A critical role for Al in regulating the corrosion resistance of nanocrystalline Mo(Si1−xAlx)2 films

Abstract

Novel nanocrystalline Mo(Si_1−xAl_x)₂ films, with differing Al contents were synthesized by double cathode glow discharge. The films exhibited a compact columnar microstructure having a pronounced (111) preferred orientation. The corrosion behaviour of these films were characterized by using various electrochemical techniques including open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution. The corrosion resistance of the films increased with increasing Al content in the as-synthesized films. The composition and chemical state of the passive layers formed on the films were investigated by X-ray photoelectron spectroscopy (XPS). It was demonstrated that the passive layer formed on the binary MoSi₂ film was highly enriched in SiO₂ with minor amounts of MoO₄²⁻, MoO₂ and SiO_x. With the increase of Al content in the films, Al₂O₃ was generated and incorporated into the passive layers, enhancing the corrosion resistance of the films by inhibiting the dissolution of Mo. Built upon the experimental results, the first-principles density-functional theory was applied to calculate the inter-atomic bonding strength in Mo(Si_1−xAl_x)₂ and elucidate the role of Al in controlling the corrosion resistance of the films. The new findings lay a solid basis for the development and application of MoSi₂ based corrosion-resistant films.