria inhibited sintering in alumina, but enhanced the sinterability of the nanocomposites. It also induced abnormal grain growth in both alumina and nanocomposites, but strongly bimodal grain size distributions could be prevented by careful choice of the composition and the sintering temperature. Fully dense (> 99%), fine-grained alumina-5 vol% SiC-1.5 wt% yttria nanocomposites were produced from uniaxially pressed powders with a yttria content of 1.5 wt% and a sintering temperature of 1600 degrees C. Reasons for this behavior are discussed, and it is suggested that the enhancement of sintering in the alumina-SiC materials is because of the reaction of silica on the surface of the silicon carbide particles with alumina, yttria, and possibly magnesia, modifying the grain boundary composition, resulting in enhanced grain boundary diffusion. scanning transmission electron microscopy/energy-dispersive X-ray data show that such cosegregation does occur in the yttria-containing nanocomposites.