The surface chemistry of chlorinated hydrocarbon extreme-pressure lubricant additives

ISSN： 1023-8883

Source： Tribology Letters, Vol.3, Iss.4, 1997-01, pp. : 303-309

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Abstract

Chlorinated or sulfurized hydrocarbons are commonly added to a base fluid to synthesize lubricants used under extreme-pressure (EP) conditions. It has been demonstrated that the interfacial temperature in the EP regime varies linearly with the applied load and that temperatures in excess of 1000 K can be attained. At these temperatures, both microbalance experiments carried out at high pressure as well as molecular beam experiments performed in ultrahigh vacuum reveal that chlorinated hydrocarbons thermally decompose forming a film that consists of a layer of iron chloride and which can also incorporate small (~50 Å diameter) carbon particles. These particles may affect the coefficient of friction of the film. The lubricant fails and seizure takes place when the film is removed sufficiently rapidly for metal-metal contact to occur so that EP lubrication is described as a dynamic phenomenon. Under appropriate circumstances, sufficient carbon can be incorporated into the iron substrate that it becomes a carbide. In this case, seizure is prevented even when the halide layer is removed because of the hardness and high melting temperature of this carbide. Ultrahigh vacuum experiments also suggest that carbon diffusion into the iron and presumably also ultimately carbide formation, is facilitated by co-adsorbed chlorine which may then explain the excellent extreme-pressure properties of carbon tetrachloride. Finally, a similar tribological model is successfully applied to dimethyl disulfide where, in this case, FeS forms the anti-seizure layer.