The Effect of TiC and WC Nanoparticles on Oxide Layers Formed by Plasma-Electrolytic Oxidation on Magnesium Alloys Containing the LPSO Phase

Abstract

Magnesium alloys with rare-earth elements are widely used in industry due to their high specific strength; however, their low corrosion and wear resistance necessitate surface protection. It is studied how the WC and TiC nanoparticles (NPs) used as the dispersed phase in the electrolyte at the plasma-electrolytic oxidation (PEO) of magnesium alloys in the system Mg–Y–Zn–Zr–Nd–Yb containing the long-period staking-ordered (LPSO) phase affect the formation, composition, and properties of oxide layers. The inert incorporation of WC and TiC NPs into the oxide (without their involvement in chemical reactions and phase transitions) is observed as well as the increased productivity of the coating formation for certain concentrations of additives, without commensurate incorporation of nanoparticles into the coating. As compared with WC NPs, the TiC NPs added into the electrolyte induce greater changes in the conditions of coating formation, as reflected in the decrease in the anodic forming voltage of the PEO process up to ~20 V and the increase in the degree of crystallinity of oxide layers to ~80 vol %. The hardness and the adhesion strength of layers increase due to their modification with WC and TiC nanoparticles. The PEO allows reducing the corrosion rate of this alloy by three—four orders of magnitude. The addition of nanoparticles into the electrolyte in concentrations of 2 and 5 g/L WС and 1 g/L TiC makes it possible to additionally increase the charge transfer resistance across the alloy/coating interface by a factor of up to 1.5. At the same time, the high (4–5 g/L) concentration of the dispersed phase in the electrolyte has a negative effect on the long-term corrosion stability of samples. Hence, these coatings are more suitable for articles with limited service life.