Chemical wear of cemented carbide tools in contact with Cu-Zn alloy – investigated using thermodynamic calculations

Cemented carbide (WC-Co) tools are worn at a very slow rate when shearing Cu-Zn alloy wire into zipper elements. Despite this, the wear is the limiting factor in the operation. Analysis of worn WC-Co from both the zipper production and tribological tests, mimicking the wear caused by the shearing action, indicate that WC grains and Co binder are worn through oxidative mechanisms. The Zn in the alloy is believed to preferentially react with oxygen in the contact. The wear mechanisms are still not fully understood since the wear occurs at a very fine scale, and consequently leaves diminutive damage. In the present study, CALPHAD based thermodynamic calculations, proven successful for studying chemical wear of tool materials in other machining operations, are performed to further investigate the chemical wear of WC-Co against Cu-Zn alloy. Equilibrium conditions are set to represent the tribological contact. This enables the stable phases and their compositions, as well as their driving forces in the contact, to be calculated and coupled, to study possible chemical wear mechanisms. The calculations indicate that wear of WC-Co is governed by oxidative mechanisms. Zn in the alloy can preferentially react with oxygen and this reduces its availability, so that both WC and Co become thermodynamically stable and thus not oxidise. This calculated effect of Zn becomes stronger with increasing Zn content and is therefore in line with the results from experimental work. This shows that CALPHAD is a suitable approach for investigating fine scale chemical wear of cemented carbides. • The interface between WC-Co and Cu-Zn alloy is modelled using equilibrium condition • Stable phases and driving forces for their formation are calculated to study trends • Calculated oxidation tendencies indicate that WC-Co is worn by oxidative mechanisms • Diffusion calculations show the opposite trend in wear, and is hence less important • Zn reduces oxidative wear of WC-Co, as observed in practice, by reacting with oxygen