Lubrication, boundary

Application of an oily or greasy substance in order to reduce friction. If a layer of lubricant is thick enough, it reduces the coefficient of friction (/u). For a very thin layer of lubricant, boundary layer lubrication takes place. 

Intricate as the theoretical calculations described in the preceding sections are, still they only approximate the conditions in a real sliding system. Real contact interfaces are composed of asperities that are subjected to a great deal of plastic deformation with the generation of heat at highly localized sites. Ordinary metal surfaces carry films of oxides and adsorbed atmospheric gases, and even the very thinnest of lubricant boundary films introduces yet another complication into the heat transmission problem. The observer of experimentally measured interfacial temperatures is thus confronted with the task of evaluating... 

LUBRICATION, BOUNDARY - A condition of lubrication in which the friction and wear between two surfaces in relative motion are determined by the properties of the surfaces and by the properties of the lubricant other than bulk viscosity. 

KEY WORDS poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG), aqueous lubrication, boundary lubrication... 

More comprehensive studies involve not only investigation of the hydrodynamics of the lubricant film, but also the generation of energy in the film and its dissipation to the lubricant, boundary solids and the environment. In addition the thermal and elastic distortions of thrust bearing pads are frequently crucial in determining the operational characteristics. The design process is thus a particularly complex one with a nunfoer of difficult analytical and practical aspects. 

Distinguish between hydrodynamic lubrication, boundary lubrication, and extreme pressure lubrication. 

It has been demonstrated elsewhere [10,13] that the mode of lubrication (boundary mixed fluid-film) plays a major role in determining the steady-state wear rate in metal-on-metal total replacement hip joints. 

Much of the classic work with boundary lubrication was carried out by Sir William Hardy [44,45]. He showed that boundary lubrication could be explained in terms of adsorbed films of lubricants and proposed that the hydrocarbon surfaces of such films reduced the fields of force between the two parts. 

Fig. XII-7. Variations of /i with molecular weight of the boundary lubricant curve for spherical slider —, curve for plane slider. (From Ref. 44.)...

It is evident that boundary lubrication is considerably dependent on the state of the monolayer. Frewing [48] found that, on heating, the value of fi rose sharply near the melting point sometimes accompanied by a change from smooth to stick-slip sliding. Very likely these points of change correspond to the transition between an expanded film and a condensed film in analogy with... 

Hardy s explanation that the small coefficients of friction observed under boundary lubrication conditions were due to the reduction in the force fields between the surfaces as a result of adsorbed films is undoubtedly correct in a general way. The explanation leaves much to be desired, however, and it is of interest to consider more detailed proposals as to the mechanism of boundary lubrication. 

Second, it is found that metal-metal contacts are still present even under normal boundary lubrication conditions where n is small. Very clear evidence... 

Fig. Xn-9. Contact region in boundary lubrication according to Hardy. (From Ref. 45.)...

For boundary lubrication, a must be on the order of 10" to account for the great reduction in metal pickup, therefore, most of the friction must be due to film-film interactions. 

The mechanism of boundary lubrication may then be pictured as follows. At the unusually prominent asperities, the local pressure exceeds the yield pressure... 

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. 

There is a breakdown of boundary lubrication under extreme pressure conditions. The effect is considered to be related to that of increasing temperature [59] this is not unreasonable since the amount of heat to be dissipated will increase with load and a parallel increase in the local temperature would be expected. 

The traditional, essentially phenomenological modeling of boundary lubrication should retain its value. It seems clear, however, that newer results such as those discussed here will lead to spectacular modification of explanations at the molecular level. Note, incidentally, that the tenor of recent results was anticipated in much earlier work using the blow-off method for estimating the viscosity of thin films [68]. 

An important aspect of the stabilization of emulsions by adsorbed films is that of the role played by the film in resisting the coalescence of two droplets of inner phase. Such coalescence involves a local mechanical compression at the point of encounter that would be resisted (much as in the approach of two boundary lubricated surfaces discussed in Section XII-7B) and then, if coalescence is to occur, the discharge from the surface region of some of the surfactant material. 

A considerable number of experimental extensions have been developed in recent years. Luckliam et al [5] and Dan [ ] review examples of dynamic measurements in the SFA. Studying the visco-elastic response of surfactant films [ ] or adsorbed polymers [7, 9] promises to yield new insights into molecular mechanisms of frictional energy loss in boundary-lubricated systems [28, 70]. 

Using friction attaclnnents (see section (bl.20.2.4)). many remarkable discoveries related to tiiin-film and boundary lubrication have been made with the SEA. The dynamic aspect of confined molecules at a sliding interface has been extensively investigated and the SFA had laid the foundation for molecular tribology long before the AFM teclnhque was available. 

The often-cited Amontons law [101. 102] describes friction in tenns of a friction coefiBcient, which is, a priori, a material constant, independent of contact area or dynamic parameters, such as sliding velocity, temperature or load. We know today that all of these parameters can have a significant influence on the magnitude of the measured friction force, especially in thin-film and boundary-lubricated systems. 

Luengo G, Israelachvlll J N and Granick S 1996 Generalized effects In confined fluids new friction map for boundary lubrication Wear 200 328-35... 

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