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Chapter 11
on steel and copper on copper. Aluminum and mild steel are soluble and have a high friction coefficient. On the other hand, white metal (babbitt), which is an alloy of tin, antimony, lead, and copper, is compatible against steel. Steel journal and white metal bearings have low dry friction and demonstrate outstanding score resistance. Roach, et al. (1956) tested a wide range of metals in order to compare their score resistance (compatibility) against steel. Table 11-1 summarizes the results by classifying the metals into compatibility classes of good, fair, poor, and very poor. Good compatibility means that the metal has good score resistance against steel. In this table, the atomic number is listed before the element, and the melting point in degrees Celsius is listed after the element. Cadmium has been found to be an intermediate between ‘‘good’’ and ‘‘fair’’ and copper an intermediate between ‘‘fair’’ and ‘‘poor.’’ The melting point does not appear to affect the compatibility with steel. Zinc, for example, has a melting point between those of lead and antimony, but has poorer compatibility in comparison to the two. It should be noted that many metals that are classified as having a good compatibility with steel are the components of white metals (babbitts) that are widely used as bearing material. Roach et al. (1956) suggested an explanation that the shear strength at the junctions determines the score resistance. Metals that are mutually soluble tend to have strong junctions that result in a poor compatibility (poor score resistance). However, there are exceptions to this rule. For example, magnesium, barium, and calcium are not soluble in steel but do not have a good score resistance against steel. Low friction and score resistance depends on several other factors. Hard metals do not penetrate into each other and do not have a high friction coefficient. Humidity also plays an important role, because the moisture layer acts as a lubricant. Under light loads, friction results only in a low temperature of the rubbing surfaces. In such cases, the temperature may not be sufficiently high for the metals to diffuse into each other. In turn, there would not be a significant score of the surfaces, although the metals may be mutually soluble. In addition, it has been suggested that these types of bonds between the atoms, in the boundary of the two metals, play an important role in compatibility. Certain atomic bonds are more brittle, and the junctions break easily, resulting in a low friction coefficient.
11.1.3
Coulomb Friction Laws
According to Coulomb (1880), the tangential friction force, Ff , is not dependent on the sliding velocity or on the apparent contact area. However, the friction