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

The tribological properties of solid lubricants such as graphite and the metal dichalcogenides MX 2 (where M is molybdenum or tungsten and X is sulphur or selenium) 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 are of technological interest for reducing wear in circumstances where liquid lubricants are impractical, such as in space technology, ultra-high vacuum or automotive transport. These materials are characterized by weak interatomic interactions (van der Waals forces) between their layered structures, allowing easy, low-strength shearing 14 , 15 . Although these materials exhibit excellent friction and wear resistance and extended lifetime in vacuum, their tribological properties remain poor in the presence of humidity or oxygen 16 , 17 , 18 , 19 , thereby limiting their technological applications in the Earth's atmosphere. But using MX 2 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 MoS 2 nanoparticles, deposited by a localized high-pressure arc discharge method, exhibit ultra-low friction (an order of magnitude lower than for sputtered MoS 2 thin films) and wear in nitrogen and 45% humidity. We attribute this ‘dry’ behaviour in humid environments to the presence of curved S–Mo–S planes that prevent oxidation and preserve the layered structure.