Plasma Electrolytic Oxidation Ceramic Coatings Research Articles

Enhancing Dry Sliding Wear Resistance of Pure Mg by Plasma Electrolytic Oxidation Ceramic Coatings Using Silver Addition

This study investigated the wear effect on ceramic coated Mg by Plasma Electrolytic Oxidation. Three series of PEO coatings were prepared; one of was the effect of silicate electrolyte (5, 8, and 10 g/L) concentrations and the other was the effect of coating time (5, 10, 15 min), last one was the effect of AgNO3 (0.1, 0.2, 0.4 g/l) additive. The coatings were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The mechanical properties were analyzed by microhardness and linear wear test. The unlubricated wear resistance was observed by linear ball-on-disc tribometer. With increasing the silicate concentration, the average pore size and porosity percentage and the surface roughness increase. On the other hand, with increasing the coating time, the surface morphology gradually becomes flat. The coating time caused more compact and homogenous appearance. The surfaces coated with the addition of silver were obtained smoother and denser. The wear resistance of the high time coated Mg was significantly improved. The best wear resistance was obtained with silver-reinforced coatings.

Effect of K2ZrF6 concentration on thermal ablation resistance of ZrO2/TiO2 composite ceramic coatings on TC4 alloy

The poor thermal ablation resistance of TC4 alloy limits its use in critical high-temperature applications. In this work, ZrO2/TiO2 composite ceramic coatings were prepared by plasma electrolytic oxidation (PEO) to enhance the ablation resistance of TC4 alloy. The phase compositions, microstructure and ablation resistance of the coatings are significantly influenced by the addition of various concentrations of K2ZrF6 to the electrolyte. The X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDS) techniques were employed to investigate the phase composition, chemical compositions, microstructure, and elemental distribution of the PEO ceramic coatings. The results showed that ZrO2/TiO2 composite ceramic coatings formed on substrate consisted of t-ZrO2, ZrTiO4, R-TiO2, and A-TiO2 phases. These coatings also exhibited a volcanic-like porous structure. The thermal ablation resistance tests were carried out at 1300 °C for 3000 s, which indicated that the presence of t-ZrO2 in the coating could improve the ablation resistance of TC4 alloys by inhibiting crack propagation. Moreover, the thermal ablation resistance of the PEO coating prepared with 15 g/L K2ZrF6 was superior to that of other coatings, as indicated by the crack density and weight loss.

Investigation of Mechanical Behavior and Surface Characteristics of Cold Spray Metallized B4C/AA7075 Composites Coated by AZ64 Alloy through Plasma Electrolytic Oxidation

Metallized cold-spray coatings were employed to make B4C/AA7075 and aluminum + plasma electrolytic oxidation (PEO) duplex coatings on AZ64. In addition, the phase structure, mechanical characteristics, wear, and PEO ceramic coatings examine the corrosion resistance. According to the findings, the PEO ceramic coating comprises α-aluminum oxide and γ-aluminum oxide, with some remnants of B4C still being preserved. PEO ceramic coatings outperformed their corresponding CS counterparts regarding mechanical characteristics and wear resistance. For example, the PEO-B4C coating achieved a hardness of 13.8 GPa and an elastic modulus of 185.5 GPa, which were 21.0% and 23.5%, respectively, more significant than the comparable values for the coating with CS. The PEO-B4C coating was 58% and 15.7% less abrasive than the equivalent CS coating due to its lower wear rate of 4.84 × 10−5 mm3/Nm and relatively lower of 0.64. The density of corrosion current in the PEO-treated B4C-AA7075 coating (3.735 × 106 A/cm2) is similar to the corrosion current density in the untreated CS coatings. Finally, compared to untreated CS B4C-AA7075, the coating’s mechanical characteristics and wear resistance are considerably enhanced by the PEO treatment.

