Plasma Electrolytic Oxidation Film Research Articles

Anodization Time Effect on Silver Particles Deposition on Anodic Oxide Coating over Al Produced by Plasma Electrolytic Oxidation

In this study, 6061 Al alloy was galvanostatically anodized under the Plasma Electrolytic Oxidation (PEO) condition. A factorial design of 22 was carried out using two variables (anodization time and presence of silver in the electrolyte) on two levels, i.e., 20 and 60 min of anodization and the absence/presence of silver ions in the electrolyte. The Al anodization was performed in sodium silicate electrolyte, applying a constant current density of 20 mA cm−2. The oxide characterization was performed by Scanning Electron Microscopy (SEM), surface roughness analysis (RMS), Energy Dispersive Spectroscopy (EDS), Rutherford Backscattered Spectroscopy (RBS), and Grazing Incidence X-ray Diffraction (GIXRD). The SEM micrographs revealed an irregular porous structure with cracks on the oxide surface composed of a thin crystalline layer of γ-Al2O3 over the Al substrate. From EDS and RBS analysis, it was possible to identify the elements Al, O, Si, Ag, and Na, demonstrating that a shorter anodization time (20 min) led to a significant amount of silver deposits on the outer layer of the oxide coating, mainly deposited in the surroundings of the pores. Conversely, the silver content on the PEO film anodized for 60 min was meager. These results demonstrated that the anodization time was the critical control variable for the amount of silver deposited over the oxide film. The shorter the anodizing time, the higher the silver content on the PEO coating.

Effect of boron carbide reinforcments on the PEO process of B4C/Al matrix composite

Plasma electrolytic oxidation (PEO) is an effective way to improve the thermal control property and service lifetime of the B4C/aluminum matrix composites (B4C/AMC) used in nuclear power field. The evolution process of B4C particles during the growth of PEO film was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) morphology observation, secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) chemical composition analysis. The results show that the B4C particles keep intact status but inhibit spark discharge in the initial oxidation stage, and then a few B4C particles are partly oxidized to small size and mixed into the film. After that, plenty of B4C particles are oxidized to B2O3 and solid C which escapes from the film as carbon oxide gas under strong spark discharge. The B4C particles disrupt the continuity of the film due to lack of sparking on their surface at the early stage of oxidation, but the film compactness is improved with decreasing B4C particles in the film under high oxidation voltage.

Tribological Behaviors of Ni-P Coatings on PEO-Treated AA1050 Alloy

Tribological behaviors of the plasma electrolytic oxidation (PEO)-treated Al1050 alloy surface without and with Ni-P deposits was examined by analyzing of the width of abrasion groove, weight loss, friction coefficient and composition at the wear track after a 100 m ball-on-disc test. The surface color, surface and cross-sectional morphologies and surface roughness (Ra) were also investigated to explain different wear behaviors of PEO films without and with Ni-P deposit. After the 100 m ball-on-disc test, a circular abrasion groove with relatively large width and shallow depth was formed when Ni-P particles cover the top surface of PEO films, but a relatively narrow and deep circular abrasion groove was formed on the Ni-P layer-covered PEO film surface. Iron was only detected at the wear track of 5 and 10 μm PEO films without Ni-P deposit, suggesting that the steel ball is worn out by the PEO films but not by the Ni-P deposit. An extremely large amount of weight loss was obtained from the Ni-P layer-covered PEO film surface, which could be related to the high density and low hardness of the Ni-P layer.

Antimicrobial properties dependence on the composition and architecture of copper-alumina coatings prepared by plasma electrolytic oxidation (PEO)

This study presents environmentally friendly and low-cost synthetic routes to produce antimicrobial coatings over 5052 Al alloy based on plasma electrolytic oxidation (PEO) technology. Two methodologies were explored: the decoration with copper and anodic doping with copper ions. The porous oxide layers produced in silicate media presented two porous layers consisting of γ-Al2O3 crystalline phase and amorphous phases of aluminosilicate, silica, and Al(OH)3. Small amounts of copper (<0.3 at.%) were detected in the PEO films. In the Cu-decorated film, copper clusters composed of Cu0 and Cu2+ species were observed visually as small black dots on the surface. In the Cu-doped film, the Cu2+ and Cu+ species were homogeneously distributed on the surface. The copper content affected the corrosion performance in aggressive corrosive media. The PEO coatings showed a remarkable antimicrobial activity after 24 h in standard tests. The antimicrobial effectiveness of the Cu-decorated sample was higher against S. aureus, while the Cu-doped sample was more effective against E. coli. The results demonstrated that differences in the PEO coating architecture can affect the material composition and, consequently, the bacterial inactivation mechanism. These findings can serve as a guide to tailor aluminum alloys for specific antimicrobial surfaces.

