Effect of the sliding orientation on the tribological properties of polyethylene in molecular dynamics simulations

The tribological behavior of oriented poly(tetrafluoroethylene) (PTFE) sliding surfaces is examined as a function of sliding direction and applied normal load in classical molecular dynamics (MD) simulations. The forces are calculated with the second-generation reactive empirical bond-order potential for short-range interactions, and with a Lennard-Jones potential for long-range interactions. The range of applied normal loads considered is 5-30 nN. The displacement of interfacial atoms from their initial positions during sliding is found to vary by a factor of seven, depending on the relative orientation of the sliding chains. However, within each sliding configuration the magnitude of the interfacial atomic displacements exhibits little dependence on load over the range considered. The predicted friction coefficients are also found to vary with chain orientation and are in excellent quantitative agreement with experimental measurements.

The role of sliding orientation on the tribological properties of polyethylene (PE) is investigated by using classical molecular dynamics simulations. Cross-linked PE surfaces slide against one another in two different directions: one that is perpendicular to and one that is parallel to the aligned direction of the polymer chains. The results indicate that sliding in the parallel direction occurs with a lower friction coefficient than sliding in the perpendicular direction. In both cases, gross level stick-slip motion is observed to be associated with the sliding of a restrained, corrugated molecular interface. In addition, the simulations demonstrate the way in which the system stores more shear strain energy during sliding in the perpendicular direction. The tribological behavior of these PE surfaces is compared to the behavior of similarly modeled polytetrafluoroethylene surfaces; the differences and similarities between the two systems are discussed.

Crosslinks occur in polymers following irradiation and are used in computational simulations to mimic the effects of chain tangling. Here, the effect of crosslink density on the tribological behavior of atomic-scale models of polyethylene and polytetrafluoroethylene is determined using classical molecular dynamics simulations. In the simulations, oriented crosslinked surfaces are slid in different directions over a range of applied normal loads. The results indicate that, at the same normal load, the friction force increases with increased crosslink density. In addition, the influence of randomized versus ordered crosslinking on the simulated tribological behavior is investigated. Finally, the influence of crosslink density on the simulated wear mechanisms of polyethylene and polytetrafluoroethylene is elucidated. The results have important implications for the atomic-scale modeling of friction at the interfaces of polymers that have been irradiated or contain entangled chains.

The tribological behavior of oriented, selfmated poly(tetrafluoroethylene) (PTFE) sliding surfaces is examined as a function of temperature from 25 to 300 K. Three distinct sliding configurations are considered: sliding perpendicular to the chain alignment in both PTFE surfaces, sliding parallel to chain alignment in both PTFE surfaces and sliding parallel to the chain alignment on one surface but perpendicular to chain alignment on the other. Our simulations reveal a general trend of increasing friction coefficient with decreasing temperature for configurations where the chains are aligned in the direction of sliding or crossed. However, for conditions where the chains are aligned but the sliding motion is perpendicular to this alignment, gross deformation and athermal friction behaviors are observed. The atomic-level processes associated with these increases in friction at low temperatures are characterized.