WS2 nanoparticles as lubricant additives

• Main Author: Niste, Vlad
• Other Authors: Ratoi, Monica
• Published: University of Southampton 2015
• Subjects: 620; T Technology (General)
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.675170

Due to their excellent tribological properties and potential to replace problematic lubricant additives currently in use, WS2 nanoparticles (NPs) have spurred considerable interest from academia and industry over the last two decades to decipher their mechanism of action. To elucidate the mechanism, this study carried out tribological tests at high pressures and low/high temperatures (40 and 100ºC) and investigated the tribofilm generated on the wear track and its wear and friction properties. It was found that WS2 NPs react with the metal substrate at high temperatures to form a chemical tribofilm covered with squashed NPs. The generation of this tribofilm accounts for their excellent tribological properties. By investigating the chemical and mechanical properties of the tribofilm, it was possible to explain the tribological properties of WS2 NPs. Based on chemical analysis results, a layered structure was proposed for the chemically formed tribofilms. The large amount of tungsten compounds in the composition may explain the excellent mechanical properties of the tribofilm, as revealed by nanoindentation tests. The importance of base oil polarity was investigated. It was found that the efficiency of the NPs is reduced in polar oils, because the oil molecules can compete with the nanoadditive by adsorbing on the metal surface in the tribological contact and impeding the formation of the tribofilm. To investigate which type of WS2 NPs (2H or IF) performs better in tribological applications and if other tungsten dichalcogenides (IF-WSe2) are also potential candidates as nanoadditives, tribological tests and analysis of the wear tracks were performed. 2H-WS2 showed superior friction and wear reducing properties in high-pressure high-temperature contacts. The tribological performance of 2H-WS2 NPs was compared to that of popular conventional additives, e.g. antiwear zinc dialkyldithiophosphates (ZDDPs) and organic friction modifiers (OFMs). At the end of three hour tests, 2H-WS2 NPs and ZDDP+OFM mixtures showed similar antiwear properties, but 2H-WS2 NPs induced a significant reduction of the friction coefficient in the mixed and boundary lubrication regimes. The ability of 2H-WS2 NPs to inhibit hydrogen permeation in high strength bearing steel used in fuel cells and wind turbines was also investigated. Thermal desorption spectroscopy revealed that the chemical tribofilm generated on the wear tracks can significantly reduce the concentration of hydrogen and water in the steel substrate after rolling contact fatigue tests performed in high-temperature high-pressure conditions.