The Frictional Behavior of Thin Halide Films on Iron

Abstract

The frictional behavior of thin (<1 µm) 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 µm 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 µm when the pin is completely supported by the film. Here, the contact behaves elastically, leading to a friction coefficient that varies as (film thickness). Amontons' law is no longer obeyed in this regime where now the friction coefficient is proportional to 1/ (normal load).

Pressure Dependence of Shear Strengths of Thin Films on Metal Surfaces Measured in Ultrahigh Vacuum

“…To calculate the lateral (frictional) force, it was assumed that the shear strengths at all asperities (that is, both those that contact the monolayercovered substrate and those that contact the film) were identical. While this accurately reproduced the general shape of the plot of friction coefficient versus film thickness, this assumption did not lead to quantitative agreement with the experimental variation of friction coefficient with film thickness [5].…”

“…The chamber was equipped with an ultrahigh vacuum-compatible tribometer, which simultaneously measures normal forces, lateral forces, and the contact resistance between the tip and substrate [1][2][3][4][5]. All tribological measurements were made using a single pass in the same direction with a sliding speed of 4 9 10 -3 m/s on a freshly prepared surface, and the tribopin was heated by electron bombardment in vacuo prior to each experiment to clean it.…”

“…This effect was shown to be due to the increase in contact area between the rough tungsten carbide ball and the growing film since, for thicker films, some of the asperities contact the monolayer-covered substrate, and others the growing film [5]. The general shape of the plot of friction coefficient with film thickness could be modeled by a modification of Greenwood-Williamson theory [6], which was used to calculate the relative contact areas both within the film and also with the substrate, assuming that plastic deformation occur at the asperity contacts.…”

“…These measurements were carried out for a tungsten carbide pin sliding over a freshly deposited film on a clean metal substrate in an ultrahigh vacuum, where it has been shown that the polished and annealed metal substrate is much smoother than the tungsten carbide ball [5]. A fundamental understanding of the frictional behavior of such thin inorganic films that are relevant to the properties of reactively formed boundary films will rely on a detailed understanding of their mechanical properties.…”

“…When deformation at the contacting asperity tips is plastic, this can only be achieved by varying the nature of the substrate and so the friction coefficient of a thin film of KCl has been measured on various substrates with differing hardnesses. These experiments were carried out for potassium chloride films since these have been the focus of the experiments described above on thicker films deposited onto iron [5].…”