Showing papers by "Judith A. Harrison published in 2005"

Odd and Even Model Self-Assembled Monolayers: Links between Friction and Structure

Abstract: The friction between an amorphous carbon tip and two n-alkane monolayers has been examined using classical molecular dynamics simulations. The two monolayers have the same packing density, but the chains comprising each monolayer differ in length by one -CH2- unit. The simulations show that the monolayers composed of C13 chains have higher friction than those composed of C14 chains when sliding in the direction of chain cant; the difference in friction becomes more pronounced as the load is increased. Examination of the contact forces between the chains and the tip, along with conformational differences between the two chain types, lends insight into the friction differences.

Contact forces at the sliding interface: mixed versus pure model alkane monolayers.

Abstract: Classical molecular dynamics simulations of an amorphous carbon tip sliding against monolayers of n-alkane chains are presented. The tribological behavior of tightly packed, pure monolayers composed of chains containing 14 carbon atoms is compared to mixed monolayers that randomly combine equal amounts of 12- and 16-carbon-atom chains. When sliding in the direction of chain cant under repulsive (positive) loads, pure monolayers consistently show lower friction than mixed monolayers. The distribution of contact forces between individual monolayer chain groups and the tip shows pure and mixed monolayers resist tip motion similarly. In contrast, the contact forces “pushing” the tip along differ in the two monolayers. The pure monolayers exhibit a high level of symmetry between resisting and pushing forces which results in a lower net friction. Both systems exhibit a marked friction anisotropy. The contact force distribution changes dramatically as a result of the change in sliding direction, resulting in an ...

Effect of filling on the compressibility of carbon nanotubes: predictions from molecular dynamics simulations.

Abstract: The compressibility of filled and empty (10, 10) carbon nanotubes (CNTs) is examined using classical molecular dynamics simulations. The filled nanotubes contain C60, CH4, Ne, n-C4H10, and n-C4H7 molecules that are covalently cross-linked to the inner CNT walls. In addition, nanotubes filled with either a hydrogen-terminated carbon nanowire or a carbon nanotube of comparable diameter is also considered. The forces on the atoms are calculated using a many-body reactive empirical bond-order potential and the adaptive intermolecular reactive empirical bond-order potential for hydrocarbons. The butane-filled system shows a unique yielding behavior prior to buckling that has not been observed previously. Cross-linking the molecules to the inner CNT walls is not predicted to affect the stiffness of the filled nanotube systems and removes the yielding response. The mechanical response of the nanowire filled CNT is remarkably similar to the response of the similarly sized multiwalled CNT.

Computational Modeling of Nanometer-Scale Tribology

Abstract: Friction and wear have long been acknowledged as limiting factors to numerous applications and many areas of technology, which has lead to significant interest in understanding and controlling these processes. Current interest in microscale and nanoscale machines with moving parts add to this interest, especially as the mechanisms that lead to friction at the atomic-scale can sometimes be quite distinct from the mechanisms that dominate at the macroscale.

The Tribology of Carbon, Hydrogen, and Silicon-Containing Solid Lubricants (Keynote)

Abstract: Constant temperature molecular dynamics simulations and the adaptive intermolecular reactive empirical bond-order potential energy function [1] (AIREBO) were used to examine the tribology of model self-assembled monolayers (SAMs) attached to diamond substrates. Two types of monolayers were examined. One was composed of alkane chains containing 14 carbons atoms and the other was composed of equal mixtures of 12 and 16 carbon-atom chains. The simulations have yielded unique insight into the origin of the friction differences between the two monolayer systems.

Effect of Atomic-Scale Surface Roughness on Friction

Abstract: Simulations of friction between clean, atomically flat surfaces indicate that the friction force per unit area decreases to zero with increasing area. In previous work, we showed that the presence of mobile atoms between surfaces suppresses this superlubric behavior. The current paper examines the effect of atomic scale roughness. Both single asperities and random self-affine surfaces are considered in two and three-dimensional geometries. We have also examined the effect of commensurability, and elastic or plastic deformation within the bounding solids. The friction force on single asperities decreases to zero as the contact area and radius of curvature increase, but can still be significant at the scale of scanning probe tips. Introducing mobile atoms leads to friction forces that rise linearly with area in large contacts. Frictional forces on random self-affine surfaces can be quite complex. If self-affine scaling extends down to atomic dimensions, the average contact between elastic solids only contains a few atoms and the friction can be large. Plasticity increases contact dimensions and may lower the friction.

Friction and Wear Studies of Hydrogenated Amorphous Carbon Films Containing Silicon

Abstract: The ratios of sp2 to sp3 content for a series of hydrogenated amorphous carbon-silicon systems were determined using molecular dynamics simulation. The values of elastic modulii were then determined for each system using constant tension and constant temperature molecular dynamics simulation method. The relationship between sp2 to sp3 content and modulii was investigated by varying Si and H content in the films. The thermal conductivity and heat capacity were also calculated for each system. A series of sliding simulations was used to determine which properties had the greatest effects on the resulting friction and wear behavior of the films.