Tribochemistry of graphene on iron and its possible role in lubrication of steel

Recent tribological experiments revealed that graphene is able to lubricate macroscale steel-on-steel sliding contacts very effectively both in dry and humid conditions. This effect has been attributed to a mechanical action of graphene related to its load-carrying capacity. Here we provide further insight into the functionality of graphene as lubricant by analysing its tribochemical action. By means of first principles calculations we show that graphene binds strongly to native iron surfaces highly reducing their surface energy. Thanks to a passivating effect, the metal surfaces coated by graphene become almost inert and present very low adhesion and shear strength when mated in a sliding contact. We generalize the result by establishing a connection between the tribological and the electronic properties of interfaces, which is relevant to understand the fundamental nature of frictional forces.Comment: 19 pages, 6 figure

Insigths into the tribochemistry of silicon-doped carbon based films by ab initio analysis of water/surface interactions

Diamond and diamond-like carbon (DLC) are used as coating materials for numerous applications, ranging from biomedicine to tribology. Recently, it has been shown that the hydrophilicity of the carbon films can be enhanced by silicon doping, which highly improves their biocompatibility and frictional performances. Despite the relevance of these properties for applications, a microscopic understanding on the effects of silicon is still lacking. Here we apply ab initio calculations to study the interaction of water molecules with Si-incorporated C(001) surfaces. We find that the presence of Si dopants considerably increases the energy gain for water chemisorption and decreases the energy barrier for water dissociation by more than 50%. We provide a physical rational for the phenomenon by analysing the electronic charge displacements occuring upon adsorption. We also show that once hydroxylated, the surface is able to bind further water molecules much strongly than the clean surface via hydrogen-bond networks. This two-step process is consistent with and can explain the enhanced hydrophilic character observed in carbon-based films doped by silicon

Ab Initio Calculation of the Adhesion and Ideal Shear Strength of Planar Diamond Interfaces with Different Atomic Structure and Hydrogen Coverage

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Ab initio insights into the interaction mechanisms between H2_2, H2_2O, and O2_2 molecules with diamond surfaces

Diamond displays outstanding chemical, physical, and tribological properties, making it attractive for numerous applications ranging from biomedicine to tribology. However, the reaction of the materials with molecules present in the air, such as oxygen, hydrogen, and water, could significantly change the electronic and tribological properties of the films. In this study, we performed several density functional theory calculations to construct a database for the adsorption energies and dissociation barriers of these molecules on the most relevant diamond surfaces, including C(111), C(001), and C(110). The adsorption configurations, reaction paths, activation energies, and their influence on the structure of diamond surfaces are discussed. The results indicate that there is a strong correlation between adsorption energy and surface energy. Moreover, we found that the dissociation processes of oxygen molecules on these diamond surfaces can significantly alter the surface morphology and may affect the tribological properties of diamond films. These findings can help to advance the development and optimization of devices and antiwear coatings based on diamond

Load-Induced Confinement Activates Diamond Lubrication by Water

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High-throughput screening of the static friction and ideal cleavage strength of solid interfaces

We present a comprehensive ab initio, high-throughput study of the frictional and cleavage strengths of interfaces of elemental crystals with different orientations. It is based on the detailed analysis of the adhesion energy as a function of lateral, \u3b3(x, y), and perpendicular displacements, \u3b3(z), with respect to the considered interface plane. We use the large amount of computed data to derive fundamental insight into the relation of the ideal strength of an interface plane with its adhesion. Moreover, the ratio between the frictional and cleavage strengths is provided as good indicator for the material failure mode \u2013 dislocation propagation versus crack nucleation. All raw and curated data are made available to be used as input parameters for continuum mechanic models, benchmarks, or further analysis

Friction by Shear Deformations in Multilayer Graphene

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Interfacial Charge Density and Its Connection to Adhesion and Frictional Forces

We derive a connection between the intrinsic tribological properties and the electronic properties of a solid interface. In particular, we show that the adhesion and frictional forces are dictated by the electronic charge redistribution occurring due to the relative displacements of the two surfaces in contact. We define a figure of merit to quantify such a charge redistribution and show that simple functional relations hold for a wide series of interactions including metallic, covalent, and physical bonds. This suggests unconventional ways of measuring friction by recording the evolution of the interfacial electronic charge during sliding. Finally, we explain that the key mechanism to reduce adhesive friction is to inhibit the charge flow at the interface and provide examples of this mechanism in common lubricant additives

Unraveling the mechanism to form MoS2 lubricant layers from MoDTC by ab initio simulations

The morphology of molybdenum disulfide (MoS2) is a crucial aspect to ensure the functionality of this remarkable 2D-material both in electronic and tribological applications. Indeed, molybdenum dithiocarbamates (MoDTCs) can be tribochemically transformed into MoS2, which is able to reduce the friction coefficient of metallic moving parts. However, this transformation is influenced by temperature, sulfur/oxygen ratio, normal and shear stresses, making the mechanism of this process particularly challenging to explain. Ab initio simulations based on density functional theory (DFT), including a quantum mechanics/molecular mechanics (QM/MM) approach, are used here to shed light on the crystallization of MoS2 promoted by mechanical stresses. Chemistry plays an important role during the reorganization of the units of MoSx obtained from MoDTC, because sulfur and oxygen atoms tend to move outside of the amorphous layer, surrounding the molybdenum atoms and creating a structure that can crystallize into MoS2. Normal load and sliding have a synergistic effect in rearranging the amorphous units into a crystalline structure, as the former helps overcoming the energy barriers associated to bonds breaking and forming, while the latter allows misplaced atoms to be pulled towards the crystalline sites. A crystalline MoS2 was obtained by ab initio calculations below 1000 K