1,720,968 research outputs found
Tribochemistry of graphene on iron and its possible role in lubrication of steel
Full text linkRecent 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
Quantum Mechanics/Molecular Mechanics (QM/MM) applied to tribology: Real-time monitoring of tribochemical reactions of water at graphene edges
Full text linkTribological phenomena, such as wear and boundary lubrication, are deeply influenced by tribochemical re- actions, i.e., chemical reactions occurring in the presence of mechanical stresses. Atomistic descriptions of these processes are still in their infancies because of the high computational costs required to properly describe the buried sliding interface by theoretical methods. In this paper we outline the application of the Quantum Mechanics/Molecular Mechanics approach to simulate tribochemical reactions during sliding and show that it can accurately describe the tribochemistry of graphene interacting with water molecules, a system which is of relevance for technological applications. Comparison with ab initio (Car-Parrinello) and classical (using the ReaxFF force field) molecular dynamics calculations highlights the advantages of this hybrid approach both in terms of computational costs and accuracy of the results
Ab Initio Insights into Zinc Dialkyldithiophosphate Linkage Isomers and Oxidative Degradation: Implications for Tribology
No full textZinc dialkyldithiophosphates (ZDDPs) are widely used as antiwear additives in lubricants, forming protective tribofilms that reduce wear and friction in metallic contacts. However, the atomic-scale mechanisms governing their performance remain poorly understood. A key factor is the role of linkage isomers─alternative molecular forms in which sulfur (S) and oxygen (O) atoms exchange positions within the ligand structure. These isomers arise through alkyl group transfers and may significantly influence tribofilm formation. Using density functional theory (DFT), we systematically characterize ZDDP linkage isomers by analyzing their stability, vibrational spectra, and dissociation pathways. Our results show that linkage isomers are more stable than standard ZDDP forms due to the greater strength of Zn–O bonds and primarily dissociate through Zn–S(O) bond cleavage under a 1 GPa load at ferrous interfaces. Additionally, we explore oxidative degradation pathways, where S atoms are replaced by O, altering the molecular stoichiometry. We find that oxidation is favorable in the gas phase, and it is exothermic when mediated by a ferrous substrate. To support experimental validation, we provide vibrational spectra for these isomers, enabling direct comparison with spectroscopic measurements. Bond strength analysis via static fragmentation and Integrated Crystal Orbital Overlap Population (ICOOP) further elucidates their structural stability
Zinc dialkyldithiophosphates adsorption and dissociation on ferrous substrates: An ab initio study
Full text linkZinc dialkyldithiophosphates (ZDDPs) have been commonly used as anti-wear additives in the automotive industry for the past 80 years. Despite their widespread use, a general agreement on their primary functioning mechanism is still lacking. The morphology and composition of the ZDDPs phosphate-based tribofilm, which is essential for its lubricant functioning, have been widely studied experimentally. However, the formation process and the relevant driving forces are still largely debated. In particular, it is unclear whether the stress-induced molecular dissociation occurs in the bulk oil or on the substrate. In this work, we employ ab initio density-functional theory simulations to compare ZDDP fragmentation in vacuum and over a reactive substrate, considering the effects of surface oxidation on the dissociation path. To do so, we developed a computational protocol to study the effects of shear stress on molecules. Our results show that the molecular dissociation is endothermic in the absence of a supporting substrate, while in the presence of an iron substrate, it becomes highly energetically favoured. Moreover, the presence of the substrate changes the reaction path, inducing the detachment of organophosphorus units from Zn-S ones. At the same time, surface oxidation reduces the molecule–substrate interaction. These findings provide valuable insights into the early stages of the formation of phosphate-based tribofilms
Graphene and MoS2 interacting with water: A comparison by ab initio calculations
Full text linkAlthough 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
Xsorb: a software for identifying the most stable adsorption configuration and energy of a molecule on a crystal surface
Full text linkMolecular 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Modeling phosphorene and interacting with iron: lubricating effects compared to graphene
Full text linkPhosphorene, a single layer of black phosphorus, is attracting interest for several applications, among which tribology. Here, we investigate its possible use as a solid lubricant for iron-based materials by comparing its friction-reduction properties with MoS2 and graphene. Through first-principle calculations, we predict that phosphorene adheres more strongly to the native iron surface than the other considered 2D materials. The higher adhesion suggests that a stable and durable coverage of reactive surface regions can be obtained with phosphorene. Furthermore, our simulation uncovers the peculiar behavior of phosphorene to exfoliate into two atomic-thin layers upon interface intercalation. This capability makes phosphorene reduce the nano-asperity adhesion very efficiently thanks to the simultaneous passivation of the surface and countersurface. These results suggest that better performances could be obtained by phosphorene than other solid lubricants at low concentrations
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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