Structure characterization and tribological properties of thick chromium coating electrodeposited from a Cr(III) electrolyte

Uniform and flat thick Cr coating (CR3 coating) was electrodeposited from a Cr(III) electrolyte containing chromium chloride and urea under the optimized process conditions. The microstructure of the coating and its tribological properties under various loads was examined. Furthermore, the performance comparison of the CR3 coating with the hard Cr coating (CR6 coating) fabricated by conventional Cr(VI) electrodepositing process, and the heat-treated thick Cr coating (CR3H coating) from the same Cr(III) electrolyte, was performed. The result shows that the main phase of the CR3 coating is amorphous, with the nanocrystals of body-centered cubic structure chromium (bcc Cr) and graphite embedded in it. The existence of the nanocrystals of bcc Cr and graphite in Cr coatings is probably the common feature of all amorphous Cr coatings from Cr(III) electrolytes. In the scope of the bearable load of the CR3 coating, its tribological properties were a little worse than those of the CR6 coating. After annealing at 180 °C for 2 h, the wear resistance of the coating was improved to a certain extent. We found the interfacial carbon film between the CR3 coating and steel substrate, which caused poor bonding and consequently insufficient load-bearing capability of the coating. There is a need to further improve the load-bearing capability and wear resistance of the CR3 coating. In view of the great environmental advantage of Cr(III) electrodeposition process and no difference of wear resistance in order of magnitude between the CR3 coating and CR6 coating, Cr(III) electrodeposition technology is still a developing direction with promising prospect.