Parallel reactive molecular dynamics simulations were used to statistically analyze chemical reactions between tri-cresyl phosphate (TCP) and an amorphous iron oxide surface. To accurately model this system, a new parameter set for Fe/P/O interactions within the ReaxFF framework was developed. Using the new parameter set, 100 parallel simulations of a single TCP molecule on an amorphous iron oxide surface were run to capture multiple possible reactions at temperatures ranging from 300 to 700 K. The frequency of TCPsurface reactions for each atom type and each unique reaction site on the TCP was analyzed across the range of temperatures. Finally, the composition of the material chemisorbed to the surface was determined and compared to results from previously reported experimental measurements of TCP films in oxygen deficient environments. The results are specifically relevant to TCP, but the parallel reactive simulation approach and statistical analysis of reaction sites can be applied more generally to a range of chemical systems, particularly those involving complex molecules and disordered surfaces where many different reactions are possible.

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.

The surfaces of lubricated mechanical components operating under extreme conditions are protected by films that form in the presence of additives in lubricant formulations. Film formation is believed to be accelerated by heat, load, and shear force in the sliding interface, but the individual contributions of these factors are poorly understood. In this study, we use reactive molecular dynamics simulations to deconvolute the effects of heat, load, and shear force on chemical reactions between di-tert-butyl disulfide, an extreme-pressure additive in lubricants, and Fe(100), a model approximation of the ferrous surfaces of mechanical components. The reaction pathway is characterized in terms of the number of chemisorbed sulfur atoms and the number of released tert-butyl radicals during heat, load and shear stages of the simulation. Chemisorption is limited by accessibility of reaction sites, so shear accelerates the reaction by facilitating movement of radicals to available sites. Analysis of tert-butyl radical release in the context of an Arrhenius-based model for mechanochemical reactions shows that shear lowers the energy barrier for reactions, implying that, in lubricated contacts, the effect of shear will be significant at lower temperatures, which are expected to arise under moderate sliding conditions.

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The properties of MoS2 can be tuned or optimized through doping. In particular, Ni doping has been shown to improve the performance of MoS2 for various applications, including catalysis and tribology. To enable investigation of Ni-doped MoS2 with reactive molecular dynamics simulations, we developed a new ReaxFF force field to describe this material. The force field parameters were optimized to match a large set of density functional theory (DFT) calculations of 2H-MoS2 doped with Ni, at four different sites (Mo-substituted, S-substituted, octahedral intercalation, and tetrahedral intercalation), under uniaxial, biaxial, triaxial, and shear strain. The force field was evaluated by comparing ReaxFF- and DFT-relaxed structural parameters, the tetrahedral/octahedral energy difference in doped 2H, energies of doped 1H and 1T monolayers, and doped 2H structures with vacancies. We demonstrated the application of the force field with reactive simulations of sputtering deposition and annealing of Ni-doped MoS2 films. Results show that the developed force field can successfully model the phase transition of Ni-doped MoS2 from amorphous to crystalline. The newly developed force field can be used in subsequent investigations to study the properties and behavior of Ni-doped MoS2 using reactive molecular dynamics simulations.