The surface and tribological chemistry of carbon disulfide as an extreme-pressure additive

The reaction of carbon disulfide with clean iron was investigated for temperatures between 623 and 776 K and pressures between 10 and 30 Torr. Film growth is limited by the thermal decomposition of CS2 at the growing interface and the activation energy for this process is 12.4±1.0 kcal/mol. The nature of the resulting film is analyzed using Raman and Mössbauer spectroscopies and by X-ray diffraction, where it is found that the film consists of a non-stoichiometric ferrous sulfide and also incorporates a carbide. This result is in accord with the tribological data where the interfacial temperature in the plateau region of a plot of seizure load vs. additive concentration is ∼1460 K, the melting temperature of FeS. The seizure load increases substantially when the additive concentration exceeds ∼2 wt.% of sulfur and, since carbide formation was detected in the film, this is ascribed to the formation of an iron carbide at higher additive concentrations.