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MM2.5.1

Granule Structure Evolution during Powder Compaction William J. Walker, Jr. New York State Center for Advanced Ceramic Technology at Alfred University, Alfred, NY 14802, U.S.A. ABSTRACT Die compaction of granulated powder is a common forming process used in the ceramics industry. Glass spheres were used as a model system to investigate granule failure during die compaction. Since glass spheres are brittle, failure results in fragmentation. Particle size analysis of the resulting fragments demonstrates the statistical nature of granule failure during compaction, with some granules failing at very low applied pressures while a large fraction persist at even the highest applied loads. The results are discussed in terms of the Andreasen, Furnas and Dinger-Funk particle packing models for continuous size distributions.

INTRODUCTION Powder compaction is a commonly used forming process in ceramics and other industries. Granulation of fine powder by spray-drying results in free flowing feed material with controlled composition and properties that can be easily used in high speed presses. The granulated material is consolidated by application of pressure into a green body, which is subsequently sintered. Artifacts of the granule structure often persist as pores and laminations after compaction, and may persist as defects in the sintered microstructure. Such defects can be detrimental to the properties of the final part [1-3]. Thus it is desirable to eliminate the granule structure as completely as possible during compaction. In this work, glass beads were used to model granule breakdown during compaction. The glass beads are spherical, as are spray-dried granules, and were selected to be of similar size distribution to granules commonly used for processing of ceramics. Glass beads are elastic, brittle, have low bulk compression and fail by brittle fracture. Granulated powder consists of porous agglomerates, which exhibit moderate bulk compression, and are elastic at low loads. At higher loads it is often desirable for granules to deform plastically, but in industrial practice brittle failure of granules is not uncommon, and may contribute to defects in ceramic parts.

EXPERIMENT Glass beads (3M Company, St. Paul, MN) with a log normal size distribution (mean diameter 85.5 µm and geometric standard deviation 1.25) were compacted in a steel die using a laboratory press (Laboratory Press Model M, Fred S. Carver, Inc., Menomonee Falls, WI.) at pressures ranging from 17.5 MPa to 1.05 GPa. The die had a cylindrical cavity with diameter 12.7 mm, and was fabricated in two halves that could be separated to facilitate removal of the specimens without an ejection step. Size distributions of the compacted beads were measured using laser scattering (Microtrac 9200, Leeds and Northrop, North Wales, PA) after dispersing the compacted material in water using ultrasound. Compacts were vacuum infiltrated with epoxy

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and polished cross-sections were prepared and evaluated with a scanning electron microscope. For specimens p