A tattoo-inspired electrosynthesized polypyrrole film: crossing the line toward a highly adherent film for biomedical implant applications

Polypyrrole (PPy) films have demonstrated promising application for implants due to their unique topographical and electronic properties. However, the limited PPy adhesiveness to metallic surfaces remains a challenge. Consequently, we propose a two-step technique for the surface modification of titanium (Ti) via a plasma electrolytic oxidation (PEO) step to serve as mechanical interlocking for the subsequent deposition of a highly adherent PPy film (PEO + PPy). Ti discs with machined and PEO-modified surfaces were used as controls. For the experimental groups, PPy film was deposited onto such surfaces by electrodeposition. Then, the role of machined and PEO surfaces in the synthesis, conductivity, microstructure, mechanical, electrochemical, microbiological, and biological properties of the PPy film was investigated. The results showed that a highly adherent “tattoo-inspired” PPy thin film was successfully achieved when the Ti surface was pretreated via PEO. PEO + PPy enhanced Ti mechanical and tribological properties by inducing a lower friction coefficient and wear loss due to the cushion effect of PPy film, besides promoting higher corrosion resistance. The “cauliflower-like” morphology of the PPy favored protein adsorption, calcium phosphate growth and demonstrated cell biocompatibility. The association between PEO and PPy film can be considered bioactive and is promising for the triggering of superior long-term stability of biomedical implants.

Formation of Bright White Plasma Electrolytic Oxidation Films with a Uniform Maze-Like Structure by Anodizing Aluminum in Ammonium Tetraborate Solutions

Bright white plasma electrolytic oxidation (PEO) films with uniform maze-like structures were obtained by anodizing Al in an ammonium tetraborate solution. High-purity Al plates were galvanostatically anodized in 0.3–2.4 M ammonium tetraborate solutions at 303–343 K and 10–100 Am−2. A PEO film consisting of an outer porous layer consisting of amorphous alumina and crystalline alumina with α-and γ-phases and an inner amorphous barrier alumina layer was obtained on the Al surface. An extremely uneven PEO film with various pore sizes and many cracks was formed in a 0.3 M ammonium tetraborate solution, whereas a relatively uniform porous PEO film with similar pore sizes was obtained in 0.9–2.4 M solutions. This difference in the PEO film morphology was due to the plasma generation behavior while anodizing. The lightness of the PEO film increased with increasing anodizing time and PEO film thickness; thus, a bright white PEO film measuring 87.5 in lightness (L*) was formed on the Al surface. The water wettability of the PEO film exhibited weak hydrophilicity. Moreover, a superhydrophobic PEO film with a contact angle of 154° was easily fabricated by self-assembled monolayer modification. Similar bright white PEO coatings were successfully fabricated on various industrial alloys.

Doping and defects: The coloring mechanism of black plasma electrolytic oxidation (PEO) films on aluminum alloys

Plasma electrolytic oxidation (PEO) can be used to fabricate black ceramic films on aluminum alloy by adding transition-metal anions into the electrolyte. Although the coloration was attributed to transition-metal oxides, a thorough explanation of the relationship between oxides and black color was required. In this study, black PEO films were created by incorporating ammonium metavanadate or sodium tungstate into the electrolyte. The research focused on alumina band-gap modification to explain the mechanism of black coloring. The absorption spectrum and X-ray photoelectron spectroscopy (XPS) valence band characterization revealed that the bandgap of alumina after doping was significantly narrowed. The density of state of alumina, including before doping, after doping, and introducing point defect was simulated by first-principles density functional theory calculation. It was discovered that doping transition metals into alumina decrease the conduction band while defects in alumina cause an upper shift of the valence band. The narrowing of the bandgap resulted in the absorption of all visible light and the appearance of black.

Enhanced tribological performance of cylinder liners made of cast aluminum alloy with high silicon content through plasma electrolytic oxidation

Aluminum-silicon (Al-Si) alloys have been increasingly used in several industries to develop novel lighter and high-performance components. Nevertheless, these novel advanced materials display poor wear resistance which hamper their application in several critical tribological components. This investigation is focused on the surface modification of Al-Si alloys with high Si content, through a breakthrough green coating technology named plasma electrolytic oxidation (PEO). The main aim is, for the first time, to study the tribological behavior of PEO-treated cylinder liners made of cast Al with high Si content, in a configuration mimicking the real contact between a cylinder liner and a piston ring. Additionally, this study is based on the development of a novel electrolyte especially designed to generate PEO films with improved tribological properties.Novel films were successfully synthesized by PEO in an innovative aluminate-based electrolyte. The morphological, topographical, and chemical features of the films were assessed through scanning electron microscopy (SEM), thickness and average roughness (Ra) measurements, energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Cross-sectional SEM images showed that PEO films developed in the novel electrolyte are denser and thicker. Additionally, the novel PEO-treated cylinder liners, presented a significantly improved resistance to wear damage when submitted to tribological actions simulating the cylinder liner/piston ring real contact movements, which was linked with the extremely higher hardness of the films. These cylinder liners presented a lower COF compared to the commercial reference, along the whole duration of sliding. Finally, tribological tests showed that PEO-treated Al cylinder liners displayed a similar level of mechanical wear damage that steel cylinder liners.The results presented in this investigation envisage the feasibility of PEO to be applied on Al-Si engine components submitted to aggressive tribological conditions, enhancing their durability requirements to levels similar to components made of steel.

