Tribological properties of nanolamellar tungsten disulfide doped with zinc oxide nanoparticles

Tribological properties of nanolamellar tungsten disulfide doped with zinc oxide nanoparticles were studied. Nanolamellar tungsten disulfide and ZnO nanoparticles produced by electrospark erosion of metal granules in an H2O2 solution were analyzed using the XRD, SEM and TEM techniques. According to the tribological measurements, ZnO nanoparticles did not significantly change the friction coefficient of nanolamellar WS2 at 25 °C in air, whereas they positively impact on wear resistance of nanolamellar WS2 at 400 °C.

The present work is therefore aimed at studying tribological properties of nanolamellar tungsten disulfide doped with zinc oxide nanoparticles.

Results and discussion
The X-ray diffraction measurements (Fig. 1) show that the main phase of the powder prepared by electrospark erosion of zinc granules in an H 2 O 2 solution is zinc oxide ZnO (zincite, PDF# 361451). The calculations according to the Scherrer's formula demonstrate that the mean size of the ZnO crystallites is about 24 nm which corresponds well to the TEM observations (Fig. 2). The synthesized ZnO powder are hexagonal particles of 15-30 nm in width which form agglomerates of several microns in width. It is also in a good agreement with the XRD data showing that the main phase is hexagonal zinc oxide. The small size and the hexagonal structure of zinc oxide nanoparticles (n-ZnO) can play an important role in lubrication processes by filling microcracks of friction surfaces. As shown in Fig. 3, the as-prepared nanolamellar WS 2 presented agglomerates of lamellar particles with a thickness of 50-150 nm. The particles were obviously well crystallized in hexagonal lattice what was confirmed by the XRD data (Fig. 4). The lamellas are 20-40 nm wide. Some lamellar particles possess multilayer structure (Fig. 3).
The additive of ZnO nanoparticles in nanolamellar WS 2 powder resulted in a low increase of the friction coefficient at 25 °C (Fig. 5) in comparison with the undoped powder. The observed effect can be explained by the difference in the hardness of zinc oxide and tungsten disulfide what results in indentation of ZnO nanoparticles in the metal disulfide nanolayer under friction according to the mechanism described in (Prasad et al. 2000). Thus, low friction of nanolamellar WS 2 doped with n-ZnO at 25 °C is provided by nanolamellar tungsten disulfide. At 400 °C, the ZnO-WS 2 composition exhibits an unstable friction coefficient (Fig. 5, rose curve) while the pure WS 2 has a low and a more stable friction coefficient (Fig. 5, red curve). After 10 min of the test, reduction of the friction coefficient up to an average value µ = 0.23 was observed in comparison with  (Prasad et al. 2000). The friction coefficient fluctuations can be explained by the more intensive tribochemical transformation of tungsten disulfide into tungsten oxide with the following interaction with n-ZnO.
Examination of the worn steel disk after the friction test at 400 °C showed a more visible effect of ZnO nanoparticles on the performance of nanolamellar WS 2 (Fig. 6). We can see a decrease in the wear track depth and degradation of the steel disk surface for the nanolamellar WS 2 doped with n-ZnO (Fig. 6a, b). Nevertheless, the wear track surface for this sample displays cavities which are caused by the use of zinc oxide.

Conclusions
The additive of zinc oxide nanoparticles showed an insignificant increase in the friction coefficient of the composite lubricant and low friction was supplied by nanolamellar tungsten disulfide at 25 °C. The nanolamellar WS 2 doped with n-ZnO showed ambiguous results in the tribological experiments in air at 400 °C which can be an object of additional studies. Apparently, doping nanolamellar WS 2 with ZnO nanoparticles can lead to a positive effect on wear at high temperature.

Experimental
ZnO nanoparticles were synthesized by electrospark erosion of zinc granules in an H 2 O 2 solution (Galanov et al. 2013). A ceramic cylinder served as a synthesis reactor. The synthesis reactor was charged with about 100 g of zinc granules of 5 mm in diameter. Zinc electrodes were placed into the reactor which was then filled with 200 ml of 40 % H 2 O 2 . The electrodes were connected to a pulse current supply with the following characteristics: pulse duration-10 µs, pulse frequency-100 Hz, voltage-500 V, and first pulse half-cycle current-250 A. The obtained suspension was dried after the process at 60 °C in air.
Tungsten disulfide was synthesized via the method reported in the previous work (Irtegov et al. 2012). After drying, the synthesized powder was examined using the X-ray diffraction (Shimadzu XRD-7000 diffractometer, CuK α radiation), SEM (JSM-7500FA, JEOL) and TEM (JEM-2100F, JEOL) techniques. The size of crystallites of as-prepared ZnO nanoparticles was calculated using the Scherrer formula: where λ is the is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, θ is the Bragg angle.
Nanolamellar tungsten disulfide and zinc oxide nanoparticles (n-ZnO) were mechanically mixed in a 1:1 weight ratio. Tribological properties of the doped WS 2 nanolamellar powder were then studied. The friction coefficient of nanolamellar WS 2 doped with n-ZnO was measured with a "ball-on-disk" PC-Operated High Temperature Tribometer (THT-S-AX0000, CSEM). The worn surfaces were studied using a non-contact profilometer (Micro Measure 3D Station, STIL, France). Medium-carbon steel disks of diameter 30 mm, height 4 mm, and surface roughness Ra = 30-50 nm were used as the body. A 3 mm hard alloy ball was used as the counterbody. The normal load was 5 N, the temperature was 25 and 400 °C, the linear speed was 5 cm/s, and the wear scar radius was 3 mm.