Se Nano-Powder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions

Abstract

Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS2, even though selenides and tellurides have demonstrated superior tribological properties. Here, we describe an innovative in-operando conversion of Se nano-powders into lubricious 2D selenides by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in shear-induced synthesis of selenide monolayers from nano-powders. The use of Se nano-powder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nano-powder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in-operando and exploit their friction- and wear reducing impact.