SURFACE-CHEMISTRY AND EXTREME-PRESSURE LUBRICANT PROPERTIES OF DIMETHYL DISULFIDE

The growth kinetics of a film formed by the thermal decomposition of d imethyl disulfide on an iron foil are measured using a microbalance wh ere the growth kinetics are parabolic (film thickness X varies with ti me as X-2 proportional to t) at high reaction temperatures and pressur es, indicating that it is limited by diffusion through the film. The a ctivation energy for this process is 54.5 +/- 0.5 kcal/mol. The growth rate becomes linear as the reaction temperature and/or reactant press ure is lowered, indicating that, under these circumstances, the reacti on rate is limited by thermal decomposition of dimethyl disulfide at t he growing interface. The activation energy for thermal decomposition at the interface is found to be 37.6 +/- 0.7 kcal/mol, and a half-orde r kinetics pressure dependence for the surface reaction rate is found consistent with a reaction limited by the rate of dimethyl disulfide d issociation. Analysis of the resulting film using Raman and X-ray phot oelectron spectroscopies as well as X-ray diffraction reveal the forma tion of FeS, which may be slightly nonstoichiometric. This film is sim ilar to that formed by methanethiol, suggesting that they may both ini tially form a surface thiolate species that further reacts to form FeS . The half-order reaction kinetics noted above are consistent with thi s. Measurement of dimethyl disulfide as an extreme-pressure (antiseizu re) additive reveals a plateau at an applied load of similar to 4000 N in the seizure load versus additive concentration curve. It has previ ously been suggested that the plateau corresponds to the load at which the interface reaches the melting point of the solid lubricant layer (in this case proposed to be FeS). Estimation of the interfacial tempe rature using a method developed previously yields an interfacial tempe rature of similar to 1480 K, in good agreement with the melting point of FeS.