Interface‐Controlled Phase Transformation and Corrosion Performance of Nb/NbN Multilayer Coatings for Bipolar Plates

Proton exchange membrane water electrolysis (PEMWE) bipolar plates face a critical trade‐off between corrosion resistance and interfacial electrical conductivity. To address this, Nb/NbN multilayer coatings with systematically varied interface densities were deposited on TA1 substrates via magnetron sputtering. Increasing the multilayer level induces a transition from substrate‐influenced to interface‐controlled growth, triggering a crystallographic phase transformation from metastable hexagonal ε ‐NbN to stable cubic δ ‐NbN. Finite element analysis and physical characterizations confirm this transformation drives microstructural densification and a pronounced increase in compressive residual stress, accompanied by pronounced transformation‐associated residual‐stress evolution and microstructural densification. Consequently, transformation‐induced densification significantly enhances corrosion resistance in simulated PEMWE anodic environments, while electrical properties remain moderate alongside stress accumulation. Notably, the Nb/NbN‐3 coating architecture achieves an optimal balance, exhibiting low interfacial contact resistance (∼6.18 mΩ·cm 2 at 1.5 MPa) and high corrosion stability (0.195 µA·cm − 2 in potentiostatic polarization). Ultimately, these results establish interface‐density engineering as a highly effective strategy for optimizing the performance of bipolar plates in advanced PEMWE systems.