The project results can be split into four interconnected areas. Simulation of friction helps to identify the main solid lubricant component, which is then applied as a coating using novel deposition methods and, finally, tested on a laboratory and industrial scale. Finally, the safety of 2D transitional dichalcogenides was assessed. We will now describe these three areas in detail.
Simulation of friction
The quantum mechanical simulations were performed on prototypical MX2 transition metal dichalcogenides (TMD). Comparison of electronic and dynamic properties among several stoichiometries allowed us to understand how to decompose the structural and the electronic contributions to nanoscale friction. 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 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) could be achieved not only by introducing structural incommensurability, but also by varying the direction of the sliding stimulus. Then, the effect of water molecules on friction was simulated as a function of surface coverage. Finally, MD was used to simulate the structural transformation of MoS2 during sliding (from amorphous to crystalline phase). These findings were often directly validated by nanoscale experiments and helped to select an optimum material for large-scale deposition processes.
Fabrication of 2D materials and solid lubricant films
To validate simulations, two types of materials were prepared – 2D transition metal dichalcogenides (Mo/W-S2/Se2) deposited by CCVD and pure magnetron sputtered TMDs. This was only a minor task, as the most effort was spent to prepare composite coatings (TMD doped with N or C). Three deposition methods were used, all belonging to the magnetron sputtering family: d.c. sputtering, r.f. sputtering, and HiTUS – High target utilization sputtering. All depositions were performed in larger (semi or fully) industrial chambers allowing coating of industrial parts. In a separate task, MoS2 and WS2 doped Ni and Ni-P coatings were deposited by electrodeposition.
Tribological testing
Friction and wear phenomena were analyzed on different scales. An atomic force microscope (AFM) was used to validate and/or support simulations, such as sliding the nanoscale flakes of 2D TMD materials on various substrates. Moreover, we deposited a thin sputtered layer directly on the top of AFM tip and could, for the first time, observe the structural transformation in situ. For macroscale tribology we used a combination of standard tribometers and, particularly, unique tailored devices harvesting in-situ analyses. Integrated Raman spectroscopy provided in situ assessment of coating structure during the sliding test, vacuum tribometer was used to measure ultra-low friction in a vacuum, and newly built tribometer inside the 3D profilometer measured actual wear rate of thin solid lubricant films with unprecedented precision.
Nanotoxicity
A general goal of the nanotoxicity field is to study the TMD nanoparticles effects on cellular viability and function. We performed several cytotoxic assays toward different cell types: human tumor cell lines, Saccharomyces cerevisiae, and Vibrio Fischeri. Our results help to understand the potential health impact represented by small TMD nanoparticles.
Dissemination and Exploitation
There were three main dissemination channels – scientific publications, conferences and workshop, and industry-oriented seminars/meetings. ESRs published published 7 papers in 2019, 18 papers in 2020, and 8 papers so far in 2021. Moreover, there are many submitted papers in various stages of review process, and many manuscripts are being prepared. We expect that the total number of peer review papers published as a result of Solution project will be close to 50. The publication activity underlines joint activities in the project. 14 papers were published by two beneficiaries and 6 papers with 3 or more beneficiaries and partners. Many papers were published in leading journals with high impact factors. All ESRs actively participated in conferences and workshops; the total number was around 30. However, this activity was significantly affected by COVID. The same applies to planned fairs and industrial workshops in 2020, which were all canceled.
