Enhanced Hardness in Transition-Metal Monocarbides via Optimal Occupancy of Bonding Orbitals

Abstract

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.

Vacancy Hardening and Ordering in Rhenium Tungsten Carbides

“…Figure 3a−d displays the band structure, total density of states (DOS), C-projected DOS, and TM-projected DOS of Re 0.5 W 0.5 C, respectively. Except for the shift of the Fermi level, its electronic structures share main features with those of other TMCs reported previously, 7,25,26 and thus, a rigid-band model can be used to explain the overall trend. In Re 0.5 W 0.5 C, each TM atom is octahedrally coordinated by six C atoms, and this crystal field leads to a splitting of the TM 5d levels into the t 2g (derived from the d xy , d yz , and d zx orbitals) and e g states (derived from the d z 2 and d x 2 −y 2 orbitals).…”

“…), but the highest stability occurs at a VEC of about 8 e/f.u. 7 Similar behaviors have been reported in other systems. For example, transition-metal carbonitrides have the largest elastic constant C 44 at a VEC of 8.4 e/f.u., 23,24 and the AlB 2 -type diborides exhibit the lowest formation energy at a VEC of 10 e/f.u.…”

“…4 Among TMCs, cubic rhenium tungsten carbide ReWC 2 (i.e., Re 0.5 W 0.5 C), originally synthesized in 1963, 5 has attracted renewed attention not only because of its superconductivity 6 but also because of its intriguing mechanical behaviors. 7 Recently, Mansouri Tehrani et al 8 have developed a machinelearning method to predict elastic moduli for ∼120,000 solids and identified Re 0.5 W 0.5 C as a potential superhard material. To validate the theoretical prediction, they attempted to synthesize this material but obtained substoichiometric ReWC 0.8 (i.e., Re 0.5 W 0.5 C 0.4 ); subsequent Vickers hardness measurements revealed that it has an extremely high hardness exceeding the superhard threshold of 40 GPa at low load.…”

“…Transition-metal carbides (TMCs) have been the focus of intense research that has led to the discovery of many novel materials with appealing properties, including strain-stiffening solids (e.g., Fe 3 C), topological metals that host a new type of triple point fermions (e.g., WC and MoC), , and high-entropy high-hardness materials (e.g., HfNbTaTiZrC 5 ) . Among TMCs, cubic rhenium tungsten carbide ReWC 2 (i.e., Re 0.5 W 0.5 C), originally synthesized in 1963, has attracted renewed attention not only because of its superconductivity but also because of its intriguing mechanical behaviors . Recently, Mansouri Tehrani et al have developed a machine-learning method to predict elastic moduli for ∼120,000 solids and identified Re 0.5 W 0.5 C as a potential superhard material.…”

Synthesis, crystal structure, and properties of stoichiometric hard tungsten tetraboride, WB4

“…Inexpensive hard and superhard (with a Vickers hardness of around 40 GPa and above) materials are in much demand for various industrial applications. Novel carbides, 1 nitrides, 2–8 borides, 9–23 and other compounds of transition metals, 24,25 which can potentially combine high hardness and promising optical, electronic, and other properties, are the focus of extensive investigations. 26–28 Nowadays, one of the most widely used hard materials for commercial applications is tungsten carbide, WC.…”