Investigation of boundary effect on elastohydrodynamic lubrication

Abstract

The present investigation aims to study the interfacial effect of elastohydrodynamic lubrication (EHL). This thesis focuses particularly on a fundamental understanding of slippage at the solid/liquid interface and its effect on film shape characteristics and on friction in the EHL contact under isothermal conditions. Boundary slip has recently been examined in tribology research, which not only indicates that it is of academic interest to researchers but also reflects its potential value in improving lubrication. According to recent findings, transforming one or two bounding surfaces of a lubricated contact into non-wetted (hydrophobic) surfaces can significantly reduce friction. Furthermore, anomalous EHL films that were obtained mainly by using optical interferometry under pure sliding conditions were reported by various researchers. Boundary slip has been hypothesised as the cause of these newly discovered phenomena. Only film shapes were described in previous experimental studies that presented anomalous EHL films by using the conventional optical EHL interferometry; friction was not discussed. In the present study, an optical EHL test rig was modified to simultaneously measure friction and map the lubricated contact. Disc and ball pure-sliding tests that use viscous polybutene oils were conducted at very slow entrainment speeds to achieve isothermal conditions. The experimentally obtained friction-speed curves were characterized by anomalous wave-like shapes with two inflection points, namely, a peak and a valley. These curves deviated from the traditional Stribeck curve which is characterized by a monotonic increase from EHL to hydrodynamic lubrication (HL) regime. Comparing to the film shapes, the two inflection points are closely related to the appearance of a dimple at the inlet of the contact and to the loss of elastic deformation of the contact surfaces, respectively. The reduced friction in the anomalous EHL film shapes support the boundary slip hypothesis. The surfaces of the ball and the disc, which were used in the optical EHL tests, were treated separately by using different coatings; that is, their surface energies were modified. The boundary slip effects were investigated. Under low load and speed conditions, no evidence of the boundary slip effect was found. However, when the load and the speed increased, the surfaces with low surface energy produced lower film thickness and friction. Thus, surfaces with low surface energy promoted boundary slip at the EHL contact, which reduced friction and film thickness. It was found that a boundary adsorption film formed on the bounding surfaces under highly pressurized and stressed conditions. The bulk fluid hardly slipped on the boundary film, which may be due to strong cohesive force. In sum, the formation of the boundary adsorption film, which is related to the magnitude of the surface energy, reduces the boundary slip effect on EHL films. A new type of EHL film shape, the "tri-dimple", was observed. The tri-dimple film shape has not been reported in any literature and is attributed to the effect of the localized adsorption film on the steel ball surface. To theoretically verify the boundary slip effect on EHL, numerical analyses were conducted. The boundary slip was simulated by using the circular non-Newtonian rheological model. In this model, a thin layer of the lubricant was adjacent to the solid surface at a very low viscosity or at an insignificant magnitude of viscous friction. The results qualitatively conformed to experimental findings of anomalous film shapes and friction-speed curves.

Date of Award	15 Feb 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pat Lam Patrick WONG (Supervisor)