Friction of metals

The idea that adhesion is responsible for metallic friction was advanced as far back as 1724 [1], but the acquisition of evidence for the role of clean surfaces in the friction of metals had its systematic beginning in the work of F. P. Bowden and his collaborators. Bowden and Young [2], who studied the cleansing of nickel surfaces by heating under vacuum pumping at 133 uPa (10 torr) and the subsequent frictional behavior vac-uo at room temperature, were able to achieve such large-... 

The amount of moisture in soils has a considerable effect on adhesion. With increasing moisture content, the adhesive force increases because the soil becomes more sticky (see Table XII.3). For soils of the chernozem type, when the moisture content is above 70%, the strong adhesion of soil to a metal surface brings about an autohesive type of detachment when tilling the soil, so that friction of metal on soil is replaced by friction of soil on soil. According to data from other sources [341] autohesive detachment for well-structured clay and loam soils is observed at a moisture content of 80-85%, and for light soils at... 

In a general description of relative motion, it is also desirable to consider the influence of higher derivatives of the relative displacement. As yet, no such specific effort is known to the author. Some experimental work has been done in which instead of prescribing a constant drive speed, a constant tangential force is applied. The ensuing motion then describes the relation between friction and speed. It has been noted by the author that the friction-speed relations obtained from the constant speed and the constant force tests are alike. A similar result has been found by the author using his theoretical model. These results suggest that the coefficient of friction is mainly dependent upon the speed. Moreover the coefficient is more likely to be a point function rather than a path function of speed. With the scant information available, the observation made here may only be looked upon as tentative. In the friction of metals, it has been stated by Rabinowicz that the past history of motion over a certain critical distance is an important factor. Such a path dependence has not yet been studied in detail. 

An interesting aspect of friction is the manner in which the area of contact changes as sliding occurs. This change may be measured either by conductivity, proportional to if, as in the case of metals, it is limited primarily by a number of small metal-to-metal junctions, or by the normal adhesion, that is, the force to separate the two substances. As an illustration of the latter, a steel ball pressed briefly against indium with a load of IS g required about the same IS g for its subsequent detachment [37]. If relative motion was set in, a value of S was observed and, on stopping, the normal force for separation had risen to 100 g. The ratio of 100 IS g may thus be taken as the ratio of junction areas in the two cases. 

B. Bhushan and B. K. Gupta, Handbook of Tribology Materials, Coatings, and Surface Treatments, McGraw-HiU Book Co. Inc., New York, 1991 P. J. Blau, Friction, Eubrication, and Wear Technology, Vol. 18 Mmerican Society of Metals (A.SM) Handbook Series, ASM International, Metals Park, Ohio, 1992. 

High temperature lubrication, as in some metal-forming processes, requites dry graphite. Although the coefficient of friction of graphite is higher than that of petroleum lubricants, it is often added as a safety measure should the carrier lubricant fail (17) (see Lubrication and lubricants). 

Below -10°C, heat is conducted away too quickly to allow this melting - and because their thermal conductivity is high, skis with exposed metal (aluminium or steel edges) are slower at low temperatures than those without. At these low temperatures, the mechanism of friction is the same as that of metals ice asperities adhere to the ski and must be sheared when it slides. The value of jl (0.4) is close to that calculated from the shearing model in Chapter 25. This is a large value of the coefficient of friction - enough... 

The frictional behaviour of rubber is quite different from that of metals. In Chapter 25 we showed that when metallic surfaces were pressed together, the bulk of the deformation at the points of contact was plastic and that the friction between the surfaces arose from the forces needed to shear the junctions at the areas of contact. 

The friction and wear of plastics are extremely complex subjects which depend markedly on the nature of the application and the properties of the material. The frictional properties of plastics differ considerably from those of metals. Even reinforced plastics have modulus values which are much lower than metals. Hence metal/thermoplastic friction is characterised by adhesion and deformation which results in frictional forces that are not proportional to load but rather to speed. Table 1.7 gives some typical coefficients of friction for plastics. 

Hausner, H. H. (1967). Friction conditions in a mass of metal powders. International Journal of Powder Metallurgy, Vol. 3, (February 1967), pp. (7-13), ISSN 0888-7462... 

The investigations on boundary lubrication used to focus on the friction elements made of metallic materials, and of steel in particular. This is, of course, due to the fact that a great majority of machines are built from metal and steel, but it is also because the hydrocarbon-based oils have been proven to be an extraordinarily good lubricant for metal surfaces. Unfortunately, the conventional oils are not so effective to lubricate the components made of other materials, like ceramics, rubbers, silicon, etc., so that the study on new types of lubricants suitable for such materials has attracted great attention in recent years. 

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. 

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