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High hardness and toughness are generally considered mutually exclusiveproperties for single-crystal ceramics. Combining experiments and ab initiomolecular dynamics (AIMD) atomistic simulations at room temperature, wedemonstrate that both the hardness and toughness of single-crystalNaCl-structure VNx/MgO(001) thin films are simultaneously enhanced through theincorporation of anion vacancies. Nanoindentation results show that VN0.8, hereconsidered as representative understoichiometric VNx system, is ~20% harder, aswell as more resistant to fracture than stoichiometric VN samples. AIMDmodeling of VN and VN0.8 supercells subjected to [001] and [110] elongationreveal that the tensile strengths of the two materials are similar.Nevertheless, while the stoichiometric VN phase systematically cleaves in abrittle manner at tensile yield points, the understoichiometric compoundactivates transformation-toughening mechanisms that dissipate accumulatedstresses. AIMD simulations also show that VN0.8 exhibits an initially greaterresistance to both {110}<1-10> and {111}<1-10> shear deformation than VN.However, for progressively increasing shear strains, the VN0.8 mechanicalbehavior gradually evolves from harder to more ductile than VN. The transitionis mediated by anion vacancies, which facilitate {110}<1-10> and {111}<1-10>lattice slip by reducing activation shear stresses by as much as 35%.Electronic-structure analyses show that the two-regime hard/tough mechanicalresponse of VN0.8 primarily stems from its intrinsic ability to transfer delectrons between 2nd-neighbor and 4th-neighbor (i.e., across vacancy sites)V-V metallic states. Our work offers a route for electronic-structure design ofhard materials in which a plastic mechanical response is triggered withloading.

Adaptive hard and tough mechanical response in single-crystal B1 VNx ceramics via control of anion vacancies