Study on electroless Ni-P-ZrO2 composite coatings on AZ91D magnesium alloys

Electroless Ni-P-ZrO2 composite coatings on AZ91D magnesium alloys was developed and their characteristics were investigated. The experimental results showed that the composite coatings were uniform and compact. The hardness of Ni-P-ZrO2 composite coatings reached the maximum value at 350 degrees C heat treatment, and then decreased at 400 degrees C. The nickel phosphor alloys provided solution strengthening and precipitation strengthening, while the ZrO2 nanoparticles provided dispersion hardening, making the composite coatings harder than that of Ni-P coatings. The wear resistance of Ni-P-ZrO2 composite coatings was obviously superior to Ni-P coatings, especially with longer wear time, which can be attributed to the dispersion strengthening and load support effect of ZrO2 in the coatings. The Ni-P-ZrO2 coatings exhibited better corrosion resistance than that of Ni-P coatings owing to codeposition of ZrO2 together with NiP leading to a more compact composite coating.