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Multi-layer Ti3C2Tx coatings have demonstrated an outstanding wear performance with excellent durability due to beneficial tribo-layers formed. However, the involved formation processes dependent on the tribological conditions and coating thickness are yet to be fully explored. Therefore, we spray-coated Ti3C2Tx multi-layer particles onto stainless steel substrates to create coatings with two different thicknesses and tested their solid lubrication performance with different normal loads (100 and 200 mN) and sliding frequencies (1 and 2.4 Hz) using linear-reciprocating ball-on-disk tribometry. We demonstrate that MXenes' tribological performance depends on their initial state (delaminated few-layer vs. multi-layer particles), coating thickness, applied load and sliding frequency. Specifically, the best behavior is observed for thinner multi-layer coatings tested at the lower frequency. In contrast, coatings made of delaminated few-layer MXene are not as effective as their multi-layer counterparts. Our high-resolution interface characterization by transmission electron microscopy revealed unambiguous differences regarding the uniformity and chemistry of the formed tribo-layers as well as the degree of tribo-induced MXenes' exfoliation. Atomistic insights into the exfoliation process and molecular dynamic simulations quantitatively backed up our experimental results regarding coating thickness and velocity dependency. This ultimately demonstrates that MXenes' tribological performance is governed by the underlying tribo-chemistry and their exfoliation ability during rubbing.

High throughput first-principles calculations, based on solving the quantum mechanical many-body problem for hundreds of materials in parallel, have been successfully applied to advance many materials-based technologies, from batteries to hydrogen storage. However, this approach has not yet been adopted to systematically study solid-solid interfaces and their tribological properties. To this aim, we developed TribChem, an advanced software based on the FireWorks platform, which is here presented. TribChem is constructed in a modular way, allowing for the separate calculation of bulk, surface, and interface properties. At present the calculated interfacial properties include adhesion, shear strength, and charge redistribution. Further properties can be easily added due to the general structure of the main workflow.

Molecular adsorption is the first important step of many surface-mediated chemical processes, from catalysis to tribology. This phenomenon is controlled by physical/chemical interactions, which can be accurately described by first principles calculations. In recent years, several computational tools have been developed to study molecular adsorption based on high throughput/automatized approaches. However, these tools can sometimes be over-sophisticated for non-expert users. In this work, we present Xsorb, a Python-based code that automatically generates adsorption configurations, guides the user in the identification the most relevant ones, which are then fully optimized. The code relies on well-established Python libraries, and on an open source package for density functional theory calculations. We show the program capabilities through an example consisting of a hydrocarbon molecule, 1-hexene, adsorbed over the (110) surface of iron. The presented computational tool will help users, even non-expert, to easily identify the most stable adsorption configuration of complex molecules on substrates and obtain accurate adsorption geometries and energies.

Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS2, even though selenides and tellurides have demonstrated superior tribological properties. Here, we describe an innovative in-operando conversion of Se nano-powders into lubricious 2D selenides by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in shear-induced synthesis of selenide monolayers from nano-powders. The use of Se nano-powder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nano-powder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in-operando and exploit their friction- and wear reducing impact.

If polytetrafluoroethylene (PTFE), commonly known as Teflon, is put into contact and rubbed against another material, almost surely it will be more effective than its counterpart in collecting negative charges. This simple, basic property is captured by the so called triboelectric series, where PTFE ranks extremely high, and that qualitatively orders materials in terms of their ability to electrostatically charge upon contact and rubbing. However, while classifying materials, the series does not provide an explanation of their triboelectric strength, besides a loose correlation with the workfunction. Indeed, despite being an extremely familiar process, known from centuries, tribocharging is still elusive and not fully understood. In this work we employ density functional theory to look for the origin of PTFE tribocharging strength. We study how charge transfers when pristine or defective PTFE is put in contact with different clean and oxidised metals. Our results show the important role played by defects in enhancing charge transfer. Interestingly and unexpectedly our results show that negatively charged chains are more stable than neutral ones, if slightly bent. Indeed deformations can be easily promoted in polymers as PTFE, especially in tribological contacts. These results suggest that, in designing materials in view of their triboelectric properties, the characteristics of their defects could be a performance determining factor.

Tribological phenomena such as adhesion, friction, and wear can undermine the functionality of devices and applications based on the diamond-silica interface. Controlling these phenomena is highly desirable, but difficult since extrinsic factors, such as the surface termination by adsorbed species, can deeply affect the reactivity of diamond and its resistance to wear. In this work, we investigate the effects of diamond oxidation by massive ab initio molecular dynamics simulations of silica sliding against diamond surfaces considering different surface orientations, O-coverages, and tribological conditions. Our findings reveal a dual role of oxygen that depends on coverage. At full coverage, the adsorbed oxygen is very effective in friction and wear reduction because the repulsion with the silica counter-surface prevents the formation of chemical bonds across the interface. At reduced coverage and high pressure, Si-O-C bonds are anyway established. In this situation the presence of oxygen results detrimental as it weakens the surface C-C bonds making the surface more vulnerable to wear. Indeed we observed atomic wear on the C(110) surface at 50% O-coverage under harsh tribological conditions. The mechanisms of friction reduction and atomistic wear are explained through the analysis of the electronic properties and surface-surface interactions. Overall, our accurate in silico experiments shed light into the effects of adsorbed oxygen on the tribological behavior of diamond and show how oxidized diamond can be worn by silica.

