Two-step sintering of alumina nano-powders: A discrete element study

Avoiding grain growth during sintering of ceramic nano-powders is of great technological interest. Although two-step sintering is an effective technique to achieve this goal, the mechanisms at play are not well under-stood. This study adapts our previous discrete model to investigate the conventional and two-step sintering of nano-powders. The densification and grain growth results agree qualitatively well with experimental data on a-alumina. Simulations confirm that faster heating rates retard grain growth in conventional sintering of nano-alumina. Our results support the hypothesis that the success of nano-alumina two-step sintering relies on the sharp increase in the activation energy of the grain boundary mobility at low temperatures. Simulations indicate a transition temperature of 1100 degrees C and that at least a 2.5-fold increase in activation energy is required to explain the suppression of grain growth. The relative weights of surface diffusion and of grain boundary motion for grain growth are clarified.