Lubrication, boundary

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. 

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 

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. 

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

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

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. 

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

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]. 

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. 

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

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 

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

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]. 

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. 

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. 

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