Sintering behaviour of gel-derived powders

85Al2O3-15ZrO2 (wt%) powders were synthesized by gel precipitation starting from AlCl3 · 6H2O and ZrCl4 solutions and dried by two different methods: (i) by spray-drying and (ii) via a sol-gel route by n-octanol in a pilot plant. The particles by process (i) were spherical granules of diameter 15 μm, and those by process (ii) were microspheres of 25 μm. The powders were characterized in terms of morphology, particle size distribution, surface area, weight loss and crystallization behaviour at different temperatures. Shrinkage, microstructure, density and pore size distribution were evaluated on compacts at different temperatures to study the sintering kinetics. Experimental observations suggest that (a) the use of controlled-geometry powders allows one to obtain high green densities; and (b) the sintering of gel-derived powders develops in two steps: during the first one, sintering mainly takes place inside each microsphere (or granule), and during the second step, mainly between the microspheres (or granules). At temperatures > 1100°C, sintering produces shrinkage of the microspheres (or granules) leading to pore formation between them, which prevents the achievement of high densities. Only by using hot pressing is it possible to obtain theoretical densities and high mechanical properties.