利用考虑微结构与微旋效应的微极流体理论来研究薄膜润滑的特性。微极效应将引起等效黏度的增加，从而影响油膜厚度。它对压力分布和膜厚形状的影响很小。数值模拟结果表明，等效黏度的增加效应与实验值有很好的类似性。

In this paper, a viscosity modification model is developed which can be applied to describethe thin film lubrication problems. The viscosity distribution along the directionnormal to solid surface is approached by a function proposed in this paper. Based on theformula, lubricating problem of thin film lubrication (TFL) in isothermal and incompressiblecondition is solved and the outcome is compared to the experimental data. In thin filmlubrication, according to the computation outcomes, the lubrication film thickness is muchgreater than that in elastohydrodynamic lubrication (EHL). When the velocity is adequatelylow (i.e., film thickness is thin enough), the pressure distribution in the contactarea is close to Hertzian distribution in which the second ridge of pressure is not obviousenough. The film shape demonstrates the earlobe-like form in thin film lubrication, whichis similar to EHL while the film is comparatively thicker. The transformation relationshipsbetween film thickness and loads, velocities or atmosphere viscosity in thin film lubricationdiffer from those in EHL so that the transition from thin film lubrication to EHL canbe clearly seen.

Thin film lubrication (TFL) is a condition in which the lubricating features between twosurfaces in relative motion are determined by the combination of the properties of the surfaces andthe lubricant and viscosity of the lubricant. The effects imposed by couple stress on lubricationcharacteristics cannot be disregarded in this regime where the ordered molecules dominate thefluid field. There are different tensor measures and constitutive equations in this case other thanNewtonian case. The lubrication of two-phase (solid phase and liquid phase) fluid is investigated inthis paper. The existence of couple stress will enhance the lubricant viscosity and hence increasethe film thickness and improve the load-carrying capability. Size-dependent effects can be seen inthe lubrication with couple stress, and the thinner the lubricating film is, the more obvious the effectwill be.

The technique of relative optical interference intensity (ROII) was used to investigate the effect of molecular ordered degree of cholesterol esters in hexadecane on lubricating properties in thin film lubrication (TFL) regime. The results indicate that transition from TFL to elastohydrodynamic lubrication (EHL) occurs when the rolling speed varies in the range from 30 to 70mm/s for pure hexadecane. However, when cholesterol liquid crystal (LC) was added into hexadecane, the tran-sition phenomenon disappears. The film thickness is closely related to the number of carbon atoms in LC alkyl chain, the LC polarity, the concentra-tion of LC in hexadecane liquid, and the applied external DC voltage. When the thickness of these lubricant films increases to about 30 nm, it nearly keeps constant and hardly changes with the volt-age. The formation mechanism of the thin lubri-cating film in the nano scale is discussed.

Partial lubrication or mixed lubrication in the nano-scale is discussed, which is constitutedfrom dry contact, boundary lubrication, thin film lubrication. A dynamic contact ratio has beenused to describe such lubrication, and the relationship between the contact ratio and its influencefactors was investigated. Experimental results indicate that the dynamic contact ratio increaseswith the decrease of film thickness by the exponential function. The decrease of speed and lubricantviscosity, and the increase of loads will enlarge the value of the contact ratio. When the polaradditives are added into the basic oil, the contact ratio decreases. In addition, the contact ratio ofthe surfaces with small roughness is larger than that of the surfaces with large roughness at verylow speed. However, the contact ratio of smooth surfaces decreases more quickly with speed thanthat of rough surfaces, and therefore, it will become smaller than that of rough surfaces after speedincreases over a certain degree.

Effects of different coatings on the tribological properties of very thin oil films in the nanoscale were investigated. The film thickness and friction force were measured by the technique of relative optical interference intensity (ROII) and a stress–strain force measuring system with high precision. The results indicate that the film thickness complies with the elastohydrodynamic theory in the high-speed region but it differs from the theory in the low-speed region. The physicochemical properties of coating surfaces have a great effect on the tribological properties of the oil film in the nanoscale. The coating with higher surface energy gives rise to a thicker film and a higher friction coefficient. The reasons for enhancement in the friction coefficient of the oil film in the nanoscale are discussed. Copyright

讨论了速度、固体表面能、滑动比、润滑剂粘度和化学性能对薄膜润滑状态下油膜厚度的影响，以及弹流润滑向薄膜润滑转化条件和液体膜失效条件。进而提出了新的润滑状态划分准则以及不同润滑机理下膜厚的变化情况。

Characteristics of a liquid lubricant film at the nanometer scale are discussed in the present paper. The variations of the film thickness in a central contact region between a glass disk and a super-polished steel ball with lubricant viscosity, rolling speed, substrate surface tension, running time, load, etc. have been investigated. Experimen. tal results show that the variation of film thickness in the thin film lubrication (TFL) regime is largely different from that in the elastohydrodynamic lubrication (EHL) regime. The critical transition point from EHL to TFL is closely related to lubricant viscosity, surface energy of substrates, "and so on. The film in TFL is much thicker than that calculated from the Hamrock-Dowson formula. An unusual behavior of the lubricant film has also been observed when the effect of the running time on the film thickness is considered. The time effect and the formation mechanism of the enhanced film in the running process have been discussed.

The determination of hydrodynamic film failure has become one of the key aspects in the study of thin film lubrication (TFL) since the hydrodynamic effect of fluid film at nano-scale can be observed with recently developed experimental techniques. In the present paper, the relative optical interference intensity, (ROH) technique with a resolution of 0.5 run in the vertical direction has been used to measure the film thickness. Experimental results show that the hydrodynamic effect can be clearly observed even at very low speed if the contact pressure is sufficiently low or if the viscosity of lubricant is comparatively high. When the pres-sure increases to a certain degree, the fihn will suddenly drop to the dimension of several layers q/molecules and this is where the failure of the fluid film has taken place. For different viscosity of lubricants, the fluid film failure occurs at different rolling speeds and pressures. In addition, when the normal load becomes high-er, a higher speed or larger viscosity is required to form the fluid film in the contact region. Finally, the effects of pressure, viscos-ity, and velocity on the occurrence of fluid film failure have been examined and a relationship involving the three parumeters is proposed.

The mechanism of the transition from elastohydrodynamic lubrication (EHL) to thin film lubrication (TFL) has been studied with a technique of relative optical interference intensity for measuring the film thickness. The resolution of the instrument could reach 0.5nm in the vertical direction and 1.5 um in the horizontal direction. The factors influencing the measurement precision and the differences between EHL and TFL are discussed. Film thicknesses with different lubricants, velocities, and loads are measured. The experimental data show that the variation of the film thickness in TFL is not the same as that in EHL. As the rolling speed decreases, a turning point in the film thickness curve is found, which distinguishes TFL from EHL. Consequently, a TFL model with a fluid layer, an ordered liquid layer, and an adsorbed layer is proposed. Finally, the functions of the layers are discussed in detail.