Nano-scale frictional properties of 2D materials

Abstract

Nano and atomic-scale frictional properties of various 2D transition metal dichalcogenide systems (monolayers, multilayers, thin-film coatings) were analysed by atomic force microscopy. All the analysed monolayer samples (MoS₂ , WS₂ , MoSe₂ , WSe₂ ) exhibited very low friction when free of surface contamination. The chalcogenide atom was found to have a larger effect on friction than the metal atom, and selenides experienced lower friction than sulphides. All the layers provided superior wear resistance, and no layer rupture could be observed even at very high loads up to 10 µN. A shift in load-dependent behaviour was observed in the multilayered samples, and the difference was contributed to the contact area gain due to adhesion and a decrease in contact shear strength due to interlayer coupling. A numerical model was used to study the problem, and a new model for fitting experimental data was developed. Nanotribological analysis of the W-S-C coatings and the corresponding wear tracks revealed a two-phase structure of the coatings. The two phases experienced entirely different frictional responses and were assigned to an amorphous solution of WS₂ and carbon (high friction phase, µ > 1) and nanocrystalline WS₂ (low friction phase, µ ≈ 0.15). The two-phase structure was confirmed by Raman spectroscopy and Raman mapping, which identified the regions with high concentrations of either carbon or crystalline WS₂ . WS₂ tribofilm in the form of multilayer flakes was identified within the wear tracks. The coverage of the wear tracks with such features was much lower than initially expected, thus indicating that the tribo-film formation is localised and occurs only when contact pressure exceeds the formation threshold.

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ORCID for Tomas Polcar: orcid.org/0000-0002-0863-6287

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