Characterization and Control of Compact Microstructure
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Powder-Compact Microstructure Under ideal conditions, a ceramic powder-compaction process would produce a defect-free, high-density compact that shrinks uniformly and predictably during sintering to produce a net-shaped part without the need for costly postfire machining (i.e., hard grinding). However in dealing with real powders and processes, practical obstacles prevent this idealized result. To improve powder compaction, critical relationships between microstructure and macrostructure need to be understood and applied. As examples, consider the following is-
(1) Minimizing defects. Whether to develop an adequate surface finish for inking operations or to minimize the size and concentration of pores in a compact to optimize mechanical properties,1 the efficient removal of porosity is a primary goal of compaction. However relying on classic hierarchical relationships between packing and particle size is complicated by the nonidealities inherent in real materials, including nonspherical particles and granules (i.e., clusters or agglomerates of smaller, primary particles), hollow agglomerates, and troublesome fines/ satellites (smaller particles and agglomerates) in the particle- and granule-size distributions. Realistic packing and compaction predictions must account for such irregularities, and new models and modeling techniques are moving in that direction. (2) Controlling Dimensional Tolerances. The relatively unexplored 2 connection between the sintering behavior of the primary particles that comprise a powder and the overall shrinkage of a compact is of considerable importance in ceramic component manufacturing. The fact that microstructural (density) variations within a body can cause differences in sintering shrinkage was recognized decades ago by Coble3 who noted that regions of higher green (i.e., as-pressed) density within a powder compact require less time to densify than regions of lower density. Until recently4'5 predictions of nonuniform shrinkage based on density gradients have been lacking, presumably because of the difficulty in accurately measuring either variable. With improved characterization of density and density gradients, as well as models and techniques that can be applied to predict dimensional change, significant advances can be expected. (3) Fast firing. Some of the demands on
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the dry-pressing process stem from the need to make the sintering process more efficient. Furnace-cycle times for all components are decreasing. For some powder-metallurgy parts, thermalexposure times as low as 5-10 min have been achieved. Led by advances made in the "dust-pressing" industry, cycle times for ceramic components are also expected to decrease. Slow firing rates have been observed to have mixed effects on the evolution of density distributions during sintering.6 However faster firing rates exaggerate the differences in densification between high-density (HD) and lowdensity (LD) zones within a compact (Figure 1), resulting in differential sintering shrinkage that produces distortions within the c
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