Fabricating exotic boron carbide ceramics by shaker milling and reactive spark plasma sintering of B and C: Application to the elusive B6C

A route based on the shaker milling of B and C powders followed by reactive spark plasma sintering (SPS) of the resulting B+C powder mixture is presented for the fabrication of exotic boron carbide monolithic ceramics with controlled stoichiometry, and applied to the elusive B6C ceramics. Firstly, it is shown that shaker milling for only 1 h refines down to the nanoscale, mechanically activates, and intimately mixes the B and C powders, making the resulting B+C powder mixture far more sinterable. Secondly, it is shown that B and C react during the heating ramp of the SPS cycles, resulting in a fully dense superhard (-30 GPa) boron carbide monolith with the desired stoichiometry (B6C in this case) and fine twinned-grain microstructure at an SPS temperature (1550 degrees C in this case) at which typical commercial B4C powders barely densify. This low-temperature densification is possible mainly due to the enhanced solid-state sinterability promoted by both shaker milling and structural ordering, with the exothermic heat released during the formation of the boron carbide contributing little. And thirdly, it is shown that there is an optimal SPS temperature above which the hardness of these exotic boron carbide monoliths decreases because there is less twinning hardening.