Oxidation-induced strength degradation of WC–Co hardmetals

Abstract The oxidation behavior at 700°C and its influence on room-temperature mechanical strength of three grades of WC–Co cemented carbides containing different binder content were studied. Oxidation kinetics, determined by thermogravimetric measurement, were found to follow a linear time dependence of weight gain per unit area in all the cases. It was also observed to rise with an increase in the oxygen concentration of the atmosphere and a reduction of the cobalt content. Three oxidation times of 10 min, 1 and 6 h were chosen to evaluate changes in the mechanical strength. Fracture resistance of oxidized samples were measured at room temperature under four-point bending and compared to that of non-oxidized specimens. In all cases, a pronounced degradation in strength was observed after oxidation. The mechanical characterization studies were complemented by detailed fractographic examination and the results were analyzed by linear-elastic fracture mechanics (LEFM). It is conjectured that strength reduction results from a combination of load-bearing section reduction (extrinsic effect) and a concomitant interplay of several factors including increasing size and severity of critical flaws, wedge-effects in the case of heterogeneous oxide penetration and residual tensile stresses associated with volume and thermal differences between the substrate and the oxide scale (intrinsic effects).