The variation of strain rate with stress in superplastic zirconia

High tensile superplastic ductilities may be achieved in yttria-stabilized tetragonal zirconia (Y-TZP) and in Y-TZP/Al2O3 composites. Nevertheless, although it is customary to relate the strain rate, epsilon, to the applied stress, sigma, through a general relationship of the form epsilon proportional to sigma(n), where n is the stress exponent, superplastic zirconia differs from superplastic metals because there appears to be no well-defined and single value for n. This paper examines the factors influencing the measured values of n and then presents some recent data, obtained by conducting strain rate cycling tests on 3Y-TZP, which demonstrate that n is a function of both stress and strain. There have been some recent reports of a threshold stress in superplastic zirconia and these reports are critically examined with reference to published experimental data for a 3Y-TZP/15% Al2O3 composite.