Special Issue "Plasma Electrolytic Oxidation"

A special issue of Metals (ISSN 2075-4701).

Deadline for manuscript submissions: closed (30 April 2018)

Special Issue Editor

Guest Editor
Prof. Dr. Tadeusz Hryniewicz

Koszalin Univ Technol, Dept Engn & Informat Syst, Div Bioengn & Surface Electrochem, Raclawicka 15-17, PL-75620 Koszalin, Poland
Website | E-Mail
Interests: Plasma Electrolytic Oxidation (PEO); Micro Arc Oxidation (MOA); aluminum; magnesium; titanium; tantalum; niobium; alloys

Special Issue Information

Dear Colleagues,

Plasma Electrolytic Oxidation (PEO), also known as Micro Arc Oxidation, is used to obtain porous coatings on metals such as aluminum, magnesium, titanium, tantalum, niobium, and their alloys. The first works on electrolyte discharge phenomena are dated as early as 1880, and the practical use of oxidation of aluminum by PEO started in 1970. These two dates should be recognized as the beginning of the Plasma Electrolytic Oxidation age. Afterwards, that technology was patented and introduced to industrial applications in the 1980s. Most important is that PEO coatings formed on metals and alloys may be enriched using selected elements, originating from the electrolytes used. The chemical and phase compositions, as well as porosity and the thickness of the PEO coating depend on the values of the DC and AC voltages used for that treatment. The expected applications of PEO coatings can be found in aerospace and space industries, as well as in the production of biomaterials and automotive catalytic converters. Investigations into PEO processing and the physical–chemical and mechanical properties of these porous coatings are the objective of this Special Issue.

A Special Issue, titled “Plasma Electrolytic Oxidation”, is expected to be prepared and edited in 2018. As a Guest Editor of this Special Issue, I invite you to submit a paper, which will be peer reviewed to be accepted for publication in Metals. The manuscripts are expected by the end of 2017, with a deadline in the beginning of 2018.

Best regards,

Prof. Dr. Tadeusz Hryniewicz
Guest Editor

Manuscript Submission Information

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Keywords

  • Plasma Electrolytic Oxidation (PEO)
  • Micro arc Oxidation (MOA)
  • Aluminum
  • Magnesium
  • Titanium
  • Tantalum
  • Niobium
  • Alloys

Published Papers (10 papers)

