The behavior of the model lubricant system molybdenum disulfide (MoS2) has been characterized as a function of thermal oxidation on the atomic to nanometer scales with scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultrahigh vacuum (UHV). STM studies of single-crystal MoS2 surfaces showed that the initial stages of thermal oxidation produce atomic-scale pits and then molybdenum oxide (MoO3) nanocrystals 1−5 nm in diameter. The densities of pits and MoO3 nanocrystals and the size of MoO3 nanocrystals increased with increasing oxidation time. UHV AFM friction measurements made on these same samples exhibited a direct relationship between defect density and friction for short oxidation times. After longer oxidation times when well-defined MoO3 nanocrystals were present, load-independent friction was observed. A model, which involves adhesion of MoO3 nanocrystals to the AFM probe tip end, has been developed to explain this first observation of load-independent friction. The implications of these results are discussed.
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