Effect of Processing Parameters On Soft Regime Behavior of Plasma Electrolytic Oxidation of Magnesium

Significance Statement

Magnesium and its alloys has a wide range of applications in industrial setups. This can be heavily attributed to the great deal of advantages that it offers such as; low density, high strength to weight ratio, good electromagnetic shielding among many more. However, magnesium and its alloys are highly susceptible to corrosion which greatly hinders their further applications, mainly in solutions containing bromide, chloride and sulphate ions. In recent times, it has been observed that coating magnesium with Zirconium oxide generates a combination of unique and desirable properties such as good mechanical strength, superior refractoriness, high ionic conductivity, excellent corrosion resistance and low thermal conductivity.

Many methods are available for the coat deposition such as plasma spraying, laser ablation, metal organic chemical vapor deposition, sputtering and sol-gel techniques. Nonetheless, these techniques have some limitations which include costly equipment, complicated process and relatively low interface bonding strength, which inhibit their development and application. Alternatively, plasma electrolytic oxidation technique offers a chance to solve these problems. Regardless, the   plasma electrolytic oxidation technique coating is not good enough for many industrial applications where dense and consistent layers with worthy mechanical properties are needed.

Researchers led by professor Chen-Chia Chou  at National Taiwan University of Science and Technology developed a novel ceramic coatings on magnesium alloy by soft plasma electrolytic oxidation regime. They aimed at fabricating high quality ceramic coatings that not only had improved corrosion resistance but also enhanced wear resistance of magnesium alloys. Their work is now published in the peer-reviewed journal, Ceramics International.

The research team examined the compositions, structure and morphologies of the oxide coatings formed using different working parameters by energy dispersive spectroscopy, x-ray diffraction, and scanning electron microscopy. They then evaluated corrosion resistance of the oxide coatings in 3.5 wt% sodium chloride solution using potentiodynamic polarization tests. Eventually, they fabricated Zirconia-containing ceramic coatings on pure magnesium by plasma electrolytic oxidation technique using two different anodic to cathodic charge quantity ratio.

The research team were able to observe that the occurrence of soft regime in CR = 0.85 may be constantly established, which strongly reduce the arcing that usually cause detrimental defects in the oxide layer. They also noted that the occurrence of soft regime in CR = 0.85 was being constantly established, which strongly reduced the arcing that was usually seen to cause detrimental defects in the oxide layer. Growth of the oxide layer can be attributed to a competition between two phenomena, sintering and etching, which induce denser coatings. Unfortunately, the corrosion resistance performance is still not enough, probably due to a big mismatch of the thermal expansion coefficients of the substrate and the ceramic layer, which produces cracks at the interface as the coating grow thicker.

Herein, it has been noted that by using a condition of anodic to cathodic charge quantity ratio smaller than 1 not only induces a “soft regime” during a plasma electrolytic oxidation process but also provides a qualitative improvement in the coating morphology and density both in surface and cross section. The negative current plays a huge role in altering the characteristics of the arc and do the etching process within the soft plasma electrolytic oxidation regime. However, the formation mechanism of the soft regime during plasma electrolytic oxidation process need further investigations.

Effect of Processing Parameters On Soft Regime Behavior of Plasma Electrolytic Oxidation of Magnesium-Advances in Engineering

Back-scattered electron micrograph using scanning electron microscopy showing a flat and uniform oxide coating on a Mg-Y-Zn magnesium alloy, which provide excellent corrosion-resistant property and rarely observed in the reported works.

About The Author

Chou, Chen-Chia was born in 1959 in Taiwan. He is currently a distinguished professor with the Department of Mechanical Engineering in the National Taiwan University of Science and Technology. He obtained his BSc degree in Mechanical Engineering from the National Chiao Tung University (1981, Taiwan), his MSc degree in Materials Science and Engineering from the Sun Yat-Sen University (1983, Taiwan), and his PhD degree in Materials Science and Engineering from University of Illinois at Urbana-Champaign, USA in 1990. After spending two years as a post-doctoral fellow and visiting assistant professor at the University of Illinois, he joined the Department of Mechanical Engineering of the National Taiwan University of Science and Technology in 1992.

His specialties are on phase transformation of materials, microstructural characterization of materials and electron microscopy. His teaching and research interests include advanced ceramic materials, their novel processing and their applications, consisting of ferroelectric/piezoelectric materials and devices, microwave dielectric materials and processing for communication devices, zirconia-based materials for mechanical and bio-medical applications.

He is also interested in novel fabrication technologies such as plasma electrolytic oxidation, laser annealing and microwave sintering of ceramics as well as precision machining for brittle materials. He is an author or co-author of about 200 journal and conference papers in the areas of ferroelectric, piezoelectric and dielectric materials, novel processing techniques using laser or microwave annealing/processing, transmission electron microscopy for materials, material phase transformation, material and mechanism design. He is associated with Chinese Society of Materials Science and Taiwanese Ceramic Society as life members.

Reference

Felix Tjiang, Li-Wei Ye, Yan-Jang Huang, Chen-Chia Chou, Da-Shiang Tsai. Effect of processing parameters on soft regime behavior of plasma electrolytic oxidation of magnesium. Ceramics International, volume 43 (2017) pages S567–S572.

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