Manufacturing Nanocomposite Parts: Present Status and Future Challenges
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Manufacturing
Nanocomposite Parts: Present Status and Future Challenges
S. Seal, S.C. Kuiry, P. Georgieva, and A. Agarwal Abstract The promises of nanotechnology are mostly based upon the ability to produce nanostructured materials with novel properties. Nanocomposites are defined here as a class of materials that contain at least one phase with constituents in the nanometer domain. This article describes the present state of knowledge of the fabrication of nanocomposite materials, with special emphasis on plasma forming of bulk parts. Future challenges facing the development of methods for consolidating nanocomposites with retained nanostructures are also highlighted. Keywords: ceramics, laser consolidation, grain growth, hot isostatic pressing (HIP), metals, nanocomposites, nanostructure, plasma forming, shock wave compaction, sintering.
Introduction
Consolidation of Nanoparticles
Composite materials containing at least one phase with constituents of less than 100 nm in size can be termed nanocomposites.1 These materials exhibit enhanced mechanical,2,3 magnetic,2 high-temperature,4 and optical5–7 properties, as well as excellent catalytic properties.8 The commercial applications of nanocomposites rely on the successful consolidation of these materials into bulk-sized components while preserving their nanostructures. Traditional consolidation techniques such as cold pressing and sintering at high temperatures, hot pressing, and hot isostatic pressing have strong limitations of not being able to retain the nanoscale grain size due to excessive grain growth during processing. This article summarizes the results of numerous studies on methods for manufacturing nanocomposites and also highlights future challenges for the successful consolidation of nanocomposite components.
The density of an unsintered compact depends on the frictional forces of the powdered particles, which originate from electrostatic, van der Waals, and surface adsorption phenomena. In the case of nanoparticles, these forces are significantly high, forming hard agglomerates and eventually leading to the creation of relatively large inter-agglomerate pores. Based on thermodynamic treatments, Mayo9 suggested that the finest pore size usually yields the highest densification rate. Therefore, in order to attain the highest densification rate during the consolidation of nanoparticles, large inter-agglomerate pores should be avoided. However, eliminating large pores requires not only higher temperatures but also prolonged sintering times; consequently, nanoscale grain sizes are difficult to retain. During sintering of nanoparticles, pores smaller than a critical size shrink,10,11 while larger pores undergo the pore-boundary separa-
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tion, which restricts the ability to attain full density in the consolidated nanoparticles.6,12 The fraction of grain boundaries in nanomaterials is large compared with that in coarse-grained materials. The density of the grain-boundary regions is less than the grain interior, due to the relaxation of atoms and the
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