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 (approximately 0.2-1.0 angstrom oxide) surfaces in UHV was approximately 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 angstrom). 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.