Role of nanotwin-confined threading dislocations in the ultra-high hardness of B₁₀C ceramics

A highly nano-twinned boron-rich boron carbide, ≈ B 10 C, has been prepared efficiently through high energy ball milling (HEBM) of elemental boron and carbon powders followed by spark plasma sintering (SPS). Prolonged mechanical activation by HEBM significantly enhances reactive SPS sinterability and leads to a submicrometric microstructure with an average grain size of 0.47 µm. A high density of nano-twins is observed in most grains, with an average twin spacing of λ = 5.1 nm. The mechanical response (H v ~34 GPa) is strongly correlated with the extensive presence of nano twining and the associated confinement of dislocations. The observed ultra-high hardness can be rationalized using the classical Foreman model, originally developed to describe threading dislocations in stressed, capped epitaxial semiconductor layers.