Transforming\nNanocrystals into Superhard Boron Carbide\nNanostructures

Boron\ncarbide (B<sub>4+δ</sub>C) possesses a large potential\nas a structural material owing to its lightness, refractory character,\nand outstanding mechanical properties. However, its large-scale industrialization\nis set back by its tendency to amorphize when subjected to an external\nstress. In the present work, we design a path toward nanostructured\nboron carbide with greatly enhanced hardness and resistance to amorphization.\nThe reaction pathway consists of triggering an isomorphic transformation\nof covalent nanocrystals of Na<sub>1–<i>x</i></sub>B<sub>5–<i>x</i></sub>C<sub>1+<i>x</i></sub> (<i>x</i> = 0.18) produced in molten salts. The\nresulting 10 nm B<sub>4.1</sub>C nanocrystals exhibit a 4-fold decrease\nof size compared to previous works. Solid-state <sup>11</sup>B and <sup>13</sup>C NMR coupled to density functional theory (DFT) reveal that\nthe boron carbide nanocrystals are made of a complex mixture of atomic\nconfigurations, which are located at the covalent structural chains\nbetween B<sub>11</sub>C icosahedral building units. These nanocrystals\nare combined with a spark plasma-sintering-derived method operated\nat high pressure. This yields full densification while maintaining\nthe particle size. The nanoscaled grains and high density of grain\nboundaries provide the resulting nanostructured bodies with significantly\nenhanced hardness and resistance to amorphization, thus delivering\na superhard material.