Challenges and advances in nanocomposite processing techniques
Introduction
Nanocrystalline materials are characterized by a grain size or particulate size of up to about 100 nm. These materials exhibit enhanced mechanical [1], [2], [3], magnetic [1], elevated temperature [4], optical [5], [6], and excellent catalytic properties [7]. The commercial applications of nanocomposite beyond the boundaries of Materials Science Laboratories lie on the successful consolidation of these materials into bulk-sized components preserving the nanostructures. The traditional consolidation techniques have a strong limitation of not being able to retain the nanograin size due to the problem of grain growth.
The density of the green compact depends on the frictional forces of the powder particles that are originated from electrostatic, van der Waals, and surface adsorption forces. These forces are significantly high in nanoparticles forming hard agglomerates and interagglomerate which are relatively large. Based on the thermodynamic treatment of the shrinkage of the pore, Mayo [8] suggested that the finest pore size usually yields the highest densification rate. Such large pores require not only higher temperature but also prolong sintering times for their successful elimination; consequently, it becomes difficult to retain the grain size in the nanometer domain. Large pores undergo pore–boundary separation that restricts attaining the full density in the consolidated nanoparticles [6], [9]. During sintering of nanoparticles, pores smaller than the critical size shrink [10], while larger pores undergo the pore–boundary separation. The fraction of grain boundaries in nanomaterials is large compared to that for coarse-grained materials. The density of the grain boundary regions is less than the grain interior due mainly to the relaxation of atoms in the grain boundaries, and they also contain other lattice defects. Therefore, consolidated nanoparticles with retained nanostructure is expected to exhibit a density lower than the theoretical density of the bulk counterpart. There are numerous conflicting views on the sintering behavior of the nanoparticles. Nanoparticles show depressed onset of sintering temperature to the range of 0.2Tm–0.3Tm compared to that of conventional powders that normally exhibit a range of 0.5Tm–0.8Tm, where Tm is the melting point (K). Such results possibly attributed to the structural instability of these particles due to the presence of high surface area. Synthesis of nanoparticles of ceramic, metallic, and their mixture has made a substantial progress in the last decade, however, consolidation of such nanoparticles into fully densified bulk components with retained nanostructures remains a difficult problem. A brief review of the various consolidation processes is presented herein.
In this review paper, latest developments, advances and challenges in processing, microstructural details, physical, and mechanical properties of nanocrystalline materials, and coatings will be dealt with. Relative merits of each processing techniques and equipment infrastructure required to do research on specific techniques have been discussed at length. Fig. 1 represents a layout of various consolidation techniques discussed in the following sections.
Section snippets
Fundamental principles
In hot pressing, there are two degrees of
Future challenges
There are several challenges ahead on successful realization of the roadmap on the commercial applications of nanocomposites. Some of the key issues are listed below:
Nanopowders synthesis: Most of the nanocomposite fabrication techniques use nanopowders as the feedstock materials. Though synthesis and production of nanopowders have developed by leaps and bounds in the last decade, it is yet to attain the maturity to produce a large amount of nanopowders at an affordable cost for their
Concluding remarks
Fabrication technology can strongly affect the morphology and materials properties in the nanometer domain, thus innovations for manufacturing of freestanding parts and composites with retained nanostructure remain a challenge. The nanocomposite materials are expected to have extensive applications in cars, ships, airplanes and even in space vehicles. Although substantial progress has been made in understanding the structure–property relationships in nanomaterials, but further progress is
Acknowledgments
Financial supports for Seal from DOD SBIR #DASG60-P-02-41, S. Seal's ONR Young Investigator Award (ONR-YIP), NSF EEC-0139614, 0136710, NSF DMR-9974129, FSGC, DURIP, NASA SBIR Phase II, DOE SBIR Phase I, Plasma Process Inc. are gratefully acknowledged. Authors would also like to thank Dr. A.K. Vasudevan for useful discussion. The authors (A. Agarwal's group) at Florida International University would like to thank financial support from Office of Naval Research (N000140510398), National Science
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