Molecular dynamic simulation and characterization of self-assembled monolayer under sliding friction

The frictional mechanisms of a self-assembled monolayer (SAM) during nano-scale sliding are studied using molecular dynamics (MD) simulation. The MD model consists of a gold slider and gold substrate with n-hexadecanethiol SAM chemisorbed to the substrate. The trajectory, tilt angles, normal forces, frictional forces, friction coefficients and potential energies per molecular chain of the SAM molecules are evaluated during the frictional process for various parameters including as sliding height, sliding direction (i.e. pro- or anti- the SAM tilt angle), sliding velocity and system temperature. The various parameters are discussed with regard to frictional forces, mechanisms and SAM structural transition. Results show that stick-slip occurs and is related to the sliding period and tilt angle of the SAM molecules. Amplitude of the stick-slip cycle increases with decreasing sliding height until reaching a critical sliding height, which is characterized such that sliding below the critical height causes irreversible changes in the SAM molecular organization and cumulative loss of SAM lubricating efficiency. Different SAM recovery mechanisms were found for different sliding directions relative to SAM tilt angle (pro- or anti-tilt). In both cases, minimum friction occurred during the SAM tilt-angle recovery phase. The friction force curves for these two cases also showed a regular phase shift above the critical height. For stick-slip sliding above the critical height, anti-tilt sliding had significantly lower average friction, but this trend inverted below the critical height. Sliding lower than the critical height cause progressive disorder of the SAM structure and the characteristic differences between pro- and anti-tilt sliding were progressively lost.