Quasi-in vivo corrosion behavior of AZ31B Mg alloy with hybrid MWCNTs-PEO/PCL based coatings

This study investigated the effects of multi-walled carbon nanotubes (MWCNTs) and polycaprolactone (PCL) on the quasi- in vivo corrosion behavior of AZ31B Mg alloy treated by plasma electrolytic oxidation (PEO). Thin (∼2 μm, PCTPCL4) and thick (∼60μm, PCTPCL6) PCL layers were applied only onto the MWCNTs-PEO coating (PCT) as it showed better corrosion performance. Findings reveal that incorporation of MWCNTs induced several structural and functional modifications in the PEO coating, such as increased roughness, a thicker inner barrier layer, and reduced hydrophilicity. Quasi- in vivo corrosion testing was carried out under controlled temperature, pH, and fluid flow in simulated body fluid (SBF) by electrochemical impedance spectroscopy (EIS) and hydrogen evolution experiments. EIS results revealed that, after 48 h immersion, a diffusion process controlled hydration of the ceramic coatings. Comparison of the collected hydrogen after 15 days of immersion in the quasi- in vivo environment revealed that the PEO and PCT ceramic coatings decreased hydrogen generation by up to 74% and 91%, respectively, compared to non-coated alloy. PCTPCL6 coating exhibited the lowest amount of collected hydrogen (0.2 mL/cm 2 ). The thick PCL layer delayed the onset of substrate corrosion for at least 120 h, reducing the corrosion rate by 85% compared with the PCT.

Influence of ceramic coating pores on the Tribological performance of PEO–PTFE composite coatings on the Ta–12W alloy

Self-lubricating composite coatings with an intermeshing structure on the Ta–12W alloy were fabricated using plasma electrolytic oxidation (PEO) and polytetrafluoroethylene (PTFE) impregnation sintering. Porous PEO ceramic coatings were formed in alkaline silicate–phosphate electrolytes using NaF and NaOH as the pore morphology adjustment agents. The tribological performances of the PEO ceramic and PEO–PTFE composite coatings were examined under 10–70-N loadings using a ball-disk wear tester. An optimized basic PEO ceramic coating with a thickness of 25 μm, a porosity of 23.8 %, and a hardness of 1410 HV is obtained in an electrolyte containing 5 g/L of NaF and 5 g/L of NaOH. A soft PTFE lubricating layer is fixed on the hard PEO ceramic coating, forming an intermeshing structure that can effectively reduce the coefficient of friction (COF) over time while enhancing the load capacity of the composite coating. In addition, the synergistic effect of the PEO ceramic coatings and self-lubricating material is proposed. The PTFE material stored in the pores will be constantly transferred to the contact interface to achieve a reduced COF. The PEO ceramic coating with a suitable porous structure acts as an “armor” because it can withstand a large load and provide physical support to the PTFE material during the friction process. Thus, the composite coating achieves a low COF with a low wear rate and a high load-bearing capacity.

Incorporating TiO2 nanoparticles to enhance corrosion resistance, cytocompatibility, and antibacterial properties of PEO ceramic coatings on titanium

This study aimed to investigate the influence of incorporating different concentrations (1, 3, and 5 g/l) of anatase-TiO2 nanoparticles (NPs) on morphology, roughness, wettability, corrosion resistance, cytocompatibility, and antibacterial activity of porous coatings produced by plasma electrolytic oxidation (PEO) process on pure titanium. Microstructural studies evidenced the successful incorporation of TiO2 NPs in the coatings. Increasing the concentration of TiO2 NPs from 1 to 5 g/l led to reduced porosity and increased wettability, with no significant change in the surface roughness. The results of the electrochemical tests after 7 days of exposure to simulated body fluid (SBF) indicated that the highest corrosion resistance (2.292 MΩ cm2) was obtained by the addition of 3 g/l TiO2 NPs. The cytocompatibility test after 3 days of cell culture showed that lower concentrations of TiO2 NPs, i.e., 1 and 3 g/l, provided the optimum conditions for adhering and flattening of MG63 cell on coatings surface. The antibacterial tests revealed that the more the TiO2 NPs concentration was, the less bacterial adhesion and therefore, the better the antibacterial properties against Staphylococcus aureus (S. aureus) bacteria were achieved. The addition of 5 g/l TiO2 NPs into the electrolyte provided the antibacterial rate of 94.47% for the coating. The incorporation of TiO2 NPs positively affects physico-chemical properties, cytocompatibility, and antibacterial behavior of pure titanium coated by PEO method.

