Improved lubricating and corrosion resistance of MAO coatings on ZK61 Mg alloy by co-doping with graphite and nano-zirconia

Abstract

The poor wear and corrosion resistance of Mg alloys are hampering wider application. Micro-arc oxidation can improve the wear and corrosion resistance, but the resulting coatings have relatively low hardness, high friction coefficient, and surface porosity, rendering them susceptible to failure in the field. Herein, ZrO2 nanocrystalline and extrinsic graphite-doped MAO coatings are prepared on the ZK61 Mg alloy using a control electrolyte containing potassium fluorozirconate and nano-graphite. The ZrO2 nanocrystals formed in situ are uniformly distributed in the amorphous coating, while the extrinsic graphite is mainly located in the amorphous structure. The size and number of pores decrease after introducing ZrO2 and graphite into the MAO coating. Compared with doping with a single element, co-doping increases the content of ZrO2 and graphite in the MAO coating. Furthermore, ZrO2 densifies the porous Mg(OH)2 corrosion product and blocks the penetration of the corrosive medium to improve the corrosion resistance. The optimized sample shows corrosion potential and corrosion current density of −1.012 V and 1.425 × 10−7 A·cm−2, which are significantly better than the singly doped MAO coating. Graphite doping decreases the friction coefficient, while ZrO2 doping decreases the wear rate. The ZrO2 and graphite co-doped C10Zr10 coating demonstrates self-lubricating properties, resulting in a significantly reduced wear rate from 2.832 × 10−4 mm3·N−1·m−1 to 1.04 × 10−5 mm3·N−1·m−1. The results reveal that co-doping improves the properties of MAO coatings on Mg alloys.

Keywords

• Mg alloy
• Micro-arc oxidation (MAO)
• ZrO2
• Graphite
• Lubrication
• Corrosion resistance