Friction between Unlubricated Surfaces

Friction between unlubricated surfaces is due to a combination of adhesion... 

This is just the empirical eqn. (25.1) we started with, with = 1/2, but this time it is not empirical - we derived it from a model of the sliding process. The value = 1/2 is close to the value of coefficients of static friction between unlubricated metal, ceramic and glass surfaces - a considerable success. 

For metals, it is usually found that the yield strength, po, is about five times the shear strength, 5, which helps explain the fact that the coefficient of friction between unlubricated metal surfaces is quite often found to be about 0.2. 

In practice, then, the friction between unlubricated metal surfaces should probably be considered that between oxide layer or composite surfaces in which there are oxide-oxide, oxide-metal, and metal-metal components. If one considers only two of the three—metal-metal and oxide-oxide friction— one may estimate surface composition from the frictional force using the relationship... 

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... 

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"... 

The most general modern model used to describe frictional phenomena assumes that the friction between two unlubricated surfaces arises from two sources. The first and generally most important is that of adhesion between points of actual contact between the surfaces. We have seen on various occasions that real solid surfaces are almost never smooth. A very smooth surface will normally have asperities of between 5 and 10 nm so that the true area of contact between surfaces will be less that the apparent area (Fig. 18.1). At those areas of contact, the two surfaces will be bound by a certain adhesion force arising from the interaction between the materials at the molecular level—the same basic forces we have encountered before plus, in some cases, more physical interactions due to mixing, interpenetration, or locking. For the two surfaces to move tangentially, the points or areas of adhesion, welds, or junctions must be sheared or broken. If the real area of contact is A and the shear strength of the weld or bond is s, then the frictional force due to adhesion will be... 

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