Lubrication thin-film

The design of precision components with ultrasmooth surfaces, for example, in the field of microelectromechanical systems and nanotechnology, boosts the use of lubricating films of molecular thicknesses. Under these conditions, the validity of continuum theories to describe the hydrodynamics of the lubricant is questionable. This is a new lubrication regime, denoted as thin film lubrication. A review on thin film lubrication as the hmiting case of elastohydrodynamic lubrication is provided in Ref [1055]. We will focus here on the aspects of molecular thin films studied in the context of nanotribology (reviews are Refs [185, 1056]). 

For film thicknesses of more than as bulk liquids [645,1058-1060]. 

Some newer experiments have, however, challenged this picture, and discrepancies between different studies have not yet been satisfactorily resolved. While in some cases, a continuous transition from liquid to solid behavior with increasing confinement was observed [1061,1062], Klein and coworkers found a sudden transition within a distance change of a single molecular layer [1060, 1063]. Such a sudden transition was predicted by molecular dynamics simulations only for commensurate orientation of the confining crystal surfaces [658]. Monte Carlo simulations by Ayappa et al. [1070] on the solidification of OMCTS between mica sheets showed freezing at a thickness of up to seven layers and phase transitions between triangular, square, and buckled phases if only one or two layers were left 

In spite of the significant number of successful experimental and numerical studies on the shear behavior of confined thin films, a consensus on the exact mechanism of friction and the stmcture and phase state of the confined liquid has not yet been reached and further investigation focusing on time-and rate-dependent effects will be necessary. 

Superlubricity, sliding friction with almost vanishing friction force, can be achieved by incommensurate alignment of crystalline surfaces, by applying vertical oscillations or by minimizing the interaction potential. 

The design of precision components with ultrasmooth surfaces, for example in the field of micro-electromechanical systems (MEMS) and nanotechnology, boosts the use of lubricating 

As we have seen in Section 6.6.1 such confined liquids may behave quite differently from the bulk lubricant. Near the surfaces, the formation of layered structures can lead to an oscillatory density profile (see Fig. 6.12). When these layered structures start to overlap, the confined liquid may undergo a phase transition to a crystalline or glassy state, as observed in surface force apparatus experiments [471,497-500], This is correlated with a strong increase in viscosity. Shearing of such solidified films, may lead to stick-slip motions. When a critical shear strength is exceeded, the film liquefies. The system relaxes by relative movement of the surfaces and the lubricant solidifies again. 

Lubricants are characterized by several parameters. The most important one is the viscosity, sometimes also called dynamic viscosity. In lubrication industry, often the kinematic viscosity is used. The kinematic viscosity is the dynamic viscosity divided by the density of the liquid % = r lp. It is given in units of m2/s or centistoke (1 centistoke = 1 mm2/s = 0.001 m2/s = 10 2stoke). 

As already mentioned, the viscosity of a base oil decreases with increasing temperature. Therefore, it is important to know, not only the viscosity at a certain temperature, but also how much it changes within a temperature range given by operating conditions. To characterize the temperature dependence of viscosity in 1929 the American Society for Testing and 

Materials (ASTM) introduced the so-called viscosity index (VI). It is based on the values of kinematic viscosity at 40° C and 100°C, which are compared to the respective viscosities of two reference oils. One reference oil is standard paraffin oil. Its viscosity only weakly depends on temperature. It was given a viscosity index of 100. The other reference is a standard naphthenic oil with a high temperature dependence. Its viscosity index is defined as zero. A low viscosity index indicates a relatively strong dependence of viscosity on temperature, a high viscosity index, a small dependence. 

---

Various other soft materials without the layer—lattice stmcture are used as soHd lubricants (58), eg, basic white lead or lead carbonate [598-63-0] used in thread compounds, lime [1305-78-8] as a carrier in wire drawing, talc [14807-96-6] and bentonite [1302-78-9] as fillers for grease for cable pulling, and zinc oxide [1314-13-2] in high load capacity greases. Graphite fluoride is effective as a thin-film lubricant up to 400°C and is especially useful with a suitable binder such as polyimide varnish (59). Boric acid has been shown to have promise as a self-replenishing soHd composite (60). 

Luo, J. B., Study on the Measurement and Experiments of Thin Film Lubrication," Ph.D. thesis, Tsinghua University, Beijing, China, 1994. 

As a major branch of nanotribology. Thin Film Lubrication (TFL) has drawn great concerns. The lubricant him of TFL, which exists in ultra precision instruments or machines, usually ranges from a few to tens of nanometres thick under the condition of point or line contacts with heavy load, high temperature, low speed, and low viscosity lubricant. One of the problems of TFL study is to measure the him thickness quickly and accurately. The optical method for measuring the lubricant him thickness has been widely used for many years. Goher and Cameron [3] successfully used the technique of interferometry to measure elastohydrody-namic lubrication him in the range from 100 nm to 1 /rm in 1967. Now the optical interference method and Frustrated Total Reflection (FTR) technique can measure the him thickness of nm order. 

Thin film lubrication (TFL), as the lubrication regime between elastohydrodynamic lubrication (EHL) and boundary lubrication, has been proposed from 1996 [3,4], The lubrication phenomena in such a regime are different from those in elastohydrodynamic lubrication (EHL) in which the film thickness is strongly related to the speed, viscosity of lubricant, etc., and also are different from that in boundary lubrication in which the film thickness is mainly determined by molecular dimension and characteristics of the lubricant molecules. 

Definition of Thin Film Lubrication and Boundary Lubrication... 

Effect of Slide Ratio on Thin Film Lubrication... 

Properties of Thin Film Lubrication with Nano-Particles... 

In the past decade, effects of an EEF on the properties of lubrication and wear have attracted significant attention. Many experimental results indicate that the friction coefficient changes with the intensity of the EEF on tribo-pairs. These phenomena are thought to be that the EEF can enhance the electrochemical reaction between lubricants and the surfaces of tribo-pairs, change the tropism of polar lubricant molecules, or help the formation of ordered lubricant molecular layers [51,73-77]. An instrument for measuring lubricant film thickness with a technique of the relative optical interference intensity (ROII) has been developed by Luo et al. [4,48,51,78] to capture such real-time interference fringes and to study the phenomenon when an EEF is applied, which is helpful to the understanding of the mechanism of thin film lubrication under the action of the EEF. 

Wen, S. Z., On Thin Film Lubrication," Proceedings of 1st International Symposium on Tribology, International Academic Publisher, Beijing, China, 1993, pp. 30-37. 

As noted before, thin film lubrication (TFL) is a transition lubrication state between the elastohydrodynamic lubrication (EHL) and the boundary lubrication (BL). It is widely accepted that in addition to piezo-viscous effect and solid elastic deformation, EHL is featured with viscous fluid films and it is based upon a continuum mechanism. Boundary lubrication, however, featured with adsorption films, is either due to physisorption or chemisorption, and it is based on surface physical/chemical properties [14]. It will be of great importance to bridge the gap between EHL and BL regarding the work mechanism and study methods, by considering TFL as a specihc lubrication state. In TFL modeling, the microstructure of the fluids and the surface effects are two major factors to be taken into consideration. 

The effective viscosity is also affected by the microrotation of the rigid particles. If the gap is much larger than the molecular dimensions, the boundary walls will have little influence on the microrotation motion. This means that if the gap between the solid walls is sufficiently large, the micropolarity can be reasonably taken out of consideration without losing precision. The microrotation in thin film lubrication will result in viscosity-enhancements and consequently higher film thicknesses, which contribute to a better performance of lubrication. 

---