Mechanism of action of WS2 nanoadditives in high-pressure contacts

Because of their excellent tribological properties and potential to replace problematic lubricant additives currently in use, WS2 nanoparticles have spurred considerable interest over the last two decades from academia and industry to decipher their mechanism of action. To elucidate the mechanism, this study carried out tribological tests at low and high temperatures andinvestigated the wear track and friction properties. It was found that in high-pressure, high-temperature sliding contacts WS2 nanoadditives react with the metal substrate to generate thick chemical tribofilmswhich account for their excellent tribological properties. Based on XPS and FIB/SIMS results, a layered structure was proposed for the chemically formed tribofilms. The large amount of W in the composition of the reacted tribofilm could explain the excellent mechanical and antiwear properties while the exfoliated squashed WS2 NPs which fill the gaps and cover the reacted tribofilm account for the striking reduction of the boundary friction