Syntheses and Sinterings of Chromium Borides via Solid-state Reactions and Mechanochemical Processes of Chromium-Amorphous Boron Mixed Powder

Kiyokata IIZUMI* and Katsuya KUDAKA

Tokyo Institute of Polytechnics, Faculty of Engineering, Department of Applied Chemistry, 1583, Iiyama, Atsugi-shi 243-0297 Japan

As a basic study for the development of chromium boride materials which are expected as refractory materials or hard materials, syntheses and sinterings of the chromium borides via solid state reactions and mechanochemical processes of chromium-amorphous boron mixed powders were studied. The single phases of Cr2B, CrB and CrB2 were obtained by the solid state reaction in the temperatures of 1400--1500 °C. These borides were so unsinterable that could be consolidated by pressure sintering only. The chromium boride of the highest relative density and hardness among the ceramics obtained was CrB2. By mechanochemical processes, the single phases of CrB2 and CrB were obtained by milling of Cr-B(molar ratio, 1:2) and Cr-B(1:1) mixed powders by using a planetary ball mill for 20--40 h, followed by annealing at 900 and 1000 °C, respectively. As-milled Cr-B(1:1) mixed powders were consolidated by both of pressureless and pressure sinterings. These are a reaction sintering because the sinterings are accelerated by the exothermic reaction heat of the unreacted Cr-B in the as-milled mixed powders. The unsinterable powder could be consolidated by pressureless sintering via mechanochemical process. At pressure sintering via mechanochemical process, the sintering temperature lowered and the hardness of ceramics obtained increased.

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