Rheological Modeling and Texture Design for Advanced Lubrication

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Tribological behaviors of lubricated interfaces are strongly affected by the interactions between mating surfaces and the rheological properties of the lubricant between them. This research aims to improve lubrication through two approaches: 1) designing surface textures for lubrication enhancement and adhesion reduction, and 2) developing lubricants of desired rheology. Molecular dynamics (MD) simulations enable us to predict the properties of a lubricant based on its molecular structures. However, due to the time- and length-scale limitations, direct viscosity calculations cannot be achieved for lubricants under a high pressure or for molecules with high molecular weights. To overcome such challenges, 1) direct Non-equilibrium molecular dynamics (NEMD) simulations and a time-temperature superposition (TTS)-based extrapolation from the NEMD at elevated temperatures were used to calculate the high-pressure viscosities of a main constituent of a synthetic base stock, 1-Decene trimer, and the pressure-viscosity coefficient was acquired; 2) The shear-thinning behavior was correlated with the molecular conformation, which is quantified using the radius of gyration, and the critical shear rates for 1-Decene trimer, squalane, and an olefin-copolymer of medium molecular weight at various pressures and temperatures were predicted using the change in the radius of gyration; 3) Viscosities of high molecular weight Polyethylene (PE) and Poly-α-olefin (PAO) fluids, representing a linear polymer and a branched polymer, respectively, were predicted using an empirical-theoretical structural-viscosity model, where the structural factors are calculated through MD only. Adhesion reduction may be accomplished by means of surface textures. A high precision manufacturing process was developed. Adequately designed surface textures were applied on the drill bits, and adhesion reduction and tool life improvements were observed for drilling of the metal material, a Titanium alloy. A flat-flat numerical model was developed for understanding the effect on the load capacity owing to the surface textures. A wide range of texture geometries was studied, and the geometric parameters were optimized based on the boundary conditions. Empirical equations were established based on the simulated results. The results in this study provide multiple tools for advancing the understanding of lubrication performances at the lubricating interfaces and designing of mating surface topography and the lubricant between them. The rheological modeling of the lubricant enables us to predict the viscosities of lubricants based on their molecular structures and the operating conditions, such as temperature, pressure, and shear rates. The predictive capabilities of the models enable us to develop and tailor the lubricants to offer optimal performances at the specific working conditions. Meanwhile, guidelines for texture design and promising texture patterns were provided. At the given working conditions, either one or both of these solutions can be utilized to seek for solutions for improving tribological performances of lubricating interfaces

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  • 02/13/2018
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