Friction studies of ‘‘clean’’ and oxygen exposed Fe surfaces in ultrahigh vacuum

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Abstract
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 Å) 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 (1s) 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 5×10−6 m/s. The friction coefficient measured for the ‘‘clean’’ (∼0.2–1.0 Å oxide) surfaces was 3.0–5.8. Large stick slips were seen during these measurements. The friction dropped monotonically for increasing oxide thickness (≤25 Å) 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 Å) moderates the friction.

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