Effect of tantalum on phase transition and thermal stability of metastable tungsten tetra-boride

Decomposition of WB4 into WB2 at temperature higher than 1200 degrees C makes the preparation of pure bulk WB4 ceramic difficult. Doping of the third element was reported to be an effective way for thermal stability improvement of ceramic materials. In the present work, the nearest element of W, tantalum (Ta), was introduced into tungsten boride synthesized by a combination of high energy ball milling and thermal treatment with (W:Ta):B = (2/3:1/3):4.5 & 5. The phase composition, chemical state of each element, and thermal stability of the as-synthesized samples were investigated. Results show that, with W:Ta molar ratio of 2/3:1/3, ei, 2:1, when the (W + Ta):B = 1: 4.5, about 49.5% mass fraction of WB4 phase was maintained successfully after being thermal treated at 1500 degrees C for 1 h, compared with pure WB(2)2 phase obatined by the Ta-free sample. Increasing B content in raw materials to (W + Ta):B = 1:5, the mass fraction of WB4 phase was increased to 97%, while the only phase can be detected in the Ta-free samples is also WB2. The significantly improved thermal stability of WB4 can be mainly attributed to the completely dissolve of Ta into the W-B system. It was also found that the oxidation resistance of the W-B system was increased with the introduction of Ta.