Plasma electrolytic oxidation of cold spray kinetically metallized CNT-Al coating on AZ91-Mg alloy: Evaluation of mechanical and surficial characteristics

The cold-spray kinetically metallized (CS) carbon nanotube-aluminum (CNT-Al) and pure Al coatings were treated using plasma electrolytic oxidation (PEO) process to obtain CNT-Al+PEO and Al+PEO duplex coatings on AZ91 Mg alloy, respectively. The phase composition, microstructure, mechanical properties, wear and corrosion resistance of the PEO ceramic coatings were studied. The results showed that the PEO ceramic coating was mainly composed of α-Al2O3 and γ-Al2O3, moreover, the small amount of CNT was also retained during the PEO process. The mechanical properties and wear resistance of the PEO ceramic coatings were significantly improved compared to the respective CS coatings. In particular, the hardness and elastic modulus of the PEO-CNT coating reached 13.9 GPa and 185.4 GPa, which were 21.0% and 23.4% higher than the respective CS coating. Moreover, the wear rate of the PEO-CNT coating was only 4.83 × 10−5 mm3/N·m with a relatively low coefficient of friction of 0.65, which were 59.8% and 15.6% lower than the corresponding CS coating. The corrosion current density of the PEO-treated CNT-Al coating was 3.734 × 10−6A/cm2, which was similar to the untreated CS coating. In short, the PEO treatment greatly improves the mechanical properties and wear resistance of the CS CNT-Al coating.

Plasma Electrolytic Oxidation Ceramic Coatings on Zirconium (Zr) and ZrAlloys: Part I—Growth Mechanisms, Microstructure, and Chemical Composition

Recently, a significant number of research projects have been directed towards designing and developing ceramic coatings for zirconium-based substrates due to their outstanding surface properties and utilization in modern technologies. The plasma electrolytic oxidation (PEO) coating is an environmentally friendly wet coating method that can be performed in a wide range of electrolytes. The surface characteristics of PEO coatings can be tailored by changing electrochemical parameters, electrolyte composition, and substrate alloying elements to adopt a conformal and adhesive PEO ceramic coating for the final demanding applications in chemical, electronics, and energy technologies. This review focuses on deriving a deeper fundamental understanding of the PEO growth mechanisms and the effect of process parameters on transient discharge behavior at breakdown, initiation, and growth of the oxide layer and incorporating species from the electrolyte. It highlights the fundamental microstructural properties associated with structural defects, phase transformation, and the role of additives.

Plasma Electrolytic Oxidation Ceramic Coatings on Zirconium (Zr) and Zr-Alloys: Part-II: Properties and Applications

A plasma electrolytic oxidation (PEO) is an electrochemical and eco-friendly process where the surface features of the metal substrate are changed remarkably by electrochemical reactions accompanied by plasma micro-discharges. A stiff, adhesive, and conformal oxide layer on the Zr and Zr-alloy substrates can be formed by applying the PEO process. The review describes recent progress on various applications and functionality of PEO coatings in light of increasing industrial, medical, and optoelectronic demands for the production of advanced coatings. Besides, it explains how the PEO coating can address concerns about employing protective and long-lasting coatings with a remarkable biocompatibility and a broad excitation and absorption range of photoluminescence. A general overview of the process parameters of coatings is provided, accompanied by some information related to the biological conditions, under which, coatings are expected to function. The focus is to explain how the biocompatibility of coatings can be improved by tailoring the coating process. After that, corrosion and wear performance of PEO coatings are described in light of recognizing parameters that lead to the formation of coatings with outstanding performance in extreme loading conditions and corrosive environments. Finally, a future outlook and suggested research areas are outlined. The emerging applications derived from paramount features of the coating are considered in light of practical properties of coatings in areas including biocompatibility and bioactivity, corrosion and wear protection, and photoluminescence of coatings

Role of incorporation of ZnO nanoparticles on corrosion behavior of ceramic coatings developed on AZ31 magnesium alloy by plasma electrolytic oxidation technique

