Lightweight polycrystalline ceramics possess promising physical, chemical, and mechanical properties, which can be used in a variety of important structural applications. However, these ceramics with coarse-grained structures are brittle and have low fracture toughness due to their rigid covalent bonding (more often consisting of high-angle grain boundaries) that can cause catastrophic failures. Nanocrystalline ceramics with soft interface phases or disordered structures at grain boundaries have been demonstrated to enhance their mechanical properties, such as strength, toughness, and ductility, significantly. In this review, the underlying deformation mechanisms that are contributing to the enhanced mechanical properties of superhard nanocrystalline ceramics, particularly in boron carbide and silicon carbide, are elucidated using state-of-the-art transmission electron microscopy and first-principles simulations. The observations on these superhard ceramics revealed that grain boundary sliding induced amorphization can effectively accommodate local deformation, leading to an outstanding combination of mechanical properties.

Research on rare-earth hexaborides mainly focuses on tuning their electronic structure from insulating-to-metallic states during high pressure experiments. However, the structural evolution that contributes to their mechanical failure is not well understood. Here, we examine the pressure-induced structural evolution of a model rare-earth hexaboride, EuB₆, during nanoindentation. Transmission electron microscopy reveals that nanoscale amorphous shear bands, mediated by dislocations, play a decisive role in deformation failure. Density functional theory calculations confirm that amorphous bands evolve by breaking boron-boron bonds within B₆ octahedra during shear deformation. Our results underscore an important damage mechanism in hard and fragile hexaborides at high shear pressures.