Plasma electrolytic oxidation coatings on AZ31 magnesium alloys with Si₃N₄ nanoparticle additives

Magnesium AZ31 alloys have been widely used in the aerospace, automotive, and personal computer industries due to their light weight and low density. However, high chemical reactivity, poor corrosion and wear resistance limit their further use in many other fields. The plasma electrolytic oxidation (PEO) process can produce a protective oxide layer on the Mg alloy to improve its mechanical property, wear resistance, and corrosion resistance. In this work, silicon nitride (Si₃N₄) nanoparticles were added into the reaction electrolyte to study their influence on the microstructure, mechanical, and anticorrosion properties of PEO coatings on AZ31 Mg alloy. The breakdown voltage for igniting the plasma discharge decreased with increasing concentration of Si₃N₄ nanoparticles. The PEO coating without Si₃N₄ additives in the reaction electrolyte had mainly MgAl₂O₄ and minor MgO phases. On the other hand, when Si₃N₄ nanoparticles were included in the PEO reaction, a Mg₂SiO₄ phase is present. In general, the coating thickness, surface roughness, hardness and elastic modulus increased with increasing Si₃N₄ nanoparticle concentration up to 3 g/L. A maximum hardness of 16.4 GPa was found in the coating fabricated with 3 g/L Si₃N₄ nanoparticles added. The PEO coating fabricated using 2 g/L of Si₃N₄ nanoparticles in its electrolyte exhibited the best corrosion resistance, high hardness, good adhesion, and low coefficient of friction in this study.