Comprehensive model for scale effects in friction due to adhesion and two- and three-body deformation (plowing)

The coefficient of friction is a sum of the adhesional component and two- and three-body deformation (plowing) components. Scale effects in dry friction at macro- to nanoscale are considered. All components depend on the real area of contact (dependent upon surface roughness and mechanical properties) and shear strength during sliding. In the adhesional component, elastic and plastic deformation and single- and multiple-asperity contacts are considered. For multiple-asperity predominantly plastic contacts, the real area of contact is scale dependent, due to the effect of strain-gradient plasticity, and decreases with decreasing scale, whereas the shear strength is scale-dependent due to the effect of the dislocation-assisted sliding, and increases with decreasing scale. The two-body deformation component increases with decreasing scale due to increasing average asperity slope, an important parameter of interest. The three-body deformation component decreases with decreasing scale, due to decreasing probability for a particle to be trapped at the interface, although the shear strength increases. Transition from elastic deformation to plastic deformation (plowing) is considered. Comparisons of the model with experimental data are presented.