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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SOLUTION (SOLUTION - Solid lubrication for emerging engineering applications)

Teaser

Summary

The SOLUTION project focuses on researching novel solid lubricant coatings for emerging engineering applications. The main goal is to understand the mechanisms of nanoscale friction in order to control the relative movement of surface in contact at the atomic level. The importance of the work is promptly understood by considering that the reduction of friction (and subsequently wear) between mechanical components lessens wasteful energy consumption, and yields longer operational lifetimes of the components, ultimately leading to significant financial savings and reduced material waste. Solid lubricants have been known and used for decades. They have been particularly successful in several demanding applications, such as the aerospace industry or nuclear reactors; however, market penetration is still very limited. Some of the limitations are related to the degradation of the solid lubricant coatings when exposed to humidity and atmospheric oxygen. The SOLUTION project brings a unique approach to the study of novel lubricant compounds by merging state-of-the-art theoretical and experimental methods towards determining optimal conditions for the synthesis, utilisation and the environmental impact of the materials.

Work performed

The work finished so far can be split into 3 major categories:

Simulations of friction
The quantum mechanical simulations planned within the project have been so far performed on prototypical MX2 transition metal dichalcogenides (TMD). Comparison of electronic and dynamic properties among several stoichiometries allowed to understand how to decompose the structural and the electronic contributions to nanoscale friction. An electro-dynamic descriptor, named cophonicity and formulated outside the project, lead to the theoretical prediction of a new Ti-doped MoS2 phase, currently under experimental investigation. The study on the effect of charge localization and load showed how to finely tune the electronic environment in order to achieve the desired local frictional force for small displacements. The study has then been generalized for larger displacements by decomposing the relative surface movements into phonon displacement patterns of the stable structure.
Classical molecular dynamics have been performed and they helped to elucidate the lubrication mechanism of molybdenum disulfide. The results showed for the first time that superlubricity (i.e., the apparent vanishing of friction) can be achieved not only by introducing structural incommensurability, but also by varying the direction of the sliding stimulus. A thoughtful investigation has been performed, including in the study ten different sliding directions and six different normal loads. These findings can have impact both on the interpretation of the material behavior observed experimentally and on the design of novel nano/micro electromechanical devices with improved friction control.

Fabrication of solid lubricant films
The experimental studies focused on the doping of transition metal dichalcogenides in order to improve the physical and chemical stability of solid lubricant films. One of the most important requirements is an increased resistance to mechanical wear which is achieved by engineering a coating with increased hardness and strong surface adhesion. The fabrication of superior high lubricity coatings is performed via physical, chemical vapour deposition and electrodeposition using both industrial scale and laboratory equipment. The resulting coatings are subjected to standardised long cycle friction, hardness and adhesion tests. The as-deposited and worn coatings are scrutinised using high resolution electron microscopy which reveals the micro and nanoscale features such as porosity, uniformity, crystal structure, chemical composition. The studies allow for the assessment of the failure mechanisms which are addressed in further experiments. So far we produced a range of WSC and MoSN coatings and investigated their structural, mechanical and tribological properties.

Nanotoxicity
A general goal of the nanotoxicity field is to study the nanoparticles effects on cellular viability and function. Toxicological studies are needed due to possible human exposure/contact with these nanoparticles. For that reason, we performed several cytotoxic assays toward different cell type: human tumor cell lines, Saccharomyces cerevisiae and Vibrio Fischeri. We split our work in two different investigations: evaluation of toxicological effects of commercial TMDs nanoparticles and evaluation of biocompatibility of TMD films. We studied the cell viability and oxidative stress after commercial micro and nano MoS2 and WS2 exposure. The results of the variety of assays conducted indicate the excellent biocompatibility of MoS2 toward mammalian tissue cells. In contrast, high decrease on cell viability and ROS production at the highest concentration were observed in yeast. Moreover, the luminescence inhibition study with the ecotoxicity model bacteria Vibrio fischeri showed a potential inhibition behaviour of micro and nano MoS2.

Final results

The theoretical studies on nanoscale friction so far produced a theoretical framework which is so general that can be exploited in fields beyond tribology, such as electronics (e.g. band-gap and metal-insulator transition tuning), photonics (e.g. optical response in photonic crystals), thermal and thermoelectric devices (e.g. anisotropic heat and electron transfer), among others. We demonstrated that ultra-low friction can be achieved for commensurate surfaces provide appropriate sliding direction is applied. We showed that MoSTi structure was likely unstable at normal condition. Finally we built on Prand-Tomlinson model and develop new statistical thermodynamic framework describing friction.

The experiments have already proven the superior lubrication and mechanical properties of doped TMD-based hard coatings which are produced by both industrial and laboratory equipment. The ongoing work focuses on developing new coating strengthening and improved adhesion mechanisms. Following the recent theoretical predictions, a number of new doping strategies have been assessed for the influence on decreasing the friction coefficient in humid air. We aim to produce a versatile and easily manufactured coating which should be able to withstand a wide range of working conditions. The major achievement so far is fabrication of MoSN coating, which is very hard but still retain solid lubricant properties.

TMDs have different interesting properties and can be utilized in a wide spectre of areas such as in medicine, energy production, materials, electronics and environmental applications. Although nano TMDs applications have great potentials, there are some concerns about the potential TMDs adverse effects on human health and the ecosystem. Therefore, the assessment of the potential toxicity of these materials is needed to guarantee their safe use. The social impact of our work is extremely important. For the first time, we evaluate the potential impact of TMDs samples from different companies using several cellular models and cytotoxicity assays.