Densification maps for nano-sized powders

Extensive work has been done to characterize the densification mechanisms acting in conventionally-sized powder materials (particle sizes greater than or equal to 1 micron) during sintering and pressure-assisted consolidation. Densification maps (Ashby HIP maps) have been developed to show the processing conditions under which particular deformation mechanisms-typically plastic yielding, power law creep and/or diffusive mass transport-are likely to be active and to predict the kinetics of these processes as a function of basic material parameters. There is a need to extend these maps to nano-scale materials in order to develop an improved understanding of deformation mechanisms and to provide a rational basis for developing cost-effective consolidation technologies. Existing densification maps predict that nano-scale powders can be densified more easily than conventionally-sized materials, primarily through surface-tension-driven diffusion. Yet this is not what has been observed in many material systems. Possible reasons for this discrepancy include the failure of current maps to incorporate powder agglomeration and surface diffusion effects, both of which ate likely to become increasingly important with decreasing powder particle size.