Influence of Mo contents on microstructures and mechanical properties of (Ti,W,Mo)(CN)–Ni cermets

The introduction of N₂ gas during a sintering and carbothermal reduction process causes the separation of the WC phase in (Ti,W)(CN)–Ni cermet. Furthermore, the addition of secondary transition-metal carbides such as Mo₂C, VC, and TaC not only promotes phase separation but also controls grain growth by the differences in their thermodynamic stabilities. Increased N₂ flow during sintering increases the precipitation and coalescence of WC particles. The addition of Mo₂C of 0.05 mol fraction suppresses the precipitation and coalescence of WC. However, increases in both secondary carbides by > 0.05 mol fraction and the N₂ flow by > 4 kPa (≒ 30 Torr), respectively, induces significant grain growth by coalescence. Consequently, the pore levels, hardness, and fracture toughness of the specimens are substantially affected by changes in the precipitation and grain growth behaviors of the WC particles. The fracture toughness of a (Ti_0.7-xW_0.3Mo_x)(C_0.7N_0.3)–20Ni cermet, sintered under 1.33 kPa (≒ 10 Torr) N₂, is significantly enhanced from 9 to 14 MPa m^0.5 by crack bridging and deflection.