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

Iron sulfide films are present in many applications, including lubricated interfaces where protective films are formed through the reactions of lubricant additive molecules with steel surfaces during operation. Such films are critical to the efficiency and useful lifetime of moving components. However, the mechanisms by which films form are still poorly understood because the reactions occur between two surfaces and so cannot be directly probed experimentally. To address this, we explore the thermal contribution to film formation of di- tert-butyl disulfide-an important extreme pressure additive-on an Fe(100) surface using reactive molecular dynamics simulations, where the reactive potential parameters are validated by comparison to ab initio calculations. The reaction pathway leading to the formation of iron sulfide surfaces is characterized using the reactive simulations. Then, the film formation process is mimicked by simulations where di- tert-butyl disulfide molecules are cyclically added to the surface and subjected to temperatures comparable to those expected due to frictional heating. The use of a reactive empirical potential is a novel approach to modeling the iterative nature of thermal film growth with realistic lubricant additive molecules.