Superlow Friction of WS₂ Coatings in Ultrahigh Vacuum at Low Temperature

Today molybdenum disulfide (MoS₂) is the most commonly used solid lubricant for vacuum applications. Its good tribological performance is considered to be due to its lamellar structure. Coefficient of friction in the millirange (so-called “superlow friction” or “superlubricity”) has already been observed in ultrahigh vacuum at ambient temperaturure for pure and stoichiometric MoS₂ coatings. On the other hand, tungsten disulfide (WS₂) is also a solid lubricant which has a lamellar structure and works as well as MoS₂ in vacuum. However, it is not widely used nor well studied due to its high cost, and until today there is no report of the achievement of the superlow friction in vacuum. In the present work, we report the superlow friction of WS₂ coatings obtained in ultrahigh vacuum and at low temperature. Pin-on-disk reciprocating friction test for RF sputtered WS2 coatings on silicon substrates mated against counterpart steel (AISI 52100) pins were conducted in ultrahigh vacuum (3×10⁻⁷ Pa) at temperatures ranging from −130 to +200°C. The origin of this superlow friction was discussed.