Superplasticity and creep in monoclinic and Y-PSZ nano-grained zirconia

A fine-grained partially stabilized tetragonal zirconia with an average grain size of about 100 nm, was prepared by Hot Isostatic Pressing of an ultrafine zirconia-yttria powder. For a given strain rate, the superplastic forming temperature (1150-1250°C) is lowered by 200°C in comparison with the standard Y-TZP materials. However, for testing temperatures below 1250°C, destabilization occurs, with a dramatic increase in the monoclinic content below 1170°C, as the tetragonal to monoclinic phase transformation takes place, in conjunction with microcracking. In the aim to overcome this difficulty, a pure monoclinic zirconia, with an average grain size of 80 nm, was prepared by hydrothermal synthesis of a monoclinic zirconia suspension. This material is stable below 1170°C, and allows to further decrease the plastic forming range down to about 1000-1100°C.