Tribological and Corrosion Properties of Coatings Produced by Plasma Electrolytic Oxidation on the ZA27 Alloy
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Tribological and Corrosion Properties of Coatings Produced by Plasma Electrolytic Oxidation on the ZA27 Alloy Guangyin Li, Yifan Mao, Zhijian Li, Linlin Wang, and Herbert DaCosta (Submitted May 17, 2017; in revised form October 26, 2017) In this paper, a continuous and dense coating was deposited on samples of the ZA27 alloy through the plasma electrolytic oxidation (PEO) process to improve its wear and corrosion performance. A nontoxic and environmentally friendly inorganic salt, Na2SiO3, is chosen as electrolytes with different concentrations. The effect of the concentration of Na2SiO3 aqueous solutions on the coating performances was investigated. The coatings with 3Al2O3Æ2SiO2 (mullite), Zn2SiO4 and Al2O3 (either crystal phase or with some amorphous SiO2 phases) were formed by the PEO processes. It was found that the coating thickness increased with the increase in electrolyte concentration. However, the wear and corrosion resistance performance of the coatings did not improve as the coatings thickness increased. This was due to the fact that the coating produced with electrolytes of 10 g/L has a porous structure with large pore size. Among all the samples, coating produced by 15 g/L Na2SiO3 has the best wear and corrosion resistance, which is attributed to its continuous and dense structure with thickness of about 47 lm. Keywords
corrosion resistance, PEO coating, wear resistance, ZA27
1. Introduction Zn-based alloys containing a certain amount of Al are denoted as ZA alloys and exhibit multiple advantages such as low casting temperature, good castability, high strength and high corrosion resistance in atmospheric conditions, to name a few. Moreover, as environmentally friendly alloys, they are easy to be recycled and have low manufacturing cost. Thus, ZA alloys have been adopted in a wide range of applications in various industries. Among the family of ZA alloys, ZA27 alloy, which contains 27 wt.% Al, is one of the most investigated for tribological applications. The ZA27 alloy can be used in bearing and bushing applications, as a replacement for bronze and aluminum alloys, due to its good tribological properties, low cost and equivalent or even superior bearing performance (Ref 1). However, ZA27 tends to present insufficient wear resistance under high speed and load, as well as low corrosion resistance, which restricts its applications in some automotive components, molds and dies (Ref 1, 2). Significant efforts have been placed in trying to enhance the wear and corrosion resistance of ZA27 alloys. For example, the microstructure of ZA27 alloys can be modified significantly by appropriate heat treatment processes (Ref 3-6) or doping with alloying elements such as Cu, Si and Mn (Ref 2, 7, 8), which improve its wear and corrosion resistance. Another effective
way is to fabricate metal matrix composites (MMC) by adding composites in form of fibers or hard particles (Ref 6, 9-13). In addition, depositing protective coatings on the surface of components is well accepted as a lo
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