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Preparation and Investigation of ZrO2 Coating on AZ31 Alloy Processed by Plasma Electrolytic Oxidation Method

Eienkhah, Feryar | 2014

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 45274 (07)
  4. University: Sharif University of Technology
  5. Department: Materials science and Engineering
  6. Advisor(s): Faghihi Sani, Mohammad Ali
  7. Abstract:
  8. Magnesium alloys are used in various industries, due to their excellent combination of low density, high strength to weight ratio and biocompatibility. However, a main drawback of Mg alloys is their poor corrosion and tribological behavior which could be improved by different surface treatments such as plasma electrolytic oxidation (PEO). Recently, PEO method as a simple and economic process is used to provide thick ceramic coating on Mg alloys. Based on some problems in zirconia coating on Mg substrates, two step PEO coating was introduced. The aim of the present study is to prepare and investigate the properties of zirconia-containing coating by two-step PEO process on the AZ31 Mg alloy and investigate its growth mechanism. The first step was carried out in alkaline phosphate electrolyte to protect the substrate from acidic potassium flourozirconate solution used in the second step. Effect of process parameters on coating including time in the first and second step, current density and frequency in the second step was evaluated. For comparison, zirconia coating was also performed through single step treatment from alkaline potassium flourozirconate electrolyte with the same instrumental condition as used in the two- step process. The surface morphology, phase composition, corrosion behavior, microhardness and tribological characteristics of the coated oxide layers were studied trough microscopy, electron dispersive spectroscopy, X-ray diffraction, potentiodynamic polarization, electrochemical impedance spectroscopy microhardness and pin-on-disc test. It was found that the two-step coating mainly composed of Mg2Zr5O12, ZrO2 and MgF2 phases. Moreover, coating formed in 1 min during the first step with Icorr of 1.02 * 10-2 µA/cm2 and CR of 0.007 mpy exhibited the best corrosion behavior among other coatings prepared at different process times due to its low surface porosity (~10.5%). It was found that thickness of coatings was increased up to ~34.6 µm with the process time to 10 min in the second step. So, coating prepared in 10 min with Icorr of 2.63*10-2 µA/cm2 and CR of 0.018 mpy was the optimum coating based on corrosion property. Results showed that there is an optimum amount of current density and applied frequency for good corrosion resistance. In fact, coating performed at current density of 150 mA/cm2 and frequency of 500 Hz with Icorr of 3.87*10-2 and 1.03*10-1 µA/cm2 Exhibited superior corrosion behavior in comparison with other coatings due to combination of low surface porosity (~19.5% and ~14.3%) and sufficient thickness (22.5 and 29.9 µm). Results illustrated that two step coating with Icorr of 7.71*10-2 µA/cm2 and CR of 0.053 mpy had better corrosion behavior in comparison with one step coating with Icorr of 1.56 µA/cm2 and CR of 1.311 mpy because of its low surface porosity. The coating with high hardness as well as low surface porosity results in better wear and mechanical properties of two step coating than the one step coating
  9. Keywords:
  10. Zirconia ; Wear ; Corrosion ; Plasma Electrolytic Oxidation ; AZ31 Magnesium Alloy

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