The Frictional Behavior of Thin Halide Films on Iron

Abstract

The frictional behavior of thin (< 1 μ m) sodium chloride, potassium chloride, and potassium iodide films deposited onto iron has been measured in ultrahigh vacuum. It has been shown previously that the friction coefficient decreases from the value for clean iron (∼2) to a minimum when the surface is covered by a monolayer film, and this is referred to as regime one. A second regime is identified in this work for films up to 0.3 μ m thick, where the friction coefficient increases with film thickness due to the increased contact area with the rough tribopin. The friction coefficient in this regime obeys Amontons' law. A third regime is found for films thicker than 0.3 μ m when the pin is completely supported by the film. Here, the contact behaves elastically, leading to a friction coefficient that varies as (). Amontons' law is no longer obeyed in this regime where now the friction coefficient is proportional to 1/).

Keywords:

• Friction Testing
• Solid Lubrication
• Iron
• Film Thickness
• Contact of Rough Surfaces
• Vacuum
• Surface Films/Coatings

ACKNOWLEDGMENT

We gratefully acknowledge support of this work by the Chemistry Division of the National Science Foundation under grant number CHE-9213988.

Manuscript received by STLE May 16, 2003

Final manuscript approved December 10, 2003

Review led by Robert Fusaro

Notes

^a Calculated using a value of 690 MPa for the hardness of iron (Wu, et al. (17)).