Boundary friction, lubrication

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. 

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. 

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. 

Under sliding conditions, films formed by ZDDPs behave in a plastic manner and boundary friction coefficients of ZDDP films are 0.13-0.15 [39]. In addition, they increase friction in the mixed lubrication region, secondary ZDDPs more so than primaries, which has an obvious impact upon energy efficiency. It may be that there is a thick viscous layer of organophosphate material on top of the solid reaction films but evidence for [40] is slightly weaker than the evidence against [41] and further studies are required to resolve this issue. 

The most important factor governing the tribochemical reactions under boundary friction is associated with the action of exoelectrons with lubricating oil components [21]. This is the basis of negative ion-radical action mechanisms, NIRAM. The general model of NIRAM assumes creation of two types of activated sites on friction surfaces, i.e. thermally activated sites and sites activated by exoelectron emission, FEE processes, Fig. 8.4. Comparison with thermally stressed solids and mechanically treated solids shows reactivity is often increased by several orders of magnitude, particularly in the low-temperature range. 

An investigation of the boundary friction of paraffin on glass in the presence of a stearic add soap intermediate layer is also very important [393]. This showed that with increase in the number of mono-layers applied, the true friction coeffident does not depend on where in the lubricating layer the shp plane is. 

In the case of boundary layer lubrication, in which the adsorption of mono-molecular films is required, the best protection is provided by materials such as fatty acids and soaps that can adsorb strongly at the surface to form a solid condensed film. Less durable but effective protection can be obtained with polar groups such as alcohols, thiols, or amines. The least effective protection is obtained with simple hydrocarbons that adsorb more or less randomly and through dispersion forces alone. For adsorbed monomolecular films, best results are obtained when the hydrocarbon tail has at least 14 carbons. In some cases fluorinated carboxylic acids and silicones may provide a lower initial coefficient of friction, but their weaker lateral interaction sometimes results in a less durable surface film that melts at a lower temperature, ultimately resulting in less overall protection. If a polar lubricant can form a direct chemical bond to the surface, as in the formation of metal soaps, even better results can be expected. 

The lubricity under the conditions of boundary friction was determined at a rotation speed of 200 rev/min under the same conditions. 

They found a smaller effective slip length (b ff = 20 10 nm) in the presence of microbubbles (Cassie state), than without any gas phase trapped at the boundary (bsff = 105 10 nm). This stresses the fact that the presence of gas trapped at an interface does not always warranty an increase of the slippage at the boundary. Super-lubricating capabilities of the Fakir surface do depend on the shape of the actual liquid interface and it must be realized that the extra friction induced by curved gas menisci may annihilate the benefits of partially coating the surface with gas. 

The other surface-active chemical components include antiwear additives, friction modifiers, and antifoam additives [15]. The functions of these other lubricant additives, as their names imply, are quite different from these two surfactants. For example, antiwear additives, such as zinc dial-kyldithiophosphates, are used to prevent wear on engine parts friction modifiers, such as glycerol monooleate, are used to reduce boundary friction in engine operation to improve fuel economy antifoam additives such as polymethylsiloxane are used to inhibit foam formation. Excessive foam formation in lubricants can cause inadequate oil supply to certain parts of the engine and a high rate of oil loss. 

Two surfaces which are in relative motion with an oil film between them are hydrodynamically lubricated. The viscosity of the oil determines the friction of motion. When the thickness of oil film is less than the surface irregularities, the asperities, then surface wear will eventually bring the two surfaces sufficiently close so as to rely on boundary layer lubrication. This is illustrated in Fig. 8.5 for a moving piston in a car engine. 

In addition, all poly glycols have good lubricating properties in boundary friction conditions. Information on corrosion and oxidation inhibition and suggestions for additives to combat wear are available in the literature [105]. 

S Korcek, R K Jensen, M D Johnson and J Sorab, Fuel efficient engine oils, additive interactions, boundary friction and wear . Lubrication at the Frontier/D Dowson et al. Editors, pp 13-24,1999... 

At the molecular scale, the surface force apparatus is being used to study lubricant behaviour in very close approach of surfaces. Atomic force microscopy can study local frictional variation due to additive deposition on surfaces in addition to giving detailed topographical maps. Nano-indenters can probe the elastic and plastic properties of surface films. These techniques are enhancing the understanding of boundary lubrication and supplement results from the multitude of more widely known boundary friction and wear testers, such as 4-ball, balTon-plate and ring-on-block. 

Improve energy efficiency and reduce CO2 emissions through viscosity optimisation in hydrodynamic lubrication, traction reduction in elastohydrodynamic lubrication, and boundary friction reduction using surface-active friction modifying additives. 

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