Co-assisted low-temperature sintering of ultrahard W2B5 composites

Fully dense and fine-grained W2B5 ceramics were successfully fabricated by spark plasma sintering at 1375 degrees C using metallic cobalt as additive. Co reacted with W2B5 to form binary (CoB) and ternary phases (W3CoB3) during sintering process. Experimental results showed that the relative density of W2B5 ceramic increased from 96.7 % (for pure W2B5 ceramic sintered at 1800 degrees C) to 99.0 % with 10 wt% Co addition and 99.5 % with 20 wt% Co addition (sintered at 1375 degrees C), respectively. Due to the lower sintering temperature, the grain size of W2B5 in W2B5-10Co decreased from 5.3 f 0.2 mu m to 3.2 f 0.1 mu m relative to pure W2B5. However, in the case of 20 wt% Co addition, W2B5 grains grew anisotropically, forming elongated strips with a length of 4.2 f 0.3 mu m and a width of 1.1 f 0.2 mu m. The W2B5-10Co composite exhibited superior mechanical properties with the Vickers hardness of 19.7 f 0.3 GPa (30 Kg), fracture toughness of 8.7 f 0.4 MPa & sdot;m1/2 and flexural strength of 783.2 f 22 MPa. Compared to pure W2B5 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.