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Investigating Tribochemical Reactions Using Reactive Molecular Dynamics Simulations

  • Author(s): Khajeh, Arash from
  • Advisor(s): MARTINI, ASHLIE
  • et al.
Abstract

Tribochemical reactions are reactions that happen at sliding interfaces due to applied pressure, shear, and heat. These reactions are responsible for the formation of tribofilms on surfaces in relative motion and affect several important tribological properties, including coefficient of friction and wear rate. In fact, the composition and structure of tribofilms can be a determining factor in the energy consumption and life of engineering components. Therefore, understanding the mechanisms by which the tribofilms form and how they interact with sliding surfaces is of great importance in many industries. This dissertation aims to explore the tribochemical reactions that are the root cause of tribofilm formation and their contribution to friction using reactive molecular dynamics simulations. To achieve this goal, we first investigated shear-induced polymerization reactions during vapor phase lubrication of α-pinene between sliding hydroxylated and dehydroxylated silica surfaces. The results suggested that the critical activation step is the oxidative chemisorption of the α-pinene molecules at reactive surface sites, which transfers oxygen atoms from the surface to the adsorbate molecule. Such activation takes place more readily on the dehydroxylated surface and, during this step, the most strained part of the α-pinene molecules undergoes partial distortion from its equilibrium geometry, which appears to be related to the critical activation volume for mechanical activation. Next, we studied the effect of temperature on the thermal film formation of tri-cresyl phosphate (TCP) on amorphous iron oxide. The statistical analysis of chemical reactions between a single TCP molecule and an amorphous iron oxide surface captured multiple possible reaction pathways. The frequency of TCP– surface reactions for each atom type and each unique reaction site on the TCP were analyzed at temperatures ranging from 300 to 700 K. The composition of the thermal film chemisorbed to the surface was mainly carbon, oxygen, and phosphorous, in agreement to the previously reported experiments performed in oxygen-deficient environments. Analysis of chemical bonding between TCP and iron oxide surfaces in the presence of nanodiamonds (NDs) showed a tribofilm comprised of NDs and TCP where the TCP was both directly and indirectly bonded to the surface. Notably, the amount of phosphorous in the film, which is important for surface protection, increased due to TCP molecules indirectly bonded to the surface via NDs, which suggested that indirect bonding is one mechanism by which NDs facilitate film growth. Lastly, we investigated the contributions of chemical and physical interactions to friction at sliding interfaces. To achieve this, chemically and topographically well-defined interfaces between silica and graphite with a single graphene step edge and basal plane were studied in various chemical environments. A range of different parameters, including applied pressure, sliding direction, chemical reactivity of surface, and environment, were shown to be important in the observed friction. Further, the findings suggest different contributions and sometimes opposite effects of chemical and physical contributors to the friction. Overall, in this series of studies, we used and developed new simulation and statistical analysis approaches for studying tribochemical reactions. The findings show that reactive molecular dynamics simulation is a robust approach that can provide insight into the conditions and properties of interfaces and so can be used as a valuable guide to improve tribological properties.

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