Reactive Molecular Dynamics Simulations of Thermal Film Growth from Di-tert-butyl Disulfide on an Fe(100) Surface

Karen Mohammadtabar ¹, Stefan J. Eder ^2,3, Pedro O. Bedolla ², Nicole Dӧrr ², Ashlie Martini ¹

¹ Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road, Merced, California 95343, United States

 ² AC2T research GmbH, Viktor-Kaplan-Strasse 2/C, 2700 Wiener Neustadt, Austria

³ Institute for Engineering Design and Logistics Engineering, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria

ABSTRACT: Di-tert-butyl disulfide is an extreme pressure additive that forms protective films to decrease friction and increase the lifetime of moving components. However, these films form between two sliding surfaces, so identification of the underlying film formation mechanisms via direct observation with experimental techniques is impossible. To overcome this limitation, we used molecular dynamics simulation with a reactive potential to model thermal reactions between di-tert-butyl disulfide and Fe(100). The potential parameters were validated by comparison of adsorption energies to density functional theory calculations. Then, simulations were used to characterize the reaction pathway, from adsorption of the di-tert-butyl disulfide on the Fe(100) until Fe-S bonding. The release of butyl groups from the tert-butyl thiyl radicals adsorbed on the surface was identified as the rate limiting step in the reaction process. Finally, to mimic the processes that occur in lubricated sliding contacts, film growth was simulated by four “waves” of di-tert-butyl disulfide molecules interacting with the iron surface. This way, the evolution of the individual species involved in the reactions and the initial stages of Fe-S film growth could be observed.