Low temperature synthesis of carbon nanotube-reinforced aluminum metal composite powders using cryogenic milling
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Brent A. Bottolfson and Luke N. Brewer Department of Mechanical and Aerospace Engineering, Naval Postgraduate School, Monterey, California 93943, USA (Received 12 May 2014; accepted 3 October 2014)
Carbon nanotube (CNT)-reinforced aluminum composite powders were synthesized by cryogenic milling. The effects of different milling parameters and CNT contents on the structural characteristics and mechanical properties of the resulting composite powders were studied. Detailed information on powder morphology and the dispersion and structural integrity of the CNTs is crucial for many powder consolidation methods, particularly cold spray, which is increasingly utilized to fabricate metal-based nanocomposites. While all of the produced composite powders exhibited particle sizes suitable for spray applications, it was found that with increasing CNT content, the average particle size decreased and the size distribution became narrower. The dispersion of CNTs improved with milling time and helped to maintain a small Al grain size during cryogenic milling. Although extensive milling allowed for substantial grain size reduction, the process caused notable CNT degradation, leading to a deterioration of the mechanical properties of the resulting composite.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] b) Current Address: School of Materials Engineering, Purdue University, 701 West Stadium Avenue, West Lafayette, IN 47907-2045, USA DOI: 10.1557/jmr.2014.300
average Young’s modulus of 1.8 TPa, while theoretical calculations predict values as high as 4.15 TPa.7 More recent measurements conducted on the outermost layers of MWCNTs using a “nanostressing stage” indicated that Young’s modulus varied from 270 to 950 GPa,10 depending on the structural variety and defects of CNT samples. Unlike traditional reinforcement agents (e.g., oxides, carbides), CNTs possess an extremely low density (1.2–1.8 g/cm3) and thus enable substantial weight saving while improving the overall performance of the matrix material.10 As with most advanced material systems, there are a number of practical challenges on the path toward successful synthesis and application of CNT–MMCs. Similar to CNT-based polymer composites, the performance of CNT–MMCs depends largely on the ability to effectively disperse the reinforcement agent in the metal matrix, to maintain the structural integrity of the CNTs during composite synthesis and processing, and to establish a suitable metal–CNT interface.11–14 Like most carbon nanostructures, CNTs exhibit a strong tendency to agglomerate, form bundles, and entangle, which can prevent an effective dispersion in the matrix material. Several attempts have been made to reduce the agglomeration of CNTs prior to composite synthesis including ultrasonication, chemical functionalization, and mechanical grinding.15–17 In addition to the agglomeration issue, due to the sp2-nature of CNTs, high synthesis or processing temperatures may lead to chemical reactions between the reinforcement agent and
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