Chapter 3: Thin Film Lubrication—Experimental Study

Abstract

OIL FILM WITH A THICKNESS IN THE NANOSCALE has been well studied from the beginning of the 1990s [1–3]. 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. In lubrication history, research has been mainly focused for a long period on two fields—fluid lubrication and boundary lubrication. In boundary lubrication (BL), lubrication models proposed by Bowdeon and Tabor [5], Adamson [6], Kingsbury [7], Cameron [8], and Homola and Israelachvili [9] indicated the research progressed in the principle of boundary lubrication and the comprehension about the failure of lubricant film. In fluid lubrication, elastohydrodynamic lubrication proposed by Grubin in 1949 has been greatly developed by Dowson and Higginson [10], Hamrock and Dowson [11], Archard and Cowking [12], Cheng and Sternlicht [13], Yang and Wen [14], and so on. The width of the chasm between fluid lubrication and boundary lubrication has been greatly reduced by these works. The research on micro-EHL and mixed lubrication has been trying to complete the whole lubrication theory system. Nevertheless, the transition from EHL to boundary lubrication is also an unsolved problem in the system of lubrication theory. Thin film lubrication [3,4] bridges the EHL and boundary lubrication [15]. Thin film lubrication (TFL) investigated by Johnston et al. [1], Wen [2], Luo et al. [3,4,16–19], Tichy [20–22], Matsuoka and Kato [23], Hartal et al. [24], Gao and Spikes [25] et al. has become a new research area of lubrication in the 1990s. However, some significant progress can retrospect to 60 years ago. In the 1940s, it had been proven by using the X-ray diffraction pattern that a fatty acid could form a polymolecular film on a mercury surface and the degree of molecular order increased from outside towards the metal surface [26]. Allen and Drauglis [27] in 1996 proposed an “ordered liquid” model to explain the experimental results of Fuks on thin liquid film. However, they thought the thickness of ordered liquid is more than 1 μm, which is much larger than that shown in Refs. [4,17,18]. The surface force apparatus (SFA) developed by Israelachvili and Tabor [28] to measure the van der Waals force and later becoming a more advanced one [29] has been well used in the tribological test of thin liquid layer in molecular order. Using SFA, Alsten et al. [30], Granick [31], and Luengo et al. [32] observed that the adsorptive force between two solid surfaces was strongly related to the distance between the two solid surfaces and the temperature of the lubricant. In 1989, Luo and Yan [33] proposed a fuzzy friction region model to describe the transition from EHL to boundary lubrication. In their model, the transition region was considered as a process in which the characteristics of lubricant changed with the variation of quantitative parameters, e.g., the film thickness. Johnston et al. [1] found that EHL phenomenon did not exist with films less than 15 nm thick. Tichy [20–22] proposed the models of thin lubricant film according to the improved EHL theory. Luo and Wen [3,4,18,34,35] have got the relationship between the transition thickness from EHL to TFL and the viscosity of lubricant, and proposed a physical model of TFL, and a lubricationmap of different lubrication regimes.