Abstract
In the plasma electrolytic oxidation (PEO) coating of light metal alloys, changing the electrical parameters and electrolytic composition can change the discharge behaviour and, ultimately, the thickness, surface morphology and porosity of the coating. In the present study a combination of cathodic and anodic current pulses with suitable Ton and Toff periods were used to control the porosity and other structural defects of PEO coatings of an AM60B magnesium alloy. In order to investigate the effect of a current mode on the plasma discharge behaviour and coating microstructure during the PEO treatment of magnesium alloy, the emission intensities of six different spectral lines from the plasma species were recorded simultaneously as a function of both time and current mode using optical emission spectroscopy (OES) system. The fluctuations in signal intensities and temperature during the coating process reflect differences in location of both the discharge initiation, and discharge type. The coating surface morphology and microstructure that are obtained can be linked to the plasma discharge behavior. These results are discussed in relation to the discharge behaviour, and how such changes in discharge behaviour relate to the coating mechanisms.
Highlights
Magnesium’s combination of high strength-to-weight ratio and lightness is especially useful in automotive andHow to cite this paper: Hussein, R.O., Northwood, D.O. and Nie, X. (2014) Processing-Microstructure Relationships in the Plasma Electrolytic Oxidation (PEO) Coating of a Magnesium Alloy
According to the evolution of voltage (for example Figure 4(b)), the PEO process can be divided into four discharge stages, which have been described in detailed by Hussein et al [18]
For hybrid1 (H1) current mode, the changing of current mode delays the transaction from stage III to stage IV (see Figure 4(c)) for H2 mode the current mode change accelerates the transition from stage III to the stage IV, which is dominated by the relatively strong discharges. 3.2
Summary
Magnesium’s combination of high strength-to-weight ratio and lightness is especially useful in automotive andHow to cite this paper: Hussein, R.O., Northwood, D.O. and Nie, X. (2014) Processing-Microstructure Relationships in the Plasma Electrolytic Oxidation (PEO) Coating of a Magnesium Alloy. (2014) Processing-Microstructure Relationships in the Plasma Electrolytic Oxidation (PEO) Coating of a Magnesium Alloy. Plasma Electrolyte Oxidation (PEO) is considered to be one of the most cost-effective and environmentally friendly ways to improve the corrosion and wear resistance of magnesium and magnesium alloys [3]. The PEO method can be used to form a thick, hard and adherent ceramic coating on the surface of Mg alloys, as well as other light weight metals Al and Ti, and their alloys [4]-[7]. Different current modes have been utilized in the PEO treatment including, DC, AC, unipolar and bipolar current modes 5 which play important roles in the consequent voltage breakdown, local melting and oxidation of the substrate, quenching and re-crystallization processes. The formation mechanisms for the coatings are complex due to the involvement of electro-, thermal-, and plasma-chemical reactions in the electrolyte [7] [11] [12]
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