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

Low friction of MoS2-based coatings in the absence of reactive gases and particularly water vapour is generally attributed to friction-induced orientation of ''easy shear'' planes of the lamellar structure, parallel to the sliding direction. It has been suggested that the substitution of sulphur by oxygen in the MoS2 structure could improve its tribological performance by increasing the basal plane distance. To check the role of the presence of oxygen in the friction of MoS2, we have developed an Auger electron spectroscopy-X-ray photoelectron spectroscopy ultrahigh vacuum tribometer, coupled with a preparation chamber, which allows the investigation of oxygen-free MoS2 sputter-deposited coatings and the performing of in-situ friction measurement in an ultrahigh vacuum. MoS2 coatings (120 nm thick) were deposited on (100) Si substrates. No trace of oxygen contamination was detected by X-ray photoelectron spectroscopy or Auger electron spectroscopy. In these conditions, reciprocating friction of the film against SiC spherical pins (normal load, 1 N; maximum hertzian pressure, 0.66 GPa; vacuum state, 50 nPa) gave extraordinary low friction coefficients below 0.005, which were often difficult to measure with the equipment available. In light of wear debris and surface analyses, the mechanisms of this superlow friction of MoS2 are discussed.