Surface Chemistry and Extreme-Pressure Lubricant Properties of Dimethyl Disulfide

Abstract

The growth kinetics of a film formed by the thermal decomposition of dimethyl disulfide on an iron foil are measured using a microbalance where the growth kinetics are parabolic (film thickness X varies with time as X 2 ∝ t) at high reaction temperatures and pressures, indicating that it is limited by diffusion through the film. The activation energy for this process is 54.5 ( 0.5 kcal/mol. The growth rate becomes linear as the reaction temperature and/or reactant pressure is lowered, indicating that, under these circumstances, the reaction rate is limited by thermal decomposition of dimethyl disulfide at the growing interface. The activation energy for thermal decomposition at the interface is found to be 37.6 ( 0.7 kcal/mol, and a half-order kinetics pressure dependence for the surface reaction rate is found consistent with a reaction limited by the rate of dimethyl disulfide dissociation. Analysis of the resulting film using Raman and X-ray photoelectron spectroscopies as well as X-ray diffraction reveal the formation of FeS, which may be slightly nonstoichiometric. This film is similar to that formed by methanethiol, suggesting that they may both initially form a surface thiolate species that further reacts to form FeS. The half-order reaction kinetics noted above are consistent with this. Measurement of dimethyl disulfide as an extreme-pressure (antiseizure) additive reveals a plateau at an applied load of ∼4000 N in the seizure load versus additive concentration curve. It has previously 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 temperature using a method developed previously yields an interfacial temperature of ∼1480 K, in good agreement with the melting point of FeS.

Shear-Induced Surface-to-Bulk Transport at Room Temperature in a Sliding Metal–Metal Interface

“…The experiments in this work were carried out using DMDS, and sulfur-containing molecules are used as lubricant additives [16,17]. Since rubbing results in the formation of a metastable film and the regeneration of a clean surface, this is available to adsorb more DMDS, which will then be tribologically transported into the subsurface region; repetition of this process will result in the formation of a sulfur-containing film.…”

Reaction of tributyl phosphate with oxidized iron: surface chemistry and tribological significance

“…It is generally agreed that AW films containing a relatively hard polyphosphate glass are formed by reaction with the phosphorus in the additive. Chlorine- [8,9], sulfur- [10,11] and phosphorus-containing [12][13][14][15][16] molecules are all used as EP additives. It has been demonstrated previously that chlorinated hydrocarbons function by reactively forming a FeCl 2 film [17][18][19] at the high temperatures (>1000 K [20]) encountered at the solid-solid interface during EP applications.…”

Lubricant Degradation and Related Wear of a Steel Pin in Lubricated Sliding Against a Steel Disc

“…Further, considering that there is only a very small amount of C found on the slid surface in stage I, we argue that the antioxidant is readily available to be adsorbed on the iron surface to form, initially, thiolates by cleavage of the SÀS bond and then FeS when the CÀS bond is cleaved. 21,22 Table 1 shows that the sulfur concentration increases with time in stage I. It may be suggested that hydrocarbon aging plays no role in stage I wear.…”