An
efficient strategy that can guide the synthesis of materials
with superior mechanical properties is important for advanced material/device
design. Here, we report a feasible way to enhance hardness in transition-metal
monocarbides (TMCs) by optimally filling the bonding orbitals of valence
electrons. We demonstrate that the intrinsic hardness of the NaCl-
and WC-type TMCs maximizes at valence electron concentrations of about
9 and 10.25 electrons per cell, respectively; any deviation from such
optimal values will reduce the hardness. Using the spark plasma sintering
technique, a number of W1–x
Re
x
C (x = 0–0.5) have
been successfully synthesized, and powder X-ray diffractions show
that they adopt the hexagonal WC-type structure. Subsequent nanoindentation
and Vickers hardness measurements corroborate that the newly developed
W1–x
Re
x
C samples (x = 0.1–0.3) are much harder than
their parent phase (i.e., WC), marking them as the hardest TMCs for
practical applications. Furthermore, the hardness enhancement can
be well rationalized by the balanced occupancy of bonding and antibonding
states. Our findings not only elucidate the unique hardening mechanism
in a large class of TMCs but also offer a guide for the design of
other hard and superhard compounds such as borides and nitrides.
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