Friction studies in ultrahigh vacuum of Fe surfaces with thin films from exposure to perfluorodiethylether

The coefficient of friction for both clean and oxidized contacting polycrystalline Fe surfaces following exposure to a perfluorodiethylether (PFDE), (CF3CF2)2O, was measured in ultrahigh vacuum (UHV) using a pin-on-flat device (0.05–0.30 N loads). This system was chosen to examine the influence of thin films on friction using a molecule having functionalities that are present in perfluoropolyether lubricants. The friction coefficient measured for the clean (∼0.2–1.0 Å oxide) surfaces in UHV was ∼3.0. The friction was measured for both oxidized and clean Fe surfaces in PFDE atmospheres (3–150 Pa). During this exposure the friction for the oxidized Fe surfaces dropped to 0.7–0.9 compared to 1.2–1.5 obtained in O2 environments. The friction for clean Fe surfaces measured in UHV following similar PFDE exposures showed two distinct friction ranges, 0.1–0.8 and 1.2–1.8. The Fe surfaces were examined in situ in the same UHV chamber using x-ray photoelectron spectroscopy and showed the formation of a Fe fluoride film (8–16 Å). High resolution profiles of these surface scars using scanning tunneling microscopy in air revealed a contact morphology dependent upon the coefficient of friction. The lower-range friction scars had multiple point contacts while the higher-range scars exhibited single point contacts with deep surface ploughing.