No abstract

The microstructure development of sintered alumina at different stages in the spark plasma sintering process has been investigated. Based on classical kinetics laws and adapted Coble creep models of hot pressing, the densification and grain growth kinetics were analyzed as a function of various parameters such as heating rate, sintering temperature, and dwell duration. It is found that sintering kinetics are greatly influenced by heating rates. The local temperature gradients at interparticle contacts during high heating rate can be from three to 10 times higher than in low heating rate cycles. Plastic yield might lead to instantaneous densification at an early stage of sintering. However, grain-boundary diffusion probably dominates at low heating rate whereas grain coarsening, in respect of thermal equilibrium establishment, is unavoidable with high heating rate during the initial-stage sintering at low temperatures. During the final-stage sintering, fast grain growth mechanisms such as surface diffusion and pore-controlled grain-boundary migration may dominate over densification controlled by grain-boundary sliding or lattice diffusion. The grain size-density trajectory corroborates that low heating rate is much favorable to achieve near full density with fine grain size at low sintering temperatures.