The recent observation of the reverse Hall-Petch relation in nanocrystalline ceramics offers a possible pathway to achieve enhanced ductility for traditional brittle ceramics via the nanosize effect, just as nanocrystalline metals and alloys. However, the underlying deformation mechanisms of nanocrystalline ceramics have not been well established. Here we combine reactive molecular dynamics (RMD) simulations and experimental transmission electron microscopy to determine the atomic level deformation mechanisms of nanocrystalline boron carbide (${\mathrm{B}}_4\mathrm{C}$). We performed large-scale (up to $\sim 3700000\mathrm{atoms}$) ReaxFF RMD simulations on finite shear deformation of three models of grain boundaries with grain sizes from 4.84 (135 050 atoms) to 14.64 nm (3 702 861 atoms). We found a reverse Hall-Petch relationship in nanocrystalline ${\mathrm{B}}_4\mathrm{C}$ in which the deformation mechanism is dominated by the grain boundary (GB) sliding. This GB sliding leads to the amorphous band formation at predistorted icosahedral GB regions with initiation of cavitation within the amorphous bands. Our simulation results are validated by the experimental observations of an intergranular amorphous GB phase due to GBs sliding under indentation experiments. These theoretical and experimental results provide an atomistic explanation for the influence of GBs on the deformation behavior of nanocrystalline ceramics, explaining the reverse Hall-Petch relation.

A previously unidentified amorphization mechanism of boron carbide yields insights into the failure of superhard materials.