Time-dependent tetragonal to monoclinic transition in hot-pressed zirconia stabilized with 2 mol % yttria

The kinetics of isothermal transformation from tetragonal (T) to monoclinic (M) structure in zirconia stabilized with 2 mol% yttria was systematically studied using thermal expansion analysis, electron probe microanalysis, X-ray diffractometry and transmission electron microscopy. Results showed that tetragonal phase obtained from rapid cooling transforms isothermally to monoclinic phase during holding at 200-400‡C for 4 h. The time-temperature-transformation (TTT) curve has a characteristic C-shape, with 300‡C as the "nose" temperature. A small amount of M phase can be detected when the holding temperature is not in the preceding temperature range. The isothermal transformation kinetics can be expressed in terms of the Johnson-Mehl-Avrami equation in which the nucleation and growth of new phase are both functions of time with the activation energy determined to be 28.67 KJ mol^-1. The metastability of T phase is associated with cooling rate, stabilizer content, as well as grain size. In situ observation of M phase propagation induced by the electron beam reveals the T → M transition to be diffusion controlled.