Effects of a self-assembled monolayer on the sliding friction and adhesion of an Au surface

The friction and adhesion mechanisms with and without a self-assembled monolayer (SAM) in nanotribology were studied using molecular dynamics (MD) simulation. The MD model consisted of two gold planes with and without n-hexadecanethiol SAM chemisorbed to the substrate, respectively. The molecular trajectories, tilt angles, normal forces, and frictional forces of the SAM and gold molecules were evaluated during the frictional and relaxation processes for various parameters, including the number of CH₂ molecules, the interference magnitude, and whether or not the SAM lubricant was used. The various parameters are discussed with regard to frictional and adhesion forces, mechanisms, and molecular or atomic structural transitions. The stick–slip behavior of SAM chains can be completely attributed to the van der Waals forces of the chain/chain interaction. When the number of CH₂ molecules was increased, the SAM chains appeared to have bigger tilt angles at deformation. The magnitude of the strain energy that was saved and relaxed is proportional to the elastic deformable extent of the SAM molecules. The frictional force was higher for long chain molecules. With shorter SAM molecules, the adhesion force behavior was more stable during the compression and relaxation processes. A surface coated with a SAM can increase nano-device lifetimes by avoiding interface effects like friction and adhesion. PACS 52.65.Yy; 81.40.Pq; 81.16; 68.35.-p