L. Rapoport

Hollow nanoparticles of WS2 as potential solid-state lubricants

Solid lubricants fill a special niche in reducing wear in situations where the use of liquid lubricants is either impractical or inadequate, such as in vacuum, space technology or automotive transport. Metal dichalcogenides MX2 (where M is, for instance, Mo or W and X is S or Se) are widely used as solid lubricants. These materials are characterized by a layered structure with weak (van der Waals) inter-layer forces that allow easy, low-strength shearing,. Within the past few years, hollow nanoparticles (HNs) of MX2 with structures similar to those of nested carbon fullerenes and nanotubes have been synthesized,. Here we show that these materials can act as effective solid lubricants: HN-WS2 outperforms the solid lubricants 2H-MoS2 and 2H-WS2 in every respect (friction, wear and lifetime of the lubricant) under varied test conditions. We attribute the outstanding performance of HN-WS2 to its chemical inertness and the hollow cage structure, which imparts elasticity and allows the particles to roll rather than to slide.

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

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.

Applications of WS2(MoS2) inorganic nanotubes and fullerene-like nanoparticles for solid lubrication and for structural nanocomposites

Nanoparticles of WS2 and MoS2 with a closed cage structure (fullerene-like) that are termed IF phases are synthesized in large amounts in a pure form. These nanoparticles were shown to play a favorable role as solid lubricants under severe conditions where fluids are unable to support the heavy load and are squeezed away from the contact area. Various tribological scenarios are presented for these superior solid lubricants, demonstrating the large scale potential for applications of these materials. The mechanism of action of these solid lubricants is briefly discussed. Various other potential applications of IF phases for nanocomposites with high impact resistance; in rechargeable batteries and in optical devices are discussed in short.

Inorganic fullerene-like material as additives to lubricants: structure–function relationship

Abstract Recently, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide MX2 (M=Mo, W, etc.; X=S, Se), materials with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. The main goal of the present work was to study the tribological properties of IF–WS2 in comparison to 2H–WS2 and MoS2 platelets over a wide range of loads and sliding velocities. The size and shape distributions of the nanoparticles were studied by transmission electron microscopy (TEM). The average size of the IF–WS2 particles was 120 nm, while that of 2H–WS2 and 2H–MoS2 was 0.5 μm and 4 μm. The chemical reactivity of the different powders in an oxygenated atmosphere was verified by heating the powders in ambient atmosphere. The friction experiments were performed in laboratory atmosphere (humidity ∼50%) using a ring-block tester. Complementary information on the state of wear of the powders in the lubricating fluid and on the metal surfaces of the specimens was obtained using a combination of TEM, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It has been established that IF–WS2 nanoparticles appear to have excellent tribological properties in definite loading range in comparison to typical metal dichalcogenides. The oxidation of the IF particles and the wear track was essentially less than with solid lubricants made of platelets of the same chemical compound (WS2). The main advantages of IF nanoparticles lie in their round shape and the absence of dangling bonds.

Superior tribological properties of powder materials with solid lubricant nanoparticles

Abstract Friction and wear of powder materials impregnated with commercially available layered (platelets) WS 2 (2H) and inorganic fullerene-like WS 2 nanoparticles (IF) were studied. Bronze–graphite, iron–graphite and iron–nickel–graphite samples were used in this experiment. The linear wear of powder materials (in situ) was measured. It was shown that the IF nanoparticles impregnated into the pores improve the tribological properties of powder materials in comparison to a reference sample or the sample impregnated with 2H solid lubricant particles. The mechanisms of friction and wear of the IF nanoparticles have been considered. The tribological role of the wear particles and nanoparticles of solid lubricants has been analyzed in the framework of a third body lubrication model. The state of the IF nanoparticles before and after the wear test was studied. It was found that the shape of the IF nanoparticles is preserved during the friction tests under high loads. Thin wear debris surrounded by spherical IF nanoparticles appear to be formed and provide easily sheared lubrication film (low friction coefficient) during friction experiments of powder materials containing IF nanoparticles.

Synthesis of fullerene-like MoS2nanoparticles and their tribological behavior

Further understanding of the growth mechanism and the detailed structure of fullerene-like MoS2 (IF-MoS2) nanoparticles was achieved by using a new kind of reactor. The annealed nanoparticles consist of >30 closed layers and their average diameter is 50–80 nm although a small (<5%) fraction of larger IF nanoparticles was discernible. The majority of the nanoparticles are found to have an oval (pitta-bread or flying-saucer) shape rather than being quasi-spherical. The (002) peak of the powder diffraction pattern reveals only a small (0.3%) shift to lower angles as compared to the bulk (2H) phase. This observation suggests that the structure of the nanoparticles produced in the present reactor is more relaxed as compared to the previously synthesized IF-MoS2 powder, which exhibited up to 2% shift. The present reactor also permitted scaling up of the production of the IF-MoS2 to more than 0.6 g/batch. Impregnation of such nanoparticles in metallic coatings is shown to endow these surfaces with excellent tribological behavior, which suggests numerous applications.

