Transforming Nanocrystals into Superhard Boron Carbide Nanostructures.

Boron carbide (B _4+δ C) possesses a large potential as a structural material owing to its lightness, refractory character, and outstanding mechanical properties. However, its large-scale industrialization is set back by its tendency to amorphize when subjected to an external stress. In the present work, we design a path toward nanostructured boron carbide with greatly enhanced hardness and resistance to amorphization. The reaction pathway consists of triggering an isomorphic transformation of covalent nanocrystals of Na _{1- x} B _{5- x} C _{1+ x} ( x = 0.18) produced in molten salts. The resulting 10 nm B _4.1 C nanocrystals exhibit a 4-fold decrease of size compared to previous works. Solid-state ¹¹ B and ¹³ C NMR coupled to density functional theory (DFT) reveal that the boron carbide nanocrystals are made of a complex mixture of atomic configurations, which are located at the covalent structural chains between B ₁₁ C icosahedral building units. These nanocrystals are combined with a spark plasma-sintering-derived method operated at high pressure. This yields full densification while maintaining the particle size. The nanoscaled grains and high density of grain boundaries provide the resulting nanostructured bodies with significantly enhanced hardness and resistance to amorphization, thus delivering a superhard material.