Inorganic Fullerene-like Materials Research Articles

Tribological behavior of nanocomposite coatings based on fullerene-like structures

This paper presents a new group of nanocomposite coatings based on integrating inorganic fullerene-like material (IFLM) structures into conventional coating matrices. Such coatings have been developed within the scope of the European funded FOREMOST project (FP6-NMP3-CT-2005-515840). Regarding the synthesis of these nanocomposite coatings based on fullerene-like components, two alternative routes were explored: introducing preformed IFLMs into the coating deposition process or forming the fullerene-like components in situ during the coating deposition process. Both methods have been demonstrated to be technically feasible, depending on the nature of the coating matrix or the desired fullerene-like structure. These new materials allow some independent control of tribological properties usually known as antagonists (very high load bearing capacity with a very low friction coefficient). In the case of unidirectional movement, under dry conditions, the best coatings developed in FOREMOST displayed a coefficient of friction in air within the range 0.04–0.10, depending on the degree of humidity and the test conditions. Pure sliding laboratory tests indicate that for some industrial applications fullerene-like nanocomposite coatings can give significant reductions in wear and friction coefficient when compared to similar coatings without fullerene-like components. The lubrication mechanisms through which these fullerene-like structures improve friction and prevent wear are also discussed in this paper.

Inorganic fullerene-like materials for increased operational life in rolling/sliding applications

The synthesis of inorganic fullerene-like materials (IFLMs) represents a major scientific breakthrough, and recently, new coatings and lubricants incorporating inorganic fullerene-like nanoparticles have been produced within the integrated European project FOREMOST. The objective of the work presented here was to assess the influence of IFLM in lubricants on the contact fatigue and wear performance of tribological contacts subjected to rolling/sliding motion such as gears. The wear tests were carried out using a double disc rig with specimens manufactured from S156 steel. The lubricants tested were two gear oils without IFLM and mixed with IFLM. The results show that a significant increase in the operational life of rolling/sliding components can be achieved using lubricants incorporating IFLMs.

Spray drying process to synthesize multiple fullerene-like MoS 2 particles

Multiple inorganic fullerene-like (IF) molybdenum disulfide hollow spheres were synthesized through spray drying method. The molybdenum trisulfide precursor, which was prepared by the mixed solution of ammonium molybdate ((NH 4) 2MoO 4) and ammonium disulfide ((NH 4) 2S) with a spray drying method, was desulfurized at 850 °C in a mixture of hydrogen and argon, and subsequently multiple fullerene-like molybdenum disulfide hollow spheres were obtained. The samples were identified and characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM), and the mechanism of formation is also discussed based on the experimental result. The spray drying process would be an effective method to synthesize large-scale of inorganic fullerene-like materials.

Behavior of solid lubricant nanoparticles under compression

Inorganic fullerene-like materials have been identified as being of potentially utmost importance for many industrial applications. MoS2 and WS2 hollow nanoparticles have been identified as strong candidates for tribological applications such as solid lubricants. The main goal of this work was to evaluate the mechanical properties of solid lubricant particles in ensemble under hydrostatic pressure. The behavior of nanopowders under compression has been described on the basis of constitutive models of continuum mechanics. The model will be applied to an isotropic compaction of copper (well-studied medium), fullerene-like (IF-WS2) nanoparticles and a natural powder of 2H-WS2 platelets. The morphology of individual nanoparticles and nanoparticle ensembles will be examined and discussed. Another aspect of this work was to study the applicability and limitations of the proposed constitutive model for the understanding of the tribological behavior of solid lubricant nanoparticles. Compression with the maximal pressure (500 MPa) showed that the shape of the IF nanoparticles is preserved. The dominant mechanism of damage was found to be the delamination or peeling-off of the external sheets of hollow nanoparticles. Strong destruction of 2H-WS2 platelets was observed under compression.

Mechanisms of ultra-low friction by hollow inorganic fullerene-like MoS 2 nanoparticles

Inorganic Fullerene-like (IF)-MoS 2 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)-MoS 2 material. Previous works made by Rapoport et al. with IF-WS 2 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-MoS 2 material appears to be a good candidate for use in various environments in regard to other MoS 2 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-MoS 2 particles are often observed after friction. In this case, low friction is thought to be due either to sliding between IF-MoS 2 external flattened planes or to slip between individual unwrapped MoS 2 sheets.

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

Recently, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide MX 2 (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–WS 2 in comparison to 2H–WS 2 and MoS 2 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–WS 2 particles was 120 nm, while that of 2H–WS 2 and 2H–MoS 2 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–WS 2 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 (WS 2). The main advantages of IF nanoparticles lie in their round shape and the absence of dangling bonds.

Bulk Synthesis of Inorganic Fullerene-like MS2(M = Mo, W) from the Respective Trioxides and the Reaction Mechanism

Recently, milligram quantities of MoS2 fullerene-like nanotubes and negative curvature polyhedra (generically called inorganic fullerene-like material, IF), were reproducibly obtained by a gas phase reaction from an oxide precursor (Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. Srolovitz, D. J.; Safran, S. A.; Homyonfer, M.; Tenne, R. Phys. Rev. Lett. 1995, 74, 1778). The present work focuses on the mechanism of the synthesis of IF-MS2 (M = W, Mo). The IF material is obtained from oxide particles smaller than ca. 0.2 μm, while larger oxide particles result in 2H-MS2 platelets. The key step in the reaction mechanism is the formation of a closed layer of MS2, which isolates the nanoparticle from its surroundings and prevents its fusion into larger particles. Subsequently, the oxide core of the nanoparticle is progressively converted into a sulfide nanoparticle with an empty core (IF). Taking advantage of this process, we report here a routine for the fabrication of macroscopic quantities of a pure IF-WS2 phase with a very high yield. As anticipated, the size distribution of the IF material is determined by the size distribution of the oxide precursor. The present synthesis paves the way for a systematic study of these materials which are promising candidates for, e.g., solid lubrication.

Scanning Tunneling Microscope Induced Crystallization of Fullerene-like MoS2

Amorphous precursors, like MoS3 (WS3), were shown before to be an ideal precursor for the growth of inorganic fullerene-like material in a rather slow crystallization process which lasts anything from 1 h at 800−900 °C1,2 to a few years at ambient conditions.3,4 Using a few microsecond short electrical pulses from the tip of a scanning tunneling microscope, crystallization of amorphous MoS3 (a-MoS3) nanoparticles, which were electrodeposited on a Au substrate into MoS2 nanocrystallites with a fullerene-like structure (IF-MoS2), is demonstrated. The (outer) shell of each nanocrystallite is complete, which suggests that the reaction extinguishes itself upon completion of the crystallization of the MoS2 layers. A completely different mode of crystallization is observed in the case of continuous a-MoS3 films. Here tiny (2−3 nm thick) 2H-MoS2 platelets are observed after the electrical pulse, suggesting a very rapid dissipation of the thermal energy through the gold substrate, in the continuous domain. Since t...