Friction boundary

On concluding this chapter, it is worthwhile to address a few issues that associate in particular with the boundary friction, i.e., the friction originated from the shear of surface films. The emphasis is given to the discussion of the shear strength and two parallel models that describe the boundary friction in terms of different mechanisms. 

In both models, the magnitude of friction can be determined in terms of the shear strength of boundary films, i.e., the critical shear stress when slip occurs, regardless of how the films are formed, by adsorption, reaction, or solidification. The existence of a critical shear stress, independent of ap- 

When boundary friction is considered as a result of sliding between the adsorbed layers, studies on the shear response of surface films are mostly concentrated on the influential 

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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. 

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The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

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. 

An application of the anion radical model to the polymerization of vinyl monomers during boundary friction has also been discussed (Kajdas 1994). The formation of anion radicals gives rise to the species that contain one radical end and one anion end ... 

Uniform flow was described earlier as it applies to hydraulic phenomena in general. In the case of open channels, uniform flow means that the water cross section and depth remain constant over a certain reach of the channel. This requires that the drop in potential energy owing to the fall in elevation along the channel be exactly consumed by the energy dissipation through boundary friction and turbulence. 

Under boundary friction conditions, the exposed metal surface is extremely reactive due to mechanical activation. Exoemission occurs when a material surface is disturbed by plastic deformation, abrasion, fatigue cracking, and phase... 

The molybdenum dialkyldithiophosphates (MoDDP) were able to reduce friction effectively in mineral oil in the absence of other additives. The MoDTC only reduces friction to boundary values of about 0.095 however, mixtures of MoDTC and ZDDP can give much lower boundary friction coefficient values, below 0.04 (Graham and Spikes, 1999 Tripaldi et al., 1999). 

Because of the availability of these new methods, devices, and purer materials, it has become more feasible to carry on effective research with adequate surface-chemical control of gas and liquid adsorption, wetting, adhesion, emulsification, foaming, boundary friction, corrosion inhibition, heterogeneous catalysis, electrophoresis, electrode surface potentials, and a variety of other subjects of interest in the surface-chemical and allied fields of research. In view of the present situation, serious investigators should now be able to report results in the scientific literature which will have much more value than ever before. There is no excuse for any investigator s taking such inadequate care in controlling surface composition or surface-active contaminants as was common in over 50% of the research publications in surface and colloid science in the past. 

Ateshian GA, Wang HQ, Lai WM (1998) The role of intestitial fluid pressurization and surface porosities on the boundary friction of articular cartilage. J Tribol 120 241-251... 

The behaviours of mono-, di- and friesters of glycerol oleate were compared as a range of commercial organic friction modifiers in a synthetic base fluid. Of the esters studied, the monoester gave the lowest friction and base fluid polarity had no discernable effect on boundary friction [18]. 

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. 

Fig. 8.3 Physical processes associated with surface enlargement during boundary friction, reproduced with permission from [21]...

A.S. Akhmatov in "Molecular Physics of Boundary Friction" Moscow 1963 Translated from Russian and available from Israel Program for Scientific Translations cited in ref. 8, 9, 19... 

The results indicate that, under boundary conditions, LCs are inferior than traditional film strength additives. Also, the order of LCs had no effect on its boundary friction properties. It is important to note that, under hydrodynamic conditions, TLCs have been found to provide with lower COF at their nematic than at their isotropic phases (5). 

TLCs have inferior boundary friction properties. This contrasts to numerous observations of superior friction properties of TLCs under hydrodynamic conditions. 

Another conclusion is that the order of TLCs had no effect on its boundary friction properties. This conclusion is based on the assumption that the phases of the TLCs observed in the bulk will be similar to that of the thin films on the rubbing surfaces, provided that the temperatures were the same. However, while the orders of LCs in the bulk can be verified easily, and are well-documented, there is no information on the orders of the thin films on various surfaces. Thus, there could be a dramatic shift in the temperature range of the various orders in thin film versus bulk. This shift could also be a function of the type of surface (i.e., ceramic, aluminum, steel), surface roughness and even the order of the liquid crystal (cholesteric, smectic, nematic, etc.). Recent work by Frommer, et al.(121 seem to support the above argument. Determination of the order of thin films of liquid cryst s on various substrates is an interesting area of research, but is beyond tiie scope of this work. 

Shchukin, E. D. et al., "On the Initiation of Sim.pie Plastic Deformations within Single Crystals of Fluorous Lithium at Boundary Friction DAN SSSR, 1971, N 6, p. 200. 

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