Experimental Study on the Functionally Graded Ni-P-Al2O3 Coating Created on AA 5xxx Series for Wear Resistance Improvement by Pulse-Electrodeposition Method

The properties such as weak wear resistance and low hardness of aluminum alloys have limited their use in various industries. In this research, it has been attempted to improve the mechanical and tribological properties of these materials by deposition of nickel-phosphorous-alumina functionally graded coating. Functionally graded coatings have been produced by a gradual change in the chemical composition and content of the nanoparticle, using continuous change in pulse parameters such as duty cycle and frequency during the coating process. So, the effect of the duty cycle and frequency has been investigated. Two types of coatings have been created with a gradual decrease in the duty cycle of 90% to 30% and a pulse frequency of 50 to 500 Hz. The result shows that the effect of frequency on the amount of phosphorus and nanoparticles is negligible, and it has mainly affected on grain size. However, in nanocomposite coats, the gradual decrease of duty cycle has led to an increase in the amount of phosphorus (5.3% to 15.5 wt. %) and alumina nanoparticles (0.7% to 2.6 wt. %) from the substrate to the top surface. With the gradual changes in chemical and microstructure, the adhesion of the coating to the substrate has improved. The results of micro-hardness have also shown that the creation of functionally graded coatings using duty cycle variation has a higher hardness than the one produced by frequency changing. Also, based on the results of the pin test on a disk against abrasive steel 52100, the wear resistance of functionally graded coatings has improved compared to single-layer coatings.