Effects of tip geometry on interfacial contact forces

Experimental techniques that utilize atomic force microscopy are routinely used to examine tribological properties of tip-sample interactions. While analysis of data obtained with these methods provides values for macroscale properties, such as interfacial shear strength, understanding nanoscale properties, such as contact radius, requires an atomic-scale approach. Molecular dynamics simulations provide the ability to numerically analyze the nanoscale origins of a wide-range of material and tribological properties. In this paper, the sliding contact between a self-assembled monolayer (SAM) and two countersurfaces (a nominally flat, amorphous carbon surface and a nearly spherical fullerene tip) is compared. By examining contact forces between the tip and monolayer atoms, large differences in monolayer behavior that occur due to tip geometry can be elucidated. The structure factor reveals that the fullerene tip creates a more disordered monolayer than the amorphous counterface. Friction forces were also studied using the atomic-level contact forces, which show that the depth at which the fullerene tip affects the SAMs substrate is much deeper than the amorphous counterface. The distribution of contact forces that contribute to friction and load were studied and show a difference in behavior between the two countersurfaces. Finally, while there are a large number of atoms that have a non-zero load during sliding, a smaller subset of 32 atoms carries similar to 96% of the load. Using this subset of atoms to compute contact radius reveals a greater agreement with the continuum mechanics models than using all atoms with a non-zero load. This paper highlights how computer simulations can yield insight into tribological interactions at the atomic scale.