On The Boundary Slippage Of Highly Pressurized EHL Films

This project aims to study the boundary slippage of highly stressedelastohydrodynamic lubrication (EHL) films.The boundary condition (BC) at the solid-liquid interface is crucial in the analyses ofliquid flow. Based on lots of experiments in macroscale fluid mechanics, it is commonlyaccepted that there is no relative motion between the solid surface and the liquid layerimmediately adjacent to it, which is referred to as the no-slippage boundary conditions.However, with the rising interests in microfluidics and related devices, the developmentof measuring techniques at micro-scale and impressed progress of molecular dynamics(MD) simulations, there are more and more experimental and theoretical evidencesshowing that liquid can slip on a solid wall, which is the boundary slippage boundarycondition.Elastohydrodynamic lubrication (EHL) is a dominating mechanism in the lubricationof rolling bearings, gears and other components of non-confornmal contacts. EHL filmsare usually subjected to high pressures, high shear rates and confined in very small gaps,which may lead to boundary slippage. In fact some observed abnormal EHL films andfriction behaviours under large slidings could be tentatively related to boundary slippage,and unfortunately these proposed boundary slippages can not be detected at present andthe lack of testing methods obstructs the research progress in the field of boundaryslippage of EHL films.Therefore this study is purposely for the measuring mehod for the boundary slippageof EHL films and its characterization. By impact EHL, it has been known that somelubricant can be entrapped and a dimple shaped film is then formed. In an optial EHL testrig, such a dimple film can generate concentric circle fringes and the flow of lubricant isvisualized by these fringes and the movement of the lunbricant at the dimple core couldbe accurately traced. An approach for quantifying slippage was proposed whereby thedifference between the actual entrainment speed and the normal entrainment speed of thedimple film core was used to infer the relative sliding at the interface. To clearlyunderstand the measuring principle, studies on the dimple core transportation under pure rolling and no-slippage conditions have been carried out first. Through the dimpletransporation measurement the occurrence of boundary slippage was detected andquantified. A model has also been proposed to clarify the experimental fundings. Theresearch work completed and conclusions can be summarized as follows:1) Movement of the dimple core of entrapped lubricant under pure rolling conditionshas been studied. It was shown that there exits a critical displacement, below which thedimple core moves at the entrainment speed and its depth keeps unchanged. However,beyond this critical value, the movement of the dimple core slows down and its filmthickness drops sharply. Some parameters such as entrainment speed, load, initial dimpledepth and initial gap can affect the critical displacement. The increase in the entrainmentspeed and the load can give a large critical displacement and the increase in the initialdepth results in a small critical displacement. Some numerical simulations were carriedout and good agreements with experimental results were obtained.2) Considering the lack of direct evidence of boundary slippage in EHL films andarguments about where the slippage occurs, experiments were performed to clarify themovement of the core of the entrapped lubricant under zero entrainment velocity (ZEV)and pure sliding conditions. It is obvious that the dimple core speed is quite differentfrom the nominal entrainment speed, indicating the boundary slippage. Especially, inZEV cases, if the bounding surfaces are of the same materials, the dimple core ismotionless while both sides are elongated. However, when the two bounding surfaces aredifferent, the dimple core moves with one of the surfaces, which has more adsorption tothe lubricant. The results suggest that the occurrence of slippage is related to the criticalshear stress of the interface and it only takes place at a single surface.3) The dependence of boundary slippage on surface properties was experimentallyobserved. The results show that if there is slippage in EHL contacts, it always occurs atthe interface of lower surface energy. Moreover, evidence of the dimple core moving onsurfaces of high surface energy at lower speeds was obtained. Under pure glass discsliding conditions, if slippage occurs on the glass samples, the dimple core gains a lowerspeed if a glass surface of relatively lower surface energy was used. In contrast, whenslippage occurs on the steel ball surface, the glass samples with higher surface energy can enhance the movement of dimple core. The above observation can be used to explain thesteady EHL films generated by different interfaces.4) A general slip length formula was defined in terms of the measured dimple coremovement to quantify slippage. The results show that slippage is suppressed afterimpact-entrainment runs several times. It is demonstrated that high pressure, highpolymer concentration and high viscosity can lead to a large slip length.5) To clarify the debate on the dependence of slip length on the shear rate, themeasured slip lengths were compared to the corresponding shear rates. Their relationdisplays highly non-linear. Under low nominal shear rates, the slip length increases withshear rate, but approaches to a constant with further increase in shear rate. This non-linearcurve is quite similar to the observation from MD simulations.6) Based on the slippage phenomena observed, a new slippage model is established.The dependence of slippage on the parameters can be interpreted by this model.