Surface film formation, boundary lubrication

Some special types of boundary lubricants, most notably the extreme pressure (EP) lubricants, react with a metallic surface, often at high temperatures, to produce a monomolecular film on the surface. This very thin film contaminates the mating surfaces and prevents metal-to-metal contact or adhesion. Extreme-pressure lubricants often contain extremely reactive constituents that re-form the film instantly if it is scraped off one of the surfaces. Film formation of this type is, in effect, corrosion when it is uncontrolled or when the film is repeatedly scraped off and re-formed, deterioration of the surface can result. 

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 ... 

Boundary lubrication describes processes which can occur at surfaces in relative motion to reduce friction and also control wear under conditions where fluid film formation is not possible. Boundary lubrication functions by several mechanisms... 

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"... 

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. 

Smeeth M, Spikes HA, Gunsel S. The formation of viscous surface films by polymer solutions boundary or elastohydrodynamic lubrication Tribol Trans 1996 39 720-5. 

Fig. 9 Huid-fllm-thickness (left axis) and 1 values (right axis) as a function of speed, calculated from the Esfahanian-Hamrock-Dowson equations [28, 29] (steel-on-glass contact) for a pin-on-disk and b MTM measurements from this study for different HEPES-glycerol mixtures. X values are indicated to enable estimation of film formation on the rough surfaces. 1 < 1 reptEsents boundary lubrication, 2 > 3 represents full-lluid-lilm lubrication and 1 < 2 < 3 represents the mixed regime...

Friction studies were carried out with a hip-joint simulator by Scholes et al. They compared the friction behavior between a number of implant materials using both carboxy methyl cellulose (CMC) solutions and biological fluids as lubricants. They found that friction coefficient for alumina/alumina was higher for biological fluids than for CMC, and attributed this to the inhibition of fluid-film formation by a protein film on the surfaces. For metal/ metal joints, the friction coefficients were lower in biological fluids, leading to the conclusion that the formation of a protein film assisted boundary lubrication. 

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

In examining the action of boundary lubricant compounds in reducing friction or wear or both between sohds in shding contact, it may be helpful to consider at least the foUowing five modes of film formation on or protection of surfaces (1) physisorption, (2) chemisorption, (3) chemical reactions with the solid surface, (4) chemical reactions on the solid surface, and (5) mere interposition of a solid or other material. These modes of surface protection are discussed in more detail in Reference 2. 

Greenberg et al. compared the effects of IF addition in oil under three lubrication regimes hydrodynamics, mixed and limit [44]. They based the lubrication mechanism of IF on a film formation on surfaces and showed that the IF are most effective at mixed Inbrication because all conditions for a good efficiency of IF are combined, hi the hydrodynamic regime, fullerenes do not have interactions with surfaces. In a boundary Inbrication regime, film formed on surfaces is quickly removed, due to contact severity. 

Surfactants and polymers used in boundary Inbrication systans adsorb on solid surfaces and form a protective film. The effectiveness of bonndary lubricants has often been attributed to the adsorption affinity and the integrity of the adsorbed film. Such adsorption is inflnenced by additives incorporated into the system to reduce thermal degradation, corrosion, sludge formation, foaming, etc. There are many interactions that can take place between the additives, the snrfactants, and the base oil, leading, in addition to adsorption effects, to a number of interfacial and colloidal phenomena such as micellization, precipitation, and solubilization as well as flocculation of particulate matter in fluid [1-3]. 

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