Microstructural evolution in nanocrystalline alumina containing silicate glasses

Pure nanocrystalline -alumina powders were coated with different fractions (5, 10, and 15 vol%) of SiO₂-SrO glass using the sol-gel technique. The isostatically cold pressed powders were pressureless sintered in air for 5 h in the temperature range of 1250°C to 1550°C. The relative densities were ranged between 60 to 90% of the theoretical and were composition dependent. The density was increased with the sintering temperature. In pure alumina, the to phase transformation went to completion by sintering at 1250°C. However, in the glass-coated samples, transition -alumina was mostly retained after sintering at the same temperature. Pure nanocrystalline alumina sintered at 1350°C exhibited vermicular structure with isolated pores. The microstructure of the low glass-containing samples exhibited nanocrystalline to submicron size grains arranged in platelet-shaped clusters. Samples with higher glass contents exhibited also micron-size needle-shape grains of strontium aluminate.