Aromatic molecules as sustainable lubricants explored by ab initio simulations

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

Importance of the catalytic effect of the substrate in the functionality of lubricant additives: the case of MoDTC

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

A comparative study on the functionality of S- and P-based lubricant additives by combined first principles and experimental analysis

Sulfur and phosphorus are key elements for the functionality of lubricant additives used in extreme pressure applications, such as synchronizer systems in cars. To understand their mechanism of action we combine first principles calculations and gas phase lubrication experiments. The surface spectroscopy analysis performed in situ after the tribological test indicates that iron sulfide (phosphide) is formed by rubbing steel-on-steel in the presence of organo-sulfur (-phosphorus) molecules. We, thus, study the effects of elemental sulfur and phosphorus on the interfacial properties of iron by spin-polarized density functional theory calculations. The results show that both the elements are very effective in reducing the adhesion and shear strength of iron. Sulfur is predicted to be more effective than phosphorus, especially at high pressure. Gas phase lubrication experiments confirm these results, indicating that the friction coefficient of iron-sulphide is lower than that of iron-phosphide and both S and P dramatically reduce the friction of steel-on-steel. These results indicate that the release of elemental sulfur and phosphorus may be the key mechanism to controlling the tribological properties of the metal interface and elucidate that the underling microscopic phenomenon is metal passivation. © 2016 The Royal Society of Chemistry.Sulfur and phosphorus are key elements for the functionality of lubricant additives used in extreme pressure applications, such as synchronizer systems in cars. To understand their mechanism of action we combine first principles calculations and gas phase lubrication experiments. The surface spectroscopy analysis performed in situ after the tribological test indicates that iron sulfide (phosphide) is formed by rubbing steel-on-steel in the presence of organo-sulfur (-phosphorus) molecules. We, thus, study the effects of elemental sulfur and phosphorus on the interfacial properties of iron by spin-polarized density functional theory calculations. The results show that both the elements are very effective in reducing the adhesion and shear strength of iron. Sulfur is predicted to be more effective than phosphorus, especially at high pressure. Gas phase lubrication experiments confirm these results, indicating that the friction coefficient of iron-sulphide is lower than that of iron-phosphide and both S and P dramatically reduce the friction of steel-on-steel. These results indicate that the release of elemental sulfur and phosphorus may be the key mechanism to controlling the tribological properties of the metal interface and elucidate that the underling microscopic phenomenon is metal passivation

Experimental and Ab Initio Characterization of Mononuclear Molybdenum Dithiocarbamates in Lubricant Mixtures

Molybdenum dithiocarbamates (MoDTCs) are a class of lubricant additives widely employed in automotives. Most of the studies concerning MoDTC take into account the dimeric structures because of their industrial relevance, with the mononuclear compounds usually neglected, because isolating and characterizing subgroups of MoDTC molecules are generally difficult. However, the byproducts of the synthesis of MoDTC can impact the friction reduction performance at metallic interfaces, and the effect of mononuclear MoDTC (mMoDTC) compounds in the lubrication has not been considered yet in the literature. In this study, we consider for the first time the impurities of MoDTC consisting of mononuclear compounds and combine experimental and computational techniques to elucidate the interaction of these impurities with binuclear MoDTC in commercial formulations. We present a preliminary strategy to separate a commercial MoDTC product in chemically different fractions. These fractions present different tribological behaviors depending on the relative amount of mononuclear and binuclear complexes. The calculations indicate that the dissociation mechanism of mMoDTC is similar to the one observed for the dimeric structures. However, the different chemical properties of mMoDTC impact the kinetics for the formation of the beneficial molybdenum disulfide (MoS2) layers, as shown by the tribological experiments. These results help to understand the functionality of MoDTC lubricant additives, providing new insights into the complex synergy between the different chemical structures

Ab Initio Molecular Dynamics Simulation of Tribochemical Reactions Involving Phosphorus Additives at Sliding Iron Interfaces

We performed, for the first time to our knowledge, fully ab initio molecular dynamics simulations of additive tribochemistry in boundary lubrication conditions. We consider an organophosphourus additive that has been experimentally shown to reduce friction in steel-on-steel sliding contacts thanks to the tribologically-induced formation of an iron phosphide tribofilm. The simulations allow us to observe in real time the molecular dissociation at the sliding iron interface under pressure and to understand the mechanism of iron phosphide formation. We discuss the role played by the mechanical stress by comparing the activation times for molecular dissociation observed in the tribological simulations at different applied loads with that expected on the basis of the dissociation barrier

Ab Initio Molecular Dynamics Simulation of Tribochemical Reactions Involving Phosphorus Additives at Sliding Iron Interfaces

We performed, for the first time to our knowledge, fully ab initio molecular dynamics simulations of additive tribochemistry in boundary lubrication conditions. We consider an organophosphourus additive that has been experimentally shown to reduce friction in steel-on-steel sliding contacts thanks to the tribologically-induced formation of an iron phosphide tribofilm. The simulations allow us to observe in real time the molecular dissociation at the sliding iron interface under pressure and to understand the mechanism of iron phosphide formation. We discuss the role played by the mechanical stress by comparing the activation times for molecular dissociation observed in the tribological simulations at different applied loads with that expected on the basis of the dissociation barrier

Wettability@Al2O3: Adsorption of lubricant additives by molecular modeling

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Adsorption of lubricant additives by molecular modeling

International audienc

Wettability@Al2O3: Adsorption of lubricant additives by molecular modeling

International audienc

Molecular adsorption on metal oxides: an in silico design of lubricants

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