Investigation of the formation mechanisms of plasma electrolytic oxidation coatings on Mg alloy AM50 using particles

The Effects of the Pre-Anodized Film Thickness on Growth Mechanism of Plasma Electrolytic Oxidation Coatings on the 1060 Al Substrate

To increase the density of the micro-arc oxide coating, AA 1060 samples were pretreated with an anodic oxide film in an oxalic acid solution. Plasma electrolytic oxidation (PEO) was performed to investigate the effect of the thickness of the pre-anodic oxide film on the soft-sparking mechanism. The experimental results revealed that the PEO coating phases with different thicknesses of the pre-anodized films contained both Al and gamma-alumina (γ-Al2O3). The pre-anodized film changes the final morphology of the coating, accelerating the soft sparking transition and retaining the soft sparking. At a pre-anodized film thickness of ≤7.7 μm, the anodized films thickened before being broken through. When the pre-anodized film thickness was ≥13.1 μm, partial dissolution of the anodized films occurred before they were struck through. Two growth mechanisms for PEO coatings with different pre-anodized film thicknesses were proposed.

Cleaning Strategies of Synthesized Bioactive Coatings by PEO on Ti-6Al-4V Alloys of Organic Contaminations

The effect of various cleaning methods on coating morphology and their effectiveness in removing organic contaminants has been studied in this research. Bioactive coatings containing titanium oxides and hydroxyapatite (HAP) were obtained through plasma electrolytic oxidation in aqueous electrolytes and molten salts. The cleaning procedure for the coated surface was performed using autoclave (A), ultraviolet light (UV), radio frequency (RF), air plasma (P), and UV-ozone cleaner (O). The samples were characterized using scanning electron microscopy (SEM) with an EDS detector, X-ray photoelectron spectroscopy (XPS), X-ray phase analysis (XRD), and contact angle (CA) measurements. The conducted studies revealed that the samples obtained from molten salt exhibited a finer crystalline structure morphology (275 nm) compared to those obtained from aqueous electrolytes (350 nm). After applying surface cleaning methods, the carbon content decreased from 5.21 at.% to 0.11 at.% (XPS), which directly corresponds to a reduction in organic contaminations and a decrease in the contact angle as follows: A > UV > P > O. This holds true for both coatings obtained in molten salt (25.3° > 19.5° > 10.5° > 7.5°) and coatings obtained in aqueous electrolytes (35.2° > 28.3° > 26.1° > 16.6°). The most effective and moderate cleaning method is ozone treatment.

Surface Modification of 6xxx Series Aluminum Alloys

Due to their superior mechanical properties, formability, corrosion resistance, and lightweight nature, 6xxx series aluminum (Al) alloys are considered as a promising structural material. Nevertheless, the successful application of these materials depends on their response to the external environment. Recently, designers considered the surface properties an equally important aspect of the component design. Due to this concern, these alloys are subjected to varieties of surface modification methodologies. Many methodologies are explored to modify the 6xxx series Al alloys surfaces effectively. These methods are anodizing, plasma electrolytic oxidation (PEO), cladding, friction stir processing, friction surfacing, melting, alloying, and resolidification using high energy beams, etc. This review work discusses some of these methods, recent research activities on them, important process variables, and their role on the final properties of the surfaces.

Fabrication and Characterization of Ceramic Coating on Al7075 Alloy by Plasma Electrolytic Oxidation in Molten Salt

The fabrication of a ceramic coating on the metallic substrate is usually applied to achieve the improved performance of the material. Plasma electrolytic oxidation (PEO) is one of the most promising methods to reach this performance, mostly wear and corrosion resistance. Traditional PEO is carried out in an aqueous electrolyte. However, the current work showed the fabrication and characterization of a ceramic coating using PEO in molten salt which was used to avoid disadvantages in system heating-up and the formation of undesired elements in the coating. Aluminum 7075 alloy was subjected to the surface treatment using PEO in molten nitrate salt. Various current frequencies were applied in the process. Coating investigations revealed its surface porous structure and the presence of two oxide layers, α-Al2O3 and γ-Al2O3. Microhardness measurements and chemical and phase examinations confirmed these results. Potentiodynamic polarization tests and electrochemical impedance spectroscopy revealed the greater corrosion resistance for the coated alloy. Moreover, the corrosion resistance was increased with the current frequency of the PEO process.

