Role of oxygen functional groups in the friction of water-lubricated low-index diamond surfaces

Abstract

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 8 H 2 O per nm 2 ) can preserve crystallinity and lower friction by localizing shear to interfacial ether groups (Regime II). Further increase in water surface density results in passivating hydrogen/hydroxyl layers (Regime IV) and finally (for more than 20 H 2 O per nm 2 ) 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 C-C bonds (Regime VI) or C-O-C 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.

Superlow Friction of a-C:H Coatings in Vacuum: Passivation Regimes and Structural Characterization of the Sliding Interfaces

“…An increase in the contact pressure to 1.2 GPa triggers a hydrogen depletion in the 3-nm-thick topmost layer and a phase transition from an sp 3 -rich a-C:H to an sp 2 -rich H-depleted lamellar phase. Analogous, tribologically-induced sp 3 -to-sp 2 transitions were observed in our previous atomistic simulations, revealing shear-induced aromatic surface reconstructions on diamond (111) surfaces lubricated with water [10,11] and ta-C surfaces lubricated with glycerol [12]. The delocalization of π-electrons on the graphenoid surfaces makes them chemically inert and, thus, shields them from cold welding, even at high contact pressures [10,13].…”

Mechano-chemical decomposition of organic friction modifiers with multiple reactive centres induces superlubricity of ta-C

“…Their aromatic nature makes them chemically inert and they can withstand high contact pressure without chemical terminations. A typical example of aromatic passivation is a Pandey-reconstructed diamond (111) surface 27,28 . For ta-C, aromatic passivation layers are not perfect graphene, but consist of a mixture of 5-, 6-, and 7-membered rings that are partially oxidized.…”

In Situ Synthesis of Graphene Nitride Nanolayers on Glycerol-Lubricated Si3N4 for Superlubricity Applications

“…Both processes are observed for boundary-lubricated tetrahedral amorphous carbon (ta-C) and nanocrystalline diamond surfaces. In these materials, ultra- , and super-low , friction can be achieved by either a dense surface passivation with hydrogen and hydroxyl functional groups (originating from lubricants) or an aromatic surface passivation due to delocalized π-bonded networks. ,, The passivation can prevent the formation of chemical bonds across the sliding interface at high contact pressures (GPa), which is a prerequisite for friction reduction. For example, some of the authors utilized tribochemical decomposition of oleic acid to form superlubricious aromatic graphenoid patches on the ta-C surface under boundary and mixed lubrication .…”