Dense Amorphous Zirconia–Alumina by Low-Temperature Consolidation of Spray-Pyrolyzed Powders

Metastable amorphous ZrO₂-Al₂O₃ powders were hot-pressed at low temperatures (873 and 923 K), under moderately high pressures (500 and 750 MPa), and amorphous pellets with 1%-8% porosity were obtained. Crystallization of the amorphous pieces in the temperature range of 1173–1673 K produced a range of ultrafine microstructures, the finest of which had grains of tetragonal (ZrO₂-40-mol%-Al₂O₃) solid solution 6–8 nm in size that formed at 1173 K. Submicrometer grain sizes of the equilibrium monoclinic ZrO₂ and alpha-Al₂O₃ were stable against coarsening at 1673 K. The new technique was applied to produce a SiC-reinforced composite with an amorphous ZrO₂-80-mol%-Al₂O₃ matrix; the high matrix sinterability overcame the reinforcement constraint. The results suggest a possible solution to the difficulties in the bulk processing of amorphous, nanocrystalline, and other novel ceramics.