Static friction between clean copper single crystal surfaces

Abstract

Static friction between two very clean oxide-free copper single crystal (001) planes is studied by applying a horizontal force in a direction parallel to either [100] or [110] to a sphere placed on the horizontal surface of a fixed cube of the same crystallographic orientation. Cross sections, longtudinal sections and layer sections of the lower crystals are examined with an optical microscope, electron microscope and microhardness tester to find the extent of the occurrence of phenomena such as deformation, adhesion, shearing and internal strain which accompany the friction, and to clarify the mechanism. Although the extraordinary growth of the contact area is chiefly responsible for the high static coefficient of friction in every case, there is a large difference between the two directions. In the case of friction direction parallel to [100], local microslip along a direction parallel to [110] or [110] superposes on a viscous flow and tends to extend the contact area later ally, so that the contact area grows in the form of an isosceles triangle with a vertical angle of about 40°. In the case of friction direction parallel to [110], however, lateral growth of the contact area scarcely occurs and the friction trace takes the form of a rod, which results in a smaller coefficient of friction.