Syntheses and sinterings of chromium borides via solid-state reactions andmechanochemical processes of chromium-amorphous boron mixed powder

As a basic study for the development of chromium boride materials which are expected as refractory materials or hard materials, syntheses and sinterin gs of the chromium borides via solid state reactions and mechanochemical pr ocesses of chromium-amorphous boron mixed powders were studied. The single phases of Cr2B, CrB and CrB2 were obtained by the solid state reaction in t he temperatures of 1400-1500 degrees 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 wa s CrB2. By mechanochemical processes, the single phases of CrB2 and CrB wer e obtained by milling of Cr-B (molar ratio, 1 : 2A) and Cr-B(1 : 1) mixed p owders by using a planetary ball mill for 20-40 h, followed by annealing at 900 and 1000 degrees C, respectively. As-milled Cr-B(1 : 1) mixed powders were consolidated by both of pressureless and pressure sinterings. These ar e a reaction sintering because the sinterings are accelerated by the exothe rmic reaction heat of the unreacted Cr-B in the as-milled mixed powders. Th e unsinterable powder could be consolidated by pressureless sintering via m echanochemical process. At pressure sintering ria mechanochemical process, the sintering temperature lowered and the hardness of ceramics obtained inc reased.