Kinetics of isothermal transition from tetragonal to monoclinic phase in ZrO2(2 mol%Y2O3) ceramic

In this paper the kinetics of the isothermal tetragonal to monoclinic transformation in ZrO2(2 mol%Y2O3) ceramic have been investigated by means of high temperature X-ray diffractometry. The present experimental results have been compared with previous results obtained by dilation experiments. It is found that the time-temperature-transformation curve for T-->M transformation is 'C degrees-shaped with 300 degrees C as the nose temperature, which is similar to the isothermal kinetics character revealed by the thermal expansion method. The incubation period is greatly shortened due to the fact that high temperature X-ray diffraction (XRD) virtually reflects the kinetics data of T-->M transformation occurring within the surface layer, several micrometers thick. In addition, the presence of pre-M phase, formed prior to holding, also plays an autocatalytic part in shortening the incubation period. The activation energy for transformation which is regressed to be 29.69 kJ mol(-1) is almost equivalent to that calculated through dilation experiment. Thermodynamic calculation has shown that the critical nucleation energy barrier for surface nucleation is significantly decreased. The proposal that T-->M transformation in ZrO2(Y2O3) ceramics possesses both displacive and nondisplacive features due to the existence of oxygen vacancies is put forward.