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 N2- to-total (N2 + Ar) flow ratios (RN). Results show that the phase structure transformed from a semi-crystal phase to a single face-centered cubic (fcc) structure with increasing RN. Atomic migration ability affected by RN 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 solidsolution strengthening and grain boundary weakening as RN increases. A high hardness of 13.71 GPa and Young's modulus of 203.3 GPa are obtained in the (CoCrFeMnNi)Nx film deposited at RN = 20 %. This work not only explores an unusual interstitial solid-solution strengthening behavior, but also sheds light on the future HECFs design.