Simulation of Thermal Stability and Friction: A Lubricant Confined Between Monolayers of Wear Inhibitors on Iron Oxide
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Introduction 1
Zinc dithiophosphates (ZnDTP) have been used for several decades to reduce wear to acceptable levels in modern engines. Although DTP is the most effective and economic antiwear agent used, there is environmental concern due to the level of phosphorous content in the formulation. Finding a ZnDTP replacement with lower phosphorus content and providing the necessary wear protection to engine surfaces is very important for the oil industry. One might consider other lubricant additives which are presently used to control friction. deposits. corrosion, oxidation, and rust (for a review, see reference 2). However, the number of possible chemical compounds is enormous, making an empirical search laborious and expensive. It should also be noted that a single complete engine test for a new wear inhibitor costs around $150,000. Another serious impediment to this search is that the factors underlying good wear performance are not well understood. Using atomistic level modelling and molecular dynamics (MD) simulation to provide guidance in prioritizing new materials for experimental testing is a viable alternative. For instance, our earlier work led to a self-assembled monolayer (SAM) model, where 3we found the calculated cohesive energy of the wear 4 inhibitor correlates inversely with wear performance. , The purpose of a lubricant is to provide a protective coating to the solid surfaces. This, in turn prevents formation of adhesive junctions, reduces frictional energy losses by acting as an interfacial layer of low shear strength. Our understanding of the friction of two solid surfaces sliding against each other with a thin liquid film between them has been greatly advanced over the last ten years. The work on a variety of 79
Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
different surfaces, such as mica, silica, metal oxide, and surfactant monolayer surfaces, sliding across different lubricants have shown that the properties of fluids confined to the nanometer scale are very different from the bulk properties. For example, the "effective" viscosity of a thin lubricant film is often much higher than bulk viscosity. 3 4 In this paper, extending our earlier work on the SAM model of wear inhibitors , , we calculated the adsorption energies and coverage for DTP and DTCs at 500 K. We found that for DTP the SAM model of adsorption energies correlate (inversely) with wear obtained from engine tests. We also present results of molecular dynamics simulations of shear stability of these SAM in the presence of lubricant (normalhexadecane). The lubricant is confined by two iron-oxide solid surfaces on which DTP molecules are chemisorbed to form a self-assembled monolayer. We have carried out the simulations for three different DTP molecules: isopropyl (i-Pr), isobutyl (i-Bu), and phenyl (Phe) to investigate the effect of the SAMs on the dynamic properties of the lubricant.
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Simulation Results and Discussion
The Force Fields for DTP, DTC and Iron Oxide.
We modified the universal force
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