Structure and corrosion properties of PVD Cr–N coatings

Abstract

PVD Cr–N coatings produced by physical vapor deposition (PVD) are increasingly used for mechanical and tribological applications in various industrial sectors. These coatings are particularly attractive for their excellent corrosion resistance, which further enhances the lifetime and service quality of coated components. PVD Cr–N coated steels in an aqueous solution are usually corroded by galvanic attack via through-coating “permeable” defects (e.g., pores). Therefore, the corrosion performance of Cr–N coated steel is determined by a number of variables of the coating properties and corrosive environment. These variables include: (i) surface continuity and uniformity; (ii) through-coating porosity; (iii) film density and chemical stability; (iv) growth stresses; (v) interfacial and intermediate layers; (vi) coating thickness; (vii) coating composition; and (viii) substrate properties. In this article, PVD Cr–N coatings were prepared, by electron-beam PVD and sputter deposition, with different compositions, thicknesses, and surface roughnesses, by changing the N2 flow rate, applying multilayering techniques and changing the substrate finish prior to coating. The microstructure of such coatings is investigated by various analytical techniques such as glancing angle x-ray diffraction and scanning electron microscopy, which are also correlated with the corrosion performance of the coated steel. Both dc polarization and ac impedance spectroscopy were employed to investigate the corrosion resistance of Cr–N coated steel in a 0.5N NaCl solution. It has been found that the N2 flow rate during reactive deposition strongly determines the microstructure of Cr–N coatings (due to the changing nitrogen content in the film) and can thus affect the corrosion resistance of coated systems. The surface finish of the steel substrate also affects the uniformity and coverage of PVD coatings; grooves and inclusions on the original substrate can raise the susceptibility of coated systems to crevice corrosion. Increased coating thickness can also greatly reduce the incidence of through-coating porosity such that the improvement in corrosion performance of thicker Cr–N coatings is significant.