Study On Corrosion Resistance, Friction And Wear Properties Of Micro-arc Oxidation Coating On Mg-Li Alloy

Mg-Li alloys as the lightest structural materials have wide application prospect in thefields of aerospace, electronic communication and automobile due to their inherent superiorproperties such as low density, high strength to weight ratio, good machining property,electromagnetic shielding and recyclability. However, the main factors hindering furtherdevelopment and industrial application of Mg-Li alloys are very poor corrosion and wearresistance. Micro-arc oxidation （MAO） coatings have been successfully fabricated on thesurface of Mg-Li alloys, followed by post-treatment of solution immersion, and theremarkable improvement in corrosion and wear resistance were obtained in the present paper.The surface morphology, microstructure, phase composition, chemical composition andformation mechanism of the coatings were characterized by means of SEM, XRD, TF-XRD,EDS and XPS techniques. The corrosion resistance was investigated by potentiodynamicpolarization curves, EIS and immersion test. The tribological properties and wear mechanismof the coatings were measured and analyzed using ball-on-disc configuration sliding frictiontest, talysurf, nanoindentation, SEM and OM.MAO coatings with high corrosion resistance were prepared on the different Mg-Lialloys by the addition of tungstate or molybdate into the alkaline silicate, using a modeinvolving an initially constant current mode, followed by a constant voltage mode. The size ofmicropores on MAO coatings was mostly found to be influenced by the speed of anodizedphase and final voltage. With the decrease in Li content of Mg-Li alloy substrate and theaddition of tungstate or molybdate into the alkaline silicate the fast speed of passivationoccurs and the dissolution of Li the is suppressed.It was demonstrated that MAO coatings have more compact microstructural features andthe impedance increases due to the penetration of H₂O and OH–into the pores and holes onMAO coatings after the immersion into alkaline permanganate or molybdate.Micro/nanoparticles containing Mn₂O₃, MnO₂（or MgO·MoO₂, MoO₃）, MgF₂and NaFcompounds are generated on the surface of the coatings. The outer layer appears to behydrated, intenerated and loosened after the immersion of the immersed coatingsW-containing into neutral NaCl for short time. By the dry sliding friction test of MAO coatings against the countermaterial Si₃N₄it wasfound that above described micro/nanoparticles or intenerated substance, which is chieflyresponsible for moderating the stress distribution and avoiding the brittle fracture of ceramiccoatings are regarded as solid lubricant. At the same time, Si₃N₄ball is insusceptible to beabraded due to its high hardness and the shear effect on MAO coating is small, which leads toreduce the friction and wear. After potentiodynamic polarization test a small amount ofcorrosion products generated on the surface of MAO coatings is favourable for improving thetribological behavior, which is independent of the partial corrosion microcracks formed in thepolarization test. The high roughness, low thickness, the surface without lubricative layer andhard ceramic particle may bring about more serious friction and wear of the coatings. Thewear morphologies of MAO coatings suggest that Si₃N₄ball results in micro-polishing,polishing, fracturation and detaching wear of the coating. By the dry sliding friction test ofMAO coatings against the countermaterial CCr15ball with lower hardness as compared withSi₃N₄ball it was found that CCr15ball is abraded and debris is transferred into the coating.With the increase of the roughness of CCr15ball, the MAO coatings are continuously shearedand a large amount of debris is generated. Although micro/nanoparticles and Fe-containingtransferred layer as solid lubricant may keep stable friction and wear, MAO coating isfractured and MAO coating/CCr15ball friction system is transferred from low friction to highfriction when the contact area and the surface roughness get higher or the friction stressexceeds the fracture stress. CCr15ball results in the adhesion wear and oxidation wear,besides the polishing, the flaking and fracturation wear of MAO coating. The wear rate andwear morphologies show that the high hardness of MAO coating results in high wear of thecountermaterials.We examined the friction and wear properties of MAO coatings before and after theaddition of titania-sol into the alkaline phosphate or silicate containing K2ZrF6electrolyte. Itwas found that the amorphous P-containing compound imparts the beneficial effect on thereducing friction of MAO coating, and the addition of titania-sol into alkaline phosphateincreases the hardness and elastic modulus of MAO coating and gets surface flat and smooth,leading to enhancing the antifriction and wear resistance. The high hardness of MAO coatingcontaining ZrO2and Mg2SiO4is beneficial for reducing wear. The addition of titania-sol intoalkaline silicate electrolyte reduces roughness and increases elastic modulus of MAO coating, it was demonstrated that the melted wear reducing friction and wear occurs at low rotatespeed.The corrosion behavior of immersed MAO coating was investigated under theimmersion into neutral NaCl for long-term, the results show that for immersed W-containingcoating more corrosive products are accumulated on the surface and micropores are sealed,hence the outer impedance rises and inner one falls. For the immersed coating without W thecorrosion solution enters inner layer due to the large pores and a large amount of corrosionproducts are accumulated in inner layer.