Improvement of corrosion resistance is one of the main goals in developing plasma electrolytic oxidation (PEO) ceramic coatings on AZ31 magnesium alloys. In this study, ceramic coatings were deposited on AZ31 magnesium alloys by PEO technique in electrolytes containing 10 g/l sodium phosphate (Na3PO4) and 1.5 g/l sodium hydroxide (NaOH) at different concentrations of ZnO nanoparticles and current density of 7.1 A/dm2 for 10 min. To investigate the corrosion behavior of the samples, they were placed in Hank's solution, while Tafel polarization and electrochemical impedance spectroscopy (EIS) were used. X-ray diffraction (XRD) together with field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS) techniques were employed to investigate the coatings. XRD results showed that the prepared PEO layers mainly consist of ZnO, Mg, and Mg3(PO4)2 phases. Corrosion test results showed that the use of ZnO nanoparticles improved the corrosion resistance of PEO ceramic coatings in Hank's solution. In contrast, the best result was obtained by using the electrolyte containing 6 g/l ZnO nanoparticles with a corrosion current density of 0.08 µA/cm2 and corrosion potential of -1.4615 VAg/AgCl. Also, EIS results showed the corrosion resistance of the mentioned coating as 1.05 × 106 Ω.cm2.

Plasma electrolytic oxidation ceramic coatings proceed by porous anodic film

Ceramic coatings were performed on aluminum through plasma electrolytic oxidation (PEO) in a Na5P3O10 solution with various concentrations of NaOH. It was revealed that the concentration of NaOH affected the types of anodic films and the activity of plasma discharges. Emphatic investigation was performed on the coatings fabricated in the electrolyte with addition of 1 g L−1 NaOH, focusing on the coating microstructure evolution and coating corrosion resistance. Some coatings were detached from the substrate, showing that the oxide particles on the anodic film C/S interface underwent profile variations from hemisphere to polygon, and its interpore distance decreased over time. The formations of pores of anodic films and plasma discharge of the PEO coatings were both easy to occur around the existing pores and the existing discharge channels. Fractured cross-section showed that a pancake-like structure corresponded to a discharge channel, a cavity, and a barrier layer protrusion whose area was similar to that of the corresponding pancake-like structure. The data of potentiodynamic polarization measurements showed that the sample treated with PEO for 10 min possessed the lowest icorr value of 4.97E-8 A cm−2, the most positive Epit value of −0.63 V, and the highest Rp value of 2.66E5 Ω cm2. EIS data of 10 min sample exhibited the highest R2 value of 1.61E6 Ω cm2, demonstrating the best corrosion resistance among these samples.

The correlation between the Na2SiO3·9H2O concentrations and the characteristics of plasma electrolytic oxidation ceramic coatings

Abstract Various plasma electrolytic oxidation (PEO) ceramic coatings were fabricated on aluminum in the electrolytes with different concentrations of Na2SiO3·9H2O. The morphology of fractured cross-section and coating/substrate (C/S) interface, phase compositions, and corrosion behavior of these ceramic coatings were characterized employing field emission scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffractometer (XRD), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The results showed that adding Na2SiO3·9H2O in the electrolyte could reduce the decrease amplitude of the voltage-time curve, and it can affect the types of the anodic film before the beginning of plasma discharges. A novel phenomenon was recorded as PEO treatment was carried out in the electrolyte with 1.85 g l−1 Na2SiO3·9H2O that the substrate was first covered by a barrier-type anodic film, and then a porous-type anodic film appeared after the barrier-type anodic film was broken down. The anodic film continued to grow for a long duration in the PEO process after being broken down. Characteristics of PEO ceramic coatings with a similar thickness acquired in various electrolytes were also evaluated. XRD results indicated that these ceramic coatings of similar thickness were mainly composed of α-Al2O3 and γ-Al2O3, and the α-Al2O3 proportion decreased with increasing Na2SiO3·9H2O concentration. The electrochemical data showed that the corrosion resistance of these PEO ceramic coatings of similar thickness decreased by adding Na2SiO3·9H2O in the electrolyte.

Influence of Al2O3 addition in NaAlO2 electrolyte on microstructure and high-temperature properties of plasma electrolytic oxidation ceramic coatings on Ti2AlNb alloy

