Boundary slippage effect on hydrodynamic lubrication

Abstract

It is well aware that efficiency of machines and engines can be increased by lubrication. Contact parts or components in relative motion in a well-lubricated machine can be totally separated by a hydrodynamic lubricating film which prevents direct solid contact and reduces friction. However, viscous friction still exists and its magnitude is proportional to the viscosity of the lubricant. Simply speaking, using thinner lubricating oil can reduce the magnitude of viscous friction, but it also reduces the thickness of the lubricating film, i.e. lubricating effect is affected. Thus, the means of using thinner oil to reduce lubricating friction is only limited and not practical. Recently, an idea for reducing lubricating friction on the basis of boundary slippage has been proposed. If the lubricant can slide on or move with a different speed of the solid surface, friction can thus be reduced. To better use the boundary slip phenomenon, this work aims to find out its effects on hydrodynamic lubrication for various operation conditions through experimental and theoretical studies. The experimental study started with a typical hydrodynamic system - squeeze film. The set up included two horizontal parallel planes that were submerged in a specimen lubricant. The lubricant was squeezed in the vertical direction. The load and the corresponding relative displacement between two parallel planes were captured. Experiments were carried out with various pairs of parallel planes. The lower plane was fixed (untreated) while the upper plane was modified with different surface treatments for various interfacial adhesion between the solid and the liquid. By comparing the experimental measurements with the hydrodynamic lubrication theory derived based on no-slip boundary conditions, genuine differences were obtained which proved the existence of boundary slippery. Based on the experimental and theoretical comparison, the degree of slip was inferred. To more accurately and systemically capture the parametric effects on boundary slip, shear film experiments were carried out with an optical slider test rig. The hydrodynamic lubricating film was generated with a rotating transparent disc and a stationary slider of adjustable tilted angles. The affinity of the specimen oil to the bounding surfaces, as quantified with contact angle, was systematically varied by the amount surfactant added to the same base oil such that a wide range of contact angles, from 3 to 52 degrees, was generated. The variation of film thickness against rotational speeds and different loads were detected by optical interferometry, which provides submicron measuring accuracy. Comparing the experimental results of load and film thickness to the hydrodynamic lubrication model with the critical shear stress criterion of slippage, the degree of slip can be illustrated and the critical shear stress can be inferred. In squeeze film experiments, the load carrying capacity was decreased with increasing the contact angle. The critical shear stress, which was inferred from the comparison of the experimental and theoretical results, was found having a negative effect on the carrying load. Moreover, it was found the load carrying capacity varied linearly with the squeeze velocity and this linear relationship was generally applied to specimen oils of different interfacial affinity. In the slider test, the contact angle was found increased with the amount of surfactant added. The experimental results illustrated that the film thickness was decreased with increasing the contact angle for a given load and speed. On the other hand, the increased contact angle would promote a reduction in the critical shear stress as well as the work of adhesion. Hence, lowering the wettability of an oil-solid system (larger contact angle) would promote slip, which, in turn, reduces the film formation capacity. Moreover, it was found that the critical shear stress is a linear function of shear rate. Lastly, the identified significant parameters that affect boundary slippage include the contact angle (or wettability), the critical shear stress, the shear rate, the speed and the work of adhesion.