A Molecular Dynamics Analysis of Surface Interference and Tip Shape and Size Effects on Atomic-Scale Friction

Molecular dynamics simulations of a rigid diamond tip sliding on a face-centered-cubic copperlike substrate were performed in order to examine the dependence of the friction coefficient on the tip–substrate interference and the shape and size of the tip. For a square-base prismatic tip, the friction force is mainly due to interactions of atoms at the front face of the tip and substrate atoms ahead of the tip, while the normal force is due to interactions of atoms at the tip base and substrate atoms under the tip. However, for a pyramidal tip, both normal and friction forces are mainly due to interactions between atoms at the front face of the tip and substrate atoms in the vicinity of the sliding tip. Consequently, the friction coefficient is either sensitive (square-base prismatic tip) or insensitive (pyramidal tip) to the tip–substrate interference distance. In addition, tip size and orientation effects on the friction coefficient were observed with square- and triangle-base prismatic tips, respectively. Lower friction coefficients were obtained with a larger base area and edge-front sliding with a triangle-base prismatic tip. The results provide insight into atomic-scale friction anisotropies due to the effects of the tip size and shape and the tip–substrate interference.