Thin films of fullerene-like MoS2 nanoparticles with ultra-low friction and wear

The tribological properties of solid lubricants such as graphite and the me tal dichalcogenides MX2 (where M is molybdenum or tungsten and X is sulphur or selenium)(1-13) are of technological interest for reducing wear in circ umstances where liquid lubricants are impractical, such as in space technol ogy, ultra-high vacuum or automotive transport. These materials are charact erized by weak interatomic interactions (van der Waals forces) between thei r layered structures, allowing easy, low-strength shearing(14,15). Although these materials exhibit excellent friction and wear resistance and extende d lifetime in vacuum, their tribological properties remain poor in the pres ence of humidity or oxygen(16-19), thereby limiting their technological app lications in the Earth's atmosphere. But using MX2 in the form of isolated inorganic fullerene-like hollow nanoparticles similar to carbon fullerenes and nanotubes can improve its performance(1). Here we show that thin films of hollow MoS2 nanoparticles, deposited by a localized high-pressure arc di scharge method, exhibit ultra-low friction (an order of magnitude lower tha n for sputtered MoS2 thin films) and wear in nitrogen and 45% humidity. We attribute this `dry' behaviour in humid environments to the presence of cur ved S-Mo-S planes that prevent oxidation and preserve the layered structure .