Structure, hardness and corrosion behavior of a gradient CrNx thick coating applied to turbine blades

In order to protect turbine blades from solid particle erosion, a gradient CrN_x coating was deposited on 10% Cr heat-resistant steel by ion plating; the thickness of the coating was about 40 μm. The chemical composition, microstructure and nano-hardness of gradient CrN_x coating were analyzed. The bonding force was determined using scratch test. The potentiodynamic polarization and high-temperature oxidation tests were respectively conducted to investigate the corrosion behavior. The results indicate that gradient variation of phase structure, chemical composition and nano-hardness in the coating is found, and the bonding force with substrate is excellent. The microstructure obtained can enhance the corrosion performance of the substrate. The corrosion resistance improvement is not only attributed to the increase of coating in thickness, but also to internal microstructure and chemical composition of coating. Based on SEM and TEM observations, the cross-sectional fracture of the coating shows nano-crystalline and fine columnar crystalline structures. There are no penetrable pinholes, which could validly reduce the electrolyte transferring to inner substrate. In addition, the corrosion resistance of coating is further improved by the formation of nitrogen and chromium rich transition layers.