The Mechanism of 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 

The radioautographic work suggests another model illustrated in Fig. XII-11. The load is supported over area A, with metal contacts of shear strength s over a portion of the area ctA and film-film contacts of shear strength Sf over the rest of the area. In analogy to Eq. XII-9, one can write the total frictional force, F as 

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. 

This second picture, while an advance over Hardy s, again encounters difficulties. It does not suggest how A could be so much greater than A i or why Sf 

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

Previous studies have indicated that no hydrodynamic lubrication occurs during CMP.28 3la There is always a physical contact between the wafer and the polishing pad asperities. In the following section, we will see that there is enough evidence to prove interactions between a wafer and a pad. The boundary lubrication associated with tribochemical interactions plays a dominant role. In order to understand the mechanisms of boundary lubrication in CMP, the physical, electrochemical, and mechanical processes of interfaces must be considered. The mechanisms can be classified into the following categories based on the surface physical chemistry of materials involved during CMP. 

Because an adsorbed monomolecular film will have a thickness on the order of 2.5 nm, while the surface asperities present on all but the finest surfaces will seldom be less than 5-10 nm, it is important to have a clear picture of the mechanism of boundary lubrication at the molecular level. A typical situation is shown schematically in Figure 18.12, where it can be seen that there are two types of contact between the two surfaces in the total contact area A contact between the adsorbed lubricant films (area cuA the figure) and that between the actual surfaces where the adsorbed film has broken down (area I3A). The total frictional force between the two will be the sum of each contribution... 

D aguin BV, ICarassev W, Zakhavaeva NN, Lazarev VP. The mechanism of boundary lubrication and the properties of the lubricating film short- and long-range action in the theory of boundary lubrication. Wear 1957 1 277-90. 

The mechanism of boundary lubrication for polymers is generally similar to that of metals. Lubricants are known to be ineffective for polymeric surfaces, partly because of plasticization, and plasticization could weaken the interfacial forces and reduce both the shear strength and the yield pressure of the polymer. This weakening effect can be illustrated with the following equations ... 

The influence of the presence of a film of liquid (water) is for various reasons regarded as an important factor. The mechanism of boundary lubrication described elsewhere has been more confined to metallic contacts and special lubricants. 

Finally, it has to be mentioned again that wear resistance has been a major subject of investigations in boundary lubrication, but this chapter is contributed mainly to the discussions of the mechanism of boundary friction. Readers who are interested in the wear process in microscopic scale are referred to Chapter 10 for further information. 

Liquid slip can be promoted by the adsorption of long chain surfactants from solution on to smooth solid surfaces. This suggests a possible new mechanism of boundary lubrication by organic friction modifiers - encouragement of local slip of the liquid lubricant against the bounding solids to produce a local reduction in shear stress. Such a mechanism would imply that friction modifiers might be able reduce friction in full film lubrication conditions. 

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. 

Nascent surface Explain the difference in the concept of liquid lubrication mechanism in (a) hydrodynamic, (b) elastohydrodynamic and (c) boundary lubrication. Which of the following characterize (a), (b), and (c) lubrication regime continuous fluid film, negligible deformation, complete separation of the surfaces, elastic and plastic deformation, no wear takes place, no contact between the sliding surfaces, involving surface topography, physical and chemical adsorption, catalysis and reaction kinetics, and tribochemical film formation ... 

The mechanisms of particle removal have been studied in the past few years. Reports show that the particles adhere to a surface primarily by van der Waals forces, electrostatic attraction, or capillary action.2 The cleaning is by hydrodynamic lubrication. The thickness of the hydrodynamic fluid layer, as estimated, was around 3.7 pm.1 On the contrary, numerical analysis concluded that the lift force in the hydrodynamic boundary layer of fluid was too small to lift particles off the surface.3 The possible removal force comes likely from the drag force between the brush and the wafer surface. Major... 

An important aspect of the function of compounded lubricants is to increase the load that can be carried by machinery without catastrophic damage to the rubbing components. Since the typical antiwear additives affect the viscosity of the carrier oil very little, it is not a fluid film effect that is responsible for the load-carrying augmentation. Examination of the various basic wear processes leads to the choice of the adhesive mechanism as the one most likely to respond to the action of boundary or extreme-pressure additives. The type of macroscopically observed severe wear which has this mechanistic process as its primary cause is generally designated as icu i ng (c(S. Chapter 13, Sections 13.4 and 13.6), and it is in this sense, as a description rather than a definition, that the term scuffing is used in the discussion to follow. 

If load is increased or speed decreased, the film between the two surfaces becomes thinner and its properties are no longer those of the bulk. The coefficient of friction rises from its lowest value, in hydrodynamic condition, to a higher value (which however is less than for unlubricated surface). This regime of boundary lubrication (Fig. 19) is of utmost interest for high performance systems and one seeks to maintain it on the largest scale at the lower level. However there are few quantitative data, "The mechanism of formation and stabilisation of the boundary film is unknown"... 

A lubricant is any material that can be placed between surfaces to lessen friction. The purpose of a lubricant is to reduce the frictional resistance between two contacting surfaces forced to slide over one another, to minimize wear, and prevent corrosion. Antiwear agents produce a surface fihn either by a chemical or by a physical adsorption mechanism to minimize friction and wear under boundary lubrication conditions. 

As shown above, the polymer PLL- -PEG functions in aqueous solution as an effective boundary-lubricating additive and this effectiveness depend upon the polymer architecture. The focus of this discussion is to consider the mechanism of this boundary-lubricating performance and thus to determine how the polymer... 

Clearly, the boundary conditions in three-body dry friction are much more difficult to express than in lubrication. A parallel analysis of powder rheology, which would consider both the changes in properties brought about by non homogeneous compacting and the different forms of velocity adaptation would lead to similar conclusions. Nevertheless, the differences observed in the modelling should not hide the basic similarities in the mechanics of both subjects which are centered on load—carrying and flow. 

Open gears operate under conditions of boundary lubrication. The lubricant can be applied by hand or via drip-feed cups, mechanical force-feed lubricators, or sprays. Heavybodied residual oils with good adhesive and fihn-strength properties are required to survive the relatively long, slow, heavy tooth pressure while maintaining some film between applications of lubricant. 

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