Microstructure and Corrosion Characterization of a MgO/Hydroxyapatite Bilayer Coating by Plasma Electrolytic Oxidation Coupled with Flame Spraying on a Mg Alloy

Thermally sprayed hydroxyapatite coatings are one of the main strategies to improve the bioactivation of metal implants. However, the naturally low corrosion resistance of these coatings is the main challenge for their use. In this study, plasma electrolytic oxidation (PEO) was used to create an intermediate layer. The anodization process was used for comparison. According to the polarization curves, the PEO layer was more effective than the anodized layer in reducing the corrosion current density (I _corr of 0.05 × 10^-9 A/cm² vs I _corr of 0.05 A/cm²). The results of electrochemical impedance spectroscopy showed higher resistance of the sample with a PEO interlayer than that of the sample with an anodized interlayer. The results of the hydrogen evolution test revealed that the PEO layer as a middle layer served as the main barrier for reducing the magnesium corrosion rate, especially during the initial immersion time.

Corrosion and Wear Behavior of PEO Coatings on D16T Aluminum Alloy with Different Concentrations of Graphene

The effect of added graphene concentration on the microstructure, phase composition, corrosion- and wear- resistance of plasma electrolyte oxidation (PEO) coatings formed on D16T aluminum alloy in silicate electrolyte with different concentrations of graphene were investigated. The results show that the morphologies of the coatings with graphene were obviously different ascribed to the mode of graphene incorporated into the coating. The coatings consisted of mainly α-Al2O3, γ-Al2O3, and Al, which were divided into an outer porous layer and a dense inner layer. The thickness of the coatings increased non-linearly with graphene concentration. The corrosion resistance of the coatings with graphene was significantly improved. The wear resistance of the coatings was also greatly improved apart from the coating with 3 g/L graphene. The coating produced in the electrolyte with 2 g/L graphene exhibited the optimal comprehensive properties because graphene successfully incorporated into the coating via the pores and spread on the surface of the coating.

Fabrication of High Temperature Oxidation Resistance Nanocomposite Coatings on PEO Treated TC21 Alloy

The effects of ZrO₂ nanoparticles in a NaAlO₂ electrolyte on the thickness, morphology, composition, structure, and high temperature oxidation resistance of plasma electrolytic oxidation (PEO) coatings on a TC21 titanium alloy were investigated. The coating thickness increased with increasing concentration of ZrO₂ nanoparticles in the electrolyte, accompanied by a decrease in the porosity of the coating surface. The PEO coatings formed in the ZrO₂ nanoparticle-free electrolyte were composed of Al₂TiO₅. ZrTiO₄, m-ZrO₂, and t-ZrO₂ were detected in the PEO coatings produced by the electrolyte that contained ZrO₂ nanoparticles, which indicated that the deposition mechanism of the nanoparticles was partly reactive incorporation. The high temperature oxidation resistance of the TC21 titanium alloy at 650 °C and 750 °C was improved by 3–5 times after PEO treatment. The oxidation mechanism involved oxygen diffusing inward to form an oxide layer at the interface of the PEO coating and substrate.

