PHASE-TRANSFORMATIONS IN MONOCLINIC ZIRCONIA CAUSED BY MILLING AND SUBSEQUENT ANNEALING

Monoclinic ZrO2 has been milled for various times down to a crystallit e size of 145 angstrom. Using the time-differential perturbed-angular- correlation technique, it was possible to associate the resulting grow ing amount of disordered material with two different, very distributed and milling-time-dependent hyperfine interactions: one (Y) of quadrup ole frequency similar to that of tetragonal zirconia after long millin g, the other (X) one of a quadrupole frequency similar to that of mono clinic zirconia at shorter milling times. Upon annealing, all samples showed the recovery of the crystalline monoclinic phase at the expense of the disordered structures. In samples milled for shorter times, th e (Y) interaction emerges as an ordered crystallization product, upon annealing at temperatures which depend nearly linearly on the crystall ite size.