Effect of current density on the mechanical properties and defects of nanocrystalline Cr-Al solid solution alloy coatings

Hard chromium coatings electrodeposited are highly valued in multiple industries for their excellent mechanical characteristics and resistance to corrosion. The general process for preparing these coatings includes Cr6+ and Cr3+ process, the Cr6+ process is highly toxic and strictly regulated. The Cr3+ process is less hazardous but can produce coatings with defects. We propose an alternative process of electroplating nanocrystalline Cr-Al alloy coating with high hardness by using anhydrous Cr2+ electrolyte. The impact of current density on mechanical properties (i.e., hardness H, elastic modulus E, fracture toughness H/E and resistance to plastic deformation H-3/E-2) and coating defects has been systematically investigated. As current density increases from 1 mA/cm(2) to 5 mA/cm(2), the coating surface morphology changes from polyhedral to nodular clusters with grain size decreasing from a few microns to several nanometers. The nanoindentation test was performed on the cross-section of the coating. The nanohardness H of the coating reaches a high of 8.26 GPa, attributed to both solid solution strengthening and grain refinement. A significant decrease in elastic modulus E of the coating compared to bulk Cr is observed and can be explained by the grain boundary mediated process and a grain coalescence model. High values H/E and H-3/E-2 indicate improved fracture toughness and resistance to plastic deformation of the coating. As current density exceeding 5 mA/cm(2), laminar cracks develop due to co-deposition of CrCl2 particles. This is attributed to the electrolyte acidity transition, caused by depletion of Al2Cl7- at the interface. This work provides a deeper understanding of the microstructural evolution, mechanical properties and defects of Cr-Al alloy coatings and also provides a new method to fabricate high-quality coatings with enhanced properties. The defect-free coatings achieved in this study have great potential for practical applications in various fields such as tribological and corrosion resistance coatings.