Tribological properties of WS2 nanoparticles under mixed lubrication

Recently, it has been established that WS2 and MoS2 nanoparticles (inorganic fullerene-like, IF) mixed with oil, and impregnated into porous matrix of powdered materials appear to enhance the tribological properties of mating surfaces in definite loading range in comparison to typical metal dichalcogenide solid lubricants. The main results have been obtained under relatively low pressures. It is important to evaluate the tribological properties of IF when the concentrated contact is obtained. The effect of the IF in oil was studied using pin-on-disk tester in the regime of mixed lubrication. The interaction between the full film and the asperity contact fractions has been considered and the time evolution of the friction force was evaluated. The states of the mating surfaces and the nanoparticles before and after the friction test were studied by transmission electron microscopy (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). It was established that the IF nanoparticles mixed with oil allow to decrease the fraction of straight asperity contact under mixed lubrication regime and thus improve their tribological properties. TEM analysis showed that the shape of the IF nanoparticles is preserved under low loads. It was found that some of the IF nanoparticles were damaged after the friction at the maximal load of 420 N. The IF nanopaticles appear to form a protective film allowing increased load capacity of the rubbed pairs. The role of the IF solid lubricants as a part of a third body is discussed in this work. The mechanism of friction and wear of the IF nanoparticles are discussed.