Gradient WC-Co hardmetals: Theory and practice

Abstract

The production of functionally graded or functionally gradient WC-Co hardmetals having a near-surface layer with a low Co content and high hardness, and not comprising the η-phase has been the objective of intensive research in the last several years. The present work is devoted to the fabrication and examination of such gradient hardmetals and consists of two parts. In the first part, the kinetics of Co drift between couples of model alloys with very similar WC mean grain sizes but different carbon contents was examined. These model alloys include (1) WC+Co+free carbon, (2) WC+Co+very little η-phase and (3) WC+Co with very low carbon content but free of η-phase. The Co drift in the model alloys consisting of the alloy 1 on the one hand and the alloys 2 and 3 on the other hand was found to be similar. Therefore, a significant Co gradient was expected can be obtained in functionally gradient hardmetals not comprising the η-phase. In the second part, the microstructure, hardness, Co contents, residual stresses and wear-resistance of functionally gradient hardmetals obtained by the selective carburisation of the near-surface layer before liquid phase sintering were examined. A significant Co gradient of up to 7wt.% and a noticeable hardness gradient of up 250 Vickers units was found to be achieved in such hardmetals. Their near-surface layer is characterised by high residual compression stresses in both the carbide phase and binder phase, which allows an exceptionally high combination of hardness and fracture toughness to be obtained. Wear-resistance of such gradient hardmetals in percussive drilling of quartzite and cutting of abrasive concrete was found to be higher compared to corresponding conventional hardmetals with uniform Co distribution by roughly 2 to 4 times.