FRICTION STUDIES OF CLEAN AND OXYGEN EXPOSED FE SURFACES IN ULTRAHIGH-VACUUM

The coefficient of friction for both clean and oxygen exposed contacting polycrystalline Fe surfaces was measured in ultrahigh vacuum (UHV) using a pin on flat device. This system was chosen in order to examine the influence of very thin oxide films (<20 A) on friction. The surfaces were then examined in situ in the same UHV chamber using both x-ray photoelectron spectroscopy (XPS) and scanning Auger microprobe (SAM). The oxide thickness was determined using O (Is) XPS intensities. The loads used were 0.12-0.16 N. The Fe pin (radius = 0.005 m) motion was linear over 0.004 m at a speed of 5X 10−6 m/s. The friction coefficient measured for the “clean” (—0.2-1.0 A oxide) surfaces was 3.0-5.8. Large stick slips were seen during these measurements. The friction dropped monotonically for increasing oxide thickness (<25 A) to 1.7-2.0. Auger line scan studies of the O/Fe ratio in several scars showed a large depletion of O in the contacted area. Even though the Fe flat surface is deformed up to depths of 3 μm during sliding and loading, the presence of the very thin oxide layer (<25 A) moderates the friction. © 1990, American Vacuum Society. All rights reserved.