Effects of process parameters during turn-milling of microstructured surfaces on the coefficient of static friction

Abstract

Increasing power densities in current technical systems as well as lightweight requirements demand the transmission of higher specific forces or torques respectively. The static friction, determining the transferable torque in friction-locked connections, is affected considerably by the properties of the surfaces in contact. A predefined microstructuring of the surface by turn-milling can increase the coefficient of static friction. Specimens of the steel 42CrMo4+QT (1.7225) are machined using single-edged TiAlN-coated cemented carbide end milling cutters. Two different tool corner geometries (sharp, chamfered 0.2mm x 45°) are applied to generate protruding microstructures with different profile tip angles. For each tool type the influence of the cutting parameters cutting speed vc, feed per tooth fztan and radial feed frad on the surface structure and the coefficient of static friction is investigated by using a 2k full factorial design with centre point. For both tool types the feed per tooth fztan is the crucial parameter with a significant influence on the coefficient of static friction. The feed per tooth fztan is responsible for the generation of superimposed surface structures and affects the real contact area with the counterpart. The maximum average coefficient of static friction is μ0.1° = 0.63 for using the sharp tool corner and μ0.1° = 0.44 for using the chamfered corner. This represents approximately a tripling or rather doubling of the values that are achieved with unstructured specimens. Furthermore a good correlation between the peak material volume Vmp and the coefficient of static friction is determined.