PHASE-TRANSFORMATIONS IN SPRAY-DRIED ALUMINA-ZIRCONIA COMPOSITE POWDERS

Hydrated alumina-zirconia composite powders containing 7.5, 10.0, 12.5 or 15.0 wt % zirconia in the final oxidized state were prepared by spray-drying the inorganic salt derived gels. These gels were prepared by hydrolysing the metal salts with ammonia. The average particle size and surface area of this powder is found to be 6 microns and 6.92 m(2)/g respectively. Infra-red spectra reveals the presence of adhesive water. In order to evaluate the phase transformation temperature, the powder samples were calcined at different temperatures ranging from 650 to 1500 degrees C for 30 minutes. X-ray diffraction patterns of the spray-dried as well as the calcined powders up to 850 degrees C revealed the powders are amorphous. Crystallized phases started forming after 900 degrees C and the first crystallized phase is tetragonal zirconia. Broad peaks of --alumina were also found. This tetragonal phase is retained even after the treatment at 1500 degrees C. Some amount of --alumina is transformed into alpha-alumina after 1000 degrees C, and this transformation is over by the formation of well crystallized alpha- alumina after 1300 degrees C. A comparison made with pure alumina shows that the presence of zirconia retards the formation of alpha-alumina, since pure alumina powder contains only alpha-alumina after 950 degrees C treatment. The increasing amount of zirconia has no profound influence on this effect in the compositions studied. DTA and TGA were also used as a complement to the X-ray diffraction in determining the transformation temperature.