Effects of Interface Affinity on Friction in Lubricated Sliding Contacts
DescriptionEnergy saving is a major global concern. The proposed project addresses the issue through friction reduction by exploring the new technology of oleophobic coatings and by understanding the interface affinity mechanism of the coating with lubricating oils.Friction models for lubricated contact 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 is valid since most engineering surfaces have good adhesion to lubricating oils and the adhesion force is much higher than the viscous resistance. In this situation, the coefficient of sliding is a function of the viscosity of the lubricant, and the friction force can be reduced simply by reducing the viscosity of the lubricant. However, there is a limitation to reduce lubricant viscosity in most engineering applications, because reducing viscosity would lead to a thinner lubricant film, which would induce more wear on the solid surface. Another way to reduce sliding friction in a lubricated condition is to allow sliding of the lubricant on the solid surface (referred to as “boundary slippage”). This requires a very low surface adhesion force between the lubricant and the solid surface. A low interface affinity may be obtained by an oleophobic coating or/and a specially defined surface.Recently, the boundary slippage of lubricant films which are confined within very small gaps was reported. Experimental results demonstrated that the friction of a hydrodynamic-lubricated system could be reduced by making one of the bounding surfaces oleophobic. However, our experiments illustrated that low interface affinity would also lead to a reduction of lubricating film thickness. To make good use of the interface affinity effect and thus enhance the efficiency of lubricating systems, we need to develop a better understanding of the relationships among the three: interface affinity, friction and film thickness. Our main focus is to extend our current experimental capability to friction measurements. Together with the simultaneously measured data of film thickness, the effects of interface affinity on lubrication as a whole can thus be appraised.The project deliverables include (i) an original optical lubricated slider test rig that can produce simultaneous measurements of friction and film thickness; (ii) an effective method of preparing interfaces of different degrees of affinity; (iii) a theoretical description of the empirical relations between the interface affinity and boundary slippage of lubricants under a wide range of operating conditions; and (iv) an understanding of the trade-off between friction reduction and the loss of film formation capacity due to low interface affinity.Upon completion of the project, we will be able to clarify how to generate positive impacts on lubrication through appropriate designs of the interface affinity.
|Effective start/end date||1/09/11 → 3/08/15|