Structural-phase transformation kinetics during sintering of alumina ceramics using metastable nanopowders

The processes taking place during pressureless sintering of nano- metastable Al2O3, compacted up to high densities (0.7 of the theoretical density) using the magnetic pulsed method were studied. The influence of MgO, TiO2 and ZrO2 additives on the kinetics of Al2O3 polymorphous transition, shrinkage and microstructure evolution during annealing at temperatures up to 1450°C has been studied. We have found that the process of annealing is two-staged starting with a polymorphous transition. Doping changes the starting temperature as well as the shrinkage depth at both stages. TiO2 and ZrO2 additives decrease the temperature of the onset of shrinkage, whereas MgO increases it. The best composition contained MgO in the series of examined types of ceramics with an α-Al2O3 matrix. The positive role of Mg addition in the production of dense and hard Al2O3 ceramics is related to the nature of Mg influence on the activation of diffusion processes in Al2O3, as well as to the way of uniform distribution of MgO dopant in the material. All these factors provide effective damping of diffusion processes and limit α-Al2O3 crystal growth. Highly dense MgO, ZrO2 and TiO2 doped Al2O3 ceramics with a grain size of 190, 220, and 250 nm and microhardness of 22, 17 and 17 GPa, correspondingly have been obtained