Boundary lubricating films

Whether this boundary layer was one molecule thick (on each surface) or thicker is not certainly known. Trillat s work6 indicates that orientation, in the case of long chain acids and similar compounds, may extend some distance from a surface into the interior of a liquid, and Andrews s electron diffraction studies7 also indicate orientation extending beyond the first layer. The heavy slider may, or may not, penetrate through to the layer of molecules next to the solid. 

According to Hardy, in general, ring compounds reduced the friction less than aliphatic compounds. In aliphatic homologous series a steady diminution in the static friction was observed with increasing length of 

When two different solid faces were used the friction of the lubricated metal was the arithmetic mean of the friction, observed with the two materials used together the value of /x0, in equation (7), for A sliding on J8, is (m + M ) where ftjf, are the values of for A sliding on A and B on B, respectively. 

Thus each solid face makes a definite quantitative contribution to the friction, irrespective of what the other solid face may be, or what lubricant is present. At one time Hardy attempted to explain this on the theory that the influence of the attraction of the solid face extended for long distances, but more recent analysis of the occurrences during sliding indicates that the process is very complex, and that the coefficient of friction is not, as a rule, a quantity capable of simple interpretation in terms of the properties of continuous surface films, and of the underlying solid. 

The coefficient of friction, as ordinarily measured, is shown clearly by the discovery of the stick-slip motion to be an average value of a rapidly 

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A reciprocating wear tester was used to investigate the nature of antiwear boundary lubrication films formed by several ZDDP additives in mineral oil. The surface films were analyzed by SEM-EDX, EPMA and scanning Auger. The antiwear film contained S, O, Zn, and P elements with trace amounts of Fe and C. Once antiwear films are formed they can be removed by enhanced roughness and the presence of hydroperoxides (Sheasby et al., 1990). 

This type of wear loses material by chemical reaction and can be very detrimental to service life, e.g. the acidic corrosion of cylinder liner materials. If it is controlled, however, it can be beneficial such as the formation of low-shear-strength boundary lubricant films by extreme pressure (EP) or anti-wear (AW) additives. A lubricated system contains many corrosive species, such as ... 

Adsorption of Surfactants on the Surface of a Friction-Pair Element Formation oe a Boundary Lubricant Film to Reduce Motion Resistance and Wear... 

It was formd that boundary lubrication films composed of Fe(0H)0, Fe304, FeP04 and compounds containing P-O bonds were formed on the worn surface, which resulted in an excellent friction reduction and also anti-wear performance (23). 

H. A. Spikes, Boundary Lubrication Films. Proc. Int. Tribology Conf., Yokohama (Japan) 1995, Satellite Forum on Tribochemistry, p. 49. To be published in Tribal. Lett. 

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. 

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. 

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. 

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. 

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. 

Antiwear Compounds. Additives are used in many lubricating oils to reduce friction, wear, and scuffing and scoring under boundary lubrication conditions, ie, when fuU lubricating films cannot be maintained. Two general classes of materials are used to prevent metallic contact. 

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... 

Boundary lubrication is perhaps best defined as the lubrication of surfaces by fluid films so thin that the friction coefficient is affected by both the type of lubricant and the nature of the surface, and is largely independent of viscosity. A fluid lubricant introduced between two surfaces may spread to a microscopically thin film that reduces the sliding friction between the surfaces. The peaks of the high spots may touch, but interlocking occurs only to a limited extent and frictional resistance will be relatively low. 

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 increasing demands being made on equipment by the requirement for increased output from smaller units create problems of lubrication, even in systems where full-fluid film conditions generally exist. For instance, at start-up, after a period of rest, boundary lubrication conditions can exist and the mechanical wear that takes place could lead to equipment failure. Anti-wear additives, by their polar... 

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