Friction between Lubricated Surfaces

The coefficient of friction between lubricated surfaces can be described by the following equation ... 

Titanium disulfide has been proposed as a soHd lubricant. The coefficient of friction between steel surfaces is 0.3, compared to only 0.2 for molybdenum disulfide. However, because it does not adhere strongly to metal surfaces, TiS2 is generally less effective than molybdenum sulfide. 

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. 

Theoretically, the diffusion coefficient can be described as a function of the disjoining pressure 77, the effective viscosity of lubricant, 77, and the friction between lubricant and solid surfaces. In relatively thick films, an expression derived from hydrodynamics applies to the diffusion coefficients. 

T ike metals minerals also exhibit typical crystalline structures. As an example, the structure of molybdenite is shown in Figure 1.17. It is hexagonal with six-pole symmetry and contains two molecules per unit cell. Each sulfur atom is equidistant from three molybdenum atoms and each molybdenum atom is surrounded by six sulfur atoms located at the comers of a trigonal prism. There are two types of bonds that can be established between the atoms which constitute the molybdenite crystal stmcture. They are the covalent bonds between sulfur and molybdenum atoms and the Van der Waals bonds between sulfur-sulfur atoms. The Van der Waals bond is considerably weaker than the covalent sulfur-molybdenum bond. This causes the bonds of sulfur-sulfur to cleave easily, imparting to molybdenite the property of being a dry lubricant. Molybdenite adheres to metallic surfaces with the development of a molecular bond and the friction between metallic surfaces is replaced by easy friction between two layers of sulfur atoms. 

Capsule formulations usually require lubricants just as do tablet formulations. Lubricants ease the ejection of plugs, reduce filming on pistons and adhesion of powder to metal surfaces, and reduce friction between sliding surfaces in contact with powder. The same lubricants are used in both tablet and capsule formulations. 

Lubricating oil a fluid lubricant used to reduce friction between bearing surfaces. 

A further use for self-assembling peptides is as lubricants to reduce joint friction between cartilage surfaces (Bell et al., 2006). The amphiphilc peptide Pn-9 which resembles hyaluronic acid (HA), the natural lubricant in joints, successfully reduced joint friction in simulated experiments. Bell and colleagues suggest that this peptide may be a superior synthetic treatment compared to injection to replace HA, which is not suitable in all osteoar-thritic cases. 

The general subject of film formation is considered in the next chapter, but at this point it will be useful to mention a few aspects of film behaviour in order to clarify the nature of friction between lubricated components. In the first place, if two surfaces slide against one another with only free molybdenum disulphide powder present as a lubricant, then initially the coefficient of friction is quite high. It is only when a smooth adherent film has formed on at least one of the surfaces that lower friction occurs. In the second place, if a smooth adherent film of molybdenum disulphide is present on only one of the surfaces, then the lowest possible friction will still not be obtained. It is only when a useful film is also present on the second surface, either formed in advance or formed by transfer from the film on the first surface, that the lowest values of friction will be found. 

Schick and Fowkes (11) studied the effect of alkyl chain length of surfactants on critical micelle concentration (CMC). The maximum lowering of CMC occurred when both the anionic and nonionic surfactants had the same chain length. It was also reported that the coefficient of friction between polymeric surfaces reaches a minimum as the chain length of paraffinic oils approached that of stearic acid (12). In order to delineate the effect of chain length of fatty acids on lubrication, the scuff load was measured by Cameron and Crouch (13). The maximum scuff load was observed when both hydrocarbon oil and fatty acid had the same chain length. Similar results of the effect of chain length compatibility on dielectric absorption, surface viscosity and rust prevention have been reported in the literature (14-16). 

To date, no attempt has been made to understand the relationship between the adsorption of biomolecules from the complex synovial fluid and the friction between the surfaces of artificial joint materials on a molecular level. There have, however, been a number of studies on the mechanisms of aqueous lubrication in the presence of synthetic macromolecules. A particularly efficient mechanism of lubrication involves the formation of hydrophilic polymer brushes at the surface, which maintain a fluid film between the opposing surfaces [16,17], The low friction between the surfaces has been attributed to the fluidity of the hydration layers around the polymers and resistance to interpenetration between the polymer brushes [18]. 

Friction on the macroscopic scale can take place either between dry contacts or between lubricated surfaces. An intermediate case called boundary lubrication is friction in which the surfaces are not separated by a thick layer of lubricant but just by surface layers such as oxide layers on metals or by a few molecular layers of adsorbed lubricants. 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

As load is increased and relative speed is decreased, the film between the two surfaces becomes thinner, and increasing contact occurs between the surface regions. The coefficient of friction rises from the very low values possible for fluid friction to some value that usually is less than that for unlubricated surfaces. This type of lubrication, that is, where the nature of the surface region is... 

Klein and co-workers have documented the remarkable lubricating attributes of polymer brushes tethered to surfaces by one end only [56], Studying zwitterionic polystyrene-X attached to mica by the zwitterion end group in a surface forces apparatus, they found /i < 0.001 for loads of 100 and speeds of 15-450 nm/sec. They attributed the low friction to strong repulsions existing between such polymer layers. At higher compression, stick-slip motion was observed. In a related study, they compared the friction between polymer brushes in toluene (ji < 0.005) to that of mica in pure toluene /t = 0.7 [57]. 

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. 

The tenn tribology translates literally into the study of nibbing . In modem parlance this field is held to include four phenomena adhesion, friction, lubrication and wear. For the most part these are phenomena that occur between pairs of solid surfaces in contact with one another or separated by a thin fluid film. Adhesion describes the resistance to separation of two surfaces in contact to while friction describes their tendency to resist shearing. Lubrication is the phenomenon of friction reduction by the presence of a fluid (or solid) film between two surfaces. Finally, w>ear describes the irreversible damage or defonnation that occurs as a result of shearing or separation. 

Lubricants protect die and punch surfaces from wear and bum-out of the compact during sintering without objectionable effects or residues. They must have small particle size, and overcome the main share of friction generated between tool surfaces and powder particles during compaction and ejection. They must mix easily with the powder, and must not excessively impede powder flow (see Lubrication and lubricants). 

Lubrication, in the generally accepted sense of the word, means keeping moving surfaces completely separated by means of a layer of some liquid. When this is satisfactorily achieved, the frictional resistance no longer depends on the solid surfaces but solely on the internal friction of the liquid, which, in turn, is directly related to its viscosity. The more viscous the fluid, the greater the resistance, but this is never comparable with that existing between non-lubricated surfaces. 

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