Processing and Microstructure of Mullite-Zirconia Composites Prepared from Sol- Gel Powders.

A high-purity stoichiometric mullite precursor was obtained by hydrolysis of the alkoxides Al(OC/sub 3/H/sub 7/)/sub 3/ and Si(OC/sub 2/H/sub 5/)/sub 4/. Fully sintered mullite ceramics can be prepared from sol-gel powders by sintering them at 1600/sup 0/C for 4 h in air with the addition of 15 to 20 vol% ZrO/sub 2/ or 1 to 3 mol% Y/sub 2/O/sub 3/ or both. Introduction of 1 to 3 mol% Y/sub 2/O/sub 3/ aids the retention of tetragonal ZrO/sub 2/; the volume fraction of t-ZrO/sub 2/ retained increases with increasing Y/sub 2/O/sub 3/ content. The maximum t-ZrO/sub 2/ retained reaches 34% in a matrix of synthetic mullite with 3 mol% Y/sub 2/O/sub 3/, but most of this t-ZrO/sub 2/ does not undergo stress-induced transformation during grinding.