EFFECT OF CUBIC PHASE ON THE KINETICS OF THE ISOTHERMAL TETRAGONAL TOMONOCLINIC TRANSFORMATION IN ZRO2(3 MOL-PERCENT Y2O3) CERAMICS

In this paper, the effect of cubic phase on the kinetics of the isothe rmal tetragonal (t) to monoclinic (m) transformation in ZrO2(3 mol% Y2 O3) ceramics was studied by means of thermal expansion analysis (TEA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). Experimental results showed that the kinetics of the isothermal t-->m transformation could be expresse d using the Johson-Mehl-Avrami equation, i.e. f = 1 -exp(-kt(K)) where f refers to the volume fraction of transformed in-phase, k is a varia ble associated with the energy barrier for critical nucleation and gro wth, and n is a constant depending on the nucleation sites. It was fou nd that both the ''nose'' temperature and the incubation periods of th e TTT curve of the ZrO2(3 mol% Y2O3) ceramics were decreased in compar ison with those of the TTT curve of the ZrO2(2 mol% Y2O3) ceramics. Th e time-temperature-transformation (TTT) curve is C-shaped, with the '' nose'' temperature determined to be 300 degrees C. Activation energy f or transformation was regressed to be 22.74 kJ/mol, which didn't chang e with temperature. TEM observation revealed that preferential growth of cubic phase grains refined t-phase grains during hot-pressing. It i s proposed that the nucleation and longitudinal growth of m-phase plat es in ZrO2(Y2O3) ceramics are displacive, while the sidewise growth th ereafter is controlled by short-range diffusion of oxygen ions and, in this sense, the t-->m transformation in ZrO2(Y2O3) ceramics possesses both displacive and non-displacive features. (C) 1997 Elsevier Scienc e Limited and Techna S.r.l.