On Chemical Wear of Tools : Thermodynamics at the Interface Level

Over the past century, the improvement of new wear resistant tool materials has been astonishing. Nowadays, tool materials for machining and forming processes, are so strong that wear mechanisms based on plastic deformation are no longer the main concern. In many cases, the main challenge is now shifted to chemical tool wear, driven by diffusive and oxidative mechanisms. Reducing chemical wear cannot be based on choosing harder tool materials, but requires an understanding of the chemical mechanisms at the sliding interface and the factors that influence them. The focus of this thesis has been to investigate chemical mechanisms important for tool wear and to create an understanding of the governing physical parameters. This has been achieved through a combinatory approach, involving simplified tribological tests, mimicking the wear in actual applications, coupled with thermodynamic modelling. The purpose of the tests has been to investigate how physical parameters affect the chemical mechanisms, while modelling has been used either to investigate the mechanisms at a deeper level, or to study whether the mechanisms could be predicted using thermodynamics. The thesis has involved two application areas. The first relates to the chemical wear of cemented carbide (WC-Co) tools for shearing Cu-Zn alloys, as used in the zipper manufacturing industry. The second study focuses on the oxide layers that form by oxidation of the work material steel that transfers onto the surfaces of turning and milling tools. It can be shown that the wear rate of WC-Co against Cu-Zn alloys is reduced by increasing the Zn content in the alloy or by reducing the oxygen availability. Thermodynamic modelling showed that the oxygen available in the tool/work interface that can oxidise the WC-Co is reduced by preferential oxidation with Zn, which in effect protects the tool from oxidative wear. Hence, measures reducing the oxygen availability are key to reducing wear in this application. Furthermore, for the studied metal cutting applications it has been experimentally verified that the proposed modelling is able predict the composition of complex oxide layers that form when the work material, the steel, transfers to and oxidises on tool surfaces – based exclusively on the steel composition. Tribological testing and thermodynamic modelling have therefore proven to be a powerful combination for studying and understanding chemical wear of tools.