Chlorine- and sulfur- and phosphorus-containing compounds are commonly added to the base fluid to synthesize lubricants used under extreme-pressure (EP) conditions. Analyzing the resulting tribological films on iron reveals that chlorinated hydrocarbons thermally decompose forming a layer that consists of iron chloride (FeCl2) or carbide (Fe3C), and that dialkyldisulfides react to form FeS and Fe3C. Alkyl phosphates thermally decompose on iron oxide to form alkyl and alkoxy, as well as POx species, on the surface. The alkyl and alkoxy species thermally decompose on heating to evolve gas-phase products and deposit carbon onto the surface. The POx species rapidly diffuse into the oxide forming a film that consists of a carbonaceous layer covering a phosphate film. The tribological properties of evaporated and reactively grown thin films have been investigated in ultrahigh vacuum. This strategy eliminates contamination and allows films of known composition and structure to be grown on well-characterized substrates. Three tribological regimes are identified depending on film thickness. In the first regime, an initial rapid decrease in friction is found when a film that is a few nanometers thick (corresponding to a monolayer) covers the surface. The friction coefficient increases once again in the second regime as the film becomes thicker, due to the increased contact area between the film and the rough tribotip, and the behavior is well described by a modified Greenwood-Williamson model. A third regime is found when the film becomes thicker than the interfacial roughness, where the surfaces are completely separated by the film. Finally, measuring the friction coefficients of thin halide films deposited onto various substrates, where the local pressure at the asperity tips depends on the substrate hardness, shows that the shear strength of the “monolayer” films depends on pressure.