Despite its unrivaled hardness, diamond can be severely worn during the interaction with others, even softer materials. In this work, we calculate from first-principles the energy and forces necessary to induce the atomistic wear of diamond, and compare them for different surface orientations and passivation by oxygen, hydrogen, and water fragments. The primary mechanism of wear is identified as the detachment of carbon chains. This is particularly true for oxidized diamond and diamond interacting with silica. A very interesting result concerns the role of stress, which reveals that compressive stresses can highly favor wear, making it even energetically favorable., Comment: 5 Figures

In the pursuit of sustainable lubricant materials, the conversion of common organic molecules into graphitic material has been recently shown to effectively reduce friction of metallic interfaces. Aromatic molecules are perfect candidates due to their inertness and possibility to form carbon-based tribofilms. Among many promising possibilities, we selected a group of common aromatic compounds and we investigated their capability to reduce the adhesion of iron interface. Ab initio molecular dynamic simulations of the sliding interface show that hypericin, a component of St. John's wort, effectively separates the mating iron surfaces better than graphene. This phenomenon is due to the size of the molecule, the reactivity of the moieties at its edges and the possibility to stack several of these structures that can easily slide on top of each other. The decomposition of the lateral groups of hypericin observed in the dynamic simulations suggests that the clustering of several molecules is possible, offering innovative paths to lubricate sliding contacts with compounds not typically employed in tribology.

Molybdenum dithiocarbamates (MoDTCs) are lubricant additives very efficient in reducing the friction of steel and they are employed in a number of industrial applications. The functionality of these additives is ruled by the chemical interactions occurring at the buried sliding interface, which are of key importance for the improvement of the lubrication performance. Yet, these tribochemical processes are very difficult to monitor in real time. Ab initio molecular dynamics simulations are the ideal tool to shed light into such a complicated reactivity. In this work we perform ab initio simulations, both in static and tribological conditions, to understand the effect of surface oxidation on the tribochemical reactivity of MoDTC and we find that when the surfaces are covered by oxygen, the first dissociative steps of the additives are significantly hindered. Our preliminary tribological tests on oxidized steel discs support these results. Bare metallic surfaces are necessary for a stable adsorption of the additives, their quick decomposition, and the formation of a durable MoS$_2$ tribolayer. This work demonstrates the importance of the catalytic role of the substrate and confirms the full capability of the computational protocol in the pursuit of materials and compounds more efficient in reducing friction.

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.

It has been recently reported that graphene is able to significantly reduce the friction coefficient of steel-on-steel sliding contacts. The microscopic origin of this behavior has been attributed to the mechanical action of load carrying capacity. However, a recent work highlighted the importance of the chemical action of graphene. According to this work graphene reduces the adhesion of iron interfaces by reducing the surface energy thanks to a passivation effect. The aim of the present work is to clarify the still debated lubricating behavior of graphene flakes. We perform pin-on-disc experiments using liquid dispersed graphene solution as a lubricant. Two different materials, pure iron and bronze are tested against 100Cr6 steel. Raman spectroscopy is used to analyze the surfaces after the friction tests. The results of these tests prove that graphene flakes have a beneficial effect on the friction coefficient. At the same time they show a tendency of graphene to passivate the native iron surfaces that are exposed during sliding as a consequence of wear., Comment: 16 pages, 4 figures

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 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.

Although very similar in many technological applications, graphene and MoS2 bear significant differences if exposed to humid environments. As an example, lubrication properties of graphene are reported to improve while those of MoS2 to deteriorate: it is unclear whether this is due to oxidation from disulfide to oxide or to water adsorption on the sliding surface. By means of ab initio calculations we show here that these two layered materials have similar adsorption energies for water on the basal planes. They both tend to avoid water intercalation between their layers and to display only mild reactivity of defects located on the basal plane. It is along the edges where marked differences arise: graphene edges are more reactive at the point that they immediately prompt water splitting. MoS2 edges are more stable and consequently water adsorption is much less favoured than in graphene. We also show that water-driven oxidation of MoS2 layers is unfavoured with respect to adsorption., Comment: 17 pages, 4 figures

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 figures

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.

The nanofriction of Xe monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapor deposition and transferred to the QCM electrodes with a polymer stamp. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of krypton and nitrogen confirm this scenario.This thermolubric behavior is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates., Comment: 5 pages, 3 figures

We studied theoretically the behavior of an injected electron-hole pair in crystalline polyethylene. Time-dependent adiabatic evolution by ab-initio molecular dynamics simulations show that the pair will become self-trapped in the perfect crystal, with a trapping energy of about 0.38 eV, with formation of a pair of trans-gauche conformational defects, three C$_2$H$_4$ units apart on the same chain. The electron is confined in the inter-chain pocket created by a local, 120$^\circ$ rotation of the chain between the two defects, while the hole resides on the chain and is much less bound. Despite the large energy stored in the trapped excitation, there does not appear to be a direct non-radiative channel for electron-hole recombination. This suggests that intrinsic self-trapping of electron-hole pairs inside the ideal quasi-crystalline fraction of PE might not be directly relevant for electrical damage in high-voltage cables., Comment: 9 pages, 3 figs; using iopart.cls