The friction, mobility and transfer of tribological films: potassium chloride and ferrous chloride on iron

Abstract

The tribological properties of FeCl 2 , measured using an ultrahigh vacuum tribometer, are compared with those of KCl. The friction coefficient of an FeCl 2 film evaporated onto iron in ultrahigh vacuum decreases to ∼0.08 after the deposition of a film ∼40 Å thick. This value is in good agreement with that measured for reactively formed FeCl 2 films in a pin and v-block apparatus. The contact resistance also increases at the same FeCl 2 thickness as that at which the friction coefficient reaches its minimum value suggesting that this corresponds to the completion of the first monolayer as found previously for KCl. Comparison of the friction coefficients and contact resistances for a second pass suggests that the film is mobile during rubbing in both cases. It is found that repeated rubbing in the case of FeCl 2 leads to a reduction in friction coefficient to ∼0.015 which is shown to be due to the formation of a transfer film on the tribotip. In contrast, no such reduction is found for a KCl film up to 40 cycles, implying that no transfer film is formed in that case. It is also found that, while friction reduction requires the surface to be completely covered by the halide film, suppression of damage in the iron requires the deposition of films thicker than ∼500 Å.

Tribological Properties of Films Formed by the Reaction of Carbon Tetrachloride with Iron

“…It has also been shown that a ferrous chloride film evaporated onto iron in ultrahigh vacuum reduces the friction coefficient to $0.08, in good agreement with the values found above for model lubricants comprising small chlorinated hydrocarbons dissolved in a poly a-olefin [10]. Transfer of the FeCl 2 from the iron to the tungsten carbide tribopin was also observed.…”

“…Shown as an inset to (a) is the variation in friction coefficient with films thickness for FeCl 2 evaporated onto iron.the completion of the first. A similar situation applies in the case of evaporated FeCl 2 layers and a similar analysis in this case reveals that the first-layer FeCl 2 film thickness is 10±2 Å[10]. Ferrous chloride forms a layer structure with the thickness of each layer being about $8.5 Å[18], in good agreement with the estimate of the first-layer thickness of the evaporated film.…”

“…The mean contact pressure during these experiments was between 500 and 1000 MPa with a linear sliding speed of 9.6 Â 10 )2 m/s. In order to gain a fundamental understanding of the frictional behavior of thin inorganic films on iron, we have studied a number of alkali halides evaporated onto clean iron in ultrahigh vacuum [6][7][8][9][10] and found, in general, that the deposition of a few tens of Å ngstroms of these halides substantially reduces the interfacial friction coefficient from $2 for clean iron to values less than $0.6 [7]. The formation of thicker films causes a slight increase in the friction coefficient [9].…”

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

“…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.…”

“…It has been demonstrated previously that a complete monolayer of an inorganic halide deposited onto an iron substrate substantially lowers the friction coefficient from *2 for the clean iron surface to less than 0.3 when the surface is covered by such a layer of KCl [1][2][3][4]. 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].…”