The tetragonal-to-monoclinic phase transformation of 3mol% and 4mol% Y
2O
3-doped ZrO
2 (Y-PSZ) was investigated by annealing at 140°C in hot water. Sintered bodies of 3Y-PSZ and 4Y-PSZ were prepared by sintering at 1350°C for 0-10h, in air, and sintering at 1400 or 1450°C, respectively. A 3mol% and 4mol% Y-PSZ fine powder, synthesized by means of the hydrolysis method was used, as a starting powder. It was revealed that the Y
2O
3 concentration within the 3Y-PSZ tetragonal phase decreased and grain size was nearly constant as the holding time at 1350°C increased. When the sintered bodies of 3Y-PSZ were subjected to annealing in hot water, the phase transformation rate increased as the holding time increased. This behavior can be explained by considering the Y
2O
3 concentration within the tetragonal phase at constant grain size. In order to confirm the effect of grain size, a similar characterization was performed on 4Y-PSZ, which shows higher grain size than the 3Y-PSZ. It is shown that the behavior upon annealing in hot water depends only on the Y
2O
3 concentration within the tetragonal phase and not on the grain size. In addition, the phase-transformation behavior abruptly changes at the critical concentration of 2.75mol% of Y
2O
3, which reveals that, for fractions >3.0mol%, the phase transformation is completely controlled by the Y
2O
3 concentration within the tetragonal phase.
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