| Magnesium alloys have many characteristics such as low cost and good recyclability,mechanical properties,formability,biocompatibility and biodegradability,which can meet the requitements of many fields.However,the high chemical activity of magnesium alloys leads to its poor corrosion resistance.This shortcoming greatly limits the application of magnesium alloys.Plasma electrolytic oxidation(referred to as PEO)is a surface engineering process,suitable for light metals(Mg,Al,Ti)and their alloys to form protective ceramics coatings.As one of the most effective methods to protect magnesium alloy from corrosion and wear,PEO technology has been widely explored by researchers.At present,many researchers are concerned about the preparation of composite coatings on the surface of magnesium alloys by combining PEO technology with multiple processes.This paper combines magnetron sputtering and PEO technology to prepare composite coatings in silicate,aluminate and silicate-hexametaphosphate electrolyte systems,respectively.At the same time,the corrosion resistance and wear resistance of the prepared coatings were explored.Coatings are prepared in silicate and aluminate respectively.The surface of coating formed in the silicate electrolyte has a“pancake”structure with horizontally distributed voids inside.The coating has a double layer structure and is mainly composed of?-Al2O3.The surface of coating formed in aluminate electrolyte has a“nodular”structures,the coating presents a single layer with more compact structure.The coating mainly composed of?-Al2O3.The performance of the coating was studied through dry sliding wear and corrosion tests.Experiments have proved that the magnetron sputtering aluminum coating can not provide good protection for the substrate.The aluminate coating exhibits excellent wear resistance under 10 N and 20N loads.The wear mechanism is adhesive wear.The wear rate of the simple under 20N load is only 3.76×10-5mm3/(N·m).The silicate coating only showed a low wear rate in the wear test under a load of 10 N,but the coating was destroyed under a load of 20N.Its wear mechanism changed from adhesive wear to abrasive wear.Corrosion tests show that the dual layer of PEO/Al significantly improves the corrosion resistance of AZ31 magnesium alloy.The corrosion current densities of the silicate and aluminate coating are 1.1×10-6(A cm-2)and 1.6×10-6(A cm-2)respectively,which is 3 orders of magnitude lower than the corrosion current density of the substrate.However,the 3.5wt%Na Cl immersion test and electrochemical impedance spectroscopy(EIS)samples show that the aluminate coating has better long-term corrosion protection performance.PEO treatment in the silicate-hexametaphosphate electrolyte system can completely consume the aluminum coating.After PEO treatment for 5 min,10 min,12 min,and 16 min,the film surface had a"pancake-like"structure.After 60min,the"pancake-like"structure disappeared and there were a lot of pores on the coating surface.The second rise of the positive potential in the time-potential curve of the PEO treatment process is caused by the exhaustion of the aluminum coating and the participation of the magnesium alloy matrix in the coating formation process.The XRD test can only detect the Al phase and the?-Al2O3 phase before the positive potential rises,and the Mg Si O3 and Mg2Si O4 phases can be detected with the extension of the PEO treatment time.At the same time,the EDS test results showed that the proportion of Mg element in the PEO film increased rapidly during the positive potential rise time period.The corrosion test proved that after the PEO treatment,the Ecorr of the sample became positive and the Icorr decreased.Except for the PEO treatment for 10 minutes,the protective efficiency of the film on the AZ31magnesium alloy is 95.01%,and the protection efficiency of the other film layers is higher than 99%.After 60 minutes of treatment,the minimum Icorr of the sample is5.680×10-6(A cm-2).At this time,the protection efficiency of the PEO film on the substrate is up to 99.92%. |
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