Friction between alkylsilane monolayers: Molecular simulation of ordered monolayers

The adhesion and friction between two ordered self-assembled monolayers of alkylsilane chains on a silicon dioxide surface are studied using molecular dynamics simulations. These simulations focus on the alkylsilanes, which are a surface coating commonly used in microelectromechanical systems. We have investigated chain lengths of 6-18 carbon atoms for systems with 100-1600 molecules on each surface. The adhesion force between monolayers at a given tail atom separation is found to increase monotonically with decreasing chain length. For each chain length the friction simulations were performed for a range of shear velocities at separation distances corresponding to pressures of 0.2 and 2 GPa. Stick-slip motion is observed at all velocities, chain lengths, and separations for the fully packed and well-ordered systems studied here. At high pressures the average friction force is independent of chain length and velocity, while at low pressures there is a weak dependence of the friction force on shear velocity. We have investigated possible system size effects in our simulations and find that larger system sizes result only in decreased noise.