This study aimed at examining the tribological properties of nanolamellar molybdenum disulfide doped with copper nanoparticles. Nanolamellar molybdenum disulfide was produced using self-propagating high-temperature synthesis via the reaction between elementary sulfur and nanosized molybdenum powder prepared by electrical explosion of wires. Copper nanoparticles were also prepared by electrical explosion of copper wires. Comparative tribological tests were carried out for nanolamellar and commercial molybdenum disulfides doped with 7 wt.% of copper nanoparticles. It was demonstrated that doping copper nanoparticles additives reduce wear of the friction body when using both commercial and nanolamellar molybdenum disulfide.
A large amount of research has been devoted to the study of tribological properties of lamellar transition metal dichalcogenides (TMDC) like molybdenum disulfide (MoS2). Remarkable antifriction and antiwear properties of molybdenum disulfide in an inert atmosphere or vacuum are explained by its lamellar structure resulting in low shear strength. Martin et al. [
Some papers are concerned with antifriction and antiwear performances of these materials in the nanostructured state. Miura et al. [
Nevertheless, some conditions, for example, high humidity and temperatures, impose limitations in its application areas. Nowadays various chemical additives are used in order to enhance operating performance of lubricants. Over the last decade, the number of studies on the effects related to doping TMDC with small additives of various metal nanoparticles has been reported [
This paper focuses on the study of tribological properties of nanolamellar molybdenum disulfide doped with copper nanoparticles. We have expected a synergistic effect by modifying nanolamellar MoS2 with copper nanoparticles due to their joint tribological action in the friction area. It is supposed that chemical stability of such a system will be better. It is also expected that the copper nanoparticles will cause a metal cladding effect of rubbing surfaces and reduction of wear.
Self-propagating high-temperature synthesis (SHS) was used for the fabrication of nanolamellar molybdenum disulfide. A similar synthesis method for fabrication of metal sulfides for tribological applications has been reported in [
Principal scheme of the setup for the fabrication of nanoparticles by electrical explosion of wires:
Copper nanoparticles were produced by electrical explosion of copper wires the diameter of which was
In order to organize an exothermal reaction for the synthesis of nanolamellar molybdenum disulfide, cylindrical pellets of 32 mm in diameter were prepared. The pellets were compacted from a stoichiometric mixture of nanodispersed molybdenum powder and pure elementary sulphur. The process of self-propagating high-temperature synthesis of nanolamellar molybdenum disulfide was carried out using the experimental setup presented in [
The as-synthesized products were easily disintegrated silvery-black agglomerates. They were ground and washed out from sulphur traces in hexane under ultrasonic treatment. After drying, the obtained powders were analyzed using an X-ray diffractometer Shimadzu XRD-7000 diffractometer (CuKα irradiation), a scanning electron microscope (JSM-7500FA, JEOL).
The current tests of the friction coefficient of the undoped, nanolamellar, and commercial molybdenum disulfides were carried out by “ball-on-disk” PC-Operated High Temperature Tribometer (TНT-S-AХ0000, CSEM). The wear scar was explored on a noncontact profilometer (Micro Measure 3D Station, STIL, France). Medium-carbon steel disks of diameter 30 mm, height 4 mm, and surface roughness
After the tribological experiments, the used disk surface was cleaned with an organic solvent (acetone) and was explored with an optical microscope. The wear track area equal to 2 mm × 1 mm was scanned and the volume wear was calculated by six points using profilometer software. After the treatment of the obtained track images, the surface areas above and below the baseline were determined. The wear volume was then calculated using the following formula (
The lubricated track asperity was also determined.
As-prepared molybdenum disulfide was subjected to X-ray diffraction analysis and scanning electron microscopy. The XRD analysis results showed that the main phase in the final products is 2H-MoS2 (Figure
XRD pattern of nanolamellar MoS2.
The nanolamellar or commercial molybdenum disulfides were mechanically mixed in a mortar with 7 wt.% of copper nanoparticles prepared by electrical explosion of wires and then treated in a hexane suspension in an ultrasonic bath and further natural drying. Nanosized copper presented particles of 50–80 nm with a spherical, slightly faceted shape (Figure
SEM images for copper nanoparticles (a), commercial molybdenum disulfide doped with nanoparticles (b), and nanolamellar molybdenum disulfides doped with Cu nanoparticles before (c) and after the tribological test (d).
Figures
Coefficient of friction for commercial MoS2 powder without (1) and with (2) copper nanoparticles and for nanolamellar MoS2 without (3) and with (4) copper nanoparticles at a friction load of 5 N.
Three-dimensional images of the wear scars for the samples lubricated with undoped (a) and doped (b) nanolamellar MoS2 (a) and undoped (c) and doped (d) commercial MoS2.
Figures
When increasing the friction load up to 10 N, nanolamellar molybdenum disulfide reveals a relatively high coefficient of friction. It can be related with the fact that nanolamellar MoS2 absorbs better water vapors from the atmosphere and can decompose at a higher friction load faster than the other samples (Figure
Coefficient of friction for undoped nanolamellar MoS2 (1), commercial MoS2 undoped (2) and doped with copper nanoparticles (3), and nanolamellar MoS2 doped with copper nanoparticles (4) at a friction load of 10 N.
Table
Profilometer results of wear track.
Sample | Depth of wear track ( |
Wear volume ( |
---|---|---|
|
0.42 | 2.0 |
|
0.57 | 3.5 |
|
0.11 | Negative value |
|
0.18 | Negative value |
|
1.22 | 624.6 |
|
0.82 | 320.3 |
|
1.12 | 421.11 |
Copper nanoparticles can make a metal cladding effect of sliding surfaces. At the same time, they can adhere to the particles surface of nanolamellar molybdenum disulfide. Copper can reveal a very good ability for adhesion to the surface of the steel disk. Figure
Schematic diagram describing the possible mechanism of the tribological action of nanolamellar disulfide doped with copper nanoparticles.
We can, therefore, observe a synergistic effect of the combined action of nanolamellar molybdenum disulfide and copper nanoparticles on the friction area. A significantly weaker similar effect is characteristic for commercial molybdenum disulfide doped with copper nanoparticles. It is necessary to take into consideration possible agglomeration of the used nanoparticles which can lead to nonuniform dispersion. This problem can be solved when using these nanoparticles in oil based lubricants with special solvents in which they can form stable suspensions.
For the first time, a comparative study of tribological properties of commercial and nanolamellar MoS2 doped with copper nanoparticles prepared by electrical explosion of wires was performed. The experiments were carried out at room temperature. It was found that, at a load of 5 N, the coefficient of friction of nanolamellar MoS2 doped with copper nanoparticles is lower than that for doped commercial MoS2. When increasing the load up to 10 N, doped and undoped nanolamellar molybdenum disulfide have demonstrated a lower coefficient of friction in comparison with that of commercial powder. It was also explored that the introduction of the copper nanoparticle additives essentially reduces wear of the friction body surface. Most likely it is due to the metal cladding effect of the wear track with the tribofilm formed by the copper nanoparticles.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported under the state assignment of the Ministry of Education and Science of Russia for 2014–2016 (Research Work no. 361). The authors would like to thank the Nano-Center at Tomsk Polytechnic University for the XRD and SEM analyses.