Isothermal phase transformation has been widely studied as a field of martensitic transition in physical metallurgy, but almost never in ceramic systems because of unusual phenomena. Zirconia ceramics have received special attention from scientists and technologists because of martensitic transformation, which is rare in ceramics [1,2]. Yttria stabilized zirconia polycrystal (Y-TZP) is expected to be a good engineering ceramic because of the high strength and fracture toughness afforded by stressinduced transformation [3,4]. However, these superior mechanical properties are degraded by thermal instability in the 100-400 Β°C temperature range, thereby restricting applications at high temperatures [5,6]. From other investigations [5-8], this behaviour has been shown to be related to isothermal martensitic transformation during ageing. Although various mechanisms of Y -T Z P degradation at low temperature were proposed, this phenomena is still not clearly explained.
To data, degradation behaviour due to isothermal phase transformation in Y -T Z P ceramics has been discovered only at temperatures in the 100-400 Β°C range [9,10]. We are currently investigating isothermal phase transformation at room temperature, using high transformable 2Y-TZP powders.
The 2Y-TZP powder used as a starting powder in this study was TZ-2Y powder which retained a metastable tetragonal phase, and was prepared by a spray-drying method at the Tosoh Co. This powder contained 2 mol % Y203 and had an average grain size of about 25 nm, and average granule size of 50-70 tLm. In order to vary the transformability of tetragonal phase, the powder was calcined in the temperature range 800-1500 Β°C for 1 h in alumina crucibles. In the case of the granular powder, densification and grain growth gradually increased with calcined temperature. Therefore, the relative magnitude of stabilization, which appeared as a particle size effect and constraint effect between tetragonal particles, varied with calcined temperature [11,12]. Ageing was carried out at room temperature in closed glass bottles. The amount of transformed monoclinic with ageing was measured by X-ray diffraction (XRD) analysis and the ratio of monoclinic phase to tetragonal phase was determined by the Garvie-Nicholson formula [13]. The structure of transformed monoclinic particles was