| While static and kinetic friction are often correlated, they arise from different mechanisms. Static friction is related to the potential energy barriers that must be overcome during sliding, while kinetic friction is related to the amount of energy dissipated once the barriers are passed. The contributions of phonon damping, elastic instability and plastic deformation to kinetic friction will be reviewed and illustrated with simulations. Studies of nanoscale single asperity contacts will be presented to contrast the behavior of bare contacts and contacts with adsorbed molecular layers. Results for bare contacts are sensitive to the detailed tip geometry and display a variety of frictional behavior. Adsorbed layers introduce a new energy dissipation mechanism associated with plastic deformation in the layer. This reduces the sensitivity to tip geometry and leads to a linear relation between friction and load. Multi-asperity contacts between rough surfaces will also be discussed. The surface roughness present on many experimental surfaces is high enough to produce plastic deformation in the substrate. The effect on friction will be illustrated for both crystalline and amorphous substrates and related to dissipation in bulk systems. |
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