  A recent experimental study reported the successful synthesis of an
orthorhombic FeB4 with a high hardness of 62 GPa, which has reignited extensive
interests on whether transition metal borides (TRBs) compounds will become
superhard materials. However, it is contradicted with some theoretical studies
suggesting transition metal boron compounds are unlikely to become superhard
materials. Here, we examined structural and electronic properties of FeB4 using
density functional theory. The electronic calculations show the good
metallicity and covalent FeB bonding. Meanwhile, we extensively investigated
stress strain relations of FeB4 under various tensile and shear loading
directions. The calculated weakest tensile and shear stresses are 40 GPa and 25
GPa, respectively. Further simulations (e.g. electron localized function and
bond length along the weakest loading direction) on FeB4 show the weak Fe-B
bonding is responsible for this low hardness. Moreover, these results are
consistent with the value of Vickers hardness (11.7 to 32.3 GPa) by employing
different empirical hardness models and below the superhardness threshold of 40
GPa. Our current results suggest FeB4 is a hard material and unlikely to become
superhard.
