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Chemical interactions between two surfaces in contact play a crucial role in determining the mechanical and tribological behavior of solid interfaces. These interactions can be quantified via adhesion energy, that is a measure of the strength by which two surfaces bind together. Several works in literature report how the presence of chemisorbed atoms at homo- and heterogeneous solid-solid interfaces drastically change their proprieties. A precise evaluation of how different species at solid contacts modulates their adhesion would be extremely beneficial for a range of different technological fields: from metallurgy to nuclear fusion. In this work we have used and high-throughput approach to systematically explore the effects of the presence of non-metallic elements, at different concentrations, on the adsorption and adhesion energies of different homogeneous metallic interfaces. Together with the databases for the adsorption and the adhesion energies, we calculated several other properties such as the charge transferred at the interface, the d-band edge shift for the substrate the Bond order and the interfacial density redistribution for the hundreds of systems analyzed. These values were used to define different trends with respect to chemical and concentration parameters that could be useful for the development of engineered interfaces with selected properties. In particular we noticed how the substrate with low filling of d-band are the most prone to adsorb ad-atoms and how the adsorption of almost all non-metallic elements decreases the adhesion energy of solid interfaces, particularly in the case of Fluorine. Carbon and Boron were the only two ad-atoms species that showed an opposite trend increasing the adhesion energy instead.

Zinc dialkyldithiophosphates (ZDDPs) have been commonly used as anti-wear additives in the automotive industry for the past 80 years. The morphology, composition and structure of the ZDDPs phosphate-based tribofilm, which is essential for its lubricant functioning, have been widely studied experimentally. However, 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 have been widely studied experimentally, but 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. Our results show that the molecular dissociation is highly 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. 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.

Boron doped diamond is extensively studied for its use in tribological and electrochemical applications due to its remarkable physical and chemical properties. However, ambient conditions play a major role to its macroscopically observed behavior. In this study, the fundamental interactions between the low Miller index (001), (110) and (111) B-doped diamond surfaces with H2O, H2 and O2 molecules, which are commonly present in ambient air and commonly involved in electrochemical reactions, are investi-gated by means of ab initio simulations. The results are presented in close comparison with previous studies on undoped diamond surfaces to reveal the impact of B on the adsorption properties. It is demonstrated that the B dopant is preferably incorporated on the topmost carbon layer and enhances the physisorption of H2O by forming a dative bond with O, while, in some cases, it can weaken the ad-sorption of O2, compared to the undoped diamond. Moreover, a noticeable displacement of the surface atoms attached to the fragment of the dissociated H2O and O2 molecules was observed, which can be associated to the first stage of wear at the atomistic level. These qualitative and quantitative results aim to provide useful insight towards the development of improved protective coatings and electrochemical devices.

Understanding frictional phenomena is a fascinating fundamental problem with huge potential impact on energy saving. Such an understanding requires monitoring what happens at the sliding buried interface, which is almost inaccessible by experiments. Simulations represent powerful tools in this context, yet a methodological step forward is needed to fully capture the multiscale nature of the frictional phenomena. Here, we present a multiscale approach based on linked ab initio and Green's function molecular dynamics, which is above the state-of-the-art techniques used in computational tribology as it allows for a realistic description of both the interfacial chemistry and energy dissipation due to bulk phonons in non-equilibrium conditions. By considering a technologically relevant system composed of two diamond surfaces with different degrees of passivation, we show that the presented method can be used not only for monitoring in real-time tribolochemical phenomena such as the tribologically-induced surface graphitization and passivation effects but also for estimating realistic friction coefficients. This opens the way to in silico experiments of tribology to test materials to reduce friction prior to that in real labs.

Diamond-based coatings are employed in several technological applications, for their outstanding mechanical properties, biocompatibility, and chemical stability. Of significant relevance is the interface with silicon oxide, where phenomena of adhesion, friction, and wear can affect drastically the performance of the coating. Here we monitor such phenomena in real-time by performing massive ab initio molecular dynamics simulations in tribological conditions. We take into account many relevant factors that can play a role, i.e. the diamond surface orientation and reconstruction, silanol density, as well as, the type and concentration of passivating species. The large systems size and the long simulations time, put our work at the frontier of what can be currently done with fully ab initio molecular dynamics. The results of our work point to full hydrogenation as an effective way to reduce both friction and wear for all diamond surfaces, while graphitization is competitive only on the (111) surface. Overall we expect that our observations will be useful to improve technological applications where the silica-diamond interface plays a key role. Moreover, we demonstrate that realistic and accurate in silico experiments are feasible nowadays exploiting HPC resources and HPC optimized software, paving the way to a more general understanding of the relationship between surface chemistry and nanoscale-tribology.

Recent nanofriction experiments of xenon on graphene revealed that the slip onset can be induced by increasing the adsorbate coverage above a critical value, which depends on temperature. Moreover, the xenon slippage on gold is much higher than on graphene in spite of the same physical nature of the interactions. To shed light on these intriguing results we have performed molecular dynamics simulations relying on ab initio derived potentials. By monitoring the interfacial structure factor as a function of coverage and temperature, we show that the key mechanism to interpret the observed frictional phenomena is the size-dependence of the island commensurability. The latter quantity is deeply affected also by the lattice misfit, which explains the different frictional behavior of Xe on graphene and gold., Comment: 9 pages, 4 figures, 2 tables

Different hypotheses have been proposed to explain the mechanism for the extremely low friction coefficient of carbon coatings and its undesired dependence on air humidity. A decisive atomistic insight is still lacking because of the difficulties in monitoring what actually happens at the buried sliding interface. Here we perform large-scale ab initio molecular dynamics simulations of both undoped and silicon-doped carbon films sliding in the presence of water. We observe the tribologically-induced surface hydroxylation and subsequent formation of a thin film of water molecules bound to the OH-terminated surface by hydrogen bonds. The comparative analysis of silicon-incorporating and clean surfaces, suggests that this two-step process can be the key phenomenon to provide high slipperiness to the carbon coatings. The water layer is, in fact, expected to shelter the carbon surface from direct solid-on-solid contact and make any counter surface slide extremely easily on it. The present insight into the wettability of carbon-based films can be useful for designing new coatings for biomedical and energy-saving applications with environmental adaptability., Comment: 22 pages, 4 figures, 1 table