EFFECTS OF ULTRASONIC AGITATION ON MICROSTRUCTURE AND PHASE-TRANSFORMATIONS IN NANOCRYSTALLINE ZRO2AL2O3

ZrO2-Al2O3 nanocomposite powders that allow a grain size of < 25 nm an d the tetragonal phase of ZrO2 to be maintained at high temperatures h ave the potential to exhibit improved mechanical and thermal propertie s compared to traditional PSZ in cyclic high temperature applications. Colloidal suspension and precipitation methods, with and without ultr asonic agitation, were used to produce 20 vol% Al2O3/80 vol%ZrO2 powde rs that were subsequently heat treated and characterized using XRD and TEM. The most successful powders were not heavily agglomerated, but r ather had a matrix of tetragonal ZrO2 grains surrounded by Al2O3 in th e grain boundaries. it was also found that the alpha-Al2O3 phase was a bsent in many of the most successful samples, even after these samples were treated above the normal temperature for transition to alpha-Al2 O3. This suggests that transition phases such as gamma-, theta-, or de lta-Al2O3 are being stabilized by a size effect and may be more desira ble for microstructural control. (C) 1998 Acta Metallurgica Inc.