Persistence of Granular Structure during Die Compaction of Ceramic Powders
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Persistence of Granular Structure during Die Compaction of Ceramic Powders William J. Walker, Jr. New York State Center for Advanced Ceramic Technology at Alfred University, Alfred, NY 14802, U.S.A. ABSTRACT Glass spheres were used as a model system to investigate granule failure during die compaction. Stresses within an assembly of spheres follow a network of pathways. When the spheres are of uniform composition, the magnitude of the stresses within a pair of contacting granules is a function of the locally transmitted stress and the diameter of the two spheres. Results obtained using glass spheres demonstrated the statistical nature of granule failure during compaction, with some granules failing at very low applied pressures while others (~40% by volume) persist at even the highest applied loads. Within a distribution of granule sizes, those granules with smaller diameter were seen to have a higher probability of failure at low pressure than were larger granules. These results are consistent with those observed during die compaction of granulated alumina powder. INTRODUCTION Die compaction of granulated powder is a common forming process used in the ceramics industry. Granulation of fine powders is necessary to produce the free flowing feed material required for die filling when high-speed presses are used. However, it is desirable to eliminate all artifacts of the granules during the compaction process in order to produce defect-free sintered components. In this work, statistical analysis of the fragmentation of glass beads during die compaction was used as a model granular system to achieving a better understanding of persistent granular structures in compacts. During compaction of granulated powder, the density increase that results from applied pressure occurs in three stages [1]. Stage I consists of granule rearrangement at low pressures resulting in a small increase in density of the granular assembly. Above an apparent yield pressure Py, which marks the onset of Stage II, the interstitial pores between granules (intergranular pores) are reduced in size as the granules break down or deform, causing a linear increase in density with log (compaction pressure). In Stage III, the intergranular pores are mostly eliminated, and particle rearrangement within the granules causes increased densification at high pressures. The types of granule-related defects that may persist after compaction include persistent intergranular pores and poorly joined interfaces between granules. Stress transmission during compaction has been modeled by a number of researchers using a continuum approach [1]. This method provides a good description of the pressure gradients that occur within compacts and the resulting density gradients that are observed. A discrete particle approach provides better insight into density gradients on a smaller scale. Granule deformation is also better understood using a discrete particle approach. Computer simulation [2] and a photoelastic disk method [3] are methods that have been used to model stress tran
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