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A special issue of Coatings (ISSN 2079-6412). This special issue belongs to the section "Plasma Coatings, Surfaces & Interfaces".

Deadline for manuscript submissions: closed (20 December 2020) | Viewed by 60924

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Dr. Marta Mohedano

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Guest Editor

Department of Chemical and Materials Engineering, Universidad Complutense de Madrid, Spain
Interests: corrosion and protection of light alloys; surface modification; plasma electrolytic oxidation coatings; active protection; biomaterials
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Dr. Beatriz Mingo

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School of Materials, The University of Manchester, Oxford Road, M13 9PL, Manchester, UK
Interests: plasma electrolytic oxidation; corrosion; hybrid coatings; light alloys; tribology

Special Issue Information

Dear Colleagues,

We would like to invite you to submit your work to a Special Issue on "Plasma Electrolytic Oxidation (PEO) Coatings". Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), functionalizes surfaces, improving the mechanical, thermal, and corrosion performance of metallic substrates, along with other tailored properties (e.g., biocompatibility, catalysis, antibacterial response, self-lubrication, etc.). The extensive field of applications of this technique ranges from structural components, in particular, in the transport sector, to more advanced fields, such as bioengineering.

The aim of this Special Issue is to present the state-of-the-art of PEO for Al, Mg, Ti, Zr alloys and steels, through a combination of original research papers, short communications and review articles from leading research groups around the world.

In particular, the topics of interest include, but are not limited to:

Fundamental understanding of PEO process: mechanistic study and modeling of coating growth;
Properties and performance of PEO coatings: corrosion, mechanical, catalytic and/or electric evaluation;
Hybrid PEO coatings;
Functionalization of PEO coatings;
Active protection based on PEO;
Bio-applications of PEO coatings;
Advanced PEO processes.

Dr. Marta Mohedano
Dr. Beatriz Mingo
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Coatings is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Related Special Issue

Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition in Coatings (9 articles)

Published Papers (12 papers)

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Editorial

Jump to: Research, Review

2 pages, 172 KiB

Open AccessEditorial

Special Issue: Plasma Electrolytic Oxidation (PEO) Coatings

by Marta Mohedano and Beatriz Mingo

Coatings 2021, 11(1), 111; https://doi.org/10.3390/coatings11010111 - 19 Jan 2021

Cited by 1 | Viewed by 2175

Abstract

The demand of modern technological society for light structural materials (Al, Ti, Mg) emphasizes a combination of good corrosion resistance with wear properties and functionalized surfaces [...] Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

Research

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11 pages, 3549 KiB

Open AccessArticle

Porous CaP Coatings Formed by Combination of Plasma Electrolytic Oxidation and RF-Magnetron Sputtering

by Anna Kozelskaya, Gleb Dubinenko, Alexandr Vorobyev, Alexander Fedotkin, Natalia Korotchenko, Alexander Gigilev, Evgeniy Shesterikov, Yuriy Zhukov and Sergei Tverdokhlebov

Coatings 2020, 10(11), 1113; https://doi.org/10.3390/coatings10111113 - 19 Nov 2020

Cited by 11 | Viewed by 2350

Abstract

The porous CaP subcoating was formed on the Ti6Al4V titanium alloy substrate by plasma electrolytic oxidation (PEO). Then, upper coatings were formed by radio frequency magnetron sputtering (RFMS) over the PEO subcoating by the sputtering of various CaP powder targets: β-tricalcium phosphate (β-TCP), hydroxyapatite (HA), Mg-substituted β-tricalcium phosphate (Mg-β-TCP) and Mg-substituted hydroxyapatite (Mg-HA), Sr-substituted β-tricalcium phosphate (Sr-β-TCP) and Sr-substituted hydroxyapatite (Sr-HA). The coating surface morphology was studied by scanning electron and atomic force microscopy. The chemical composition was determined by X-ray photoelectron spectroscopy. The phase composition of the coatings was studied by X-ray diffraction analysis. The Young’s modulus of the coatings was studied by nanoindentation test. RF-magnetron sputtering treatment of PEO subcoating resulted in multileveled roughness, increased Ca/P ratio and Young’s modulus and enrichment with Sr and Mg. Sputtering of the upper layer also helped to adjust the coating crystallinity. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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12 pages, 34716 KiB

Open AccessArticle

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

by Alexander Sobolev, Tamar Peretz and Konstantin Borodianskiy

Coatings 2020, 10(10), 993; https://doi.org/10.3390/coatings10100993 - 17 Oct 2020

