Solid Boundary Lubrication

R is an alkyl or phenyl group. The disulphide molecule becomes a thiol and then ejects hydrogen at the surface to become a thiolate. This then leads to the formation of iron sulphide (the desired solid boundary lubricant). This reaction therefore should yield some hydrogen during the 4-baII tests. 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

Even when solid surfaces are protected by oxide films and boundary lubricants, some solid-to-solid contact occurs at regions where the oxide film breaks down under... 

A variety of chemical additives can be incorporated in lubricating oils to improve their properties under boundary lubrication conditions. Some of these additives react with the surfaces to produce an extremely thin layer of solid lubricant, which helps to separate the surfaces and prevent seizure. Others improve the resistance of the oil film to the effect of pressure. 

The definition of different lubrication regimes is a historic problem [41 ]. In boundary lubrication, molecules will be absorbed on a solid surface of a tribo-pair and form a monomo-lecular absorbed layer as described by Hardy [42] as shown in Fig. 1 (a). If the film thickness of lubricants in the contact region is from a few nanometres to tens of nanometres, different layers will be formed as shown in Fig. 1 (b) proposed by Luo et al. [3,4]. The layer close to the surfaces is the adsorbed film that is a monomolecular layer. The layer in the... 

The failure of TFL only means a loss of mobility here, but monolayers can stay on solid surfaces to separate the solid surfaces in relative motion, and subsequently sustain a feasible boundary lubrication state [10]. Because the film thickness of TFL is of the nano scale or molecular order, from a mechanical point of view, TFL is the last one of the lubrication regimes where the Reynolds equation can be applied. 

Boundary lubrication (BL) can also evolve into TFL in a bottom-up way [11]. Compared to BL, TFL has a thicker film. In the vicinity of solid walls, the liquid molecules take the states of that of the boundary film, the ordered one, and the disordered one, from the wall surfaces to the center of the gap. From a mechanical point of view, the existence of an ordered film makes the lubricant film differ strongly from the boundary film, which can form a glassy state or solid-like... 

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. 

Finally, there is another category of lubricants, including the laminated materials, highly ordered organic mono-layers, and various thin solid hlms, which provides effective lubrication via their properties of low shear strength or high wear resistance. Lubrication via ordered molecular films and other solid lubricants, which have been considered by some investigators as a sub-discipline of boundary lubrication, will be discussed more specifically in Section 4. 

One of the major themes of boundary lubrication is to transfer the shear stress at the interface of direct solid contact to somewhere inside the lubricating layer, to achieve low friction and high wear resistance. In this sense, materials with low shear strength, such as liquid films, soft metals, and lamella solids, can be employed as candidate lubricants. 

In the studies that attribute the boundary friction to confined liquid, on the other hand, the interests are mostly in understanding the role of the spatial arrangement of lubricant molecules, e.g., the molecular ordering and transitions among solid, liquid, and amorphous states. It has been proposed in the models of confined liquid, for example, that a periodic phase transition of lubricant between frozen and melting states, which can be detected in the process of sliding, is responsible for the occurrence of the stick-slip motions, but this model is unable to explain how the chemical natures of lubricant molecules would change the performance of boundary lubrication. 

Numerous simulations of boundary lubricants have shown that cto is indeed small, even at pressures close to the yield strengths of solids.14,21 Thus, it seems that Amontons s law should hold in a wide range of cases. However, exceptions are observed when adhesive interactions are strong, which leads to large values of cto. 

In the example shown in Figure 5, c is positive and the exponent y is unity however, neither of these statements are universal. For example, the Prandtl-Tomlinson model can best be described with y = 2/3 in certain regimes,26 27 whereas confined boundary lubricants are best fit with y = l.25 28 Moreover, the constant c can become negative, in particular when junction growth is important, where the local contact areas can grow with time as a result of slow plastic flow of the opposed solids or the presence of adhesive interactions that are mediated by water capillaries.29,30... 

Special cases of solid lubrication arc boundary anil IIP (extreme pressure) lubrication In both cases ihe solid lubricant is formed by chemical reaction of special compounds, usually applied as oil solutions, with the metallic rubbing surfaces Typical boundary lubricants are the fatty acids which react with the metal surface to form metallic soaps which then carry the load. Strongly adsorbed hut nonreacting substances of linear structure, such as long chain tally alcohols, can also act as boundary lubricants but only under very mild conditions... 

Zinc dialkyldithiophosphates (ZDDPs) function mainly as antioxidants and antiwear additives. Molecules of ZDDPs adsorb on metal surface to participate in surface tribofilm formation under conditions of boundary lubrication. The solid tribofilms are formed at the metal surface to protect even under conditions of coarse contact under load (Bom et al., 1992). 

It appears that MoS2 forms, and friction is reduced, only when direct solid-solid contact occurs, i.e., in true boundary lubrication conditions (Graham et ah, 2001a). 

A detailed derivation of Eq. (1.6a) using the no-slip boundary condition is provided in Section A.2 of the Appendix. If we were to generalize the analysis above with the partial-slip boundary condition, that is, Sv/Sz = pv (p = slip parameter) instead of the no-slip condition in Eq. (A.2) at the lubricant/solid boundary with q (z) = p, we would obtain... 

At the interface between solids and organic solvents, however, specific attractions between the solid and the adsorbed substances may come into play and produce considerable adsorption. Some such cases are of importance in lubrication long-chain fatty acids, and some of their salts, are adsorbed from solution in hydrocarbon oils at the surface of many metals, and the result is a boundary lubricating layer (see Chap. VI). 

FIGURE 4.6 Values of /I (tribological mechanism indicator) describing boundary lubrication for an IC1400 K-groove pad at 25% solids and partial lubrication for an ICIOOO flat pad at 12.5% solids (from Ref 6). 

Feng, I. Ming, Perilstein, W.L. and Adams, M.R., Solid Film Deposition and Non-Sacrificial Boundary Lubrication, ASLE Trans, 6, 60, (1963). 

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