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Research

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Open AccessArticle Study of Plasma Electrolytic Oxidation Coatings on Aluminum Composites
Metals 2018, 8(6), 459; https://doi.org/10.3390/met8060459
Received: 29 April 2018 / Revised: 12 June 2018 / Accepted: 13 June 2018 / Published: 15 June 2018
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Abstract
Coatings, with a thickness of up to 75 µm, were formed by plasma electrolytic oxidation (PEO) under the alternating current electrical mode in a silicate-alkaline electrolyte on aluminum composites without additives and alloyed with copper (1–4.5%). The coatings’ structure was analyzed by scanning
[...] Read more.
Coatings, with a thickness of up to 75 µm, were formed by plasma electrolytic oxidation (PEO) under the alternating current electrical mode in a silicate-alkaline electrolyte on aluminum composites without additives and alloyed with copper (1–4.5%). The coatings’ structure was analyzed by scanning electron microscopy, X-ray microanalysis, X-ray photoelectron spectroscopy, nuclear backscattering spectrometry, and XRD analysis. The coatings formed for 60 min were characterized by excessive aluminum content and the presence of low-temperature modifications of alumina γ-Al2O3 and η-Al2O3. The coatings formed for 180 min additionally contained high-temperature corundum α-Al2O3, and aluminum inclusions were absent. The electrochemical behavior of coated composites and uncoated ones in 3% NaCl was studied. Alloyage of aluminum composites with copper increased the corrosion current density. Plasma electrolytic oxidation reduced it several times. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessArticle Combined Galvanostatic and Potentiostatic Plasma Electrolytic Oxidation of Titanium in Different Concentrations of H2SO4
Metals 2018, 8(6), 386; https://doi.org/10.3390/met8060386
Received: 29 April 2018 / Revised: 18 May 2018 / Accepted: 21 May 2018 / Published: 26 May 2018
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Abstract
We report on plasma electrolytic oxidation of titanium, employing a technique with combined potentiostatic and galvanostatic control. The effect of different H 2 SO 4 electrolyte concentrations on the titanium oxide formation was studied sytematically. The titanium oxide consisted of two distinguishable layers.
[...] Read more.
We report on plasma electrolytic oxidation of titanium, employing a technique with combined potentiostatic and galvanostatic control. The effect of different H 2 SO 4 electrolyte concentrations on the titanium oxide formation was studied sytematically. The titanium oxide consisted of two distinguishable layers. The upper layer is porous, up to few micrometers thick and primarily rutile, while the interlayer is compact, comparatively thin and is associated to anatase formation. The electrolyte concentration changed substantially layer thickness, porosity and phase composition, as deduced from scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessFeature PaperArticle The Influence of the Electrolyte Nature and PEO Process Parameters on Properties of Anodized Ti-15Mo Alloy Intended for Biomedical Applications
Metals 2018, 8(5), 370; https://doi.org/10.3390/met8050370
Received: 26 April 2018 / Revised: 15 May 2018 / Accepted: 18 May 2018 / Published: 21 May 2018
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Abstract
Plasma electrolytic oxidation (PEO) of Ti-15Mo alloys conducted in electrolytes containing Ca and P compounds can be an efficient process with which to obtain bioactive coatings. This paper reports on the influence of the nature of the electrolyte, its concentration, and PEO process
[...] Read more.
Plasma electrolytic oxidation (PEO) of Ti-15Mo alloys conducted in electrolytes containing Ca and P compounds can be an efficient process with which to obtain bioactive coatings. This paper reports on the influence of the nature of the electrolyte, its concentration, and PEO process parameters on the properties of anodized layers on Ti-15Mo. A wide range of Ca- and P-containing alkaline and acidic solutions was employed to incorporate Ca and P ions into the anodized layer. The efficiency of the incorporation was evaluated by the Ca/P ratio in the coating as compared to that in the electrolyte. It was found that alkaline solutions are not suitable electrolytes for the formation of good quality, uniform PEO coatings. Only acidic electrolytes are appropriate for obtaining well-adherent homogeneous layers on Ti-15Mo. However, the maximum Ca/P ratios reached in the coatings were rather low (close to 1). The variation of electrical signal (negative-to-positive current ratio, frequency) and time of electrolysis do not result in a substantial change of this value. The processing time, however, did influence the coating thickness. Despite their low Ca/P ratio, the anodized layers demonstrate good biological activity, comparable to pure microrough titanium. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessArticle Plasma Electrolytic Oxidation of High-Strength Aluminium Alloys—Substrate Effect on Wear and Corrosion Performance
Metals 2018, 8(5), 356; https://doi.org/10.3390/met8050356
Received: 12 April 2018 / Revised: 7 May 2018 / Accepted: 8 May 2018 / Published: 15 May 2018
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Abstract
With the progress in materials science and production technology and the establishment of light-weight design in many fields of the industry, the application of light metals no longer requires only mechanical strength, but also a significant protection of the material against wear and
