Preparation Methods and Superplastic Properties of Fine-Grained
Zirconia and Alumina Based Ceramics
Yoshio SAKKA* and Keijiro HIRAGA
National Research Institute for Metals, 1-2-1, Sengen, Tsukuba-shi 305-0047 Japan
Superplasticity provides the possibility of high-temperature deformation processing of dense ceramics and has the advantages of greater shape form ability with better dimensional accuracy. Tensile ductility in fine-grained ceramics has been widely studied since large elongation was reported for fine-grained yttria-doped tetragonal zirconia (Y-TZ). CuO- or SiO2-doping effectively improves the ductility of Y-TZ, such that the maximum elongation exceeds 1000% for 5 wt% SiO2 doped Y-TZ. However, the tensile ductility of Al2O3-based ceramics is limited to 140%. Low ductility in fine-grained alumina has been attributed to rapid dynamic grain growth accompanied by large strain hardening and heavy intergranular cavitation. The addition of MgO or ZrO2 is known to be effective in suppressing dynamic grain growth by solute drag (MgO) or second phase pinning (ZrO2), but the resultant tensile ductilities are still very small as compared with that obtained in TZ. The cavitation incurred during high temperature deformation is examined in a MgO-doped alumina, a ZrO2-dispersed alumina and Y-TZ. A quantitative analysis of the cavity densities and cavity growth rates reveals that the damage accumulation in both the MgO-doped and ZrO2-dispersed alumina is controlled strongly by a cavity nucleation process, whereas the damage in Y-TZ is controlled by cavity growth.
Preliminary studies of fine-grained ZrO2, Al2O3 and their composites have shown that certain requirements must be met to achieve superplasticity: fine grain size, homogeneous microstructure and the inhibition of grain growth during high-temperature deformation. These requirements are being met both through advances in powder preparation and through novel developments in powder processing, such as colloidal processing. Particle dispersion control is the most important factor in colloidal processing. The dispersion of particles in aqueous suspensions can be stabilized by electrostatic repulsion or electrosteric repulsion. Slurries of tetragonal zirconia, silica-zirconia and alumina-zirconia fine particles were prepared by adjusting the pH value or adding appropriate amounts of polyelectrolyte. Their dense bodies were obtained through colloidal filtration, followed by cold isostatic pressing (CIP) and low-temperature sintering. Excellent superplastic properties were observed for Y-TZ, Al2O3-doped TZ, SiO2-doped TZ and ZrO2-dispersed Al2O3 systems as a result of dense, fine-grained, and homogeneous microstructures. In particular, large tensile elongation exceeding 550% can be obtained for 10 vol% ZrO2-dispersed Al2O3 when the initial grain size is maintained below 0.5 μm.
Y-TZ with a grain size of 0.3 μm was successfully prepared by sintering chemically synthesized powder. To obtain a dense Y-TZ with a grain size less than 0.1 μm, however, a special procedure is necessary. We processed fine-grained (1) monoclinic zirconia polycrystal using a monoclinic zirconia sol prepared by wet processing, and (2) CuO-doped TZ using a Cu-adsorbed Y-TZ suspension, where both suspensions were directly consolidated by pressure filtration without a dry powdering process.
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