The Effect of BN or SiC Addition on PEO Properties of Coatings Formed on AZ91 Magnesium Alloy

Currently, due to the economic and ecological aspects, light alloys are increasingly important construction material, in particular in the transport industry. One of the popular foundry magnesium alloys is the alloy AZ91, which among others due to mechanical properties and technological features, is used, for example, for light structural parts. The paper presents the results of research on modification of the AZ91 alloy surface layer in the plasma electrolytic oxidation process. The change of usable properties of the produced coatings was obtained by introducing additions of silicon carbide or boron nitride. The thickness and hardness of the protective layers produced, resistance to scratches and corrosion resistance were determined. Moreover, the friction coefficient of the coating-steel pair was investigated. The quality of the connections made between the coating and the substrate, i.e. the magnesium alloy, was also evaluated. The results obtained for coatings with silicon carbide or boron nitride additives were always compared to the results obtained for unmodified samples. On the basis of the obtained results, it was shown that the introduction of boron nitride additive to the AZ91 alloy coating produced in the plasma electrolytic oxidation process significantly improves the resistance to: (i) corrosion and (ii) abrasive wear of the coating.