TIME-DEPENDENT TETRAGONAL TO MONOCLINIC TRANSITION IN HOT-PRESSED ZIRCONIA STABILIZED WITH 2 MOL-PERCENT-YTTRIA

The kinetics of isothermal transformation from tetragonal (T) to monoc linic (M) structure in zirconia stabilized with 2 mol % yttria was sys tematically studied using thermal expansion analysis, electron probe m icroanalysis, X-ray diffractometry and transmission electron microscop y. Results showed that tetragonal phase obtained from rapid cooling tr ansforms isothermally to monoclinic phase during holding at 200-400-de grees-C for 4 h. The time-temperature-transformation (TTT) curve has a characteristic C-shape, with 300-degrees-C as the ''nose'' temperatur e. A small amount of M phase can be detected when the holding temperat ure is not in the preceding temperature range. The isothermal transfor mation kinetics can be expressed in terms of the Johnson-Mehl-Avrami e quation in which the nucleation and growth of new phase are both funct ions of time with the activation energy determined to be 28.67 KJ mol- 1. The metastability of T phase is associated with cooling rate, stabi lizer content, as well as grain size. In situ observation of M phase p ropagation induced by the electron beam reveals the T --> M transition to be diffusion controlled.