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

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.

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.

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.

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.