The Contact and Friction of Clean Surfaces

The fact that the surfaces of materials exposed to the ordinary atmosphere are not clean is well established. Water vapor and the fixed atmospheric gases are ubiquitously adsorbed physically on even non-reactive surfaces. As for the ordinary metals, almost all of these are chemically reactive, and after substantial exposure to the atmosphere their surfaces carry a layer of oxide. The adhesive theory of friction implies true contact of the putative material of composition at asperity 

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- 

TABLE 9-1. FRICTION AND ADHESION OF CLEAN COPPER IN VACUUM 

Form and orientation Adhesion coefficient before sliding Coefficient of friction during sliding Adhesion coefficient after sliding 

Rider Platen or disk Coefficient of friction Vacuum, Pa  

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