Tribological Interfaces and Fluid Flows Containing Particles and Chemically Designed Additives

Abstract

This work investigates bi-phase flows in a series of experimental and computational studies with a tribological focus. Three silver-based complexes are chemically designed, through collaboration with the Marks group, for use as additives in high-temperature lubrication. Mixtures of engine oil and various concentrations of these nanoparticle complexes lubricate sliding steel surfaces in ball-on-disk tests. Friction and wear measurements demonstrate that the silver complexes provide beneficial wear reduction over a range of temperatures and loads. The silver pyrazole-pyridine complex in particular provides excellent friction performance at high temperatures (> 200°C) and high concentration (20 wt%). In a similar set of experiments, hexyltrimethoxy silane is added to polyalphaolefin (PAO) oil contaminated with sand. As debris, dirt, and dust are commonly present in lubricants, it is desirable to find additives which lessen the abrasive damage of these hard contaminant particles. Chemical reaction between the sand and organosilane is found to be low, which results in minimal improvements in friction and wear. A tin catalyst is recommended to improve the reactivity of the sand and organosilane for future testing. Finally, a computational model of fluid-solid flow is developed to examine the characteristics of particle-laden viscous flows. Existing computational techniques utilizing a distributed Lagrange multiplier (DLM) method and a mechanistic collision model are extended for multi-particle collisions and elastohydrodynamic lubrication, accounting for surface deformations through the solution of elasticity equations. This deterministic numerical model allows for various sizes and shapes of particles, as well as dilute to dense suspensions in viscous, laminar flow. Furthermore, the model is computationally fast and applicable to a wide range of thin-film flows. The numerical model is utilized in a study of particle-fluid flow through narrow channels with wall features of increasing size. Particles are found to have different equilibrium positions based on the flow Reynolds number, with minimal effects from the wall features. In addition, results for rigid particle motion entrapped in a deformable channel are presented as a preliminary investigation. The location of maximum surface pressure and deformation is predicted to shift slightly from the point of contact in the direction of motion of the moving particle