  High-entropy materials often outperform their lower-entropy relatives in
various aspects, such as thermal stability and fracture toughness. While there
are extensive research activities in the field of high-entropy alloys,
comparably little is performed for high-entropy ceramics, and especially for
high-entropy diborides. Here we show, that not only the hardness of ZrB2 layers
can be improved from 43.2 to 45.8 to 47.2 GPa through the formation of solid
solution ternary diborides (Zr0.61Ti0.39B2) and high-entropy diborides
(Zr0.23Ti0.20Hf0.19V0.14Ta0.24B2), respectively, but especially their thermal
stability against structural rearrangements and decomposition towards the
constituting binary diborides.