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With the progress in materials science and production technology and the establishment of light-weight design in many fields of the industry, the application of light metals no longer requires only mechanical strength, but also a significant protection of the material against wear and corrosion. Hard and wear-resistant oxide coatings on aluminium are produced by plasma electrolytic oxidation (PEO). During PEO, a conversion of the aluminium substrate to a ceramic oxide takes place. While the role of strength-giving alloying elements like Cu, Mg/Si, Zn, and Zn/Cu on the PEO process has selectively been subject of investigation in the past, the significance of the alloy composition for the service properties of the coatings is still unknown. Therefore, the performance of PEO coatings produced on the widely used commercial high-strength alloys AlCu4Mg1 (EN AW-2024), AlMgSi1 (EN AW-6082), and AlZn5.5MgCu (EN AW-7075) is examined with regard to their behaviour in the rubber-wheel test according to ASTM G65 and the current density-potential behaviour of the substrates with undamaged and worn coatings in dilute NaCl solution. To give a reference to the unalloyed material the testings were carried out also on Al 99.5 (EN AW-1050) which was treated in an adjusted PEO process. Although differences in the conversion of intermetallic phases during PEO and the phase composition of the coatings on the various substrates are determined, the service properties are hardly depending on the alloying elements of the investigated aluminium materials. The wear rates in the rubber-wheel test are low for all the alloyed samples. The current density-potential curves show a decrease of the corrosion current density by approximately one order of magnitude compared to the bare substrate. Eventually, previous wear of the coatings does not deteriorate the corrosion behaviour. PEO layers on technically pure aluminum can resist the testing regimes if they are prepared in an electrolyte with an elevated silicate content and without additional hydroxide ions, during a longer process time. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessFeature PaperArticle Characterization of the Micro-Arc Coatings Containing β-Tricalcium Phosphate Particles on Mg-0.8Ca Alloy
Metals 2018, 8(4), 238; https://doi.org/10.3390/met8040238
Received: 4 March 2018 / Revised: 2 April 2018 / Accepted: 3 April 2018 / Published: 4 April 2018
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Abstract
The characterization of the microstructure, morphology, topography, composition, and physical and chemical properties of the coatings containing β-tricalcium phosphate (β-TCP) particles deposited by the micro-arc oxidation (MAO) method on biodegradable Mg-0.8Ca alloy has been performed. The electrolyte for the MAO process included the
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The characterization of the microstructure, morphology, topography, composition, and physical and chemical properties of the coatings containing β-tricalcium phosphate (β-TCP) particles deposited by the micro-arc oxidation (MAO) method on biodegradable Mg-0.8Ca alloy has been performed. The electrolyte for the MAO process included the following components: Na2HPO4·12H2O, NaOH, NaF, and β-Ca3(PO4)2 (β-TCP). The coating morphology, microstructure, and compositions have been studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). With increasing of the MAO voltage from 350 to 500 V, the coating thickness and surface average roughness of the coatings increased linearly from 6 to 150 µm and from 2 to 8 µm, respectively. The coating deposited at 350 V had more homogeneous porous morphology with numerous pores similar by sizes (2–3 µm) than the coatings formed at 450–500 V. The β-TCP isometric particles were included in the coating surface. The XRD recognized the amorphous-crystalline structure in the coatings with incorporation of the following phases: β-TCP, α-TCP, MgO (periclase) and hydroxyapatite (HA). The corrosion experiments showed that the biodegradation rate of the Mg-0.8Ca alloy coated by calcium phosphates is almost 10 times less than that of uncoated alloy. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessFeature PaperArticle Characterization of Porous Phosphate Coatings Enriched with Magnesium or Zinc on CP Titanium Grade 2 under DC Plasma Electrolytic Oxidation
Metals 2018, 8(2), 112; https://doi.org/10.3390/met8020112
Received: 27 December 2017 / Revised: 26 January 2018 / Accepted: 1 February 2018 / Published: 6 February 2018
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Abstract
The aim of the paper is to study and determine the effect of voltage increasing from 500 up to 650 VDC on chemical and electrochemical properties of the obtained porous coatings with plasma electrolytic oxidation (PEO) processes, known also as micro arc
[...] Read more.
The aim of the paper is to study and determine the effect of voltage increasing from 500 up to 650 VDC on chemical and electrochemical properties of the obtained porous coatings with plasma electrolytic oxidation (PEO) processes, known also as micro arc oxidation (MAO). In the present paper, the chemical and electrochemical characterization of porous phosphate coatings enriched with magnesium or zinc on commercially pure (CP) Titanium Grade 2 under DC-PEO obtained in electrolytes based on concentrated 85% analytically pure H3PO4 (98 g/mole) acid with additions of 500 g·L−1 of zinc nitrate Zn(NO3)2∙6H2O or magnesium nitrate Mg(NO3)2∙6H2O, are described. These materials were characterized using scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectroscopy (GDOES). It was found that the voltage of PEO process has influence on the chemical composition and thickness of the obtained porous coatings as well as on their electrochemical behavior. The higher the potential of PEO treatment, the higher the amount of zinc-to-phosphorus ratio for zinc enriched coatings was obtained, whereas in magnesium enriched coatings, the average amount of magnesium detected in PEO coating is approximately independent of the PEO voltages. Based on XPS studies, it was found out that most likely the top 10 nm of porous coatings is constructed of titanium (Ti4+), magnesium (Mg2+), zinc (Zn2+), and phosphates PO43 and/or HPO42− and/or H2PO4 and/or P2O74−. On the basis of GDOES studies, a four-sub-layer model of PEO coatings is proposed. Analysis of the potentiodynamic corrosion curves allowed to conclude that the best electrochemical repeatability was noted for magnesium and zinc enriched coatings obtained at 575 VDC. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessArticle Alkali Treatment of Anodized Titanium Alloys Affects Cytocompatibility
