Coating growth behavior during the plasma electrolytic oxidation process

Abstract

In this study, aluminum oxide was deposited on an Al alloy substrate to produce hard ceramic coatings using a plasma electrolytic oxidation (PEO) process working at atmospheric pressure. The process utilizes dc and unipolar pulsed dc in the frequency range 0.2–20 kHz. Optical emission spectroscopy was employed to study the species and electron temperature of the plasma. The morphology and microstructure of the coatings were investigated using scanning electron microscopy. It was found that in the first 12 min of the PEO process, the plasma electron temperature increased with the applied voltage during the experiments, the plasma electron temperature was found to be in the range 4000–9000 K, and the applied voltage to the electrodes ranged up to 550–600 V for the different current modes. The plasma temperature profile exhibits a wider peak temperature spike for the dc power mode than for the pulsed dc mode, indicating that the dc plasma discharges would provide longer sintering time. The pulsed dc mode increases the spike temperature up to 8700 K but does not necessarily enhance the coating growth. The high spike temperature generated by strong discharges likely melts the oxide and then traps gas into the melt pool, resulting in some porosity at the interface. By eliminating the high temperature spike, a denser interface layer and homogenous coating morphology are produced.

The effect of current mode and discharge type on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloy AJ62

“…under only positive polarization of the metal electrode, operating at a 2 kHz and duration time of 80% duty cycle. The specific frequency was chosen based on the dependence of coating growth rate and coating characteristics to current frequency [25]. -A bipolar current mode, comprising two components, i.e., a positive component and a negative component.…”

“…Different current modes have been utilized in the PEO treatment including, DC, AC, unipolar and bipolar current modes [21][22][23]. Compared to a unipolar current operating mode, the application of bipolar current pulse regimes usually produces a denser PEO coating on Al-and Mg-alloys [24][25][26]. The properties of the plasma discharges themselves in the bipolar current mode differ from those of the discharges for the unipolar current mode.…”

“…In these previous studies, the collective behavior of the plasma discharge over the entire surface, rather than the individual discharges, was reported. Based on the OES results, and with an assumption of local thermodynamic equilibrium (LTE) [27], plasma electron temperatures for an Al 1100 alloy have been calculated using the relative intensities of spectral lines of the same atomic or ionic species [24,25]. Evaluations based on the spectroscopic method for an Al 1100 alloy show the electron temperatures to be in the range of 4000-7000 K for the unipolar current mode and 4000-5500 K for the bipolar current mode [20].…”

Evolution of the Three-Dimensional Structure and Growth Model of Plasma Electrolytic Oxidation Coatings on 1060 Aluminum Alloy

“…Plasma electrolytic oxidation (PEO) is a surface-modification technique for producing ceramic coatings on light metals and their alloys (such as aluminum, magnesium, and titanium) [1,2]. PEO coating is considered to be amongst the most promising protective coatings for application in a wide range of industry sectors because of its high microhardness and its good wear and corrosion resistance [3,4].…”

Influence of Applied Frequency on Microstructural and Electrochemical Characteristics of Ceramic Coating Produced on Al-Mg Alloy by Pulsed Plasma Electrolytic Oxidation

“…Yerokhin et al and R.O. Hussein et al related to the influence of electrical parameters on coating characteristics in pulsed PEO coating process for Al and Al alloys [1][2][3]. However, the materials in their investigations were limited to 1000 or 6000 series Al alloy, and only a few studies have dealt with Al-Mg alloy.…”