BEHAVIOR OF THE TETRAGONAL TO MONOCLINIC TRANSITION IN HOT-PRESSED ZIRCONIA DOPED WITH 2 MOL-PERCENT YTTRIA

The behaviour of the tetragonal (t) to monoclinic (m) transition in ho t pressed zirconia stabilized by 2 mol% yttria was investigated, with attention particularly paid to its time dependence. Thermal expansion analysis system (TEAS), X-ray diffractometry (XRD) and transmission el ectron microscopy (TEM) were employed in order to thoroughly understan d the effect of cooling rate on the amount of m phase, as well as the effect of pre-m phase on the kinetics of the isothermal t-->m transiti on. Experimental results demonstrate that the amount of m phase increa ses with decreasing cooling rate; the transformation temperature of t- ->m and m-->t is influenced by the cooling and heating rate, respectiv ely; and the existence of pre-m phase not only can obviously shorten t he incubation period of isothermal transformation, but also can enhanc e the overall content of m phase at ambient temperature as well. The i sothermal kinetics of the t-->m transformation can be characterized by the Avrami equation in its usual form, in which the exponent n for th e case with pre-m phase is slightly different from n without pre-m pha se. An argument for classical heterogeneous nucleation is proposed, as confirmed by TEM observation. The substructure of the m phase, which consists mainly of twins, is relatively unaffected by the thermal hist ory.