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<p>Tool protective films operate under extreme service conditions, requiring exceptional hardness. Transition metal diborides (TMB<sub>2</sub>), with strong covalent TM–B and B–B bonds, are promising candidates, but achieving superhardness while preserving their simple binary structure remains challenging. Here, we use HfB<sub>2</sub> as a model system to reveal how boron vacancy filling and in-plane compressive stress synergistically enhance hardness, through combined experimental synthesis and first-principles calculations. (001)-oriented HfB<sub>2</sub> thin films were fabricated, including sub-stoichiometric HfB<sub>2−<i>x</i></sub>, stoichiometric HfB<sub>2</sub>, and stoichiometric HfB<sub>2</sub> under compressive stress. Nanoindentation shows the hardness increases from 33.0 ± 1.1 GPa in HfB<sub>1.90</sub> to 40.5 ± 0.4 GPa in stoichiometric HfB<sub>2</sub>, and further to 45.7 ± 1.1 GPa under −3.67 GPa stress. Calculations reveal that vacancy filling increases the number of load-bearing bonds and strengthens B–B bonding via charge accumulation, while compressive stress shortens B–B bonds to further enhance their strength. These findings clarify the atomic-scale mechanisms of vacancy and stress engineering in TMB<sub>2</sub>, and propose a simple, scalable pathway to superhard protective films without alloying or doping, addressing a long-standing challenge in coatings for extreme environments.</p>

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Atomic-scale mechanisms for superhard HfB2 films via vacancy engineering and compressive stress

  • 期刊论文

  • 发表日期：

    2025年11月24日

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  • Kaiwen Wang,
  • Xinxin Gao,
  • Mao Wen,
  • Weitao Zheng,
  • Kan Zhang

摘要

Tool protective films operate under extreme service conditions, requiring exceptional hardness. Transition metal diborides (TMB2), with strong covalent TM–B and B–B bonds, are promising candidates, but achieving superhardness while preserving their simple binary structure remains challenging. Here, we use HfB2 as a model system to reveal how boron vacancy filling and in-plane compressive stress synergistically enhance hardness, through combined experimental synthesis and first-principles calculations. (001)-oriented HfB2 thin films were fabricated, including sub-stoichiometric HfB2−x, stoichiometric HfB2, and stoichiometric HfB2 under compressive stress. Nanoindentation shows the hardness increases from 33.0 ± 1.1 GPa in HfB1.90 to 40.5 ± 0.4 GPa in stoichiometric HfB2, and further to 45.7 ± 1.1 GPa under −3.67 GPa stress. Calculations reveal that vacancy filling increases the number of load-bearing bonds and strengthens B–B bonding via charge accumulation, while compressive stress shortens B–B bonds to further enhance their strength. These findings clarify the atomic-scale mechanisms of vacancy and stress engineering in TMB2, and propose a simple, scalable pathway to superhard protective films without alloying or doping, addressing a long-standing challenge in coatings for extreme environments.