An all-atom force field for dry and water-lubricated carbon tribological interfaces

We present a nonreactive force field for molecular dynamics simulations of interfaces between amorphous carbon surfaces and their interaction with water. The force field is tailored to surfaces with hydrogen, hydroxyl, and aromatic surface passivation and enables large-scale simulations of dry and lubricated tribological contacts. To favor its compatibility with existing force-field parameterizations for liquids and allow a straightforward extension to other types of surface passivation species, we adopt the functional form of the Optimized Potentials for Liquid Simulations. The optimization of the force-field parameters is systematic and follows a protocol that can be reused for other surface-molecule combinations. Reference data are calculated with gradient- and dispersion-corrected density functional theory and include the bonding geometry and elastic deformation of bulk and surface species as well as surface adhesion and water adsorption energy landscapes. The conventions adopted to define the different force-field atom types are based on the hybridization of carbon orbitals and enable a simple and efficient parameter optimization strategy that only requires quantum-mechanical simulations of crystalline reference structures. After testing the force field on amorphous interfaces, we present two examples of application to tribological problems. Namely, we investigate the relationships between dry friction and the corrugation of the contact potential energy surface and the dependency of friction on the thickness of interface water films. We finally discuss the limitations of the force field and propose strategies for its improvement and extension.