In situ synthesis, mechanical properties, and microstructure of reactively hot pressed WB4 ceramic with Ni as a sintering additive

In this study, tungsten tetraboride (WB4) ceramics were synthesized in situ from powder mixtures of W and amorphous B with Ni as a sintering aid by reactive hot pressing method. The as-synthesized ceramics exhibited porosity as low as 0.375% and ultra-high Vickers hardness (Hv), as much as 49.808 ± 1.683 GPa (for the low load of 0.49 N). It was seen that the addition of Ni greatly improved the sinterability of WB4 ceramic. Besides, the flexural strength and fracture toughness of WB4 ceramic were measured for the first time to be 332.857 ± 36.763 MPa and 4.136 ± 0.259 MPa m1/2, respectively, suggesting that the ceramic has good mechanical properties. The effects of sintering temperature and holding time on the densification, Vickers hardness, and mechanical properties of WB4 ceramics were also investigated systematically as part of our study. The results indicated that increasing the sintering temperature can obviously improve the densification and mechanical properties of the ceramics. The bulk density and Vickers hardness of WB4 ceramic sintered at 1650 °C for 60 min under 30 MPa revealed the highest values of 6.366 g cm−3 and 27.948 ± 0.686 GPa (for the high load of 9.8 N), respectively. The flexural strength increased to the highest value of 332.857 ± 36.763 MPa for sintering temperature up to 1550 °C, but decreased slightly as the sintering temperature further increased to 1650 °C. On the other hand, the fracture toughness increased gradually with increasing temperature. It was also found that Vickers hardness showed a similar trend as the densification of the samples with increasing temperature and holding time. Besides, no obvious improvements in the densification, mechanical properties, and Vickers hardness of the samples with sintering time were observed in this study. The microstructure and fracture behaviours of the as-synthesized WB4 ceramic were also revealed, and the toughening mechanism has been discussed.