Molecular dynamics simulations of atomic-scale friction behavior

The friction behaviors between rigid spherical tips with different atomic scale rough topography and the elastic flat substrate are simulated using large-scale molecular dynamics method. The relationships between the friction and load, real contact area and load, friction and real contact area are analyzed for both non-adhesion and adhesion cases to study the friction behavior laws on nanometer scale. The results show that the relations between real contact area and load of the studied systems all agree well with the corresponding continuum contact models, such as Hertz smooth contact model, Greenwood-Williamson rough contact model and Maugis-Dugdale adhesive contact model. For the non-adhesive contacts, the friction-load relations are linear for both smooth and rough surface system, while there is no simple relationship between the friction and real contact area. However, the relationships between the friction and real contact area are linear for the adhesive contacts, while the friction-load relations are sublinear and agree well with the prediction from Maugis-Dugdale model. Our research shows that when it is changed from non-adhesive to adhesive surface, the decisive factor of friction will transfer from load to contact area, and the friction behavior will transfer from load-controlled friction to adhesion-controlled friction.