| Magnesium and its alloys are the lightest structural engineering materials and are made as the best choice for replacing the traditional metals in the view of manufacturers due to their low density and high specific strength. They have gradually been applied in the fields of automotive, electronic, aerospace, and so on. However, the poor corrosion and wear resistance of magnesium alloys limit their application severely.Micro-arc oxidation (MAO), also known as plasma micro-arc oxidation (PMAO), anodic spark deposition(ASD), spark discharge anodic oxidation(ANOF) and spark anodizing process(SAP), is a new environmentally friendly technology of obtaining high quality matrix metal oxide-based ceramic coating on aluminum, magnesium, titanium and their alloy surfaces based on the normal anodic oxidation and getting more and more attention in the field of surface treatment. The micro-arc oxidation layers are formed in the electrolyte used specialized oxidation power and relied on the arc discharge generated on the workpiece anode with the action of instantaneous high temperature, high voltage and so on. The wear resistance, corrosion resistance and mechanical strength of magnesium alloy can be improved by micro-arc oxidation treatment. For promoting the development of micro-arc oxidation technology, it is considerably significant to discuss the growth characteristic of the ceramic coating and study the influence of electrical parameters on micro-arc oxidation.There always exist microporous in the MAO coatings which is detrimental to corrosion and wear resistance. It is important for improving the corrosion and wear resistance of magnesium alloys by sealing technology. In this thesis, firstly a dense ceramic coating was prepared on the surface of AZ91D magnesium alloy by micro-arc oxidation method in the silicate-phosphate, and then composite coatings with good performance were formed on the oxidation film surface through sol-gel sealing technology. Scanning electron microscope (SEM) for surface morphology observation, X-ray diffraction (XRD) for phase components analysis, polarization curves for corrosion resistance testing, reciprocating wear experiment for tribological performance and UV spectrophotometer for photocatalytic properties investigation were used. The main results are shown as follows:(1) An insulating oxidation layer was formed with many pores like crater in the surface after micro-arc oxidation. With oxidation time increasing the pores diameter and the molten particles increases gradually while the total number decreases. The ceramic coating thickness increases while the growth rate slows down. (2) The change of the current density has a significant influence on the cross-section morphology of the ceramic coating. When the current density is lower, the thickness of the ceramic quickly increases with the current density. However, the thickness of the ceramic reduces when the current density is beyond a certain value. The highest coating thickness can be obtained when the current density is l0A/dm2.(3) The ceramic coatings prepared in silicate-phosphate composite system with different oxidation time and current density are composed of MgSiO3、 Mg2SiO4、Mg3(PO4)2、MgO and Mg phases and mainly Mg2Si04、MgSiO3. After sealing by sol-gel technology, a new phase of titanium oxide appears on the ceramic coating.(4) After micro-arc oxidation, the corrosion current density and corrosion rate of the magnesium alloy are improved remarkably. However, the corrosion resistance dose not always increase with oxidation time and current density and it is related to the density of the coatings. When the oxidation time is20minutes and the current density is10A/dm2, the coating shows the best corrosion resistance.(5) The wear rate of AZ91D magnesium alloy decreases significantly after micro-arc oxidation. It reduces with the oxidation time extension. When the oxidation time is30minutes, the thickness of the coating is the biggest and the wear resistance is the best, which makes the wear rate is one tenth of the one of the magnesium alloy matrix. (6) The pores on coating surface reduce obviously and the corrosion resistance of magnesium alloy increases remarkably after the sol-gel sealing treatment. The corrosion potential of the composite film increases with a value of206mV and the corrosion rate decreases with two magnitudes comparing to the ceramic coating. The wear rate of the composite film is lower than the ceramic coating and is only one seventh of the one of the magnesium alloy matrix.(7) The composite film shows a certain photocatalytic properties of more than20%photo-degradation rate after sol-gel sealing treatment. |
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