Metals 2018, 8(1), 29; https://doi.org/10.3390/met8010029
Received: 31 October 2017 / Revised: 27 December 2017 / Accepted: 28 December 2017 / Published: 3 January 2018
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Abstract
In this paper, the surface modification of titanium alloys Ti-15Mo, Ti-13Nb-13Zr, and Ti-6Al-7Nb is presented as a material for dental implants. The conditions of the plasma electrolytic oxidation process and alkali treatment were designed in this way to enhance the biological properties of
[...] Read more.
In this paper, the surface modification of titanium alloys Ti-15Mo, Ti-13Nb-13Zr, and Ti-6Al-7Nb is presented as a material for dental implants. The conditions of the plasma electrolytic oxidation process and alkali treatment were designed in this way to enhance the biological properties of the surface of promising Ti alloys. The differences in their surface morphology and, consequently, in their biological properties were discussed. The bioactivity of the samples was examined in vitro using simulated body fluid, and Saos-2 osteoblast cells. On all the samples, characteristic apatite particles were formed. However, compared to as-ground, natively-oxidized bare alloys, the plasma electrolytic oxidation (PEO)-modified surface of the Ti-13Nb-13Zr alloy showed the highest cytocompatibility for Saos-2 osteoblast cells, and a beneficial gain of cytocompatibility was also achieved in the treated sample of Ti-6Al-7Nb. In contrast, the modification of the Ti-15Mo alloy did not influence the adhesion and proliferation of osteoblast cells. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessArticle Characterisation of Calcium- and Phosphorus-Enriched Porous Coatings on CP Titanium Grade 2 Fabricated by Plasma Electrolytic Oxidation
Metals 2017, 7(9), 354; https://doi.org/10.3390/met7090354
Received: 1 August 2017 / Revised: 16 August 2017 / Accepted: 1 September 2017 / Published: 8 September 2017
Cited by 4 | PDF Full-text (16239 KB) | HTML Full-text | XML Full-text
Abstract
In the paper, Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and Glow Discharge Optical Emission Spectroscopy (GDOES) analyses of calcium- and phosphorus-enriched coatings obtained on commercial purity (CP) Titanium Grade 2 by plasma electrolytic oxidation (PEO), known also
[...] Read more.
In the paper, Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and Glow Discharge Optical Emission Spectroscopy (GDOES) analyses of calcium- and phosphorus-enriched coatings obtained on commercial purity (CP) Titanium Grade 2 by plasma electrolytic oxidation (PEO), known also as micro arc oxidation (MAO), in electrolytes based on concentrated phosphoric acid with calcium nitrate tetrahydrate, are presented. The preliminary studies were performed in electrolytes containing 10, 300, and 600 g/L of calcium nitrate tetrahydrate, whereas for the main research the solution contained 500 g/L of the same hydrated salt. It was found that non-porous coatings, with very small amounts of calcium and phosphorus in them, were formed in the solution with 10 g/L Ca(NO3)2·4H2O, whereas the other coatings, fabricated in the consecutive electrolytes containing from 300 up to 650 g/L Ca(NO3)2·4H2O, were porous. Based on the GDOES data, it was also found that the obtained porous PEO coating may be divided into three sub-layers: the first, top, porous layer was the thinnest; the second, semi-porous layer was about 12 times thicker than the first; and the third, transition sub-layer was about 10 times thicker than the first. Based on the recorded XPS spectra, it was possible to state that the top 10-nm layer of porous PEO coatings included chemical compounds containing titanium (Ti4+), calcium (Ca2+), as well as phosphorus and oxygen (PO43− and/or HPO42− and/or H2PO4, and/or P2O74−). Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Open AccessFeature PaperArticle Influence of Alkali Treatment on Anodized Titanium Alloys in Wollastonite Suspension
Metals 2017, 7(9), 322; https://doi.org/10.3390/met7090322
Received: 12 July 2017 / Revised: 3 August 2017 / Accepted: 17 August 2017 / Published: 23 August 2017
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Abstract
The surface modification of titanium alloys is an effective method to improve their biocompatibility and tailor the material to the desired profile of implant functionality. In this work, technologically-advanced titanium alloys—Ti-15Mo, Ti-13Nb-13Zr and Ti-6Al-7Nb—were anodized in suspensions, followed by treatment in alkali solutions,
[...] Read more.
The surface modification of titanium alloys is an effective method to improve their biocompatibility and tailor the material to the desired profile of implant functionality. In this work, technologically-advanced titanium alloys—Ti-15Mo, Ti-13Nb-13Zr and Ti-6Al-7Nb—were anodized in suspensions, followed by treatment in alkali solutions, with wollastonite deposition from the powder phase suspended in solution. The anodized samples were immersed in NaOH or KOH solution with various concentrations with a different set of temperatures and exposure times. Based on their morphologies (observed by scanning electron microscope), the selected samples were investigated by Raman and X-ray photoelectron spectroscopy (XPS). Titaniate compounds were formed on the previously anodized titanium surfaces. The surface wettability significantly decreased, mainly on the modified Ti-15Mo alloy surface. Titanium alloy compounds had an influence on the results of the titanium alloys’ surface modification, which caused the surfaces to exhibit differential physical properties. In this paper, we present the influence of the anodization procedure on alkali treatment effects and the properties of obtained hybrid coatings. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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Review

