Isothermal Tetragonal-to-Monoclinic Phase Transformation in a Zirconia–Yttria System

Abstract

The isothermal tetragonal-to-monoclinic phase transformation in zirconia-0.5–4 mol% yttria ceramics with and without alumina addition is summarized. This transformation proceeds sigmoidally with increasing isothermal aging time. C-shaped T-T-T curves are obtained, and their nose positions shift to shorter times and higher temperatures as the yttria content decreases. The apparent activation energy obtained from the TT-T curves is almost coincident with that for diffusion of oxygen ions in zirconia–yttria ceramics, irrespective of the environment in water, air or vacuum. The isothermally transformed laths advance from the grain boundaries to the grain interiors and then propagate adjacent grains with increasing aging time. Furthermore, this growth is controlled by the diffusion of oxygen ion vacancies in zirconia. The alumina addition acts to reduce the anisotropy of the transformed monoclinic phase.