Mechanism of friction of fullerenes

In the past few years, inorganic fullerene‐like (IF) supramolecules of metal dichalcogenide WS2 and MoS2 with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. Recent experiments showed that IF added to oil and impregnated into the porous matrixes possess lubricating properties superior to those of layered WS2 and MoS2 (2H platelets). The main goal of this work was to analyze the mechanism of friction of fullerene‐like nanoparticles. Friction and wear behavior of IF in different contact conditions is studied. Third body model is considered. Sliding/rolling of the IF nanoparticles in the boundary of the first bodies and in between the wear particles (third body) is supposed to facilitate the shear of the lubrication film. Broken and oxidized 2H‐WS2 small pieces adhered to wear debris do not provide high tribological properties especially under high loads.

The tribological behavior of type II textured MX2 (M=Mo, W; X=S, Se) films

Abstract The tribological properties of textured WS 2 , MoS 2 and WSe 2 films, which were prepared using an ultra-thin interlayer of Ni/Cr (van der Waals rheotaxy technique) on quartz substrate, were determined in ambient conditions. Using scanning force microscope adapted for tribological measurements, very low (0.04 and below) friction coefficients and little wear were measured on flat areas of the films. Macroscopic (engineering) tribological measurements, using the reciprocating ball on flat tribometer, exhibit somewhat higher friction coefficients. Compactization of the films under the load and little wear were observed for the films even after a few hundred cycles. X-ray photoelectron spectroscopy of the WS 2 film after the wear experiment confirmed that some oxidation took place within the wear track, but the overall integrity of the film was preserved. These measurements indicate that highly textured films of this kind are promising candidates for tribological coatings, where oil-free lubrication is required.

Controlled Doping of MS2 (M=W, Mo) Nanotubes and Fullerene‐like Nanoparticles

Doping of semiconductor nanocrystals and nanowires with minute amounts of foreign atoms plays a major role in controlling their electrical, optical, and magnetic properties. In the case of carbon nanotubes, subsequent doping with oxygen and potassium leads to a p-type and n-type behavior, respectively. In another work, VOx nanotubes were transformed from spin-frustrated semiconductors to ferromagnets by doping with either electrons or holes. Calculations indicated that nand p-type doping of multiwall MoS2 nanotubes (INT) could be accomplished by substituting minute amounts of the Mo lattice atoms with either Nb (p-type) and Re (n-type), respectively. Substituting (< 0.1 at%) molybdenum by rhenium atoms and sulfur by halogen atoms was shown to produce n-type conductivity in MoS2 crystals. To synthesize rhenium-doped nanoparticles (NP) and nanotubes both in situ and subsequent doping methods were used. Figure 1a shows the quartz reactor used for in situ synthesis of rhenium doped MoS2 NP with fullerene-like structure (Re:IF-MoS2). The formal Re concentration was varied from 0.02 to 0.7 at%. The precursor RexMo1 xO3 (x< 0.01) powder was prepared in a specially designed auxiliary reactor (see Supporting Information). Evaporation of this powder takes place in area 1 at 770 8C (Figure 1 a). The oxide vapor reacts with hydrogen gas in area 2 (Figure 1a) at 800 8C which leads to a partial reduction of the vapor and its condensation into Re-doped MoO3 y nanoparticles. The resulting NP react with H2/H2S gas in area 3 at 810–820 8C to produce reduced oxide nanoparticles engulfed with a few closed layers of Re:MoS2, which protect it against ripening into bulk 2H-MoS2. [7] To complete this oxide to sulfide conversion a long (25–35 h) annealing process at 870 8C in the presence of H2S and forming gas (H2 10 wt %; N2) was performed. At the end of this diffusion-controlled process a powder of Re-doped MoS2 NP with a fullerene-like (IF) structure (Re:IF-MoS2) was obtained. In addition, doping of IF-WS2 NP and INT-WS2 was subsequently carried out by heating the pre-prepared IF/INT in an evacuated quartz ampoule also containing ReO3, or ReCl3 and iodine. In the case of ReCl3, both the rhenium and the chlorine atoms (substitution to sulfur atoms) served as ntype dopants. Typical high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) micrographs of the Re-doped fullerene-like NP are shown in Figure 1b. The Re:IF-MoS2 consists of about 30 closed (concentric) MoS2 layers. No impurity, such as oxides, or platelets (2 H) of MoS2 could be found in the product powder. The line profile and the Fourier analyses (FFT) (inset of Figure 1b) show an interlayer spacing of 0.627 nm (doped). Furthermore, the layers seem to be evenly folded and closed with very few defects and cusps, demonstrating the Re-doped NP to be quite perfectly crystalline. HRTEM did not reveal any structural changes even for the samples with high Re concentration (0.71 at%). However, owing to its quasispherical shape and size, this analysis cannot completely rule-out the presence of a small amount of the ReS2 phase in the IF NP. Figure 1c shows a typical TEM image of Re(Cl) (post synthesis) doped multiwall WS2 nanotube. There is no [*] L. Yadgarov, Dr. R. Rosentsveig, Dr. A. Albu-Yaron, Prof. R. Tenne Department of Materials and Interfaces, Weizmann Institute Rehovot 76100 (Israel) E-mail: reshef.tenne@weizmann.ac.il

Fabrication of self-lubricating cobalt coatings on metal surfaces

Composite coatings of Co?+?fullerene-like WS2 nanoparticles on stainless steel substrate were obtained through electroless deposition, using DMAB (dimethyl borane complex, 97%) as the reducing agent, and by electroplating in acidic solution. Phase analysis results show that the coatings consist of Co and the fullerene-like WS2 nanoparticles alone. Tribological measurements show reduced wear and friction of the composite coatings as compared with the pure cobalt film or the stainless steel substrate.