Research On Electrodeposition And Tribological Properties Of Ni-BaCr₂O₄ Composite Coatings

BaCr₂O₄ chromate powders were synthesized with the high-temperature solid-state reaction process using equal molar BaCO₃ and Cr₂O₃ powders as starting materials. A variety of pure nickel coatings and Ni–BaCr₂O₄ composite coatings were electrodeposited on the surfaces of Inconel 718 superalloys. The influences of the BaCr₂O₄ incorporation and electrodeposition parameters on the microstructure, microhardness and tribological properties of electrodeposited Ni–BaCr₂O₄ composite coatings were investigated by means of X–ray diffraction (XRD), scanning electron microscopy (SEM), microhardness tester and friction and wear tester, respectively, to obtain a better understanding on the wear mechanisms.After suitable pretreatments and a variety of single–factor electrodepositing experiments, pure nickel coatings with an excellent bonding strength were electrodeposited on the surfaces of Inconel 718 superalloys. The optimized parameters are as follows: pH value of 3, electrodeposition time of 1h, current density of 1.5–6A/dm², temperature of 30–60℃. The Inconel 718 superalloy exhibit a friction coefficient of 0.5–0.65 and a wear width of 500–550 m, however, the pure nickel coating has a friction coefficient of 0.35 to 0.4 and a wear width of 300 m. Adhesive wear and abrasive wear are the dominant wear mechanism of Inconel 718 superalloys, however, oxidation wear and mild delamination wear are responsible for material removal of pure nickel coating during wear tests. The results show that the current density have little effect on tribological properties and wear mechanisms of pure nickel–coatings, but with increasing the temperature, the friction coefficient of pure nickel coating increases.Orthogonal experimental methods are employed to investigate the influences of current density, temperature and BaCr₂O₄ powder concentration in the electrolyte on the BaCr₂O₄ content in electrodeposited composite coatings. With increasing the current density and temperature, the volume fraction of BaCr₂O₄ in electrodeposited composite coating increases at first and then decreases. With increasing the BaCr₂O₄ powder concentration, the volume fraction of BaCr₂O₄ in electrodeposited composite coating increases. The optimum parameters for electrodepositing Ni–BaCr₂O₄ composite coatings are as the following: current density of 3A/dm², BaCr₂O₄ powder concentration of 70g/L, temperature of 50℃, pH value of 3, electrodeposition time of 1h.The incorporation of BaCr₂O₄ particles into the Ni–BaCr₂O₄ composite coating improves the morphology of the coating, including grain refinement and surface roughness. Owing to dispersion strengthening mechanism of BaCr₂O₄ particles in the composite coatings, it effectively enhances the microhardness of Ni–BaCr₂O₄ composite coatings. Pure nickel coating and Ni–16.6%BaCr₂O₄ composite coating have microhardness values of 279HV and 746HV, respectively.Owing to dispersion strengthening mechanism and grain refinement effect of BaCr₂O₄ particles, it effectively improves tribological properties of Ni-BaCr₂O₄ composite coatings. Low friction coefficients and small wear rate are obtained with increasing the BaCr₂O₄ content in Ni–BaCr₂O₄ composite coatings as contrasted with pure nickel coatings. The friction coefficient of the Ni–16.6%BaCr₂O₄ composite coatings is about 0.3. The wear rates of Inconel718 and Ni–16.6%BaCr₂O₄ composite coatings are 3.47×10^{–}5mm³/Nm and 2.79×10^{–6}mm³/Nm, respectively. Besides, the incorporation of BaCr₂O₄ particles reduces the surface roughness of worn surface, and effectively avoids the appearance of adhesion and spallation of composite coating.