Microstructural dependence of aging-induced phase transformation in Y2O3-partialiy-stabilized ZrO2 polycrystals

Abstract

One drawback of YOs-partially stabilized zirconia (Y-PSZ) ceramics is the low-temperature degradation, that is, when the material is aged or used at around 500 K, particularly in water-containing environments, the strength and/or fracture toughness remarkably decrease. The degradation 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 decreasing the tetragonal grain size for a fixed composition. In the present study, 2mol% Y2O3-ZrO2 polycrystals with average grain sizes of 0.51 to 0.96 Jim were prepared by sintering in air at 1673 K for 2 to 100 h. The influence of grain growth on the aging-induced phase transformation during annealing in water and in vacuum was investigated. It has been found that the presence of water causes the t-to-m transformation even at temperature as low as 353K at which the transformation was difficult to only be thermally activated during annealing in vacuum. The amount of t-to-m phase transformation during annealing in water at 353 K firstly decreased and reached a constant value as the sintering time was prolonged (i.e., with the grain growth), and then increased with the increase in sintering time (or grain size). On the other hand, the amount of transformation in vacuum shows a monotonously sharp increase with sintering time. This interesting result has been discussed from two aspects of the thermally activated transformation and low-temperature degradation mechanisms of Y-PSZ.