Effect of the temperature and heating rate modes on the microstructure and mechanical properties of aluminum oxide: analysis of Spark Plasma Sintering mechanisms

The effect of temperature and heating rate on the shrinkage kinetics of submicron and fine-dispersed aluminum oxide powders was investigated. The objects of the study were (1) submicron powder α-Al2O3, (2) submicron powder α-Al2O3 with an amorphous layer on the particle surface, (3) fine powder α-Al2O3. Ceramic samples were obtained by spark plasma sintering (SPS). Based on a comparison of powders (1) and (2) of the same dispersity the influence of the amorphous layer on the sintering kinetics was analyzed. Based on the comparison of powders (1) and (3) the effect of the initial particle size on the shrinkage kinetics was analyzed. Shrinkage curves were analyzed using the Young-Cutler model and the Coble model. It is shown that the sintering kinetics of submicron powders is determined by the intensity of the grain boundary diffusion process, and of fine powders by simultaneously proceeding processes of volume and grain boundary diffusion. It was found that the presence of an amorphous layer on the surface of submicron α-Al2O3 powder influences the grain boundary rate and the parameters of the Coble equation at the final stage of SPS. It was suggested that the abnormal characteristics of ceramics SPSed from submicron powder with an surface amorphous layer are related to the presence of an increased density of defects and excess free volume on the grain boundaries formed in the process of crystallization of the amorphous layer.