Better Sintering through Green‐State Deformation Processing

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Aqueous colloidal suspensions of alumina were processed in the dispersed state and the flocculated state by controlling the double‐layer interactions between the particles. Repulsive particle forces led to high packing densities but the green bodies were mechanically so weak that they were unable to retain their shape (the dispersed case). Attractive forces led to good green strength but the packing density was low and the particles were agglomerated (the flocculated case). The agglomerated structure of the flocced specimens could be fragmented by mechanical deformation of the green compact; the deformation was carried out under a superimposed hydrostatic pressure of less than 1 MPa. The flow stess of the flocculated structures depended on the deformation rate, and on the magnitude of the superimposed hydrostatic pressure. The flow stress was 2.5 kPa at a strain rate of 0.1 s−1. Deformation processing of the flocced structures increased the green (relative) density from 0.51 to 0.62. The sintering behavior of underformed and deformation‐processed flocced structures was studied. Deformation‐processed green bodies sintered more rapidly and yielded a final grain size that was smaller and more uniform than that obtained from the undeformed specimens. The ability to homogenize and densify the packing of flocculated structures by deformation processing suggests new opportunities in green‐state processing, for example (i) uniform mixing of more than one kind of particle or particles and fibers, and (ii) net shape forming by injection molding or extrusion, without the use of organic binders.