Enhancing mechanical properties of (HfMoNbZrTa)₁₋ₓNₓ films through multi-phase structures in substoichiometric compositions

Elemental composition design is a common strategy for tuning the crystallographic structure of films, thereby optimizing their mechanical properties. The evolution of film structure is highly dependent on both selection and concentration of cation and anion elements. In this work, we utilized strong nitride-forming elements (Hf, Nb, Zr, and Ta) and the weak nitride-forming element Mo as the metallic cations in a multi-principal element nitride system. The nitrogen content in the (HfMoNbZrTa)₁₋ₓNₓ film was manipulated by varying the N₂/Ar flow ratio (RN) during reactive magnetron sputtering. The results showed that the HfMoNbZrTa metallic film crystallized weakly into a body-centered cubic (BCC) structure, yielding a hardness (H) of approximately 10.7 ± 0.2 GPa. The incorporation of nitrogen immediately induced a transformation from BCC to a face-centered cubic (FCC) predominated multi-phase structure at substoichiometric regimes (10% ≤ RN ≤ 20%). A further increase of RN to ≥25%, i.e., at near-stoichiometric regimes, resulted in the formation of single-phase FCC structure. Hardness and wear rates both reached their optimal values at RN = 20–25%. The strengthening mechanism was elucidated through density functional theory (DFT) calculations, which suggest that higher H of the substoichiometric film is mainly associated with the increased local lattice distortion and the formation of a multi-phase structure.