The effect of sintering temperature on the kinetics of isothermal tetragonal-to-monoclinic transformation in ZrO₂(2 mol% Y₂O₃)

In this paper, the effect of sintering temperature on the kinetics of isothermal transformation from tetragonal (T) to monoclinic (M) phase in hot-pressed zirconia stabilized with 2 mol% yttria has been investigated by means of thermal expansion analysis (TEA), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Results show that transformation behaviour of T → M in yttria-containing zirconia is quite different from that in pure zirconia. The tetragonal phase transforms isothermally to the monoclinic phase during holding at 200-400°C for 4h. The time-temperature transformation (TTT) curve is characteristic of a C shape with 300°C as the nose temperature irrespective of density. The effect of porosity on the shape of TTT is to shorten the incubation period, the underlying cause of which has been explored. The kinetics data follow the Johnson-Mehl-Avrami equation, f{hook} = 1 - exp(-ktⁿ), where f{hook} is the volume fraction of the M phase at an instant t; k and n are constant at a given temperature. For fully dense material sintered at 1600°C, n decreases with increasing isothermal temperature, i.e. from 1·2 to 0·8; for material with porosity, n remains almost constant, i.e. 0·8. It has been shown that in the framework of the Johnson-Mehl-Avrami theory, n below 1 is impossible to exist; the implication of n is therefore discussed.