Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH4OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat‐treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO2 in these ultrafine particles was studied by X‐ray diffractometry. Growth of ZrO2 crystallites at elevated temperatures was strongly inhibited by Al2O3 derived from aluminum hydroxide. The monoclinic‐to‐tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al2O3, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO2 is lower than that of the monoclinic form. With increasing A12O3 content, however, the transformation temperature gradually increased, although the growth of ZrO2 particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO2. The reverse tetragonal‐to‐monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO2 particles was stabilized by the presence of water vapor, which probably decreases the surface energy.