Effect of Bias Power on the Corrosion and Electrical Conductivity of Nbn Coating on Ti Bipolar Plates for Pemfc

Abstract

Metallic niobium nitride (NbN) is a promising material for protective coatings on PEMFC metal bipolar plates. However, poor surface quality, characterized by cracks and defects, significantly impair NbN coating ability to meet the dual performance requirements of electrical conductivity and corrosion resistance. This study examines the influence of substrate bias on the morphology and surface roughness of NbN coatings, as substrate bias directly affects sputtering particle energy, surface mobility, and ion bombardment effects. The findings reveal that in the absence of applied bias, sputtered particles exhibit low energy and diffuse non-directionally, resulting in NbN coatings with a columnar cross-sectional morphology and a rough, granular surface. Such coatings demonstrate poor corrosion resistance but relatively high electrical conductivity. At an optimal substrate bias, enhanced atomic migration and increased ion bombardment facilitate the formation of a dense, low-roughness coating with superior corrosion resistance. However, excessive substrate bias leads to intensified ion bombardment, causing grain coarsening and increased surface roughness. This results in reduced corrosion resistance but improved conductivity. The study underscores how variations in substrate bias alter the microstructure and roughness of NbN coatings, thereby influencing the dual mechanisms of corrosion resistance and conductivity. This straightforward and efficient approach provides a viable strategy for optimizing both the corrosion resistance and conductivity of PEMFC bipolar plates.

Keywords: NbN Coatings, Bias Power, Surface Roughness, Corrosion resistance, Interfacial Contact Resistance