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TTMWNT2005 - Frictional anisotropy of oriented carbon...

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Unformatted text preview: Frictional anisotropy of oriented carbon nanotube surfaces P.L. Dickrell a , S.B. Sinnott b , D.W. Hahn a , N.R. Raravikar c , L.S. Schadler c , P.M. Ajayan c and W.G. Sawyer a,b,* a Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA b Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA c Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Received 5 April 2004; accepted 21 May 2004 This report examines highly anisotropic tribological behavior of multi-walled nanotube films oriented in mutually orthogo- nal directions. The average values of coefficient of friction varied from extremely high values ( l ¼ 0.795) for vertically aligned nanotubes grown on rigid substrates to very low values ( l ¼ 0.090) for nanotubes dispersed flat on the same substrates. The results were insensitive to humidity, in contrast to graphite materials, and indicate that nanotubes could be utilized as both low and high frictional surfaces. KEY WORDS: carbon nanotubes, coefficient of friction, micro-tribology, engineered surfaces Carbon nanotubes have unique properties, such as high tensile and flexural strengths, high elastic modu- lus and a high aspect ratio, that make them attractive for tribological applications [1–6]. Particularly, films up to several microns in thickness can be easily depos- ited on select substrates by chemical vapor deposition (CVD) [7], to configure novel nanostructured surfaces in tribology. Nanotube filled polymer [8,9], ceramic [10–12], and metal nanocomposites [13–15] have already been considered. In these composites, it was observed that the friction and wear rates were reduced through the addition of nanotubes. In a study with arrays of vertically aligned amorphous carbon nanofi- bers, low friction coefficients ( l < 0.1) were measured in a humid laboratory air environment, consistent with a graphitic surface [16]. However, the overwhelming argument for the reduction of friction in nanotube reinforced composite structures has been based on the hypothesis of an intrinsic self-lubricity of nanotubes [9,14] at the tribological interface. The ability of indi- vidual carbon nanotubes to both slide and roll on their surfaces has been demonstrated in both atomic force microscopy-based experiments [17,18] and atomistic simulations [19,20] and their use as low friction nano- scopic bearings has been proposed [21]. Considering the highly anisotropic geometric struc- ture of individual carbon nanotubes, it is expected that the tribological properties of nanotubes are orientation dependent. In the present study, the coefficient of friction for a bed of vertically aligned capped multi- walled nanotubes and a layer of multi-walled nanotubes randomly oriented in the plane of the substrate (trans- versely aligned) is experimentally evaluated. The coeffi- cient of friction was measured under a glass pin where...
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