A Study Of The Plasma Electrolytic Oxidation Of AZ31 Magnesium Alloy And The Effect Of SiC Nanoparticles On The Properties Of The Coatings

Magnesium alloy has a series of advantages such as light weight and rich resources.However,the poor corrosion and wear resistance limit its further application.The ceramic coating can be formed on the surface of magnesium alloy by plasma electrolytic oxidation(PEO),and the surface properties can be significantly improved.In order to further improve the performance of PEO coatings on magnesium alloy and to better understand the mechanism of magnesium alloy PEO process,the AZ31 magnesium alloy in phosphate&silicate mixed system and aluminate system was studied,respectively.The effects of SiC nanoparticles on the behavior of PEO and the properties of the coating were investigated.Furthermore,the film growth mechanisms and different microstructure of AZ31 magnesium alloy were analysed in different electrolytes.In the system of 12 g/L Na2SiO3+15 g/L(NaPO3)6,the influence of current density and the addition of silicon carbide(SiC)nanoparticles on the PEO process and film perfoamnce were studied by PEO under a high current density(HC)or low current density(LC)with or without the presence of 2 g/L SiC suspension.The coating growth rate under HC is faster than that of LC condition(The rate for HC-SiC samples with the same addition of silicon carbide are faster than LC-SiC samples as well).At the same current density,the sample with SiC addition can increase the thickness of PEO coating.Besides,the thickness of HC and HC-SiC sample are 121 and 126 ?m,respectively;LC and LC-SiC sample are 76 and 88 ?m,respectively.XRD results show that the coatings formed in different conditions contain a large amount of amorphous phase,which may be related with the silicon and phosphorus elements in the films.In addition,the crystalline phases in the coatings are mainly Mg2SiO4 and MgO.The results of XRD also confirmed that SiC has been incorporated into the coatings.Furthermore,the presence of SiC particles in the coatings were also verified by the scanning electron microscope observation,which was in accordance with the XRD results.The wear resistance is tested by fretting wear at the sliding time of 1200 s.The results show that the wear resistant of LC coatings is higher than that of HC coatings,which may correlate with the decreasing in the compactness of the HC coating.Under the two different current densities,SiC addition can significantly improve the wear resistance of the coatings,which may be ascribed to the increased coating thickness and homogeneity.According to the experimental results,the corrosion resistance of AZ31 magnesium alloy is enhanced by all of the different coatings.In aluminate solution system,the growth rate of PEO coating increased with the increase of the concentration of NaAlO2 electrolyte.Especially,the growth rate of the coating was significantly increased after adding hexametaphosphate(10 g/L NaAlO2+10 g/L(NaPO3)6+1 g/L KOH),the thickness of the coating reaches-74?m at the treatment time of 1200 s.The phase compositions of oxide coatings of the aluminate electrolyte system are MgO and MgAl2O4.At the low concentration electrolyte(2 g/L NaAlO2 +1 g/L NaOH),PEO coating is a nodular structure.While the concentrations were increased to 10 g/L NaAlO2 +1 g/L NaOH and 20 g/L NaAlO2+1 g/L NaOH,the coating morphology transformed into a "pancake"structure with large holes.The newly structure can be explained by the former PEO model in literature.The PEO in 10 g/L NaAlO2+10 g/L(NaP03)6+1 g/L KOH electrolyte under the reaction time of 1200 s generated the best coating corrosion resistence,and the corrosion current density is decreased two orders of magnitude than that of the substrate in diferrent NaAlO2 systems.