Size effects in powder compaction
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t is well known that great difficulties are encountered in the cold compaction of ultrafine powders. Such difficulties have been qualitatively attributed to several origins (e.g., increasing relative contribution of oxidized layers to particle resistance as particle size decreases). The main densification stage during compaction is governed by plastic deformation at interparticle contacts under pressure. On account of the strength enhancement of plastic resistance in presence of plastic strain gradients (physically resolved by “geometrically necessary dislocations”) a contribution to the size effect on powder compaction efficiency is here predicted. Some quantitative experimental data available are in good agreement with this explanation.
The process of-low temperature powder compaction (without simultaneous sintering) by application of uniaxial or hydrostatic pressure is customarily divided in four stages:1,2 (i) rearrangement of the loose powders from the low density derived from pouring and shaking until a mechanically stable aggregate with sufficient mutual interparticle contacts is formed; (ii) densification by local plastic deformation at the interparticle contacts; (iii) densification by general plastic deformation of the particles after merging of the individual plastic zones of the contacts of a particle; (iv) bulk deformation of the compact once a state of closed porosity is reached. The initial relative density of the loose powders can be very low. It is affected by the size of the powders: small particles (less than 1 m) tend to form low-density aggregates, the weak interparticle attractive forces counteracting more easily the gravitational forces and allowing for the existence of dangling bonds. Ideally, the first densification stage occurs merely by rearrangement of the particles involving sliding, rotation and recollocation, without interactions requiring any plastic deformation of the particles. According to German,2 the lower bound of *, the fractional packing density (FPD) of monosized spheres is 0* ⳱ 0.35 for mechanical equilibrium. It corresponds to 4.75 average contacts per particle. The upper bound FPD for random “dense” packing of monosized spheres is most frequently quoted to be 0* ⳱ 0.64. However, the existence of a “random closepacking” state is now discredited. It seems to be more appropriate to speak of a range of aggregates in “jammed states” (without any loose particles and capable of sustaining nonnegligible stresses) between a lower bound and the maximum FPD of 0.74 for a fully ordered close packing.3 1238
http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 5, May 2001 Downloaded: 18 Mar 2015
Once a jammed state is reached, the second densification stage, where most of the mechanical compaction takes place, sets in. The macroscopic stress applied to the powder aggregate is locally transmitted through the interparticle contacts where local plastic deformation occurs once the compressive stress surpasses the indentation hardness H of the particle material. As far as the pl
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