Formation process of tungsten borides by solid state reaction between tungsten and amorphous boron

Various kinds of tungsten borides were synthesized by solid state reaction between tungsten and amorphous boron powders. The mixed powders with various compositions (B/W = 0.4 to 13.0) were treated at 800 to 1550° C for 0 to 120 min in a stream of argon. Four kinds of boride phases such as W₂B, WB, W₂B₅ and WB₄ were formed, although the boride phase having the composition of the highest boride, WB₁₂, did not appear. The formation of W₂B was initiated approximately at 1000° C in excess of tungsten content. On the other hand, in the excess boron content, the formation of WB, W₂B₅ and WB₄ was initiated approximately at 800, 950 and 1200° C, respectively. The maximum formation amount and crystallinity of WB and W₂B₅ was found in nearly 10 at % excess boron content in their own stoichiometric compositions. The only crystalline phase of WB₄ was prepared with a large excess boron content. However, the formation behaviour of WB₄ showed that WB₄ is metastable above 1400° C. The stability of WB₄ phase could be increased by the presence of excess boron.