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·cm² 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.