Preparation Process On The Densification And Microstructure Of Nano-alumina

Decreasing the sintering temperature and improving the fracture toughness of alumina have been one of important research subjects among the ceramic scientists. Alumina powders with nanometer scales have been considered as most promising alternative to obtain the fine-grained dense ceramic with sufficient mechanical performances at low sintering temperature. Presently, nanosized alumina are generally formed with cold isostatic pressure, super high pressure, followed by being sintered with hot pressing, super high pressing, spark plasma et al. Few attempts, however, have been systematically made to study their sintering kinetics employing the conventional manufacturing processes. The present work is mainly aimed at the densification behaviours and microstructure evolutions of nanosized alumina powders as functions of grain size distributions, ball-milling time, softly grinding, forming pressures, heating or cooling rates, and sintering aids, respectively.Based upon high driving force for sintering of fine grains, 5～25wt% of nm-Al₂O₃ was incorporated intoμm-Al₂O₃ to promote the densification. Experimental results have shown that nm-Al₂O₃ decrease the packing factors of green compacts, but increase the bulk densities of sintered bodies when prolonging the holding time at 1600°C. Grain size and distributions were also modified by ball-milling in a planetary mill. Milling time to maximize densification is found to be 5hours, beyond which sintering kinetics are slightly inhibited presumably due to increasing larger grains and broaden grain size distributions. On the other hand, packing factors of green compacts are enhanced with increasing times of softly grinding and forming pressures. For sintered bodies, however, grinding for 2 times or forming under 300MPa has been confirmed to be a favorable condition for sintering. In addition, densification and performances of samples are also dependent on sintering schemes. Usually, fast rather than slow heating rates can more obviously deteriorate densification and mechanical properties of sintered bodies, which is due in part to being preferentially densified at surfaces of nm-Al₂O₃ during sintering.Sintering behaviours of TiO₂/CaO-Al₂O₃-SiO₂-incorporated nanosized alumina have been systematically investigated using combination design. The experimental results have shown that TiO₂ can greatly enhance the densification rates of nm-Al₂O₃ but trigger severely abnormal grain growth when incorporation levels of TiC>2 reaches up to 0.6wt%. On the other hand, small amounts of CAS can effectively inhibit abnormal grain growth, but provoke an abnormal densification owing to the formations of in situ pores left by liquid phase. With additional 0.50wt% of CAS incorporated into 0.60wt% TiO₂-doped nm-Al₂O₃, the aspect-ratios of anisotropic growth grains increase by reduced growth rates of flat boundaries, accomplished by basal planes much smoother and straighter. This microstructure subsequently disappears when CAS incorporation is up to 4.0wt%, substituted by that with low aspect-ratio and fine-equiaxed grains. Such occurrences are discussed in term of levels of liquid phase and 2-dimention nucleation processes.