The mechanical properties and corrosion resistance of microarc oxidation coatings on Al18B4O33w/AZ91 Mg matrix composite
- Paper number
IAC-06-C2.P.1.14
- Author
Ms. Yanqiu Wang, Harbin Institute of Technology, China
- Coauthor
Dr. Kun Wu, Harbin Institute of Technology, China
- Coauthor
Dr. Min Zhao, Harbin Institute of Technology, China
- Coauthor
Dr. Mingyi Zheng, Harbin Institute of Technology, China
- Year
2006
- Abstract
In recent years, there has been an increasing interest in researching and developing magnesium matrix composites for applications in aerospace field because of their low density, high specific strength, high specific stiffness and low coefficient of thermal expansion etc. The high properties and high cost-effective aluminum borate whisker reinforced Mg matrix composites have a great potential for application. However, the poor corrosion resistance of Mg matrix composites limits their applications, which means surface treatment before application is necessary. However till now, the information about surface treatment of Mg matrix composites is very limited. Microarc oxidation (MAO) is a relatively new surface treatment technique, by which a ceramic coating can be obtained on valve metals and their alloys including Mg and Mg alloys. In fact, MAO is an advancement of the conventional electrochemical anodizing, and that it combines electrochemical oxidation with a high-voltage spark discharge treatment. Therefore it was also called anodic spark deposition (ASD) at its early development stage. MAO is attracting increasing attention among many surface treatment techniques due to its simple process, low cost and high comprehensive performances of the ceramic coatings. However, no research work on MAO of Mg matrix composites reinforced by Al18B4O33 whisker has been reported. In the present study, in order to improve the corrosion resistance of Al18B4O33w/AZ91 Mg matrix composite, the protective coating was fabricated by MAO technique on the composite in an alkali-silicate electrolyte solution. Scanning electron microscope (SEM) was employed to characterize the microstructure of MAO coatings. The phase composition of the coatings was analyzed by X-ray diffraction (XRD). A nanoindentation testing system with a berkovich diamond indenter was employed to measure the mechanical properties of the MAO coating including nanohardness and elastic modulus. Corrosion resistance enhancement of the composite after MAO treatment was evaluated by electrochemical method. The results show that a protective coating with a uniform and compact microstructure can be successfully fabricated on Al18B4O33w/AZ91 composite by MAO technique. Nanohardness and elastic modulus of the coating were greatly higher than those of the composite substrate. The coated composite exhibited excellent corrosion resistance. Corrosion current of the composite decreased by about 5 orders of magnitude after MAO treatment, and that the coated composite showed spontaneous passivation trend during electrochemical polarizing.
- Abstract document