Abstract
Tetragonal zirconia polycrystals stabilized by Y2O3 (Y-TZP) are recognized as a high strength material. However, Y-TZP has a fatal degradation at relatively low temperatures (100-300°C) according to the transformation from tetragonal to monoclinic phase. This transformation was reported to depend on the grain size in the sintered body. The purpose of the present study was to clarify the relationship between the properties of the starting powder and the low-temperature degradation of sintered specimens. The process and the results are as follows.
(1) The fine raw powders of zirconia with specific surface area of 10, 20 and 30m2/g were prepared by hydrolysis of ZrOCl2·8H2O and Zr(OH)4, coprecipitation of hydrated ZrO2 and Y(OH)3 by NH4OH, and calcination in the temperature range of 800-1100°C.
(2) These raw powders having specific surface area of 20 and 30m2/g could be sintered easily at 1300°C and yielded the sintered bodies with small grain sizes. In these samples, the degradation could be scarcely observed even under a 200°C hydrothermal condition.
(3) The degradation in bending strength of Y-TZP by aging in hot water of 200°C showed good correspondence with the increase of monoclinic phase by the aging.
(4) When Y2O3 content and bulk density were fixed, the degradation behavior was closely related to the grain size of the sintered body. The critical grain size of the phase transformation from tetragonal to monoclinic was about 0.3-0.4μm.
