  The search of new superhard materials has received a strong impulse by
industrial demands for low-cost alternatives to diamond and $c$-BN, such as
metal borides. In this Letter we introduce a new family of superhard materials,
"fused borophenes", containing 2D boron layers which are interlinked to form a
3D network. These materials, identified through a high-throughput scan of
BxC1-x structures, exhibit Vicker's hardnesses comparable to those of the best
commercial metal borides. Due to their low formation enthalpies, fused
borophenes could be synthesized by high-temperature methods, starting from
appropriate precursors, or through quenching of high-pressure phases.
