26.01.2022 - Thomas Reichenbach "Steric Effects Control Dry Friction of H- and F‑Terminated Carbon Surfaces"

Steric Effects Control Dry Friction of H- and F‑Terminated Carbon Surfaces

A stable surface passivation of surface dangling bonds underlies the outstanding friction properties of hard carbon coatings. However, the number of possibilities to passivate the surfaces are manifold, and there are no established structure-property relationships between the surface chemical passivation and friction.

To shed light on this topic, we develop a quantum-mechanical-based force field for non-reactive interactions in dry hydrogenated and fluorinated diamond and diamond-like carbon [1]. Molecular dynamics simulations reveal a linear correlation between friction and the corrugation of the contact potential energy. At odds with what is often proposed in the literature, the latter is almost exclusively determined by the steric properties of the passivation species, i.e., by the density of terminations, by the C−H and C−F bond lengths, and by the H and F atomic radii, while the atomic charges only play a minor role. These results are consistent with the rationalization of polar hydrophobicity in fluorocarbons [2]. The presented multiscale approach provides a general strategy to establish design rules for optimally terminated systems with minimal friction. As an outlook, the implications of the results are discussed for the solid lubrication of mechanical components by PTFE [3].

[1] Reichenbach et al., ACS Appl. Mater. Interfaces 12, 7, 8805 (2020)

[2] Mayrhofer et al., J. Am. Chem. Soc. 138, 12, 4018 (2016)

[3] von Goeldel et al., Tribol. Lett. 69, 136 (2021)