Defects and friction in alkylsilane self-assembled monolayers

The adhesion and friction between pairs of self-assembled monolayers (SAMs) of alkylsilane chains on a silicon dioxide surface are studied using molecular dynamics simulations. We study chains with n=6, 8, 12, and 18 carbons in the backbone for both fully packed and defected monolayers. The defects are introduced by the random removal of chains from a well-ordered crystalline substrate. The adhesion force between monolayers at a given separation is found to increase monotonically with chain length and with coverage for a fixed chain length for the crystalline substrate. Friction simulations were performed at a relative shear velocity of 2 m/s at constant applied loads between 200 and 600 MPa. Stick slip motion is observed at full coverage, but disappears with the inclusion of 10% defects. We find that with the addition of random defects, the friction becomes insensitive to both chain length and defect density.