arXiv subjects

M. Clelia Righi

Publications and source records attributed to M. Clelia Righi.

6 recordsLinked to original sources

arXiv2025

Phosphorus-based lubricant additives on iron with Machine Learning Interatomic Potentials

Phosphorus-based lubricant additives are used for protecting metallic contacts under boundary lubrication by forming surface films that reduce wear and friction. Despite their importance, the molecular mechanisms driving their friction-reducing effects remain unclear, especially for phosphate esters, whose molecular structure critically impact tribological behavior. In this study, we use machine learning-based molecular dynamics simulations to investigate the tribological performance of three representative phosphorus-based additives, Dibutyl Hydrogen Phosphite (DBHP), Octyl Acid Phosphate (OAP), and Methyl Polyethylene Glycol Phosphate (mPEG-P), on iron surfaces. The mPEG-P family is further analyzed by varying esterification degree and chain length. DBHP exhibits the lowest friction and largest interfacial separation, resulting from steric hindrance and tribochemical reactivity, as indicated by P-O bond cleavage and enhanced O-Fe interactions. In contrast, OAP and mPEG-P monoesters produce higher friction due to limited steric protection and reduced resistance to shear, leading to partial loss of surface coverage under extreme conditions. Within the mPEG-P family, multi-ester and longer-chain molecules significantly lower friction by maintaining larger separations, demonstrating that steric effects can outweigh surface reactivity under severe confinement. Overall, these results provide atomistic insights into how molecular architecture controls additive performance and support the design of phosphorus-based lubricants combining reactive anchoring with optimized steric structures for durable, low-friction interfaces.

cond-mat.mtrl-sci↗

arXiv2025

On the role played by electrons in the stress-strain curves of ideal crystalline solids

The mechanical properties of a solid, which relate its deformation to external applied forces, are key factors in enabling or disabling the use of an otherwise optimal material in any application, strongly influencing also its service lifetime. Intrinsic crystal deformation mechanisms, investigated experimentally on single crystals with low dislocation densities, have been studied theoretically through atomistic simulations, mainly focusing on lattice-induced instabilities. Here, instead, we employ density functional theory and a thermodynamic analysis to probe and analyze the way in which the electronic charge of crystalline solids (Cu, Al and diamond) responds to uniaxial strain and affects their mechanical properties. Indeed, despite the very simple nature of our models, and in the presence of minimal atomic displacements, we find that the stress strain curves of Cu and Al deviate from a simple linear elastic behavior. Within a thermodynamics perspective, the features of such curves can be interpreted in terms of first and second order phase transitions, which originate from Van-Hove singularities of the electronic density of states crossing the Fermi level and electron redistribution within the solid, respectively.

cond-mat.mtrl-sci↗

arXiv2024

Solid-Lubrication Performance of Ti3C2Tx -- Effect of Tribo-Chemistry and Exfoliation

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.

cond-mat.mtrl-sci↗

arXiv2023

Se Nano-Powder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions

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.

cond-mat.mtrl-sci↗

arXiv2023

TribChem: a Software for the First-principles, High-Throughput Study of Solid Interfaces and their Tribological properties

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.

cond-mat.mtrl-sci↗

arXiv2023

Xsorb: a software for identifying the most stable adsorption configuration and energy of a molecule on a crystal surface

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.

cond-mat.mtrl-sci↗