Densification of composite powder compacts in pressure cycling

The effect of pressure cycling on the consolidation of green composites from powders is investigated. Mixed powders of lead and various amounts of alumina were consolidated under static and cyclic pressure at room temperature in constrained ''uniaxial'' consolidation experiments. The experiments show that in static compaction, density dramatically decreases with increasing ceramic content. In a separate series of experiments, the compacts were subjected to a large number of loading cycles where the pressure varied from zero to a constant maximum value. Virtually identical results are obtained when the powder mixtures were dilute in ceramic. However, at high ceramic contents much higher densities were obtained in stress cycling. Increasing the compaction pressure cycle amplitude and the number of cycles significantly increases the compacted density, but changes in the frequency have relatively little effect at the same number of cycles. Green composites fabricated with cyclic pressure also have significantly increased hardness and rupture strength relative to those produced in static compaction. The improved consolidation is explained and qualitatively analyzed in terms of the volumetric mismatch between the metal and ceramic that is naturally produced in a pressure change. This produces deviatoric stresses in the metal phase. These in turn facilitate plastic deformation and consolidation. This effect of volumetric mismatch creating deviatoric stresses which aid plastic flow and, hence, densification is believed to be quite general.