Effect of the Temperature-Rate Regimes in Electric Pulse Plasma Sintering on Microstructure and Mechanical Properties of Aluminum Oxide: The Role of Sintering Mechanisms

The temperature and heating rate affecting the shrinkage kinetics are studied for cylindrical workpieces obtained from submicron and fine aluminum oxide powder. The studies involve the powder from three batches: (1) submicron (similar to 0.15 mu m) alpha-Al2O3 powder, (2) submicron (similar to 0.2 mu m) alpha-Al2O3 powder having an amorphous layer deposited on the particle surface, and (3) fine (similar to 1 mu m) alpha-Al2O3 powder. It is established that the powder particles in all batches has a monocrystalline structure. The powder workpieces are sintered using the electric pulse (spark) plasma sintering (SPS) technique. The shrinkage curves are analyzed using the Young-Cutler and the Coble models. The kinetics of sintering workpieces is shown to depend on diffusion developing between the powder particles. The sintering kinetics of workpieces made from submicron powder depends on intensity of the grain-boundary diffusion. In the sintering workpieces made of finely dispersed powder, the kinetics is additionally dependent on simultaneously developing volumetric and grain-boundary diffusion. It is established that the presence of an amorphous layer on the surface of particulate alpha-Al2O3 having submicron size affects the rate of migration of grain interfaces and the parameters of the Coble equation at the final SPS stage. It is assumed that the accelerated growth of grains and an increase in the microhardness of samples obtained through sintering workpieces made from submicron powder with an amorphous layer on the particle surface is caused by a higher density of defects at the grain interfaces. The elevated density of defects at grain interfaces can result from crystallization of the amorphous layer.