The classical molecular dynamics simulations presented here examine the tribology associated with the sliding of a hydrogen-terminated diamond counterface across a monolayer of n-alkane chains that are covalently bound to a diamond substrate. Two systems using chains of fixed length (18 carbon atoms per chain) on diamond (111) are examined:  a tightly packed (2 × 2) arrangement and a loosely packed system with approximately 30% fewer chains. Both systems give a similar average friction at low loads. Under high loads, the tightly packed monolayer exhibits significantly lower friction than the loosely packed monolayer. While the movement of chains is greatly constricted in both systems, the tightly packed monolayer under high loads is clearly more uniform in geometry and more constrained with respect to the movement of individual chains than the loosely packed monolayer. This suggests that efficient packing of the chains is responsible for the lower friction for tight packing under high load. This is supported by the fact that sliding initiates larger bond-length fluctuations in the loosely packed system, which ultimately lead to more energy dissipation via vibration.

Classical molecular dynamics simulations have been conducted to investigate the atomic-scale friction and wear when hydrogen-terminated diamond (111) counterfaces are in sliding contact with diamond (111) surfaces coated with amorphous, hydrogen-free carbon films. Two films, with approximately the same ratio of sp(3)-to-sp(2) carbon, but different thicknesses, have been examined. Both systems give a similar average friction in the load range examined. Above a critical load, a series of tribochemical reactions occur resulting in a significant restructuring of the film. This restructuring is analogous to the "run-in" observed in macroscopic friction experiments and reduces the friction. The contribution of adhesion between the probe (counterface) and the sample to friction was examined by varying the saturation of the counterface. Decreasing the degree of counterface saturation, by reducing the hydrogen termination, increases the friction. Finally, the contribution of long-range interactions to friction was examined by using two potential energy functions that differ only in their long-range forces to examine friction in the same system.

The effect of filling nanotubes with C60, CH4, or Ne on the mechanical properties of the nanotubes is examined. The approach is classical molecular dynamics using the reactive empirical bond order (REBO) and the adaptive intermolecular REBO potentials. The simulations predict that the buckling force of filled nanotubes can be larger than that of empty nanotubes, and the magnitude of the increase depends on the density of the filling material. In addition, these simulations demonstrate that the buckling force of empty nanotubes depends on temperature. Filling the nanotube disrupts this temperature effect so that it is no longer present in some cases.

Molecular dynamics simulations were used to examine the mechanical and tribological properties of amorphous-carbon thin films with and without surface hydrogen. The simulations showed that the three-dimensional structure, not just the sp3-to-sp2 carbon ratio, is paramount in determining the mechanical properties of the films. Particular orientations of sp2 ringlike structures create films with both high sp2 content and large elastic constants. Films with graphite-like top layers parallel to the substrates have lower elastic constants than films with large amounts of sp3-hybridized carbon. The layered structure of the hydrogen-free films caused them to have novel mechanical behavior, which also influenced the shape of the friction versus load data. The atomic-scale structure of the film at the interface was of critical importance in determining the load at which tribochemical reactions (or wear) between the counterface and films were induced.

Scanning force microscopies ͑SFM͒ are being routinely used to examine the mechanical and tribological properties of materials with the goal of obtaining information, such as Young's Moduli and shear strengths from the experimental data ͓Unertl, J. Vac. Sci. Technol. A 17, 1779 ͑1999͔͒. Analysis of data obtained from an SFM experiment typically requires the use of continuum mechanics models to extract materials properties. When applying these models care must be taken to ensure that the experimental conditions meet the requirements of the model being applied. For example, despite many successful applications of the Johnson-Kendall-Roberts ͑JKR͒ model to SFM data, it does not take into account the presence of a compliant layer on the sample surface. Recent AFM experiments that examined the friction of self-assembled monolayers ͑SAMs͒ have confirmed that friction versus load data cannot be fit by the JKR model. The authors suggest that the penetration of the SAM by the tip gives rise to an additional contribution to friction due to "plowing" ͓Flater et al., Langmuir 23, 9242 ͑2007͔͒. Herein, molecular-dynamics simulations are used to study atomic contact forces between a spherical tip in sliding contact with a SAM. These simulations show that different regions around the tip contribute in unanticipated ways to the total friction between the tip and the monolayer and allow for the number and location of monolayer atoms contributing friction to be determined. The use of atomic contact forces within the monolayer, instead of forces on the rigid tip layers, allows for the contributions to friction force ͑and load͒ to be deconvoluted into forces that resist ͑repel͒ and assist ͑attract͒ tip motion. The findings presented here yield insight into the AFM experiments of SAMs and may have important consequences for the adaptation of continuum contact models for the contact between a sphere and surface where penetration into the sample is possible.