The structure and sliding friction of diamond-like carbon surfaces from molecular dynamics simulations

Using the reactive force field in molecular dynamics (MD) simulations, the three-dimensional structure of amorphous carbon at densities ranging from 2.4-3.4 g/cm³ has been predicted. The structures for these solid amorphous systems were generated by melting a cell with 512 carbon atoms, followed by rapid quenching from the liquid phase. At the density of 3.24g/cm³, we find that 70% of the atoms have sp³ character, in good agreement with our experiment. Simulation results show that all of the sp³ atoms connect to form a percolating tetrahedral network, to which are attached isolated sp² atoms or short chains of sp² atoms. Hydrogen-free DLC surfaces were constructed by determining the lowest energy surface for cutting the bulk amorphous carbon cell. It is found that the surface C atoms react readily with glycerol to form a carbon surface containing OH-terminated groups, which is enhanced by sliding. Using MD simulations, we examined the friction properties for various DLC surfaces: bare, H-terminated, OH-terminated, and the passivated surfaces after reaction with glycerol and H₂O₂. Simulation shows that the bare DLC surface has friction coefficient of about 0.8, whereas the DLC surface passivated with OH/H by reacting with H₂O₂ leads to friction coefficients down to 0.01. These results suggest that the origin of the superlubricity observed in the DLC system arises from the passivation of carbon surface by OH groups, which is consistent with experiment. The relationship between the friction and interfacial adhesion has also been investigated. The MD simulations suggest that friction is determined by variations in the adhesion during sliding, rather than the absolute value of the adhesion between interfaces. Larger variations (energy barriers) induce larger deformations of the sliding objects, leading to higher friction.