Mechanical and Tribological Properties of Carbon Nanotubes Investigated with Atomistic Simulations
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Mechanical and Tribological Properties of Carbon Nanotubes Investigated with Atomistic Simulations Boris Ni and Susan B. Sinnott The University of Florida, Department of Materials Science and Engineering, 154 Rhines Hall, Gainesville, FL 32611-6400 ABSTRACT Atomistic simulations are used to better understand the behavior of bundles of singlewalled carbon nanotubes that have been placed between two sliding diamond surfaces. A many-body reactive empirical potential for hydrocarbons that has been coupled to a Lennard-Jones potential is used to determine the energies and forces for all the atoms in the simulations. The results indicate that the degree of compression of the nanotube bundle between the nanotubes has a significant effect on the responses of the nanotubes to shear forces. However, no rolling of the nanotubes is predicted in contrast to previous studies of individual nanotubes moving on graphite. INTRODUCTION It has been shown that while most carbon nanotubes have an exceptionally high Young’s modulus in the direction of the nanotube axis,1,2 they also have a low Young's modulus perpendicular to the nanotube axis.3 This makes them susceptible to shear, although simulations and experiments also show that nanotubes can withstand significant compressive1,2,4,5 and tensile6 forces prior to failure because of their great flexibility. In this work the responses of bundles of single-walled, (10,10) carbon nanotubes to shear forces is investigated. The bundles are placed between two hydrogen-terminated diamond (111) surfaces, one of which slides relative to the other that approximates typical tribology and wear experimental conditions. Several previous studies have examined the movement of carbon nanotubes on graphite surfaces in response to shear forces and found that they slide along the surface except when the helical structure of the nanotubes align with the substrate which allows them to roll.7,8,9 This study is motivated by the fact that nanotube-based composites have been proposed for use in high wear applications such as break pads. Their use in these applications would benefit from a fundamental understanding of the responses of nanotubes in more controlled environments such as are present in these simulations. The objective of this work is to determine the effect of the amount of compression of the carbon nanotubes between the sliding surfaces on their response to shear. COMPUTATIONAL DETAILS The atomistic approach taken is classical molecular dynamics simulations10 where Newton's equations of motion are numerically integrated using a timestep of 0.2 femtoseconds (fs). The forces on the atoms are calculated using an analytic reactive empirical bond-order potential (REBO) developed by Brenner11 coupled to a long-range Lennard-Jones potential as described in detail elsewhere.12 This many-body potential has A17.3.1
been extensively used to study the mechanical properties of carbon nanotubes4,5,13,14 in addition to tribology and lubrication at diamond surfaces.15,16 It has been shown to provide reasonable pr
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