Mechanochemical synthesis and annealing of tungsten di‐ and tetra‐boride

Tungsten diboride and tetra‐boride were synthesized by a combination of mechanochemical method and the subsequent heat treatment. To clarify the formation process of tungsten diboride (WB2) and tetra‐boride (WB4) using W‐B mixture system with variable B concentration, the effects of ball‐to‐powder ratio, W to B molar ratio on mechanochemical process were studied, and the relationship between phase composition and annealing temperature was built. The results show that, the synthesis of WB2 with W:B = 1:5 can be improved by increasing ball‐to‐powder ratio from 4:1 to 6:1, but no WB4 phase can be directly synthesized by mechanochemical method. WB2 (AlB2‐WB2, P6/mmm) phase can be only synthesized with W to B molar ratio of 1:4 and 1:5. Further increase in B content to W:B = 1:6 or less, W was present as the main phase with high crystallinity (>71%). WB4 phase was formed as an annealing temperature as low as 1000°C, which was not stable and may decompose to WB2 (WB2‐WB2, P6/mmm) phase with the increase in temperature. The decomposition temperature of WB4 can be improved by increasing the B content. Compared with the W‐B mixtures (W:B = 1:12) being milled for 1 hour, the powders milled for 40 hours present 20% higher of WB4 weight fraction after being annealed at 1400°C for 2 hours.