Plasma electrolytic oxidation (PEO) has become an effective surface modification method to form ceramic coatings on orthorhombic Ti2AlNb alloy due to its high productivity, economic efficiency and ecological friendliness. In the present work, different contents of Al2O3 additive were introduced into a NaAlO2 basic electrolyte with the aim of improving the high-temperature properties of PEO ceramic coatings on Ti2AlNb alloy. The influence of Al2O3 addition in NaAlO2 electrolyte on microstructure, adhesive strength, isothermal oxidation behavior and tribological properties of PEO coatings at elevated temperatures was investigated by means of X-ray diffraction, scanning electron microscopy, the pull-off method, isothermal oxidation tests as well as ball-on-disc friction and wear tests. The introduction of Al2O3 into the basic electrolyte promotes the formation of Al2TiO5 phase in the PEO coatings. Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings with Al2O3 addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L−1 Al2O3 additive exhibits the least mass gain of only 1.33 mg cm−2. The micro-hardness of the PEO coatings prepared with Al2O3 additive is improved due to the presence of Al2TiO5 phase, however, the adhesive strength of PEO coating is slightly weakened due to the formation of defects inside ceramic coating. The wear resistance of Ti2AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coatings prepared with Al2O3 addition is comparable to those formed in the basic electrolyte at both room temperature and 600 °C.

Effects of anodic and cathodic current densities on microstructure, phase composition and properties of plasma electrolytic oxidation ceramic coatings on 6063 aluminum alloy

Ceramic coatings on 6063 aluminum alloy were prepared by plasma electrolytic oxidation (PEO) using pulsed bipolar power supply under different anodic and cathodic current densities. The scanning electron microscope (SEM), x-ray diffraction (XRD), potentiodynamic polarization curves analysis and tribometer were used to investigate the microstructure, phase composition, corrosion and wear resistance properties of PEO coatings. It is found that with the anodic and cathodic current densities increasing, the whole layer thickness increased while the rate of inner compact layer increased and then decreased. The proportion of α-Al2O3 phase in PEO coatings increased when anodic current density increased. However, higher cathodic current density would result in less α-Al2O3 phase in the coating. All these phenomena can be explained by the effects of anodic and cathodic current on PEO process. Higher anodic current density would enhance the discharge reaction and generate more heat and higher temperature, which are the essential conditions for γ-Al2O3 phase transforming to α-Al2O3 phase. But it also damages the ceramic coatings, causing large voids and microcracks in PEO coatings. Certain level of cathodic current can drive Al3+ losing in the electrolyte back to the oxide-electrolyte interface and inhibit the large size sparks, which favors the growth of ceramic coatings. However, too high cathodic current would inhibit the anodic process and heat generation, so that the rate of compact layer and proportion of α-Al2O3 phase decrease. Furthermore, higher rate of compact layer and larger proportion of α-Al2O3 phase in the ceramic coatings significantly improve the corrosion and wear resistances. It is possible to obtain the desired structure, composition and properties of PEO coatings by controlling anodic and cathodic current densities.

Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO2–Na2CrO4 electrolyte

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti2AlNb alloy in various NaAlO2 electrolytes containing 2 g L−1, 4 g L−1, and 6 g −1 Na2CrO4 additive, respectively. The influence of Na2CrO4 additive in NaAlO2 electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na2CrO4 additive in the NaAlO2 electrolyte not only promotes the formation of γ-Al2O3 phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr3O and (Al0.948Cr0.052)2O3 phases. The PEO ceramic coatings formed in NaAlO2 electrolytes with 2 g L−1 and 4 g L−1 Na2CrO4 additive show better oxidation resistance than those PEO coatings formed in a NaAlO2 basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO2 electrolyte with 4 g L−1 Na2CrO4 additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO2 basic electrolyte without any Na2CrO4 additive.

Formation of Ca/P ceramic coatings by Plasma Electrolytic Oxidation (PEO) on Ti6Al4V ELI alloy

The formation of PEO ceramic coatings on Ti6Al4V ELI alloy was investigated using a phosphate/calcium containing electrolyte at 300 and 400V at 310K for different times. The Plasma Electrolytic Oxidation (PEO) coated specimens were then heat treated at 873 and 1073K for 2 hours. Scanning electron microscopy, Energy Dispersive X-Ray Spectroscopy (EDS) and X-ray diffraction analysis were used to study the composition and the morphology of the ceramic coatings. The corrosion behaviour of the coatings was studied by Electrochemical Impedance Spectroscopy (EIS) in Simulated Body Fluid (SBF). The PEO-treated specimens primarily revealed a porous structure with thickness between 4 and 12μm, according to the voltage and process time used. The coatings are mainly composed of hydroxyapatite; however, as the voltage and anodizing time increase, the Ca/P ratio decreases. Generally, the corrosion resistance of the alloy was improved by the PEO-treated coatings, although the specimens treated at 1073K showed the presence of cracks that reduced the protective effect of the coatings.

