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Understanding Surface and Bulk Properties of Lubricants


Tribology is the study of friction, wear, and lubrication of surfaces in relative motion. In mechanical components, friction and wear lead to loss of energy efficiency. Lubricants control friction and wear by creating a film that separates contacting surfaces, which consequently reduces energy consumption and prolongs machine life. The effectiveness of a lubricating film is highly dependent on the physical and chemical properties as well as the composition of the lubricant used. Minor changes in chemical properties or composition can significantly influence the functionality of the lubricating film. Therefore it is necessary to understand the factors and mechanisms that influence the performance of lubricants at the molecular level.

The aim of this thesis is to investigate the surface and bulk properties of liquid lubricants using molecular dynamics (MD) simulation. This thesis is divided into two parts, where Part I focuses on understanding the properties of lubricated surfaces, and Part II explores the properties of bulk lubricants, specifically the pressure-viscosity and temperature-viscosity response of lubricating fluids. We explored the surface coverage and stability of thin functionalized polymer films using coarse-grain models, where we quantified the change in disjoining pressure with film thickness. We also studied the mechanochemical process occurring at boundary lubricated sliding interfaces and demonstrated that a distribution of forces is present at the sliding interface. Only a small percentage of the molecules at the sliding interface experienced forces large enough to initiate mechanochemical reactions. Next, we developed a novel method for predicting the pressure-viscosity response of fluids using an empirical equation and MD-predicted material properties. Using this method, the pressure-viscosity response of fluids was predicted from the variation of volume with pressure, which can be obtained relatively easily using atomic simulations. The temperature-viscosity response of liquid lubricants was also investigated to understand the mechanisms underlying the functionality of viscosity index improving (VII) polymers. Here, we studied the coil expansion mechanism of typical VII chemistries and found that the presence of specific molecular features influences the functionality of the polymer. In general, this work provides an in-depth analysis of several key properties that govern the functionality of surface and bulk lubricants. A clear understanding of functionality can lead to better lubricating capabilities through the design of novel application-specific lubricants, thus leading to an increase in energy efficiency and decrease in energy consumption.

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