Preparation methods and superplastic properties of fine-grained zirconia and alumina based ceramics

Superplasticity provides the possibility of high-temperature deformation pr ocessing of dense ceramics and has the advantages of greater shape form abi lity with better dimensional accuracy. Tensile ductility in fine-grained ce ramics has been widely studied since large elongation was reported for fine -grained yttria-doped tetragonal zirconia (Y-TZ). CuO- or SiO2-doping effec tively improves the ductility of Y-TZ, such that the maximum elongation exc eeds 1000% for 5 wt% SiO2 doped Y-TZ. However, the tensile ductility of Al2 O3-based ceramics is limited to 140%. Low ductility in fine-grained alumina has been attributed to rapid dynamic grain growth accompanied by large str ain hardening and heavy intergranular cavitation. The addition of MgO or Zr O2 is known to be effective in suppressing dynamic grain growth by solute d rag (MgO) or second phase pinning (ZrO2), but the resultant tensile ductili ties are still very small as compared with that obtained in TZ. The cavitat ion 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 accumulat ion 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 b y cavity growth. Preliminary studies of fine-grained ZrO2, Al2O3 and their composites have s hown 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 po wder processing, such as colloidal processing. Particle dispersion control is the most important factor in colloidal processing. The dispersion of par ticles in aqueous suspensions can be stabilized by electrostatic repulsion or electrosteric repulsion. Slurries of tetragonal zirconia, silica-zirconi a and alumina-zirconia fine particles were prepared by adjusting the pH val ue or adding appropriate amounts of polyelectrolyte. Their dense bodies wer e obtained through colloidal filtration, followed by cold isostatic pressin g (CIP) and low-temperature sintering. Excellent superplastic properties we re observed for Y-TZ, Al2O3-doped TZ, SiO2-doped TZ and ZrO2-dispersed Al2O 3 systems as a result of dense, fine-grained, and homogeneous microstructur es. 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 mu m. Y-TZ with a grain size of 0.3 mu m was successfully prepared by sintering c hemically synthesized powder. To obtain a dense Y-TZ with a grain size less than 0.1 mu m, however, a special procedure is necessary. We processed fin e-grained (1) monoclinic zirconia polycrystal using a monoclinic zirconia s ol prepared by wet processing, and (2) CuO-doped TZ using a Cu-adsorbed Y-T Z suspension, where both suspensions were directly consolidated by pressure filtration without a dry powdering process.