Preparation And Properties Study Of Electrodeposition Nano-composite Coatings On AZ91HP Mg Alloy Surface

In recent years, the industrialization of Mg alloy has been rapidly developed. AZ91HP Mg alloy becomes the most widely used casting Mg alloy because of its excellent casting performance and low cost. But poor hardness, wear resistance and corrosion resistance restrict the extensive applications of AZ91HP Mg alloy. In this paper, AZ91HP Mg alloy was used as substrate, n-SiO₂ and n-TiO₂ particles were used as the second phase particles, and n-SiO₂/Ni and n-TiO₂ composite coatings on AZ91HP Mg alloy surfaces were prepared using electrodeposition method. The micro-morphology, microhardness, friction and wear behavior, and corrosion resistance of coatings were researched.During preparing electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces, the parameters included cathode current 30mAcm-2, electrolyte pH 5.0, and bath temperature 55℃. The dispersing mode of the electrolyte was ultrasonic vibration along with the mechanical agitation.Through comparing and analyzing the micro-morphology and mechanical properties of electrodeposition pure Ni coating, n-SiO₂/Ni and n-TiO₂/Ni composite coatings, it was found that the grains of nano-composite coatings were smaller, and the nano-composite coatings were more compact and smooth. The coatings combined well with AZ91HP Mg alloy matrix. After the scratch test, the coatings did not peel and exfoliate.The microhardnesses of n-SiO₂/Ni and n-TiO₂/Ni composite electrodeposition coatings on AZ91HP Mg alloy surface were harder than AZ91HP Mg alloy matrix and pure Ni coating. The microhardness of coatings increased and then decreased with increasing contents of n-SiO₂/Ni and n-TiO₂/Ni in the electrodepositionelectrolyte. Electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surface had the highest microhardness when the contents of n-SiO₂ and n-TiO₂ particles were 10g/l-15g/l. The wear resistance, friction coefficient of AZ91HP Mg alloy matrix, electrodeposition pure Ni coating and n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surface were compared. The wear mechanisms of coatings were studied. Compared with AZ91HP Mg alloy matrix and electrodeposition pure Ni coating, the wear resistances of electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surface were obvious improved. With the increase of the content of nano materials in the electrolyte, the wear resistances of electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces increased, and the friction coefficient decreased. However, when the contents of n-SiO₂ and n-TiO₂ were too high and over the best value (10g/l), the wear resistances of electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces decreased, and the friction coefficients increased. But their wear resistances were still better than electrodeposition pure Ni coating on AZ91HP Mg alloy surface. The friction coefficients of electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces decreased with increasing load. In the same condition, the wear resistance of electrodeposition n-SiO₂/Ni composite coating on AZ91HP Mg alloy surface is better than that of n-TiO₂/Ni composite coating. The wear mechanism of AZ91HP Mg alloy was adhesive wear, the wear mechanism of electrodeposition pure Ni coating on AZ91HP Mg alloy surface was peeling wear, and the wear mechanisms of electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces were abrasive wear. The corrosion resistances of AZ91HP Mg alloy, electrodeposition pure Ni coating, and n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces were compared. The corrosion resistances of electrodeposition n-SiO₂/Niand n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces were much better than those of the other two.Electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces were dipped in 3.5wt.% NaCl and Mg (OH) 2 solution for 5 days, and no corrosion was found, which indicated good corrosion resistance.Electrodeposition n-SiO₂/Ni and n-TiO₂/Ni composite coatings on AZ91HP Mg alloy surfaces, whose contents of n-SiO₂ and n-TiO₂ in the electrolyte were 10g/l and 15g/l, had the best corrosion resistance. The results of polarization curves were accordance with those of the dipping test.