Load-independent friction: MoO3 nanocrystal lubricants

The behavior of the model lubricant system molybdenum disulfide (MoS2) has been characterized as a function of thermal oxidation on the atomic to nano meter scales with scanning tunneling microscopy (STM) and atomic force micr oscopy (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 diamete r. The densities of pits and MoO3 nanocrystals and the size of MoO3 nanocry stals increased with increasing oxidation time. UHV AFM friction measuremen ts made on these same samples exhibited a direct relationship between defec t density and friction for short oxidation times. After longer oxidation ti mes when well-defined MoO3 nanocrystals were present, load-independent fric tion was observed. A model, which involves adhesion of MoO3 nanocrystals to the AFM probe tip end, has been developed to explain this first observatio n of load-independent friction. The implications of these results are discu ssed.