Cited by 19 | Viewed by 3327

Abstract

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. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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11 pages, 5637 KiB

Open AccessArticle

Plasma Electrolytic Oxidation of Titanium in H₂SO₄–H₃PO₄ Mixtures

by Bernd Engelkamp, Björn Fischer and Klaus Schierbaum

Coatings 2020, 10(2), 116; https://doi.org/10.3390/coatings10020116 - 30 Jan 2020

Cited by 9 | Viewed by 2694

Abstract

Oxide layers on titanium foils were produced by galvanostatically controlled plasma electrolytic oxidation in 12.9 M sulfuric acid with small amounts of phosphoric acid added up to a 3% mole fraction. In pure sulfuric acid, the oxide layer is distinctly modified by plasma discharges. As the time of the process increases, rough surfaces with typical circular pores evolve. The predominant crystal phase of the titanium dioxide material is rutile. With the addition of phosphoric acid, discharge effects become less pronounced, and the predominant crystal phase changes to anatase. Furthermore, the oxide layer thickness and mass gain both increase. Already small amounts of phosphoric acid induce these effects. Our findings suggest that anions of phosphoric acid preferentially adsorb to the anodic area and suppress plasma discharges, and conventional anodization is promoted. The process was systematically investigated at different stages, and voltage and oxide formation efficiency were determined. Oxide surfaces and their cross-sections were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The phase composition was determined by X-ray diffraction and confocal Raman microscopy. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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12 pages, 19971 KiB

Open AccessArticle

Correlation between Defect Density and Corrosion Parameter of Electrochemically Oxidized Aluminum

by Hao-Ren Lou, Dah-Shyang Tsai and Chen-Chia Chou

Coatings 2020, 10(1), 20; https://doi.org/10.3390/coatings10010020 - 27 Dec 2019

Cited by 11 | Viewed by 3327

Abstract

It has been recognized that a connection may exist between defects of oxide coating and its corrosion protection. Such a link has not been substantiated. We prepare two coatings of anodized aluminum oxide (AAO) and plasma electrolytic oxidation (PEO), and analyze them with Mott-Schottky plots and potentiodynamic polarization scans. The as-grown and annealed AAO coatings exhibit both p-type and n-type semiconductor behaviors. Polarization resistance of the AAO coating increases from (1.8 ± 1.7) × 10⁸ to (4.3 ± 0.5) × 10⁸ Ω·cm², while corrosion current decreases from (6.1 ± 3.6) × 10⁻⁷ to (2.3 ± 0.9) × 10⁻⁷ A·cm⁻², as annealing temperature increases from room temperature to 400 °C. The parameter analysis on AAO indicates a positive correlation between corrosion current and donor density, a negative correlation between polarization resistance and donor density. The attempt on correlating corrosion potential gives rise to considerable deviation from a linear fit. The results suggest protection of AAO hinges on its donor density, not acceptor. On the PEO coatings, only the n-type behavior is observed. Intriguingly, the donor density of PEO coating is influenced by the annealing temperature of its pre-anodized layer. The most resistant PEO coating, with pre-anodized and 400 °C annealed AAO, exhibits polarization resistance (2.1 ± 0.4) × 10⁹ Ω·cm² and corrosion current (1.7 ± 0.4) × 10⁻⁸ A·cm⁻². Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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12 pages, 21154 KiB

Open AccessArticle

Production of Phosphorescent Coatings on 6082 Aluminum Using Sr_0.95Eu_0.02Dy_0.03Al₂O_4-δ Powder and Plasma Electrolytic Oxidation

by Krisjanis Auzins, Aleksejs Zolotarjovs, Ivita Bite, Katrina Laganovska, Virginija Vitola, Krisjanis Smits and Donats Millers

Coatings 2019, 9(12), 865; https://doi.org/10.3390/coatings9120865 - 16 Dec 2019

Cited by 7 | Viewed by 3450

Abstract

In this study, a new approach for producing phosphorescent aluminum coatings was studied. Using the plasma electrolytic oxidation (PEO) process, a porous oxide coating was produced on the Al6082 aluminum alloy substrate. Afterwards, activated strontium aluminate (SrAl₂O₄: Eu²⁺, Dy³⁺) powder was filled into the cavities and pores of the PEO coating, which resulted in a surface that exhibits long-lasting luminescence. The structural and optical properties were studied using XRD, SEM, and photoluminescence measurements. It was found that the treatment time affects the morphology of the coating, which influences the amount of strontium aluminate powder that can be incorporated into the coating and the resulting afterglow intensity. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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17 pages, 6032 KiB

