A longstanding controversy remains whether $\gamma \text{−}{\mathrm{B}}_{28}$ is intrinsically superhard or not, i.e., $H_v\ge 40\mathrm{GPa}$. Here we perform comprehensive investigations on the mechanical properties of $\gamma \text{−}{\mathrm{B}}_{28}$ to reveal the plasticity and failure mode of $\gamma \text{−}{\mathrm{B}}_{28}$ through the unique combination of microindentation experiment, the ideal strength approach, and the ab initio informed Peierls-Nabarro model. A low load-invariant hardness of ∼30 GPa is found for both polycrystalline and monocrystalline $\gamma \text{−}{\mathrm{B}}_{28}$. By carefully checking the strength anisotropy and strain facilitated phonon instability, a surprising ideal strength of 23.1 GPa is revealed along the (001)[010] slip system for $\gamma \text{−}{\mathrm{B}}_{28}$, together with an inferior Peierls stress of 3.2 GPa, both of which are close to those of ${\mathrm{B}}_6\mathrm{O}$ and $\mathrm{Zr}{\mathrm{B}}_{12}$ yet much lower than those of diamond and c-BN. These results suggest that $\gamma \text{−}{\mathrm{B}}_{28}$ could not be intrinsically superhard. Atomistic simulation and electronic structure analysis uncover an unprecedented plastic flow channel through the specific ultrasoft bonding, which causes a dramatic softening of $\gamma \text{−}{\mathrm{B}}_{28}$. These findings highlight an approach to quantifying the realistic hardness by means of two plasticity descriptors beyond the elastic limit, i.e., the ideal strength approach and the Peierls-Nabarro model.

• Received 19 March 2018
• Revised 9 August 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.123602