The Use Of Surface Texturing And Microspheres In Aqueous Based Lubrication
Several billion tons of lubricants are consumed yearly to reduce friction and wear. Modern lubricants consist either of base oils that are derived from refining crude oil (i.e., mineral base oil) and/or chemically synthesized oil (synthetic base oil). Lubricants also contain a variety of additives to impart desired characteristics such as anti-oxidation properties, corrosion resistance, etc. It is estimated that over 40% of waste lubricants end up being disposed improperly, which can potentially cause a negative impact on the environment. Hence, there is an emerging interest in formulating biodegradable lubricants or novel low friction surfaces. Aqueous based lubricants provide an eco-friendly alternative. In this thesis, two novel methods relying on (i) surface texturing, and (ii) microspheres are explored to reduce friction under aqueous conditions. Typically, a liquid medium such as oil is used to reduce friction between two surfaces. The latter prevents two shearing surfaces from achieving intimate contact, thereby reducing van der Waals interactions. There has been extensive research in the development of lubricants. However, relatively less research has focused on engineering low friction surfaces. This thesis first describes the engineering, fabrication and implementation of low friction polymer surfaces. These surfaces are inspired by the "u201cweeping"u201d lubrication mechanism of articular cartilage, which exhibits superior tribological properties (i.e., minimal surface wear and ultra-low friction). We show for the first time, that it is possible to shift the lubrication mechanism from a boundary to a hydrodynamic regime, even at a low shear velocity. This is achieved by creating vertical pores in a compliant polymer, which undergoes the necessary deformation. We hypothesize that the compressed, pressurized liquid in the pores produces a repulsive hydrodynamic force as it extrudes from the pores. The presence of the fluid between two shearing surfaces shifts the lubrication mechanism to the hydrodynamic/mixed regime thereby providing low friction. Tribological properties of these surfaces are studied for a range of applied loads and shear velocities, both relevant parameters in practical applications. Probes made of different materials are used to show that the load induced lubrication mechanism can be observed for a range of materials. The use of a traditional boundary lubricant, when combined with an optimized surface texture, has the potential to provide a novel means to attain unprecedented low friction. The next section of the thesis explores the tribological properties of iron-core hard carbon microspheres (HCS), which act as micro-sized ball bearings in aqueous media. Previous work in the Pesika lab demonstrated that HCS, dispersed in an aqueous medium, are effective lubricants. However, under prolonged shearing, friction forces were found to increase. This was attributed to the squeezing out of the HCS from the contact zone region causing the two shearing surfaces to make direct contact. We hypothesize that the use of a magnetic field and iron-core HCS will maintain the particles between the two shearing surfaces for prolonged times. Iron-core HCS are successfully synthesized and preliminary studies demonstrate that the iron-core HCS are effective at lowering friction. Ongoing research seeks to determine the influence of particle size, particle concentration, and magnetic field strength on the effectiveness of magnetic iron-core HCS. The exploratory work presented in this thesis provides results to provide an initial proof of concept in using surface texturing of soft polymers and microspheres to reduce friction in aqueous medium. By improving the design and material properties of the porous polymers, we can create surfaces with ultra-low friction that can be potentially used as a coating in total joint replacement implants. Through systematic studies on the effect of magnetic field over the tribological properties of HC/Fe microspheres, it is possible to create systems in which friction can be tuned by changing the external magnetic field.