Steric effects control dry friction of H- and F-terminated carbon surfaces

A stable passivation of surface dangling bonds underlies the outstanding friction properties of diamond and diamond-like carbon (DLC) coatings in boundary lubrication. While hydrogen is the simplest termination of a carbon dangling bond, fluorine can also be used as a monoatomic termination, providing an even higher chemical stability. However, whether and under which conditions a substitution of hydrogen with fluorine can be beneficial to friction is still an open question. Moreover, which of the chemical differences between C-H and C-F bonds are responsible for the change in friction has not been unequivocally understood yet. In order to shed light on this problem, we develop a density functional theory-based, nonreactive force field that describes the relevant properties of hydrogen- and fluorine-terminated diamond and DLC tribological interfaces. Molecular dynamics and nudged elastic band simulations reveal that the frictional stress at such interfaces correlates with the corrugation of the contact potential energy, thus ruling out a significant role of the mass of the terminating species on friction. Furthermore, the corrugation of the contact potential energy is almost exclusively determined by steric factors, while electrostatic interactions only play a minor role. In particular, friction between atomically flat diamond surfaces is controlled by the density of terminations, by the C-H and C-F bond lengths, and by the H and F atomic radii. For sliding DLC/DLC interfaces, the intrinsic atomic-scale surface roughness plays an additional role. While surface fluorination decreases the friction of incommensurate diamond contacts, it can negatively affect the friction performance of carbon surfaces that are disordered and not atomically flat. This work provides a general framework to understand the impact of chemical structure of surfaces on friction and to generate design rules for optimally terminated low-friction systems.