Open AccessArticle

Evaluation of the Corrosion Resistance and Cytocompatibility of a Bioactive Micro-Arc Oxidation Coating on AZ31 Mg Alloy

by Shun-Yi Jian, Mei-Ling Ho, Bing-Ci Shih, Yue-Jun Wang, Li-Wen Weng, Min-Wen Wang and Chun-Chieh Tseng

Coatings 2019, 9(6), 396; https://doi.org/10.3390/coatings9060396 - 20 Jun 2019

Cited by 18 | Viewed by 3798

Abstract

Magnesium alloys have recently been attracting attention as a degradable biomaterial. They have advantages including non-toxicity, biocompatibility, and biodegradability. To develop magnesium alloys into biodegradable medical materials, previous research has quantitatively analyzed magnesium alloy corrosion by focusing on the overall changes in the alloy. Therefore, the objective of this study is to develop a bioactive material by applying a ceramic oxide coating (magnesia) on AZ31 magnesium alloy through micro-arc oxidation (MAO) process. This MAO process is conducted under pulsed bipolar constant current conditions in a Si- and P-containing electrolyte and the optimal processing parameters in corrosion protection are obtained by the Taguchi method to design a coating with good anti-corrosion performance. The negative duty cycle and treatment time are two deciding factors of the coating’s capability in corrosion protection. Microstructure characterizations are investigated by means of SEM and XRD. The simulation body-fluid solution is utilized for testing the corrosion resistance with the potentiodynamic polarization and the electrochemical impedance test data. Finally, an in vivo testing shows that the MAO-coated AZ31 has good cytocompatibility and anticorrosive properties. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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14 pages, 8182 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Degradation Behaviour of Mg0.6Ca and Mg0.6Ca2Ag Alloys with Bioactive Plasma Electrolytic Oxidation Coatings

by Lara Moreno, Marta Mohedano, Beatriz Mingo, Raul Arrabal and Endzhe Matykina

Coatings 2019, 9(6), 383; https://doi.org/10.3390/coatings9060383 - 13 Jun 2019

Cited by 15 | Viewed by 4144

Abstract

Bioactive Plasma Electrolytic Oxidation (PEO) coatings enriched in Ca, P and F were developed on Mg0.6Ca and Mg0.6Ca2Ag alloys with the aim to impede their fast degradation rate. Different characterization techniques (SEM, TEM, EDX, SKPFM, XRD) were used to analyze the surface characteristics and chemical composition of the bulk and/or coated materials. The corrosion behaviour was evaluated using hydrogen evolution measurements in Simulated Body Fluid (SBF) at 37 °C for up to 60 days of immersion. PEO-coated Mg0.6Ca showed a 2–3-fold improved corrosion resistance compared with the bulk alloy, which was more relevant to the initial 4 weeks of the degradation process. In the case of the Mg0.6Ag2Ag alloy, the obtained corrosion rates were very high for both non-coated and PEO-coated specimens, which would compromise their application as resorbable implants. The amount of F⁻ ions released from PEO-coated Mg0.6Ca during 24 h of immersion in 0.9% NaCl was also measured due to the importance of F⁻ in antibacterial processes, yielding 33.7 μg/cm², which is well within the daily recommended limit of F⁻ consumption. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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16 pages, 4157 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

LDH Post-Treatment of Flash PEO Coatings

by Rubén del Olmo, Marta Mohedano, Beatriz Mingo, Raúl Arrabal and Endzhe Matykina

Coatings 2019, 9(6), 354; https://doi.org/10.3390/coatings9060354 - 30 May 2019

Cited by 21 | Viewed by 4631

Abstract

This work investigates environmentally friendly alternatives to toxic and carcinogenic Cr (VI)-based surface treatments for aluminium alloys. It is focused on multifunctional thin or flash plasma electrolytic oxidation (PEO)-layered double hydroxides (LDH) coatings. Three PEO coatings developed under a current-controlled mode based on aluminate, silicate and phosphate were selected from 31 processes (with different combinations of electrolytes, electrical conditions and time) according to corrosive behavior and energy consumption. In situ Zn-Al LDH was optimized in terms of chemical composition and exposure time on the bulk material, then applied to the selected PEO coatings. The structure, morphology and composition of PEO coatings with and without Zn-Al-LDH were characterized using XRD, SEM and EDS. Thicker and more porous PEO coatings revealed higher amounts of LDH flakes on their surfaces. The corrosive behavior of the coatings was studied by electrochemical impedance spectroscopy (EIS). The corrosion resistance was enhanced considerably after the PEO coatings formation in comparison with bulk material. Corrosion resistance was not affected after the LDH treatment, which can be considered as a first step in achieving active protection systems by posterior incorporation of green corrosion inhibitors. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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19 pages, 4648 KiB

