Chapter 14 Studies on friction and transfer layer using inclined scratch

Abstract

Friction influences the nature of transfer layer formed at the interface between die and sheet during forming. In the present investigation, basic studies were conducted using Inclined Scratch Tester to understand the effect of surface topography on friction and transfer layer formation. A tribological couple made of an Al-Mg alloy pin against steel flat was used in the tests. Tests were conducted at a sliding speed of 2 mm/sec in ambient conditions under both dry and lubricated conditions. The inclination angle of the steel flat was 1.0 ± 0.10. Normal loads varied from 0 to 135 N during the scratch test. Two surface parameters of steel flats – roughness and texture – were varied in tests. Care was taken to ensure that the surface roughness, measured along the scratch direction, had similar values for different textures, namely Uni-directional, 8-ground, and Random. Grinding the EN8 flat in a uni-directional manner and a criss-cross manner on emery sheets produced the uni-directional and 8-ground surfaces. While the random surfaces were produced by polishing the EN8 flats using various abrasive powders. Scanning Electron Microscopy was used to reveal the pin damage and morphology of the transfer layer formed on flats. It is observed that the coefficient of friction, and the transfer layer formation, is controlled by the nature of surfaces and is independent of surface roughness. Moreover, the coefficient of friction, which has two components – the adhesion and plowing, is controlled by the nature of surfaces. The plowing component of friction was highest for the surface that promotes plane strain conditions near the surface and was lowest for the surface that promotes plane stress conditions near the surface.