Improvement of Mechanical Properties of Hard Metals Using Solid Solution Strengthened Metallic Binder (Co_1-xW_x)

The metallic binder in WC-Co hard metals was effectively strengthened using the solid solution phases of Co and W. These metallic phases of Co and W (Co_1-xW_x, x<1), which consist of two kinds of structures (FCC and HCP), were successfully formed by hydrogen reduction of milled oxides mixtures (Co₃O₄ and WO₃) at over 1000 ^oC. When hard metals are fabricated by pressureless sintering of mixed WC and Co_1-xW_x, the hard metals containing the WC₂ and M₆C phases (Co₂W₄C and Co₄W₂C) have brittleness, which degrades their mechanical properties, like hard metals fabricated from mixtures of WC, Co, and W. By rapidly sintering the WC-Co_1-xW_x hard metals for 5 min the W₂C and M₆C phases were eliminated, and a two-phase (WC and the metallic phase of Co and W) region was successfully obtained. The mechanical properties of the WC-Co_1-xW_x hard metals showed higher values for both hardness (max. 18.8 GPa) and fracture toughness (8.5 MPa·m^1/2) than conventional WC-Co hard metal (H_V: 15.9 GPa, K_IC: 6.9 MPa·m^1/2). The enhancement in toughness was attributed to the solid solution strengthening of the metallic binder and the elimination of the WC₂ and M₆C phases. The suppression of grain growth due to the short duration of sintering also played a positive role in improving the hardness of the WC-Co_1-xW_x hard metals. The phase-controlled solid solution metallic binder could be the key material to enhance the hardness and toughness of hard metals.