SUPERFLOW FRICTION OF OXYGEN-FREE MOS2 COATINGS IN ULTRAHIGH-VACUUM

Low friction of MoS2-based coatings in the absence of reactive gases a nd particularly water vapour is generally attributed to friction-induc ed orientation of ''easy shear'' planes of the lamellar structure, par allel to the sliding direction. It has been suggested that the substit ution of sulphur by oxygen in the MoS2 structure could improve its tri bological performance by increasing the basal plane distance. To check the role of the presence of oxygen in the friction of MoS2, we have d eveloped an Auger electron spectroscopy-X-ray photoelectron spectrosco py ultrahigh vacuum tribometer, coupled with a preparation chamber, wh ich allows the investigation of oxygen-free MoS2 sputter-deposited coa tings and the performing of in-situ friction measurement in an ultrahi gh vacuum. MoS2 coatings (120 nm thick) were deposited on (100) Si sub strates. No trace of oxygen contamination was detected by X-ray photoe lectron spectroscopy or Auger electron spectroscopy. In these conditio ns, reciprocating friction of the film against SiC spherical pins (nor mal load, 1 N; maximum hertzian pressure, 0.66 GPa; vacuum state, 50 n Pa) gave extraordinary low friction coefficients below 0.005, which we re often difficult to measure with the equipment available. In light o f wear debris and surface analyses, the mechanisms of this superlow fr iction of MoS2 are discussed.