MICROSTRUCTURAL DEPENDENCE OF AGING-INDUCED PHASE-TRANSFORMATION IN Y2O3-PARTIALLY-STABILIZED ZRO2 POLYCRYSTALS

One drawback of Y2O3-partially stabilized zirconia (Y-PSZ) ceramics is the low-temperature degradation, that is, when the material is aged o r used at around 500 K, particularly in water-containing environments, the strength and/or fracture toughness remarkably decrease. The degra dation is caused by the tetragonal-to-monoclinic phase transformation accompanying micro- and macro-cracking. It was generally believed that such an aging-induced transformation can be effectively retarded by d ecreasing the tetragonal grain size for a fixed composition. In the pr esent study, 2 mol% Y2O3-ZrO2 polycrystals with average grain sizes of 0.51 to 0.96 mu m were prepared by sintering in air at 1673 K for 2 t o 100 h. The influence of grain growth on the aging-induced phase tran sformation during annealing in water and in vacuum was investigated. I t has been found that the presence of water causes the t-to-m transfor mation even at temperature as low as 353K at which the transformation was difficult to only be thermally activated during annealing in vacuu m. The amount of t-to-m phase transformation during annealing in water at 353 K firstly decreased and reached a constant value as the sinter ing time was prolonged (i.e., with the grain growth), and then increas ed with the increase in sintering time (or grain size). On the other h and, the amount of transformation in vacuum shows a monotonously sharp increase with sintering time. This interesting result has been discus sed from two aspects of the thermally activated transformation and low -temperature degradation mechanisms of Y-PSZ.