Boron carbide samples exhibiting nanometric and submicronic microstructure were sintered by Spark Plasma Sintering to investigate the effect of grain size on mechanical properties. The mechanical properties of sintered monoliths were characterized at the grain and macroscopic scales. Although nanostructured material exhibits finer grains than the submicronic material (i.e. mean diameter of 82 vs. 474 nm), its apparent rigidity and hardness are found to be reduced by 6.8 % and 8.4 % respectively. This contradiction with the Hall-Petch law is linked to the chemical compositions of both materials, which show significant difference in terms of B/C ratio and higher structural oxygen content especially for nanostructured material.

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