Discovery of Al2O3 particles incorporation mechanism in plasma electrolytic oxidation of AM60B magnesium alloy
Highlights
► Al2O3 particles were added into the electrolyte for plasma electrolytic oxidation (PEO) of AM60B magnesium alloy. ► A new chemical reaction mechanism for the incorporation of Al2O3 in the PEO coating was proposed based on our studies. ► It provides another possible mechanism during PEO processes depending on the operating conditions, in addition to the conventionally known mechanical trapping mechanism.
Introduction
Plasma electrolytic oxidation (PEO) offers an effective way to improve surface properties of Al, Mg, Ti, and their alloys, such as micro-hardness, corrosion resistance, and wear resistance. In order to improve the properties of PEO coating, different particles suspended in the electrolyte were incorporated into the coating to improve the microstructure and corrosion resistance of magnesium and its alloys [1], [2], [3], [4], [5]. Examples of such particles include silica, titania, zirconia, and alumina (Al2O3) [6], [7], [8]. Mechanical trapping of particles have been extensively reported in the conventional coating processes from liquid solution to form a composite coating, such as electroplating and anodizing [9]. Unfortunately, the Al2O3 particle incorporation mechanism in PEO process has hardly been reported, with the exception by Liu et al. that also proposed a simple mechanical trapping mechanism [10]. However, plasma electrolytic oxidation is a unique process when compared to those conventional electrochemical coating processes in that it involves the creation of high temperature micro-arc. From the curve fitting of the spectrum of thermal radiation with Planck's law, the temperature of plasma arc was determined by Klapkiv et al. to be about 6000 °C [11]. Others have reported arc temperature of 2000–3000 °C [12]. Under certain operating conditions (such as applied voltage, frequency, etc.) and electrolyte chemistry, the arc temperature could exceed the melting temperature of particles, 2072 °C in the case of Al2O3 [13]. The Al2O3 melt could then participate in chemical reactions to form a coating. With that, we allow three possible ways for Al2O3 particles to participate in the coating formation depending on the deposition conditions: (i) Al2O3 is trapped mechanically in the coating; (ii) Al2O3 is chemically reacting with other species during coating formation, (iii) or the combination of the two. To explore the Al2O3 incorporation mechanism, coatings formed using electrolytes with and without Al2O3 particle suspension were investigated.
Section snippets
Experimental
Samples of AM60B magnesium alloy with dimensions of 20 mm×20 mm×7 mm were cut from an ingot. The nominal composition of AM60B alloy was: 5.6–6.4 wt% Al, 0.26–0.50 wt% Mn, 0.2 wt% Zn, 0.05 wt% Si, 0.004 wt% Fe, 0.008 wt% Cu and balance of Mg. The samples were first polished using sand paper up to 600 grit, degreased in 2-propanol in an ultrasonic bath for 5 min and then dried in warm air before PEO treatment.
The main electrolyte used was an aqueous solution containing 18 g/l NaAlO2 and 0.5 g/l KOH. In
Results and discussion
The voltage versus time curve is shown in Fig. 1. Compared to the curve without Al2O3 suspension, the addition of Al2O3 particles in the solution appeared to slow down the voltage increase and prolonged the constant current time from 160 s to 240 s. This implies that the coating thickness growth rate is lower with Al2O3 addition. Since the coating formed contains both MgO and MgAl2O4, the decreased coating growth rate could be caused by the melting of MgO phase chemically formed in the coating
Conclusions
With Al2O3 particles suspended in an aluminate electrolyte, the ratio of MgAl2O4/MgO in the PEO coating formed on an AM60B alloy increased and no Al2O3 particles were detected in the coating. A new chemical reaction mechanism for the incorporation of Al2O3 in the PEO coating was proposed, as opposed to the mechanical trapping mechanisms previously reported. However, it ought to be mentioned that this is the case under the experimental conditions used in our work. This does not exclude the
Acknowledgment
The present work was supported by the National Research Council Canada.
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