Isothermal T-to-M transformation nucleated at grain boundaries in zirconia-yttria ceramics

The tetragonal-to-monoclinic (T-M) phase transformation in zirconia containing 0.5-4 mol% yttria during isothermal aging at various temperatures between 75 and 300 degreesC was studied by means of X-ray diffraction, SEM observations and atomic force microscopy (AFM), The amount of transformation increases sigmoidally with an increase in aging time in all specimens, which indicates that the. transformation proceeds with a typical nucleation and growth mode. The rate. of sigmoidal curves depends on the grain size and environments, i.e., a shift to shorter aging time occurs with a larger grain size and also after aging in a water environment. The kinetics analysis indicates that these factors have an effect on the nucleation stage of transformation but not on the growth stage. AFM observations suggest that the transformation products nucleate at grain boundaries and advance into grain interiors.