KINETICS OF ISOTHERMAL TRANSITION FROM TETRAGONAL TO MONOCLINIC PHASEIN ZRO2(2 MOL-PERCENT-Y2O3) CERAMIC

In this paper the kinetics of the isothermal tetragonal to monoclinic transformation in ZrO2(2 mol%Y2O3) ceramic have been investigated by m eans of high temperature X-ray diffractometry. The present experimenta l results have been compared with previous results obtained by dilatio n experiments. It is found that the time-temperature-transformation cu rve for T-->M transformation is 'C degrees-shaped with 300 degrees C a s the nose temperature, which is similar to the isothermal kinetics ch aracter revealed by the thermal expansion method. The incubation perio d is greatly shortened due to the fact that high temperature X-ray dif fraction (XRD) virtually reflects the kinetics data of T-->M transform ation occurring within the surface layer, several micrometers thick. I n 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 expe riment. Thermodynamic calculation has shown that the critical nucleati on energy barrier for surface nucleation is significantly decreased. T he proposal that T-->M transformation in ZrO2(Y2O3) ceramics possesses both displacive and nondisplacive features due to the existence of ox ygen vacancies is put forward.