Co-assisted low-temperature sintering of ultrahard W2B5 composites

Fully dense and fine-grained W 2 B 5 ceramics were successfully fabricated by spark plasma sintering at 1375 °C using metallic cobalt as additive. Co reacted with W 2 B 5 to form binary (CoB) and ternary phases (W 3 CoB 3 ) during sintering process. Experimental results showed that the relative density of W 2 B 5 ceramic increased from 96.7 % (for pure W 2 B 5 ceramic sintered at 1800 °C) to 99.0 % with 10 wt% Co addition and 99.5 % with 20 wt% Co addition (sintered at 1375 °C), respectively. Due to the lower sintering temperature, the grain size of W 2 B 5 in W 2 B 5 -10Co decreased from 5.3 ± 0.2 μm to 3.2 ± 0.1 μm relative to pure W 2 B 5 . However, in the case of 20 wt% Co addition, W 2 B 5 grains grew anisotropically, forming elongated strips with a length of 4.2 ± 0.3 μm and a width of 1.1 ± 0.2 μm. The W 2 B 5 -10Co composite exhibited superior mechanical properties with the Vickers hardness of 19.7 ± 0.3 GPa (30 Kg), fracture toughness of 8.7 ± 0.4 MPa·m 1/2 and flexural strength of 783.2 ± 22 MPa. Compared to pure W 2 B 5 ceramics, the hardness remained almost unchanged, while the fracture toughness and flexural strength were significantly improved by 61.8 % and 49.8 %, respectively. These improvements benefitted from the combination of high densification degree, fine-grained strengthening and in-situ CoB phase formation improving interfacial adhesion via semi-coherent interfaces. However, excessive addition of Co (20 wt%) induced stress concentration via brittle CoB coalescence, outweighing the densification benefit and drastically reducing hardness and flexural strength.