Electrolyte–electrode interface and surface characterization of plasma electrolytic nitrocarburizing

A plasma electrolytic nitrocarburizing (PEN/C) process was performed on the Q235 steel in an aqueous solution of urea to modify the surface properties. The decomposing particles of organic compounds in the electrolyte adjacent to the electrode were obtained using ultraviolet spectrum during the process. The outer surface (2–5 nm) was analyzed using X-ray photoelectron spectroscope (XPS). The cross-section and the surface morphologies were investigated using optical microscope and scanning electron microscope (SEM) equipped with energy dispersive spectroscope (EDS). The microstructure was determined using X-ray diffraction (XRD). Hardness and roughness were evaluated using micro-hardness tester and roughness meter. The wear of the samples were measured using pin-disk friction and wear testing machine in dry condition. It is indicated that urea electrolyte is ionized into conjugated double bonds consisting of carbon, nitrogen and oxygen. The micro-pores are uniformly distributed on the surface of the PEN/C samples. Iron carbide, iron nitride, CC hydrocarbon and iron oxide are formed on the outer surface of the PEN/C samples. The content of carbon and nitrogen in the diffusion layer is higher than that in the substrate. The diffusion layer mainly contains phases in Fe3C and Fe4N. The micro-hardness of the diffusion layer is 3 to 4 times higher than that of the substrate. The roughness of the surface is increased with prolong of time. The wear of the PEN/C sample is improved greatly, and this will enhance the service life of steel.

► The formation, adsorption and diffusion processes of the PEN/C particles are studied.
► The XPS of the nanometer scale of the outer surface is carried out.
► The micro-pores are uniformly distributed on the surface of the PEN/C sample.
► The performance of the surface on the PEN/C sample is improved.