An Analytical and Experimental Investigation of Nanoparticle Lubricants

Abstract

This work investigated the tribological effects of nanoparticle additives on lubrication through experimental and analytical investigations on colloidal lubricants. The focus of this dissertation was to investigate and elucidate the enhancement mechanisms of nanoparticles. Different types of particles including copper oxide, silver and diamond nanoparticles were used as lubricant additives. Various fluids were also used as the base lubricant in the experiments including mineral base oil, dodecane, polyethylene glycol 600, polyalphaolefin base oil and fully formulated SAE 5W20. The friction experiments were performed using a pin on disk and a disk on disk test setup. Studies were carried out using dry nanoparticles in powder form and lubricated experiments in the boundary, mixed and elasto-hydrodynamic lubrication regimes. Three dimensional surface metrology were performed using a stylus profilometer and an optical profilometer. Wear measurements were done through analysis of the wear grooves. Various surface analysis including scanning electron microscope (SEM), energy-dispersive x-ray spectroscopy (EDX) and auger electron spectroscopy (AES) was performed to assess the stoichiometry of the elements on the tested surfaces. X-ray photoelectron spectroscopy (XPS) surface analysis was also performed to detect the bonds between the elements on the surface and to confirm the stoichiometry. Analytical analysis was also adopted to study the system in depth. A contact model for nanoparticles in contact between rough surfaces was developed. This contact model was also adapted to the general case of particles that are of the same scale as the roughness. This model is useful for the contact of surfaces with contaminants such as sand or dust between them. Based on the results, the mechanism of “the reduction in area of contact” for nanoparticles in boundary and mixed lubrication was proposed. The third body contact model was used to further verify this mechanism. The results showed that the nanoparticles reduce the friction force in the thin film elasto-hydrodynamic lubrication (EHL) regime. A mechanism was discovered and proposed to explain the effect of nanoparticles on friction in the EHL regime. That is the nanoparticles induce a plug flow and localize the shear to the layers adjacent to the walls which in turn reduce the friction force. Results also showed that the dry nanoparticles in powder form are effective in reducing friction and wear. Also, it was demonstrated that there is a critical concentration of nanoparticles below which they can’t sustain a reduced friction force. Based on all the results it was proposed that the nanoparticles have a dual effect on a contact’s tribology. A direct and an indirect effect which need to be added to find the overall effect of particles on the system. The results showed that diamond nanoparticles in fully formulated oil can effectively reduce friction and impose a positive polishing effect on the surfaces. Silver nanoparticles in fully formulated oil showed significant reduction in both the friction and wear.