Corrosion Behavior of AZ91D Magnesium Alloy with a Calcium–Phosphate–Vanadium Composite Conversion Coating

A novel self-healing calcium–phosphate–vanadium (Ca–P–V) composite coating on Mg alloy was successfully fabricated through a chemical conversion method. The effects of the vanadium concentration on the anticorrosion property of the substrate were also tested. The Ca–P–V coating with the main composition of CaHPO4, Ca3(PO4)2, and Mg3(PO4)2, with some hydroxides of V(V) dispersed into it has a similar morphology to the single vanadium coating. The corrosion behaviour of the Ca–P–V coating was studied through the electrochemical tests and the scratch immersion test in 3.5 wt % NaCl solution. The results showed that the Ca–P–V coated samples not only exhibit good corrosion resistance property, but also show self-healing ability. The ions of Ca, P, and V released from the coating can migrate in the corrosion solution and form a new compound layer on the damaged zone.

Effect of Duty Cycle on Properties of Al₂O₃ Ceramic Coatings Fabricated on TiAl Alloy via Cathodic Plasma Electrolytic Deposition

In order to study the effect of duty cycle during the cathodic plasma electrolytic deposition (CPED) process, Al₂O₃ ceramic coatings were fabricated via the CPED technique on prepared TiAl alloy in an Al(NO₃)₃ electrolyte with different duty cycles. Microstructure, morphology, and chemical compositions of coatings were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The mechanical properties, such as thickness, hardness, and binding strength, were also characterized, and heat-resistance and wear-resistance tested. The results indicated that duty cycle mainly affected the relative crystallinity of CPED coatings. As the duty cycle increased, the crystallinity of CPED coatings increased, the content of Al(OH)₃ and γ-Al₂O₃ decreased, and the content of α-Al₂O₃ increased. The thickness and bonding strength both increased firstly and then decreased, while hardness increased as duty cycle increased. Heat-resistance and wear-resistance of TiAl alloy with CPED coating was highly improved compared to that of TiAl alloy substrate without CPED coating.

Novel Porous Phosphorus⁻Calcium⁻Magnesium Coatings on Titanium with Copper or Zinc Obtained by DC Plasma Electrolytic Oxidation: Fabrication and Characterization

In this paper, the characteristics of new porous coatings fabricated at three voltages in electrolytes based on H₃PO₄ with calcium nitrate tetrahydrate, magnesium nitrate hexahydrate, and copper(II) nitrate trihydrate are presented. The SEM, energy dispersive spectroscopy (EDS), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS), and XRD techniques for coating identification were used. It was found that the higher the plasma electrolytic oxidation (PEO) (micro arc oxidation (MAO)) voltage, the thicker the porous coating with higher amounts of built-in elements coming from the electrolyte and more amorphous phase with signals from crystalline Ca(H₂PO₄)₂∙H₂O and/or Ti(HPO₄)₂∙H₂O. Additionally, the external parts of the obtained porous coatings formed on titanium consisted mainly of Ti4+, Ca2+, Mg2+ and PO₄3-, HPO₄2-, H₂PO₄-, P₂O₇4- as well as Zn2+ or copper Cu⁺/Cu2+. The surface should be characterized by high biocompatibility, due to the presence of structures based on calcium and phosphates, and have bactericidal properties, due to the presence of zinc and copper ions. Furthermore, the addition of magnesium ions should accelerate the healing of postoperative wounds, which could lead to faster patient recovery.

Plasma Electrolytic Oxidation of Magnesium Alloy AZ31B in Electrolyte Containing Al₂O₃ Sol as Additives

Plasma electrolytic oxidation (PEO) coatings were produced on AZ31B magnesium alloys in alkaline electrolytes with the addition of various concentrations of Al₂O₃ sols. Effects of Al₂O₃ sol concentrations on the microstructure, phase composition, corrosion resistance and hardness of PEO coatings were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness testing and potentiodynamic polarization measurements, respectively. It was revealed that the Al₂O₃ sol mostly participated in the formation of the ceramic coating and transferred into the MgAl₂O₄ phase. With the increase of the Al₂O₃ sol concentration in the range of 0–6 vol%, the coating performance in terms of the microstructure, diffraction peak intensity of the MgAl₂O₄ phase, corrosion resistance and microhardness was improved. Further increase of Al₂O₃ sol addition did not generate better results. This indicated that 6 vol% might be the proper Al₂O₃ sol concentration for the formation of PEO coatings.

