Design of superplastic cubic yttria stabilized zirconia

Microstructural design concepts were utilized to create a superplastic 8 mol% yttria stabilized cubic zirconia. Initial studies on the wetting behavior of various amorphous phases with cubic zirconia and the kinetics of grain growth were undertaken to control grain growth in order to achieve superplasticity. Optimal process parameters included a rapid heating rate (100{dollar}spcirc{dollar}C/min) and hot isostatic pressing in order to achieve a small grain size and full density for cubic zirconia. It was found that grain growth rates could also be controlled by the deliberate addition of intergranular phases of various amorphous silicate or inert crystalline phases such as alumina. Silicate phases with the highest solubility for yttria and zirconia enhanced grain growth compared to control samples without grain boundary phases. Amorphous intergranular phases with the lowest solubility for yttria and zirconia were the most effective among glassy phases in suppressing grain growth and enhancing superplasticity. Activation energies for grain growth of glass containing samples were in the range of 400 kJ/mol.; In samples containing 5 wt% silica or 10 wt% alumina, the presence of a secondary phase not only proved effective against grain coarsening of the material, but also resulted in a relatively cavitation free high-temperature deformation of cubic zirconia. Results of high temperature deformation of both 10 wt% alumina and 5 wt% silica containing samples under compression indicate strain rates similar to those observed in superplastic tetragonal yttria stabilized zirconia.; Strain hardening was evident in alumina containing samples but no strain hardening was observed in silica containing samples. Strain rates as high as {dollar}4times10sp{lcub}-3{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar} was obtained at 1450{dollar}spcirc{dollar}C for 10 wt% alumina containing samples and strain rates as high as {dollar}5times10sp{lcub}-3{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar} was obtained at 1500{dollar}spcirc{dollar}C for 5wt% silica containing samples. Strain rate sensitivity exponent was calculated to be about 0.5. Activation energy for superplastic deformation of 683-597 kJ/mol was obtained at 10-35 MPa for 10 wt% alumina containing samples, decreasing with increasing stress. The values of activation energy for superplastic deformation of 5 wt% silica containing samples ranged from 341-411 kT/mol at 10-45 MPa, increasing with increasing stress. The use of inert amorphous or crystalline phases is a successful approach to limit grain growth and to promote superplasticity in oxide ceramics.