Properties and behavior of zirconia oxide coatings on Mg-Y-Zn alloy using plasma electrolytic oxidation
| Autor: | Li-Wei Ye, 葉力瑋 |
|---|---|
| Rok vydání: | 2017 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 105 The purpose of this study is to coating zirconia oxide layer on Mg-Y-Zn alloy using Plasma Electrolytic Oxidation (PEO) to strengthen surface and corrosion-resistance properties. In some previous studies, when the positive and negative current ratio is less than 1 in the constant current mode, the microarc discharge reaction may switch from “arc” to “soft” regime. In the soft regime, the reaction may reduce damage and the cracks of the oxide layer will be fixed, and this phenomenon may be affected by the parameters of Plasma Electrolytic Oxidation. In this thesis, we will also study the occurrence and mechanism of soft regime in this experiment. Firstly, in order to find the suitable parameters to coat oxide layers on Mg alloy, we use AZ91D for the micro-arc oxidation coating, and change the electrolyte parameters to achieve process stabilization and smooth coating reaction. After the process was stabilized, the nano-zirconia particles were added to the electrolytes for experiment. It was found that there was a significant pore-filling effect in the coating surface after adding nano zirconia, but the trend of voltage decline at the soft regime was not observed all the time and voltage instability was seen. We also found that without nano-particle addition the PEO process grow denses MgO oxide layer at the interface region of the Mg alloy substrate so that the deposited bottom layer with defects and pores close to the substrate interface is separated by the MgO dense layer. On the other hand, after adding the ZrO2 nanoparticles, the particles will be filled into the pores at the interfacial region to achieve denser coating. In order to reduce the instability caused by the addition of nano-zirconia particles, the first part of the experiment demonstrates that the electrolyte and the working parameters can be used for plasma electrolytic oxidation coating on the surface of Mg-Y-Zn alloy. At the second part, we prepare three groups of electrolytes with a iv constant K2ZrF6 / NaOH molar ratio of 1/4 and change the weight ratios of 6: 3.4, 7.5: 4.25 and 9: 5.1 in 2 liter solution of 5 g/l (NaPO3)6 and 20 ml/l nano ZrO2 particles and demonstrated appropriate zirconate concentration and PH value of the electrolyte can solve the problem of nano-particles process instability. With increasing zirconate concentration, the soft regime would occur quickly, and switch the reaction from strong to weak earlier. Surface hardness of the coatings were improved, but if compared with the previous research results, they are lower, probably attributed to the short duty cycle (10%) induced low plasma. If the coating time is not enough or plasma is weak, the surface mechanical properties contributed by the coating layer may be limited. Corrosion resistance tests show that the first group and the third group of the solution after 52.5 minutes PEO treatment exhibit a downward trend in corrosion resistance, which may be attributed to the difference of the thermal expansion coefficients of phases between the substrate and coatings, causing detrimental thermal stress on the oxide layer. The corrosion resistance of the second group of electrolytes (K2ZrF6 / NaOH weight ratio of 7.5: 4.25) was found to exhibit higher corrosion resistance value with the working time and larger workable time region, and the voltage trend was relatively stable. Therefore, it was considered that the suitable electrolyte parameters of the coating on the Mg-Y-Zn alloy. Using the current electrolytes, phase structure analysis of the coating showed that the coating time before and after 60 minutes exhibit a Rhombohedral/Cubic phase change phenomenon of the zirconia-based film, implying the phase transition due to the reaction of MgO and ZrO2 phases. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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