Effect of microstructural evolution on the measured creep parameters of yttria-stabilized tetragonal zirconia polycrystals

From the early work of Wakai et al., many studies have been devoted to the structural superplasticity on fine-grained zirconia-based polycrystals, especially when doped with yttria. Superplastic deformation is provided by grain boundary sliding without appreciable deformation of the grains themselves; the strain rate var-epsilon is usually controlled by diffusion necessary to accommodate the motion of the grains one each other. High temperature deformation data are analyzed through the standard equation for steady state deformation: var-epsilon = A/T · σn · d-p · exp(-Q/RT) where A is a constant, σ the stress, d the grain size and T the temperature. The parameters n, p and Q are characteristic of the deformation and diffusion mechanisms. The dependence of var-epsilon on the oxygen partial pressure (PO2) has not been taken into account in eq. (1) because a PO2 exponent m = 0 was found in the creep of Y2O3-stabilized ZrO2 single and polycrystals between 0.21 and 10-9 atm at 1,200--1,600 C [4,5], and also in this work between 0.21 and 10-5 atm at 1,200--1,400 C. The stress and temperature exponents can be measured by two different techniques, which are not equivalent when the microstructure evolves during the tests. In the conventional method, experiments are performed on different specimens changing only one variable and keeping the others constant; on the contrary, in the differential method, one variable is changing during the test on the same sample