Reinforcing effects of carbon nanotubes in structural aluminum matrix nanocomposites

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Junsik Park Division of Advanced Materials Engineering, Hanbat University, Daejoen 305-719, Korea

Donghyun Baea) Division of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea (Received 15 December 2008; accepted 12 May 2009)

The reinforcing effects of carbon nanotubes (CNTs) are investigated for aluminum matrix composites. The composites present a strong bonding between CNTs and the aluminum matrix using a controlled mechanical milling process, producing a network structure of aluminum atoms around CNTs. At the same time, CNTs that are dispersed during the milling process can be located inside aluminum powders, thereby providing an easy consolidation route via thermomechanical processes. A composite containing 4.5 vol% multiwalled CNTs exhibits a yield strength of 620 MPa and fracture toughness of 61 MPamm1/2, the values of which are nearly 15 and seven times higher than those of the corresponding starting aluminum, respectively.

I. INTRODUCTION

The discovery of carbon nanotubes (CNTs) has garnered great attention for structural and functional applications.1–12 While significant outcomes have been reported on functional applications of carbon nanotubes, reports covering substantial developments using CNTs for structural applications have yet to be made; the exceptional properties of CNTs have not been translated to bulk composites. It has been well documented that the critical limitations for a direct application of CNTs originates from both (i) difficulties in distribution of agglomerated CNTs through the matrix, and (ii) poor interface bonding properties between CNTs and the matrix, when CNTs are incorporated as a reinforcing material in a metal matrix. Without solving the two critical factors, the giga-level ultra strength of the CNTs would not work in any structural composite applications.1–12 There have been a fair number of studies that attempted to resolve the problems by using the solid1,2 or liquid state technique.3–12 The specimens produced by casting show entangled CNTs and some carbides or oxides formed at the interface between CNTs and the matrix,1,2 leading to detrimental effects on the mechanical properties due to the poor nature of the interfaces. Although the specimens produced by the powder metallurgy method exhibit enhanced strength,3–6 difficulties in a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0318

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http://journals.cambridge.org

J. Mater. Res., Vol. 24, No. 8, Aug 2009 Downloaded: 11 Jun 2014

consolidation, poor bonding between CNTs and the metals, and distortion of the original structure of CNTs have been considered as still vital issues to be overcome. Alternative effort such as molecular level mixing,7 the in situ reduction approach method,8 plasma spray formation,9 and high pressure torsion10 have broadened the scope for material strengthening by addition of CNTs. However, they have also resulted in a weak bonding between CNTs and the metal matrix. Furthermore, since CNTs are transformed into carbid

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