The surface chemistry of chlorinated hydrocarbonextreme-pressure lubricant additives

Chlorinated or sulfurized hydrocarbons are commonly added to abase fluid to synthesize lubricants used under extreme-pressure(EP) conditions. It has been demonstrated that the interfacialtemperature in the EP regime varies linearly with the appliedload and that temperatures in excess of 1000 K can be attained.At these temperatures, both microbalance experiments carried outat high pressure as well as molecular beam experiments performedin ultrahigh vacuum reveal that chlorinated hydrocarbonsthermally decompose forming a film that consists of a layer ofiron chloride and which can also incorporate small (~50 Ådiameter) carbon particles. These particles may affect the coefficient of friction of the film. The lubricant fails andseizure takes place when the film is removed sufficientlyrapidly for metal-metal contact to occur so that EP lubricationis described as a dynamic phenomenon. Under appropriatecircumstances, sufficient carbon can be incorporated into theiron substrate that it becomes a carbide. In this case, seizureis prevented even when the halide layer is removed because ofthe hardness and high melting temperature of this carbide.Ultrahigh vacuum experiments also suggest that carbon diffusioninto 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 appliedto dimethyl disulfide where, in this case, FeS forms the anti-seizure layer.