Densification of powder compacts: An unfinished story

The focus of this review is to summarize the understanding of why the coarsening of grains and pores accompanies the densification of a powder compact. The review will initiate with a summary of the thermodynamic studies of simple particle arrays, which suggests that the growth of necks between the initial touching particles stop before the compact is dense. Results of analytical and computer studies concerning how grain growth occurs in partially dense powder compacts will be reviewed to show that grain coarsening will reinitiate the neck growth process responsible for further shrinkage. Experimental results will then be reviewed and related to the analytical studies. It will be shown that isolated pores connected to the largest number of grain boundaries, namely, avenues for rapid mass transport, will disappear first. Finally, the lack of pore periodicity will constrain the shrinkage of one region relative to another.