Enhanced Hardening Effects on Molybdenum-Doped WB2 and WB2–SiC/B4C Composites

Tungsten diboride (WB2) solid solutions with increasing molybdenum (Mo) substitution were synthesized by resistive arc-melting from the pure elements and characterized for their mechanical properties. The WB2-type structure is maintained up to 30 atomic percent (at%) Mo substitution. W0.70Mo0.30B2 achieved a maximum Vickers hardness of 45.7 ± 2.5 GPa at 0.49 N, resulting in the hardest WB2 solid solution to date. In agreement with this fact, high-pressure radial diffraction studies indicate that substitution of Mo into WB2 strengthens metal–boron bonding, as the solid solution supports high differential stress and has a bulk modulus of 355 ± 2 GPa. WB2 and W0.70Mo0.30B2 composites were then synthesized with increasing additive content (0–30 wt%) of B4C or SiC to study extrinsic hardening effects through multiphase formation. These composites show extrinsic effects on the Vickers hardness because of secondary-phase precipitation. While WB2–30 wt% B4C exhibited the highest hardness (53.8 ± 6.0 GPa at 0.49 N), WB2–30 wt% SiC demonstrated the slowest oxidation rate. This work offers new insights for tailoring transition-metal boride systems with optimized hardness, grain morphology, and thermal stability.