Mikio Obayashi

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.

Synthesis of TiB2 powder from a mixture of TiN and amorphous boron

TiB2 powder was synthesized by solid state reaction using amorphous boron and TiN as a source of titanium. The TiB2 formation did not occur at all in a nitrogen atmosphere even at 1400° C. TiB2 formed above 1100° C in argon and hydrogen atmospheres. The only crystalline phase of TiB2 powder was favourably synthesized at 1400° C for 360 min in an argon atmosphere from a starting powder with a composition containing excess boron (B/Ti = 2.2). The synthesized powder was well dispersed and had a particle size of 0.5 to 2 µm. The powder activity was evaluated by sintering at 4 G Pa and 1300 to 1600° C for 15 min.

Effect of atmosphere of heat treatment on synthesis of tungsten borides.

The effect of atmosphere of heat treatment on the formation of tungsten borides was investigated by the solid state reaction between tungsten and amorphous boron, in which the formation ranges and synthetic conditions of WB and W2B5 solid solutions were determined.The low temperature activation treatment at 500°C in a hydrogen stream promoted the boride formation in the subsequent heat treatment. Since a coexisting phase tended to from in the case of WB synthesis, the pretreatment at about 1000°C in an argon stream prior to the final heat treatment, was required to obtain a homogeneous composition of specimen.WB solid solution had a narrow formation range of B/W=1.0-1.2, while W2B5 solid solution had a wide formation range of B/W=1.75-2.75. The formation amount of WB or W2B5 solid solution showed the maximum at the composition of mixed powder which contains 10at% boron in excess of the stoichiometric composition of each boride. The synthesized tungsten boride powder with uniform grain size of 0.5-2μm was expected to have a good sinterability.

Synthesis and reaction sintering of WB

The conditions for preparing WB by solid state reaction between tungsten and amorphous boron powders were investigated. The effect of added titanium or zirconium on the sinterability of the synthesized WB powder was examined under high pressure and temperature conditions. WB was obtained as a single crystalline phase, when a mixed powder containing boron 10 at% in excess of the stoichiometric composition WB, was pretreated at 500°C for 60min in a stream of hydrogen and subsequently treated at 1400°C for 60min in a stream of argon. The crystallinity and the amount of WB increased with increasing both heat treatment time and temperature. The atomic ratio B/W remained constant after the heat treatment below 1500°C. Sintering of the synthesized powder at 4GPa and 1500°C for 15min resulted in the density of 13.3g/cm3 and microhardness of 1400kg/mm2. For 5 at% titanium addition, higher density (14.0g/cm3) and improved microhardness (2800kg/mm2) were attained by sintering at a lower temperature of 1300°C at 4GPa. The densif ication of reaction-bonded WB is promoted possibly by the formation reaction of TiB2 from free amorphous boron and added titanium. Similar behavior was observed with the zirconium addition also.