Research on Boundary Slippage in Thin Film Hydrodynamic Lubrication

Abstract

The traditional lubrication models are usually based on the viscous flow of the lubricant. It is assumed that there is no relative motion at the interface of the lubricant and the solid surfaces. This assumption has been proved by many engineering examples and theoretical studies. In this situation, the sliding friction coefficient is a function of the viscosity of the lubricant, and the friction force can be decreased by reducing the viscosity of the lubricant directly. However, there is a limitation to reduce lubricant viscosity in most engineering applications, because reducing viscosity would lead to the drop of lubricant film thickness, which may induce wear in contact areas. Recently, a new idea for reducing friction force has been proposed by using boundary slippage. This requires a very low adhesion force between the solid surface and the lubricant. To better use the slippage, the effect of three parameters, interfacial affinity, sliding speed and lubricant's viscosity on hydrodynamic lubrication performance was studied through experiments and theories in this project.
A self-developed slider-on-disc test rig was applied for this study. In order to realize the simultaneous measurement of lubricant film thickness and friction, a dichromatic optical interference method was developed. By simultaneously projecting two light beams of different wavelengths on the lubricating contact, an envelope of intensity-difference and film thickness variation equivalent to an intensity and film thickness curve with a longer cycle can be obtained. Using the equivalent intensity curve, the measurement range of film thickness can be enhanced compared with monochromatic interferometry. The moving rate of the equivalent intensity curve corresponding to a rapid change in film thickness was fairly slow, which facilitates the on-line measurement of lubricant thickness and realizes the simultaneous measurement of lubricant film thickness and friction.
Two parameters, contact angle and contact angle hysteresis were compared for their correlation with the hydrodynamic lubricating effect of a slider bearing. Five very smooth slider surfaces of different materials and three lubricants, polar and non-polar, which provided contact angles ranging from 40° to 110°, were used in the bearing tests. Contact angle hysteresis, but not the contact angle, was found to closely correlate with the hydrodynamic effect. The finding is also supported by an existing theory based on thermodynamic principles that the adhesive strength, which can be quantified by potential energy barrier, between the contacting solid and liquid molecules is a strong function of contact angle hysteresis but not the contact angle if its value falls into the range of 20° to 140°.
More samples of different interfacial affinity were obtained with Silicon oil 201-500 containing different concentrations of perfluoroenathic acid (C6F13COOH) and a steel surface. The influence of roughness on hydrodynamic lubrication could be ignored. Experiments proved again that the contact angle hysteresis determines the hydrodynamic lubrication behavior. Besides, monotonous correlation between the boundary yield stress and the potential energy barrier was confirmed based on the experimental results for the two parameters describing similar phenomena.
The effect of sliding speed of glass disc and viscosity of the lubricant on boundary slippage was studied. Two types of lubricants, glycerol solutions and PAO oils, were applied for the study of the viscosity effect. The strong linear relationship between lubricant's viscosity and boundary yield stress was found through the experimental results. Besides, it is also proved that the boundary yield stress increases with sliding speed. These conclusions are consistent with the slip model proposed by Spikes and Granick.

Date of Award	10 Dec 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pat Lam Patrick WONG (Supervisor)