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.

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.

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.

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。

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