Role of oxygen functional groups in the friction of water-lubricated low-index diamond surfaces
Large-scale quantum molecular dynamics simulations unveil eight friction regimes of water-lubricated low-index diamond surfaces. Four of these friction regimes are universal, i.e., they occur on diamond (111), (001), as well as (110). Dry sliding leads to immediate cold welding accompanied by amorphization (regime I). Small amounts of water (less than 8H2O per nm2) can preserve crystallinity and lower friction by localizing shear to interfacial ether groups (regime II). A further increase in water surface density results in passivating hydrogen/hydroxyl layers (regime IV) and finally (for more than 20H2O per nm2) in free water layers between hydrogen/hydroxyl passivated diamond surfaces (regime V). The other four friction regimes are special, i.e., they occur only on certain surfaces. An ultralow friction regime is established by aromatic Pandey surface passivation on diamond (111) surfaces (regime III). On diamond (110) surfaces, regime II coexists with three other regimes: while partial cold welding via CC bonds (regime VI) or COC bonds (regime VII) leads to frictional shear stresses that are in-between the cold-welding regimes (I and II) and the non-cold-welding regimes (IV and V), the formation of an oxidized carbon monolayer consisting of keto and ether groups results in ultralow friction (regime VIII). Regime VIII is also observed for diamond (001) surfaces. These findings are rationalized by the structural and energetic peculiarities of the different low-index surfaces. Our study provides guidelines for nanoscale control and manipulation of oxygen functional groups on carbon surfaces in boundary lubrication with water or other oxygen-containing lubricants.