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To reduce friction coefficient, hardness is increased by using a nanometer-period multilayer structure and shearing strength is decreased by applying of low-shearing-strength tungsten disulfide and molybdenum disulfide layers. In nanoindentation testing, multilayer films showed a higher hardness and a lower modulus of dissipation energy than single-layer films. In nanoscratch and microwear testing using an atomic force microscope (AFM), both the scratching force and wear rate of multilayer films could be decreased. The friction coefficient of the nanoperiod multilayer film was as low as µ=0.04 in the ball-on-disk tribotest in 52 to 58% humidity conditions. The friction endurance life of the nanoperiod multilayer films evaluated from the cycles in which the friction coefficient increases rapidly is nine times longer than those of single-layer films.

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.