Scale effects in dry friction at macro- to nanoscale were considered. According to the adhesional model of friction, the friction force depended upon the real area of contact and the average shear strength of asperity contacts during sliding. The scale dependence of the so-called geometrically necessary dislocations caused enhanced hardness with decreasing scale. In the case of plastic contacts, enhanced hardness results in a decrease in the real area of contact. The average shear strength at the interface was associated with dislocation-assisted sliding (micro-slip) and increased with decreasing scale, from geometrical considerations. In cases of single-asperity and multi-asperity elastic or plastic contact, the scale dependence of the real area of contact and shear strength results in scale-dependent friction. Comparisons of the model with experimental data were also presented.

Scale Effects in Friction using Strain Gradient Plasticity and Dislocation-Assisted Sliding (Micro-Slip). B.Bhushan, M.Nosonovsky: Acta Materialia, 2003, 51[14], 4331-45