Atomistic Molecular Dynamics Study of Structural and Thermomechanical Properties of Zdol Lubricants on Hydrogenated Diamond-Like Carbon

Using atomistic (all-atom) molecular dynamics simulations with COMPASS force-field, we study the structural and thermomechanical properties of Zdol lubricants accommodated on a hydrogenated diamond-like carbon substrate. It is found that the lubricant molecules form a distinct layered structure near the substrate surface. The thickness of each layer and the location of each density maximum depend on temperature. The computed radius of gyration shows a strong anisotropy near the substrate surface and both parallel and perpendicular components of the radius of gyration increase with increasing temperature. In addition, it is found that the lubricant diffusion coefficient is also anisotropic with the component parallel to the surface being larger than the perpendicular one, and both increase with the lubricant film thickness. Using the calculated diffusion coefficients, we extract the activation energies for lubricant diffusion. We also qualitatively compare the present all-atom simulations with the previous coarse-grained simulations.