Coarsening, densification, and grain growth during sintering of nano-sized powders-A perspective

Sintering is one of the main approaches, among limited options, for building bulk nanomaterials from bottom up. A primary challenge of sintering nanosized powders is to control grain growth while achieving full densification. Considerable literature is now available in the search for unique mechanisms that could assist in achieving densification with minimum grain growth. It is critical to understand the detailed mechanistic steps, in order to design processes that might lead to bulk nanomaterials with maximum density and minimum grain size. Using experimental data of sintering nanosized tungsten carbide and tungsten powders, as well as selected data on other materials in the literature, this article examines the unique characteristics of nano-sintering, including the driving forces, kinetics, and intertwined processes of neck growth, coarsening, densification, and grain growth. When the density of the powder compact of nanosized particles are very low, coarsening of particles is responsible for most of the observed initial growth of grain sizes as well as densification. Because the mechanism of the initial coarsening depends on surface diffusion, surface diffusion thereby contributes to densification indirectly. Although the initial grain growth is only a small fraction of the total grain growth that occurs by the completion of sintering, it is the most critical part of grain growth, determining whether the grain size of the material can be controlled within the nanoscale regime.