Microstructure of electroplated hard chromium coatings after plasma nitrocarburizing

Plasma nitrocarburizing was carried out on the hard chromium coating deposited on SM45C mild carbon steel substrate by electroplating. After plasma nitrocarburizing at 720 °C for 20 h, a 6–7-μm-thick nitride layer consisting of CrN and Cr2N was formed on the surface of Cr coating with the microhardness of about 950–1100 HV0.1. Due to the effect of annealing caused by plasma nitrocarburizing process at higher temperature and the Cr being a strong carbide-forming element, the carbon in steel substrate diffused outward into the Cr coating and reacted with Cr forming Cr carbide interface layer between the Cr electroplating and substrate. The nitrogen going into the microcracks and the volume increase accompanied by Cr nitride and carbide formation would cause the microcracks inherent to hard chromium plating disappear and improve its corrosion resistance. The microstructures of nitride and carbide layers were studied using X-ray diffraction and cross-sectional transmission electron microscopy.