• • Filling boron vacancies in HfB2−x raises hardness from 33.0 ± 1.1 GPa (HfB1.90) to 40.5 ± 0.4 GPa (stoichiometric HfB2), achieving superhardness (>40 GPa) without alloying or doping, offering a scalable route for protective coatings.
• • Introducing in-plane compressive stress of −3.67 GPa further increases hardness to 45.7 ± 1.1 GPa, demonstrating that stress engineering can push binary TMB2 films beyond the superhard threshold.
• • First-principles calculations reveal that vacancy filling increases the number of load-bearing B–B bonds and enhances charge accumulation on initially weak bonds (e.g., B8–B9), while compressive stress shortens B–B bond lengths, collectively strengthening the covalent network.
• • The study provides a mechanistic understanding of how vacancy and stress engineering affect indentation shear strength under large strain, guiding the design of superhard TMB2 films for extreme environment applications.