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Open AccessReview Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation
Metals 2018, 8(2), 105; https://doi.org/10.3390/met8020105
Received: 14 December 2017 / Revised: 27 January 2018 / Accepted: 29 January 2018 / Published: 1 February 2018
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Abstract
A dense inner layer is highly valued among the surface coatings created through plasma electrolytic oxidation (PEO) treatment, because the PEO coating has been troubled by inherent porosity since its conception. To produce the favored structure, a proven technique is to prompt a
[...] Read more.
A dense inner layer is highly valued among the surface coatings created through plasma electrolytic oxidation (PEO) treatment, because the PEO coating has been troubled by inherent porosity since its conception. To produce the favored structure, a proven technique is to prompt a soft sparking transition, which involves a sudden decrease in light and acoustic emissions, and a drop in anodic voltage under controlled current mode. Typically these phenomena occur in an electrolyte of sodium silicate and potassium hydroxide, when an Al-based sample is oxidized with an AC or DC (alternating or direct current) pulse current preset with the cathodic current exceeding the anodic counterpart. The dense inner layer feature is pronounced if a sufficient amount of oxide has been amassed on the surface before the transition begins. Tremendous efforts have been devoted to understand soft sparking at the metal–oxide–electrolyte interface. Studies on aluminum alloys reveal that the dense inner layer requires plasma softening to avoid discharge damages while maintaining a sufficient growth rate, a porous top layer to retain heat for sintering the amassed oxide, and proper timing to initiate the transition and end the surface processing after transition. Despite our understanding, efforts to replicate this structural feature in Mg- and Ti-based alloys have not been very successful. The soft sparking phenomena can be reproduced, but the acquired structures are inferior to those on aluminum alloys. An analogous quality of the dense inner layer is only achieved on Mg- and Ti-based alloys with aluminate anion in the electrolytic solution and a suitable cathodic current. These facts point out that the current soft sparking knowledge on Mg- and Ti-based alloys is insufficient. The superior inner layer on the two alloys still relies on rectification and densification of aluminum oxide. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation)
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