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Advancing Solid Interfaces and Lubricants by First Principles Material Design

Periodic Reporting for period 1 - SLIDE (Advancing Solid Interfaces and Lubricants by First Principles Material Design)

Periodo di rendicontazione: 2020-11-01 al 2022-04-30

Friction and wear have massive energy environmental costs. By improving the tribology technologies, a huge amount of energy could be saved, with consistent reduction of fuel consumptions and carbon dioxide emissions. These technologies are nowadays fully based on materials, thus great efforts should be made to understand the physics and chemistry that rule the frictional behavior of surfaces in contact. This is not an easy task as friction is governed by atomistic processes that occur at the sliding buried interface, which is very difficult to monitor in real time by experiments. Simulations can play a crucial role here, in particular those based on a quantum mechanical approach, which is essential for an accurate description of the conditions of enhanced reactivity imposed by the mechanical stresses applied.
The goal of SLIDE is to port the material design paradigm based on First Principles Material Discovery to the field of Tribology by the development and applications of i) a protocol for harnessing tribochemical reactions to reduce interface friction. SLIDE will focus, in particular, in the development of environmental- friendly alternatives to commercial additives used in engine oils; ii) a workflow for high throughput screening of solid interfaces. A public database for the intrinsic adhesion and shear strength of a wide number of materials pairs will be created. Such database will constitute a source of realistic parameters for continuum models, paving the way for serial multiscale approaches to tribology, from the electronic- to the macro scale.
The progresses made in the period covered by the report in achieving the two objectives outlined above are summarized in the following.
i) We considered tribochemical processes involving the three classes of molecular complexes that have been planned: molybdenum dithiocarbamate (MoDTC), Zinc dialkyldithiophosphates (ZDDP), and C-based additives. MoDTC is a well-known lubricant additive, which, in tribological conditions, is capable of forming layers of MoS2 with excellent friction reduction properties. By means of ab initio calculations, we elucidated the mechanisms of MoDTC adsorption and dissociation on iron and iron oxyde. By employing static and dynamic density functional theory (DFT) simulations, including a quantum mechanics/molecular mechanics (QM/MM) approach, we shed light into the tribochemistry of MoSx and its crystallization of MoS2.
Lightweight alloys should substitute steel in a wide range of industrial applications for reducing the fuel consumption. It is still unclear whether the lubricant additives currently employed to reduce friction of sliding metallic parts are also efficient on non-ferrous substrates. We studied the interaction of one of the most common additives, zinc dialkyldithiophosphates (ZDDPs), with Al, Mg and Mg17Al12 alloy. Our calculations indicate that molecular fragments originated from ZDDP chemisorb more strongly on the alloy with respect to pure aluminum and magnesium, due to the higher surface energy. AIMD simulations show that the kinetics of the additive decomposition is significantly higher in the mixed phase. These results are supported by atomic force microscopy sliding tests and suggest that the tribological performance of lightweight alloys can be enhanced by alloys that increase the additive-surface chemical interactions.
In the pursuit of sustainable lubricant materials, we studied the conversion of common organic molecules into graphitic material that 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. Ab initio molecular dynamic simulations of the sliding interface show that hypericin, a component of St. John’s wort, is very effective in reducing iron friction. The decomposition of the lateral groups of hypericin observed in the dynamic simulations suggests that the polymerization of several molecules is possible, offering innovative paths to lubricate sliding contacts with compounds not typically employed in tribology.
In addition to the three molecular complexes above described, we considered graphene, diamond and MXenes.
ii) A robust, modular, and high-throughput workflow is presented to automatically match and characterize solid–solid interfaces using density functional theory calculations with automatic error corrections. The potential energy surface of the interface is computed in a highly efficient manner, exploiting the high-symmetry points of the two mated surfaces. The surfaces are matched according to user-specified maximal cross-section area and mismatches. The software, written in Python, is based on the Fireworks platform and uses different packages from the Materials Projects and relies on the Vienna Ab initio Simulation Package (VASP) for DFT calculations. The software has been applied to calculate the adhesion of one hundred interfaces obtained by mating different metals of technological relevance. The obtained data were analyzed using a regression algorithm with aim at identifying a relation between the adhesion of homogenous interfaces and the heterogenous ones has been identified.
i) The studies on tribochemical processes are particularly relevant as they allowed us to describe in real time novel phenomena difficult to be observed by experiments. Very few research groups in the world are tackling MD simulations of tribological interfaces by considering chemical reactivity and, to our knowledge, none is applying fully AIMD. Most of the tribochemistry studies are, in fact, based on force-fields (FFs). We have proven our approach to be very powerful. The mechanism for the tribologically-induced formation of lubricious MoS2 layers from MoDTC molecular additives has been longley debated in the tribology community. Our simulations allowed us to provide a clear picture of the process, which ends with the layer transformation from amorphous to crystalline. This observation is very relevant not only for applications, but also for a fundamental understanding of the mechanical activation of structural phase transitions. The results obtained on the tribologically-induced formation of graphene from aromatic molecules are also very promising for the design of novel lubricant materials not harmful for the environment.
ii) For the first time to our knowledge, we apply a high throughput approach to tribology. The advanced software we produced is able to screen hundreds of solid interfaces in an automatized way and store the results in a database. The software can be used by non-experts and thanks to its modular structure can be extended to study several interfacial properties.
The database we produced on the adhesion of metals will be extended tp covalent/metallic interfaces and will constitute a source of realistic parameters for continuum mechanics models, paving the way to serial multiscale approaches to tribology. Moreover, with the aid of machine learning algorithms, general trends will be identified among the data and predictive models will be developed
Materials modeling and tribology