Lubricant additives are chemical compounds that are added to lubricant formulations to enhance their performance. Sulfur-containing additives are an important family of these compounds that increase the life of mechanical components by creating a protective film on surfaces in relative motion to prevent direct metal-metal contact. These additives are widely used in the industry whenever a mechanical system is subject to extreme pressure or wear, but the mechanisms and conditions by which the protective films form are not yet fully understood. This dissertation seeks to address this from an atomistic point of view by investigating the tribochemical reactions between additives and surfaces using reactive molecular dynamics simulations. First, the reactions between a representative sulfur-containing additive (di-tert-butyl disulfide) and an iron(100) surface is modeled. Results show that film formation reaction proceeds through three main steps, S--S bond cleavage, Fe--S bond formation, and S--C bond dissociation. These are start of the formation of an iron sulfide layer that can provide protection for the iron surface. The effect of mechanical force on activating chemical reactions is investigated. It is found that mechanical force assists the initiation and the acceleration of these chemical reactions by lowering the reaction energy barrier. Next, a model base oil, dodecane, and an iron oxide surface are modeled to increase the physical realism of the simulations of tribochemical reactions on ferrous surfaces. Results show that this base oil does not chemically interfere with the film formation reaction, however, the physical presence of the base oil molecules impedes the reaction. Conversely, the oxide surface directly changes the reaction pathways that lead to film formation. The oxidation of the additive molecules, their decomposition products, and the surface introduces intermediate steps and reduces the rate at which reactions occur. The kinetics of the reactions are studied in the context of the Bell model and results reveal that the reactions between additives and ferrous surfaces are mechanochemical in nature, and that mechanical force facilitates these reactions by moving the reactants laterally on the surface, weakening their chemical bonds. Finally, another sulfur-containing lubricating material, MoS2, is studied. First, a ReaxFF force field is developed for Ni-doped MoS2. The force field is shown to accurately capture material parameters such as bond lengths, lattice constants, and dopant relocation in the presence of vacancies. Lastly, the applicability and accuracy of the force field is demonstrated by modeling the process of deposition and annealing Ni-doped MoS2 in a reactive molecular dynamics simulation. Overall, the results presented in this dissertation contribute to the understanding of the mechanisms by which sulfur-containing additives function and are an important step towards rational design of more energy efficient and longer lasting lubricated mechanical systems.