Influence of SiO2 Particles on the Corrosion and Wear Resistance of Plasma Electrolytic Oxidation-Coated AM50 Mg Alloy

The influence of SiO2 particles on the microstructure, phase composition, corrosion and wear performance of plasma electrolytic oxidation (PEO) coatings on AM50 Mg was investigated. Different treatment durations were applied to fabricate coatings in an alkaline, phosphate-based electrolyte (1 g/L KOH + 20 g/L Na3PO4 + 5 g/L SiO2), aiming to control the incorporated amount of SiO2 particles in the layer. It was found that the uptake of particles was accompanied by the coating growth at the initial stage, while the particle content remained unchanged at the final stage, which is dissimilar to the evolution of the coating thickness. The incorporation mode of the particles and phase composition of the layer was not affected by the treatment duration under the voltage-control regime. The corrosion performance of the coating mainly depends on the barrier property of the inner layer, while wear resistance primarily relies on the coating thickness.

Surface modification of valve metals using plasma electrolytic oxidation for antibacterial applications: A review

Plasma electrolytic oxidation (PEO) is an advance technique to develop porous oxidation layer on light metals, primarily to enhance corrosion and wear resistance. The oxidation layer can also offer a wide variety of mechanical, biomedical, tribological, and antibacterial properties through the incorporation of several ions and particles. Due to the increasing need of antimicrobial surfaces for biomedical implants, antibacterial PEO coatings have been developed through the incorporation of antibacterial agents. Metallic nanoparticles that have been employed most widely as antibacterial agents are reported to demonstrate serious health and environmental threats. To overcome the current limitations of these coatings, there is a significant need to develop antibacterial surfaces that are not harmful for patient's health and environment. Attention of the readers has been directed to utilize bioactive glasses as antibacterial agents for PEO coatings. Bioactive glasses are well known for their excellent bioactivity, biocompatibility, and antibacterial character. PEO coatings incorporated with bioactive glasses can provide environment-friendly antimicrobial surfaces with exceptional bioactivity, biocompatibility, and osseointegration. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 590-605, 2018.

An Investigation of Oxide Coating Synthesized on an Aluminum Alloy by Plasma Electrolytic Oxidation in Molten Salt

Plasma electrolytic oxidation (PEO) is a surface treatment process for obtaining oxide coatings with a high performance on valve metals. PEO is mostly performed in an aqueous solution electrolyte that limits the size of treated parts due to the fact that the system is heated. Therefore, the coating of large surfaces cannot be synthesized in an aqueous electrolyte. In the current work, an alternative approach of PEO treatment, whereby an aluminum 1050 alloy in nitrate molten salt at a temperature of 280 °C is applied, was investigated. The microstructure, phase and chemical compositions, and micro-hardness were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and micro-hardness tests. The obtained results show that formed coating contains from two sub-layers: one is the outer sub-layer of the α-Al2O3 phase and the second is its inner sub-layer. It was found that the formed coating was free of any contaminants originating from the electrolyte and had no cracks or pores, which are usually present in coatings formed by PEO treatment in an aqueous solution electrolyte.

Antibacterial effect of PEO coating with silver on AA7075

In this work, plasma electrolytic oxidation (PEO) coatings were produced on AA7075 using alkaline solution containing silicates compounds and silver micrometric particles in order to give to the coating an antimicrobial effect. In the optic of circular economy, silver chloride derived from the acid pre-treatment of electronic scraps was used as raw material and successively silver powders were synthesized from silver chloride solution using glucose syrup as reducing agent. The coatings were characterized by scanning electron microscope (SEM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization test and antimicrobial tests. The results evidenced that the obtained coatings were homogenous and give to the samples higher corrosion resistance than untreated alloy. The silver particles, found both inside and outside of the pores that characterize the PEO layer, produced an efficacious antimicrobial effect both against E. coli and S. aureus.