Optical observations of ?junction growth? in asperities of copper, aluminium, PTFE and nylon under combined normal and tangential stresses

Abstract

An experimental investigation is described in which the phenomenon of “junction growth” in model conical and spherical asperities of copper, aluminium, PTFE and nylon has been examined optically. The test asperity was first normally pressed against a smooth flat surface of soda-lime glass and the contact observed through the glass surface. In the presence of the normal load the asperity deformed plastically. When a tangential force was applied, the size of the real area of contact between the two solids, as measured in situ, was found to increase for all the asperities used. Depending upon the geometry and material of the test asperity, the increase in the size of the contact area was up to 40%. The behaviour was compared with an analytical expression for junction growth in the case of a right circular cylindrical asperity. The closest agreement between the measurements and the theory was found for the 60° conical asperities of work-hardened copper and for nylon spheres. The sliding of the asperity on the glass plate caused transfer of the material of the former to the latter. This occurred for all the asperities used. Moreover, the sliding of the metallic asperities resulted in up to ∼5 μm deep grooves on them as well as on the glass plate. These observations have also been briefly discussed.