Structure and Properties of WC–Co Composites with Different CrB2 Concentrations, Sintered by Vacuum Hot Pressing, for Drill Bits

We prepared samples of composites based on tungsten carbide and cobalt with different concentrations (0–10 wt %) of chromium diboride, 10 mm in diameter and 8 mm in thickness, by cold pressing followed by vacuum hot pressing. A comprehensive study of composites using conventional methods of testing mechanical properties, combined with digital optical microscopy, transmission microscopy, and scanning atomic force microscopy, revealed stable correlations between the concentration of CrB2 additive with the average WC grain size and microstructure parameters, hardness, and fracture toughness of the composites. A coarse-grained structure is observed in WC–6Co (wt %) composites, with direct contact of WC grains and large regions of a cobalt binder. The introduction of CrB2 into the composite, on the contrary, ensures the formation of thin (~100 nm) and extended layers of a cobalt binder, even between fine WC grains. The CrB2 additive yields a finer-grained structure, the parameters of which can be purposefully controlled by changing the additive concentration. The addition of 4 wt % of CrB2 into the composite leads to a more than twofold increase in fracture toughness, that is, from 4.4 to 9.8 MPa m1/2, with a slight decrease in hardness from 15.1 to 13.0 GPa. With a further increase in the CrB2 concentration from 4 to 10 wt %, fracture toughness and hardness gradually decrease.