Ž . Wear 239 2000 77–82 www.elsevier.comrlocaterwear The surface and tribological chemistry of carbon disulfide as an extreme-pressure additive J. Lara a , K.K. Surerus a , P.V. Kotvis b , M.E. Contreras c , J.L. Rico c , W.T. Tysoe a, ) a Department of Chemistry and Laboratory for Surface Studies, UniÕersity of Wisconsin — Milwaukee, Milwaukee, WI 53211, USA b Benz Oil, 2724 Hampton AÕenue, Milwaukee, WI 53209, USA c Instituto de InÕestigaciones Metalurgicas, UniÕersidad Michoacana de San Nicolas de Hidalgo, Morelia, Michoacan, Mexico ´ Received 25 August 1999; received in revised form 2 December 1999; accepted 15 December 1999 Abstract 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 CS at the growing interface and the activation energy for this 2 process is 12.4 "1.0 kcalrmol. The nature of the resulting film is analyzed using Raman and Mossbauer 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. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Carbon disulfide; Iron; Raman spectroscopy; Mossbauer spectroscopy; X-ray diffraction; Film growth kinetics; Extreme-pressure lubrication ¨ 1. Introduction We have previously demonstrated that chlorinated hy- drocarbons thermally decompose during extreme-pressure lubrication at the hot, lubricated interface to deposit a film w x that consists of FeCl and carbon 1,2 . This is continually 2 removed during the frictional process but simultaneously replenished by reaction with the surface. Key to under- standing this chemistry was the ability to calibrate the temperature at the interface where it was found that tem- wx peratures in excess of 1000 K could be attained 3 . It was also shown that the interfacial coefficient of friction de- pended on the chlorinated hydrocarbon used as an additive with films incorporating larger amounts of carbon appar- ently having larger interfacial coefficients of friction. Ra- man spectroscopy revealed that the carbon was present, in ˚ Ž . these cases, as small ; 50 A diameter particles and the presence of these particles was proposed to increase the ) Corresponding author. Tel.: q 1-414-229-5222; fax: q 1-414-229- 5036. Ž . E-mail address: wtt@csd.uwm.edu W.T. Tysoe . shear strength of the halide film thereby increasing the wx friction coefficient 4 . Carbon tetrachloride was found to be a particularly effective lubricant additive where much higher loads were sustained, and therefore much higher interfacial tempera- tures attained, than for other chlorinated hydrocarbon addi- wx tives such as methylene chloride or chloroform 5 . It was demonstrated that this effect was due to the formation of a carbide film at the interface and that the tendency of iron to form carbides increased with the presence of additional wx chlorine at the surface 6 . Similar carbides were found when chloroform was used as an additive at higher concen- trations and was manifest by a subsequent increase in wx seizure load 7 . This work has been extended to examining another important class of extreme-pressure lubricant additives; w x those containing sulfur 8–22 . This was initiated by exam- w x ining the chemistry of dimethyl disulfide 23 since it contains sulfur–sulfur linkages common in commercial sulfur-containing additives. The same general model, as found for chlorinated hydrocarbons, applies equally well to this sulfur-containing molecule. The dimethyl disulfide thermally decomposes at the hot iron interface to deposit a film that consists of FeS. It was also shown that the 0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0043-1648 99 00368-3