Study of tribological behavior of Cu–MoS2 and Ag–MoS2 nanocomposite lubricants
© An et al. 2016
Received: 25 October 2015
Accepted: 11 January 2016
Published: 22 January 2016
Tribological behavior of Cu–MoS2 and Ag–MoS2 nanocomposite lubricant was studied. Cu nanoparticles produced by electrical explosion of copper wires and Ag nanoparticles prepared by electrospark erosion were employed as metal cladding modifiers of MoS2 nanolamellar particles. The tribological tests showed Cu–MoS2 and Ag–MoS2 nanocomposite lubricants changed the friction coefficient of the initial grease and essentially improved its wear resistance.
Molybdenum and tungsten disulfides due to their anisotropic layered crystal structure are characterized by unique properties. These materials are good solid lubricants and antifriction additives to oil and greases (An and Irtegov 2014), moreover MoS2 is a promising material for lithium ion batteries (Wang et al. 2010). With respect to application of molybdenum and tungsten disulfides as lubricants, the synthesis and the appropriate state of dispersed materials or films play an important role. For improving tribological properties of MoS2 several methods are used: decreasing the particle size (Hu et al. 2010), creation of adaptive lubricants (Prasad et al. 2000), a composite mixture with other lubricants etc. As concerns composite lubricants, Sb2O3–MoS2 (Zabinski et al. 1993), Ag–MoS2 (Zhang et al. 2012), Ti–MoS2 (Renevier et al. 2001; Ilie and Tita 2007), Ni–WS2 (Wang et al. 2008) composites have shown a positive effect on tribological properties in comparison with pure compounds. Copper and copper alloys are well known lubricant materials due to the zero-wear friction effect discovering in 1956 (Garkunov 2000) and widely used in composite lubricants with molybdenum disulfide, especially for applications in vacuum (Kolesnichenko et al. 1986; Merstallinger et al. 2007; Kato et al. 2003). However, a synergetic effect of excellent antiwear properties of copper and antifriction behavior of MoS2 is observed in air at room temperature (An et al. 2014). The present paper is devoted to the study of the composition dependence on tribological properties of greases doped with Cu–MoS2 and Ag–MoS2 nanocomposites.
Results and discussion
Wear and roughness of the steel disks after the friction tests
Wear (μm3 10−6)
Roughness of the track (nm)
Litol grease +5 % n-MoS2
Litol grease + 5 % (n-MoS2 + 7 % n-Cu)
Litol grease 5 % (n-MoS2 + 7 % n-Ag)
VNIINP grease + 5 % (n-MoS2 + 7 % n-Cu)
Composite greases containing nanolamellar MoS2 doped with Cu and Ag nanoparticles were successfully prepared for tribological tests. The performed tribological tests showed the better antifriction performance for both the solid n-MoS2 lubricant doped with copper nanoparticles and the Litol and VNIINP greases doped with n-MoS2 with copper nanoparticles. Electroexplosive copper and electroerosive silver nanoparticles can improve the n-MoS2 tribological performance due to a visible rise in antiwear characteristics. At the same time, we can expect essential improvement of oxidation stability of the greases doped with the studied metal nanoparticles, especially with n-Ag. Another explanation for the improvement of the properties is related to a synergetic effect in using nanolamellar molybdenum disulfide and metal cladding additives of Cu and Ag nanoparticles.
MoS2 nanolamellar particles (n-MoS2) produced by self-propagating high-temperature synthesis (SHS), as well as copper (n-Cu) and silver nanoparticles (n-Ag) obtained by electrical explosion of wires (EEW) and electrospark erosion, respectively, were used for preparing a composite lubricant. SHS of metal sulfides from metal nanopowders is discussed in Irtegov et al. (2012). Conditions and parameters of electrical explosion of copper wires are presented in An et al. (2014). The initial powders were analyzed using an X-ray diffractometer Shimadzu XRD-7000 diffractometer (CuK α irradiation) and a scanning electron microscope (JSM-7500FA, JEOL). In order to minimize agglomeration, the nanoparticles were subjected to ultrasonic treatment in an organic solvent before the preparation of the greases. For tribological tests MoS2 nanolamellar particles and Cu nanopowder are mechanically mixed during 30 min. Copper content in composite lubricant was 2, 7, 25 and 50 wt%, respectively. Besides, a solid lubricant, complex soap based greases (LITOL and VNIINP) with Cu–MoS2 additives were produced by dispersing using ultrasonic bath. Before dispersing, viscosity of greases was decreasing by addition of hexane. After dispersing composite greases are dried at room temperature during 24 h. Tribological investigations of pure nanolamellar MoS2, composite Cu–MoS2 lubricants and greases were carried out by “ball-on-disk” PC-Operated High Temperature Tribometer TXT-S-AH0000, CSEM. The wear scar was explored on a noncontact profilometer Micro Measure 3D Station, STIL. All tests were carried out using a 30 mm diameter medium-carbon steel disks as the friction body, and a vanadium-cobalt ball of diameter 3 mm was used as the counterface. The tests were run using a load of 5 N and sliding speed of 5 cm/s, with track diameter 3 mm, duration of tests was 30 min. The mean contact pressure was 0.56 N/mm2. After friction tests surface of wear scars were analyzed using an atomic force microscope Ntegra Aura (NT-MDT, Russia).
VA carried out the main conception and the main tribological experiments, participated in the analysis and interpretation of data. EA and IS carried the main tribological experiments and participated in the analysis interpretation of the data obtained. VD, NB, MK participated in the development of the main conception and its interpretation. All authors read and approved the final manuscript.
This work was supported under the state assignment of the Ministry of Education and Science of Russia for 2014–2016 (Research Work No. 361) and by the Russian Foundation for Basic Research (Project No. 15-38-50081). The authors would like to thank the Nano-Center and Scientific Analytical Centre at Tomsk Polytechnic University for the XRD, TEM and SEM analyses. Tribological tests were done using the equipment of the Material Properties Measurements Centre of TPU.
The authors declare that they have no competing interests.
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