Model of two-step sintering conditions for yttria-substituted zirconia powders

A method is described to assess optima two-step sintering schedules of ceramic materials that combines predictions of the sintering kinetics based on shrinkage data with Taguchi fractional experimental plans. The study is based on commercial nanopowders (∼50 nm) of the partially stabilized tetragonal zirconia (ZrO2)0.97(Y2O3)0.03, and the fully stabilized cubic phase (ZrO2)0.92(Y2O3)0.08. The controlled processing parameters are the peak temperature (Tp), the temperature and dwell time of the isothermal step (Td and td) and the cooling rate (β) between Tp and Td. Dense (ZrO2)0.97(Y2O3)0.03 ceramics were obtained with an average grain size of ∼100 nm under the conditions Tp = 1320 °C, Td = 1250 °C, td = 12 h and β = 20 K min−1. Grain growth is inhibited to a lesser extent for the (ZrO2)0.92(Y2O3)0.08 ceramics, with the lowest average grain size of 450 nm obtained for Tp = 1320 °C, Td = 1270 °C, td = 12 h and β = 20 K min−1. The predictions based on Herringʹs scaling law applied to non-isothermal densification data are in reasonable agreement with isothermal results obtained for the tetragonal zirconia ceramics, whereas they fail for the cubic phase, probably due to the grain growth occurring up to Tp. The total ionic conductivity values at 1000 °C are 0.052 S cm−1 for (ZrO2)0.97(Y2O3)0.03 and 0.145 S cm−1 for (ZrO2)0.92(Y2O3)0.08. These values are comparable to those reported for samples with three to four times larger grain size.