A molecular beam study of the tribological chemistry of dialkyl disulfides

Molecular beam studies carried out in ultrahigh vacuum show that dimethyl d isulfide reacts with initially clean iron to evolve methane. The reaction i s proposed to proceed via a methyl thiolate intermediate. Reaction ceases a t similar to 600 K, an effect that is proposed to be due to the surface bei ng blocked by an overlayer of sulfur and carbon. Reaction recommences above similar to 950 K as sulfur diffuses into the iron. The activation energy f or the film-forming reaction is 52.5 +/- 2.1 kcal/mol, in good agreement wi th the activation energy for the growth of FeS films from dimethyl disulfid e at higher pressures measured using a microbalance. A depth profile of the film grown in ultrahigh vacuum shows that the sulfur-containing film grows on a Fe + C underlayer. Similar molecular beam experiments with diethyl di sulfide suggest the formation of an intermediate ethyl thiolate species whi ch decomposes via a B-hydride elimination reaction to evolve ethylene. The activation energy for film growth, in this case, is 60 +/- 2.4 kcal/mol. Th e results of tribological experiments using a pin and v-block apparatus are consistent with FeS forming the anti-seizure film.