  Revealing the hardening and strengthening mechanisms is crucial for
facilitating the design of superhard and high-strength high-entropy ceramics
(HECs). Here, we take high-entropy diborides (HEB$_2$) as the prototype to
thoroughly investigate the hardening and strengthening mechanisms of HECs.
Specifically, the equiatomic 4- to 9-cation single-phase HEB$_2$ ceramics
(4-9HEB$_2$) are fabricated by an ultra-fast high-temperature sintering method.
The as-fabricated 4-9HEB$_2$ samples possess similar grain sizes, comparable
relative densities (up to ~98%), uniform compositions, and clean grain
boundaries without any impurities. The experimental results show that the
hardness and flexural strength of the as-fabricated 4-9HEB$_2$ samples have an
increasing tendency with the increase of metal components. The first-principles
calculations find that lattice distortion is essential to the hardness and
strength of HEB$_2$. With the increase of metal components, an aggravation of
lattice distortion accompanied by B-B bond strengthening is determined,
resulting in the enhancement of the hardness and flexural strength. Moreover,
the correlation between other potential indicators and the hardness/flexural
strength of HEB$_2$ has been disproved, including valence electron
concentration, electronegativity mismatch, and metallic states. Our results
unravel the hardening and strengthening mechanisms of HECs by intensifying
lattice distortion, which may provide guidance for developing superhard and
high-strength HECs.
