Tribological properties of solid lubricant nanocomposite coatings on base of tungsten disulphide nanoparticles
Keywords:Solid lubricant coatings, nanoparticles, nanocomposite, XRD, SEM
Tungsten disulphide (WS2) and molybdenum disulphide (MoS2), which belong to the family of transition metal dichalcogenides, are well known for their solid lubricating behavior. Thin films of WS2 and MoS2 exhibit extremely low coefficient of friction in dry enviroments, and are typically applied by sputter deposition, pulsed laser ablation, evaporation or chemical vapor deposition, which are essentially either line- of-sight or high temperature processes. In this paper we have investigated the tribological properties of embedded solid lubricant coatings from WS2 nanoparticles and compared them to monolithic and alloy films of the same constituents.
The WS2/metal (Ti, W) nanocomposite were deposited on alloy substrate by magnetron sputtering. The patterning methods were used to create two component coatings in which WS2 were embedded in a Ti matrix. Coatings were deposited using sputter deposition, and co-deposition of WS2 and Ti was used to deposit the alloy coatings. A pin-on-disk (POD) test was used to examine the frictional behavior and mechanical stability of the coatings, and was carried out in both low and high humidity conditions.
The morphology and microstructure of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). The tribological properties of WS2/metal (Ti, W) nanocomposite coatings were investigated using a POD tribometer in ambient air and in humidity conditions.
XRD and SEM was used to examine morphology of the wear track after the POD test. The nanoparticles WS2 can decrease the friction coefficient of lubricant obviously. However the results showed that their friction reductions have not obvious difference by the POD tribometer.
The WS2/Ti nanocomposite coating showed lower frictions coefficient and higher wear resistance as compared to the pure WS2 coating, which were caused by the microstructure of the composite coating that serve as perfect intermediate lubricants between the contact surfaces.
The analyses of surfaces composition coating conducted by XRD and SEM images showed that nanoparticles form a protective film (WO3) allowing an increase the load capacity of friction (rubbed) pairs. The main advantage of the nanoparticles is ascribed to the release and furnishing of the nanoparticles from the valley onto the rubbing metal surface and their confinement at the interface.
Coatings of WS2 alone were found to perform well under low-humidity conditions, but poorly under high- humidity. Alloying of WS2 with Ti was found to provide some improvement under high humidity. The patterned film, which consisted of columns of WS2 embedded in a Ti film was found to exhibit friction and wear properties superior to either Ti or WS2 alone. The coating investigated here have potential applications for cutting tools and metal forming dies that will enhance tool life and reduce the energy expended due to friction forces; as well is used in various tribological fields such as seals, bearings or electrical motor brushes and, also for applications needing excellent lubrication and wear-reducing properties.