Mechanisms of ultra-low friction by hollow inorganic fullerene-like MoS2 nanoparticles

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Abstract

Inorganic Fullerene-like (IF)-MoS2 nanoparticles were tested under boundary lubrication and ultra-high vacuum (UHV) and were found to give an ultra-low friction coefficient in both cases compared to hexagonal (h)-MoS2 material. Previous works made by Rapoport et al. with IF-WS2 revealed that the benefit effect of the inorganic fullerene-like materials decreases at high loads and sliding velocities. Nevertheless, under the conditions used in our experiments using high contact pressure (maximum pressure above 1.1 GPa in oil and 400 MPa in high vacuum) and slow sliding velocities (1.7 mm/s in oil test and 1 mm/s in high vacuum), friction always decreases and stabilizes at about 0.04 for 800 cycles in both cases. Therefore, IF-MoS2 material appears to be a good candidate for use in various environments in regard to other MoS2 crystal structures. Wear mechanisms were investigated using both High Resolution TEM and surface analyses (XPS) on the wear tracks. Wear particles collected from the flat wear scar show several morphologies, suggesting at least two lubricating mechanisms. As spherical particles are found in the wear debris, rolling may be a possible event. However, flattened and unwrapped IF-MoS2 particles are often observed after friction. In this case, low friction is thought to be due either to sliding between IF-MoS2 external flattened planes or to slip between individual unwrapped MoS2 sheets.

Introduction

Lamellar compounds such as MoS2 are used both as solid lubricants and additives in liquid lubricants but it is especially well known for its successful application in space technology and in a variety of dispersions in grease, oil and lacquers. In ultra-high vacuum, pure and stoichiometric sputtered MoS2 coatings give an extraordinary low friction coefficient (in the 10−3 range). To allow MoS2 coatings to slide relative to each other, the crystallites reorient with their basal planes parallel to the direction of sliding [1]. Pure h-MoS2 powder, however, has a short lifetime and considerably higher friction (more than 0.1) when it is used in humid, oxygen-containing environments. In ambient atmosphere (i.e. 50% of relative humidity) platelets of the 2H polytype tend to stick to the mating metal pieces through the reactive dangling bonds, which leads to their rapid deterioration through burnishing and oxidation. Besides being a fascinating topic of research, the development of better and more efficient lubricants is important for both economic and environmental reasons. Due to advances of fullerene chemistry in carbon systems, yielding and observations of such structures have been developed in metal dichalcogenides MX2. Hollow inorganic fullerene (IF) nanoparticles with spherical shape and the absence of dangling bonds (edge effects) were thus synthesized [2]. As nanoparticles replaced molecules in tribological systems, it is necessary to understand what effect they have on wear and lifetime of systems in severe conditions i.e. in boundary lubricantion regime. Tribological behavior of IF-nanoparticles was studied in several cases including: (i) additives in oil [3], [4], [5]; (ii) solid lubricant as coating and (iii) by impregnated composites with IF lubricants [6]. In this study IF-MoS2 nanoparticles are used as a friction-modifying additive in a base oil and as coatings in UHV friction tests. The aim of this work is to show first the capability of this product to remain chemically pure whatever the preserving conditions are and second the relationship between the chemical and physical structure of these fullerene nanoparticles and their tribological behavior. In particular, the substitution of sulfur by oxygen in the MoS2 structure have a very bad influence on the friction coefficient and we would like to determine if this particular structure could protect molybdenum of oxygen influence even during tribological tests.

Section snippets

Chemical and structural characterization of IF-MoS2

IF-MoS2 was synthesized by a gas phase reaction in a reducing atmosphere at elevated temperature (800–950 °C). The growth of MoS2 was obtained by the reaction between the solid MoO3 and the gas H2 giving gas MoO3−x. The sublimation of this last compound by H2S lead to form IF-MoS2 in a gas-phase reactor allowing a turbulent flow regime. This synthesis is well described elsewhere [7], [8]. The size of the particles was measured by Transmission Electron Microscopy (HRTEM). A drop of highly

Experimental procedure

IF-MoS2 material was tested at ambient temperature either in a lubricated contact or as a deposited coating in ultra-high vacuum. Two different tribometers were used. The first one is a pin-on-flat tribometer [11] working in ambient air and the lubricant is a dispersion of IF-MoS2 at various concentrations in mineral base oil (150NS) or a polyalphaolefin (PAO). Nanoparticles are dispersed by an ultrasonic bath and two droplets of the lubricant are deposited on the flat just before starting the

Lubricated tests

The evolution of friction in lubricated tests as a function of the number of cycles is shown in Fig. 5. Although there is some noisy signal, the difference in the friction coefficient remains significant. At the end of the test, friction for IF-MoS2 is 0.04±0.005 in both tests (test #1 and test #2). It is a very low friction coefficient compared to ones obtained with both pure base oils (in the range of 0.09–0.15) in the same experimental conditions (see Fig. 5). Additive concentration in test

Discussion

MoS2 is a well-known lamellar solid lubricant with a hexagonal structure. Covalent bonds exist between sulfur and molybdenum atoms in planar arrays of basal planes. Van der Waals forces between planes explain low interplanar shear strength. The ultra-low friction coefficient needs the absence of adventitious elements [16]. The low friction obtained with these nanoparticles could be explained by several phenomena:

  • i

    The first one involves rolling of nanoparticles in the contact between the two

Conclusion

Hollow inorganic-fullerene-like (IF-MoS2) nanoparticles exhibited ultra-low friction coefficients (close to 0.04) as well in lubricated conditions as deposited coating in UHV environment. First of all, the fullerene structure brings the chemical stability of MoS2 nanoparticles (i.e. chemical stoichiometry and inertness) even if they are in contact with ambient air, in particular with oxygen. During friction, the structure of hollow IF-MoS2 particles is modified. HRTEM studies show that

Acknowledgements

The authors would like to thank the ‘Région Rhône-Alpes’ (France), TotalFinaElf (Dominique Faure), Condat (Nguyen Truong Dinh) and HEF (Chrisotphe Heau) for financial supports.

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