Hard chromium coatings used where high wear resistance is needed. These Coatings have disadvantages such as environmental hazards of hexavalent chromium ions, require high energy usage and low deposition rate. In recent years, many researchs has been done to replace hard chromium coating with a healthier and lower power consumption such as trivalent chromium nanostructured coatings and nanostructured nickel based coatings. Reviews showed that Ni-Cr coatings with trivalent chromium bath is a good candidate for replacement of hard chromium coatings because of its high hardness and wear resistance. In present study, hrad chromium coating, the uniform and functionally graded (FG) Ni-Cr coatings were produced via electrodeposition. To create this coatings, all kinds of variables, such as current density, temperature, pH, bath composition, additives and complexing agents were investigated. Also electrodeposition mechanism of coatings was studied by cyclic voltametry (C.V) technique. To examine the crystalline microstructure and texture, SEM and XRD methods were used. For the studying the mechanical wear and corrosion properties of coatings, the micro-hardness, potentio-dynamic polarization, EIS and pin-on-disk wear tests were applied.
In the case of uniform coatings, it was observed by C.V method that using of formic acid as complexing agent, causes higher chromium ions reduction. Addition of optimum concentration of 30 g/L formic acid to bath not only improved uniform distribution of particles but also led to improving hardness, corrosion and wear resistance of coating. Electroplating carried out at temperature of 20, 40 and 60 ° C. Plating at 20 ° C improves the microstructure, wear and corrosion resistance of the coatings also. Addition of 30 g/L of formic acid and plated at 20 ° C reduces the grain size of 100 nm to about 40 nm and a significant increase was observed in hardness of coating about 500 to 1020 Hv. Annealing at temperatures of 500, 600 and 700 ° C for half an hour was done. Annealing increased grain size and decreased wear resistance, corrosion resistance and micro-hardness of uniform Ni-Cr coatings.
In continue the FG coatings were produced to achieve maximum values of adhesion strength and wear resistance. In this attribution the variation of grain size and Ni and Cr content were attended via changing current density and temperature of bath. The gradual variation of current density from 10 to 40 A/dm2 and temperature from 60 to 20 °C led to changing of grain size and Cr content, respectively from 110 nm and 12 wt.% near interface to 40 nm and 83 wt.% at the outer surface of the coating. In this case, further improvement in microhardness, wear resistance and corrosion resistance was achieved in comparison to uniform Ni-Cr and hrad chromium coatings.