Effects of electric parameters on structure and thermal control property of PEO ceramic coatings on Ti alloys

Abstract The aim of this work is to analyze the thermal control performance of the coatings prepared on Ti6Al4V alloys by plasma electrolytic oxidation (PEO) under different electric parameters in zirconate electrolytes. The composition and structure of the coatings were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), thickness measurement gauge and roughness measuring instrument, respectively. The infrared emissivity and solar absorptance of the ceramic coatings were evaluated by the UV–vis–NIR spectrophotometer and infrared reflectometer. The results show that the coatings have typical porous structure and the main phase composition is KZr2(PO4)3 phase. With the increase of current density, extension of duty ratios and decrease of working frequency, the absorptance (αs) decreases and the emissivity (e) increases, due to the increase of the real surface area for radiating energy of coating and the thicker coating composed of nonmetallic inhibiting the absorptance of solar energy. Besides, the thermal shock tests and repeated experiments show outstanding thermal stability of coatings and satisfactory reusability of the electrolyte. In general, the coatings have better thermal control performance than the other coatings in the previous reports, and the above results indicate that the coatings prepared in this work have significantly promising industrialization potential in spacecraft field.

Effects of current density on microstructure and properties of plasma electrolytic oxidation ceramic coatings formed on 6063 aluminum alloy

Plasma electrolytic oxidation (PEO) ceramic coatings were fabricated in a silicate-based electrolyte with the addition of potassium fluorozirconate (K2ZrF6) on 6063 aluminum alloy, and the effects of current density on microstructure and properties of the PEO coatings were studied. It was found that pore density of the coatings decreased with increasing the current density. The tribological and hardness tests suggested that the ceramic coating produced under the current density of 15 A/dm2 showed the best mechanical property, which matched well with the phase analysis. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves proved that the coating obtained under 15 A/dm2 displayed the best anti-corrosion property, which was directly connected with morphologies of coatings.

Discharge behaviors during plasma electrolytic oxidation on aluminum alloy

A plasma electrolytic oxidation (PEO) process was performed on the 2024 aluminum alloy in silicate electrolyte to fabricate ceramic coatings under a constant voltage. Optical emission spectroscopy (OES) was employed to evaluate the characteristics of plasma discharge during PEO process. The plasma electron temperature and density were obtained by analyzing the spectral lines of OES, and the atomic ionization degree in discharge zone was calculated in terms of Saha thermal ionization equation. The illumination intensity of plasma discharge and the temperature in the interior of alloy were measured. Combining the surface morphology and cross-sectional microstructure with the optical emission spectra and illumination at different discharge stage, a discharge model in the growth of PEO ceramic coatings was proposed. It is found that there are two discharge modes of type A with small spark size and type B with large spark size, and the latter only appears in the intermediate stage of PEO process. The illumination intensity has a maximum value in the initial stage of oxidation with many sparks of discharge type A. The electron temperature in plasma discharge zone is about 3000 K–7000 K and atomic ionization degree of Al is about 2.0 × 10−5–7.2 × 10−3, which depend on discharge stage. The discharge type B plays a key role on the electron temperature and atomic ionization degree. The electron density keeps stable in the range of about 8.5 × 1021 m−3–2.6 × 1022 m−3.

Fabrication and Electrochemical Corrosion Behavior of PEO Coatings on Strip-Cast AZ31Mg Alloy in 3.5% NaCl Solution

The generation of compact plasma electrolytic oxidation (PEO) coatings with outstanding corrosion resistance is essential for the widespread application of Mg alloys. In the present investigation, PEO ceramic coatings formed at different silicate concentration and oxidation time on AZ31 Mg alloy were studied, and the resultant surface structures of the oxide films were observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis. X-ray photoelectron spectroscopy and thin film-X-ray diffraction analysis of ceramic coatings showed that the surface coating is mainly composed of Mg2SiO4, and MgO with different amount based on oxidation time. Further, the potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) measurements were used to characterize corrosion behavior of PEO coated substrates in 3.5% NaCl solution. The results revealed that PEO coatings processed with 0.2 M silicate showed better performance due to its dense and compact coating with fewer structural imperfections in comparison to others.