Phase formation and unusual interstitial solid-solution strengthening behavior of (CoCrFeMnNi)Nx high-entropy ceramic films.

High-entropy ceramic films (HECFs) have become promising industrial materials due to their excellent mechanical and functional properties. Achieving the precise design of HECFs has been a hot topic in recent years. Herein, the CoCrFeMnNi alloy, one of the most classic high-entropy systems, is chosen as the model to reveal the phase formation, growth, and properties of HECFs. Films were fabricated by magnetron sputtering at various N 2 -to-total (N 2 + Ar) flow ratios (R N). Results show that the phase structure transformed from a semi-crystal phase to a single face-centered cubic (fcc) structure with increasing R N. Atomic migration ability affected by R N drives different growth processes, thereby constructing different microstructures, such as loose fiber structure and stacked particle structure. Moreover, the hardness exhibits a nonlinear relationship with interstitial solid solubility. Such unusual interstitial solid-solution strengthening behavior is attributed to the offset between solid-solution strengthening and grain boundary weakening as R N increases. A high hardness of 13.71 GPa and Young's modulus of 203.3 GPa are obtained in the (CoCrFeMnNi)N x film deposited at R N = 20 %. This work not only explores an unusual interstitial solid-solution strengthening behavior, but also sheds light on the future HECFs design. • Phase transforms from semi-crystal phase to fcc phase with increasing N content. • It is proved that an interstitial solid-solution structure was constructed in HECFs. • Different growth processes drive the formation of different microstructures. • An unusual interstitial solid-solution strengthening is observed in HECFs. [ABSTRACT FROM AUTHOR]