Open AccessArticle

Characterization, Bioactivity and Antibacterial Properties of Copper-Based TiO₂ Bioceramic Coatings Fabricated on Titanium

by Salih Durdu

Coatings 2019, 9(1), 1; https://doi.org/10.3390/coatings9010001 - 20 Dec 2018

Cited by 30 | Viewed by 4246

Abstract

The bioactive and anti-bacterial Cu-based bioceramic TiO₂ coatings have been fabricated on cp-Ti (Grade 2) by two-steps. These two-steps combine micro-arc oxidation (MAO) and physical vapor deposition–thermal evaporation (PVD-TE) techniques for dental implant applications. As a first step, all surfaces of cp-Ti substrate were coated by MAO technique in an alkaline electrolyte, consisting of Na₃PO₄ and KOH in de-ionized water. Then, as a second step, a copper (Cu) nano-layer with 5 nm thickness was deposited on the MAO by PVD-TE technique. Phase structure, morphology, elemental amounts, thickness, roughness and wettability of the MAO and Cu-based MAO coating surfaces were characterized by XRD (powder- and TF-XRD), SEM, EDS, eddy current device, surface profilometer and contact angle goniometer, respectively. The powder- and TF-XRD spectral analyses showed that Ti, TiO₂, anatase-TiO₂ and rutile-TiO₂ existed on the MAO and Cu-based MAO coatings’ surfaces. All coatings’ surfaces were porous and rough, owing to the presence of micro sparks through MAO. Furthermore, the surface morphology of Cu-based MAO was not changed. Also, the Cu-based MAO coating has more hydrophilic properties than the MAO coating. In vitro bioactivity and in vitro antibacterial properties of the coatings have been investigated by immersion in simulated body fluid (SBF) at 36.5 °C for 28 days and bacterial adhesion for gram-positive (S. aureus) and gram-negative (E. coli) bacteria, respectively. The apatite layer was formed on the MAO and Cu-based MAO surfaces at post-immersion in SBF and therefore, the bioactivity of Cu-based MAO surface was increased to the MAO surface. Also, for S. aureus and E. coli, the antibacterial properties of Cu-based MAO coatings were significantly improved compared to one of the uncoated MAO surfaces. These results suggested that Cu-based MAO coatings on cp-Ti could be a promising candidate for biomedical dental implant applications. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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11 pages, 6028 KiB

Open AccessArticle

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

by Xiaopeng Lu, Yan Chen, Carsten Blawert, Yan Li, Tao Zhang, Fuhui Wang, Karl Ulrich Kainer and Mikhail Zheludkevich

Coatings 2018, 8(9), 306; https://doi.org/10.3390/coatings8090306 - 29 Aug 2018

Cited by 19 | Viewed by 4664

Abstract

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. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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Review

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19 pages, 5957 KiB

Open AccessReview

Introduction to Plasma Electrolytic Oxidation—An Overview of the Process and Applications

by Frank Simchen, Maximilian Sieber, Alexander Kopp and Thomas Lampke

Coatings 2020, 10(7), 628; https://doi.org/10.3390/coatings10070628 - 30 Jun 2020

Cited by 164 | Viewed by 18294

Abstract

Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), is an innovative method in producing oxide-ceramic coatings on metals, such as aluminum, titanium, magnesium, zirconium, etc. The process is characterized by discharges, which develop in a strong electric field, in a system consisting of the substrate, the oxide layer, a gas envelope, and the electrolyte. The electric breakdown in this system establishes a plasma state, in which, under anodic polarization, the substrate material is locally converted to a compound consisting of the substrate material itself (including alloying elements) and oxygen in addition to the electrolyte components. The review presents the process kinetics according to the existing models of the discharge phenomena, as well as the influence of the process parameters on the process, and thus, on the resulting coating properties, e.g., morphology and composition. Full article

(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings)

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