| Acting as one of the green environmental-protective engineering materials in the 21 st century,the excellent performances make Mg alloys available in a wide range of applications including aerospace, automotive, electronic industries, etc. However, poor corrosion resistance of magnesium and its alloy seriously limits their widespread applications. Improving the corrosion resistance of magnesium and its alloy is the most important issue in the current area of research. One of the most effective ways to develop a new protecting coating with high corrosion resistance and long service life is surface modification by protective coatings and to change the structure and component of the protective coatings. The main research directions are:(1) Develop facile and general methods to produce low cost magnesium alloy prevention coatings with novel structures for excellent corrosion resistance performance;(2) Study the structural and componential factors on corrosion prevention coatings performance;(3) Using the ideas of materials genome initiative and the method of simulation forecast to speed up the development of new high performance coatings.The object of the dissertation is set to partly solve the limitations in the present magnesium alloy corrosion prevention coatings research area, for which MAO/DLC composite coatings are prepared by Micro-Arc oxidation(MAO) technology and radio frequency reactive magnetron sputtering(RF-MS) on AZ31 substrates. This article mainly discusses the effect of technological parameter on MAO coating properties and DLC film properties, respectively. The electrochemistry technological parameters include the ratio of electrolyte solution, current density of oxidation and oxidation time, and magnetron sputtering technological parameters contain target power, sputtering time and gas flow. At last, the good MAO/DLC composite coatings are prepared at the optimized technological parameters. The morphology, chemical state and performance of the film system were characterized by Surface Profilometer, dual beam focused ion beam-scanning electron microscope, Raman spectroscopy, micro-scratch tester and electrochemical workstation. The thickness, roughness, surface morphology and microstructure, chemical components, and the corrosion resistance of MAO/DLC composite coatings were studied. The main innovations and results are as follows:AZ31 magnesium alloy was treated by Micro-Arc Oxidation(MAO) in electrolyte containing Na2SiO3,KF,and KOH. Its concentrations were 0.15 M, 0.1 M and 0.2 M respectively. The MAO treatment was conducted at a constant current density of 60 mA?cm-2 for 30 min. The prepared MAO coatings have best corrosion resistance in 3.5wt.% NaCl solution and the threshold-thickness is about 640 nm.When the sputtering power of graphite target, the total pressure, the Ar flow rate, the C2H2 flow rate and the sputtering time of magnetron sputtering technological parameters are 180 W, 1.0Pa, 40 sccm, 20 sccm and 2h, respectively, the prepared DLC film provided the best performance results. The film is uniform and compact with no defects such as cracks and pores. The measured intensity ratio of the D peak to the G peak(ID/IG) of DLC is only 0.68.The corrosion current densities of MAO/DLC coated AZ31 Mg alloy decrease from 1.75×10-5 A cm-2 to 3.323×10-9 A cm-2, which is ~ 4 orders of magnitude lower than the uncoated substrates, and the corrosion potential is increased from-1.56 V to-1.37 V, which is shifted to nobler direction about 0.19 V compared to the uncoated AZ31 Mg substrate. These results show the MAO/DLC coatings provide significantly corrosion protection to the AZ31 magnesium alloy. The DLC coating seals the surface pores of MAO coating and blocks the infiltration of aggressive ions such as Cl- to the substrate, resulting in a significant improvement on corrosion protection of Mg alloys. After 28 days of continuous immersion in 3.5 wt% NaCl solution, It still has good barrier property of MAO/DLC composite coating. |
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