ALUMINA/YAG 20 VOL% COMPOSITES PREPARED BY THE DAWSONITE THERMAL DECOMPOSITION

The co-precipitation of alumina/YAG precursor from the aqueous solutions of aluminium and yttrium nitrates with ammonium carbonate resulted in ammonium dawsonite (NH4Al(OH)2CO3) with yttrium built into its structure. The decomposition of dawsonite at elevated temperatures led to the formation of gamma-Al2O3 with delta- and theta-Al2O3 modifications appearing at higher temperatures. Full transformation to alpha-Al2O3 and YAG occurred above 1230 C. The starting powder for sintering experiments was prepared using the coprecipitated precursor calcination at 600 C. Seeding of this powder with 5 wt% alpha-Al2O3 was necessary to achieve dense composites at 1500 C. The dense composites had lower wear susceptibility when compared with dense pure alumina polycrystals. The unseeded powder led to samples with high porosity and showed signs of discontinuous grain growth.