Analyses of electrochemical behavior of plasma electrolytic oxidation film on Zirlo alloy in lithium borate buffer solution at 25–300 °C

The electrochemical behavior of plasma electrolytic oxidation (PEO) film on Zirlo alloy in 25–300 °C lithium borate buffer solution was evaluated by in-situ electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. As the solution temperature rose to a critical value (150 °C), a new relaxation process related to the distinguishable maximum in phase angle vs. frequency curves was observed at medium frequency and became more pronounced with increasing temperature. This relaxation process almost disappeared after a short exposure time under 300 °C. The transition of the relaxation process was closely related to the impedance of PEO film. The new relaxation process was ascribed to the diffusion of corrosion product through the PEO film. In addition, the degradation of the protective properties of PEO film was observed with increasing temperature, which resulted from the enhanced ionic conductivity of doped zirconia under high temperature. Finally, the PEO film hardly changed in morphology and microstructure after in-situ EIS tests at 25–300 °C. • Electrochemical behavior of PEO film was evaluated by in-situ electrochemical impedance spectroscopy. • A new relaxation process was observed at medium frequency. • The new relaxation process was ascribed to the diffusion of corrosion product. • The conduction mechanism of corrosion species through PEO film was analyzed based on a mix-conduction model.

Significance of plasma electrolytic oxidation treatment on corrosion and sliding wear performances of selective laser melted AlSi10Mg alloy

In this investigation, Plasma Electrolytic Oxidation (PEO) process was used to prepare an alumina (Al 2 O 3 ) coating for corrosion and wear protection of a Selective Laser Melted (SLM) AlSi10Mg alloy radiator. The phase compositions, microstructures, mechanical properties, corrosion properties and wear resistance of the SLM AlSi10Mg alloy and the PEO coatings were investigated systematically. It was observed that the AlSi10Mg alloy prepared by SLM displayed a denser microstructure, finer grains and excellent mechanical properties with an average ultimate tensile strength of 402 MPa and an elongation of 9.9%. In addition, a double-layered coating with Al 2 O 3 as the top layer having a thickness of ~20 μm and a hardness of 1051HV 0.01 was realizable on the SLM AlSi10Mg alloy by PEO treatment. The corrosion current density and wear rate of the PEO Al 2 O 3 coatings were 5.7 and 279.3 times lower compared to the plain SLM AlSi10Mg alloy. The results indicate that the PEO treatment can significantly improve the wear and corrosion resistance of the AlSi10Mg alloy prepared by the SLM process. • AlSi10Mg alloy prepared by SLM exhibits dense microstructure and fine grain size. • AlSi10Mg alloy prepared by SLM process has excellent mechanical properties. • PEO film has dense outer layer and less dense inner one whose main phase is γ-Al 2 O 3. • PEO can provide excellent corrosion and wear protection for the SLM AlSi10Mg parts.

Synthesis, microstructure and biodegradation behavior of MgO-TiO2-PCL nanocomposite coatings on the surface of magnesium-based biomaterials

In this study, a magnesium oxide-titanium dioxide (MgO-TiO2) nanocomposite film was successfully applied to pure magnesium (Mg) surface using the plasma electrolytic oxidation (PEO) technique in an electrolyte solution containing TiO2 nanoparticles. A uniform top-layer polycaprolactone (PCL) coating was eventually formed on the PEO-treated Mg through a dip-coating process. The results indicated that the porous nature of the PEO-coated samples could not provide the desired corrosion resistance in simulated body fluid (SBF). However, the deposition of the top-layer PCL coating (utilizing 8wt% PCL) over the MgO-TiO2 nanocomposite coating greatly improved the corrosion resistance of the Mg substrate. The proposed PCL top-layer coated MgO-TiO2 nanocomposite film could be considered as a promising candidate for the surface modification of Mg-based biodegradable implants. Due to the microstructural and macrostructural porous nature of the PEO film, this coating can be further used on the surface of biodegradable implants not only to control the degradation rate but also to deliver therapeutic molecules.

Steam oxidation behavior of ZrO2/Cr-coated pure zirconium prepared by plasma electrolytic oxidation followed by filtered cathodic vacuum arc deposition

Plasma electrolytic oxidation (PEO) and filtered cathodic vacuum arc deposition (FCVAD) techniques were employed to fabricate a ZrO2/Cr composite coating on pure zirconium. The steam oxidation properties of bare and ZrO2/Cr-coated pure Zr were performed in water vapor environment at 900–1100 °C. Their morphologies, microstructures and phase constituents before and after steam oxidation were analyzed. Meanwhile, their composition depth profiles were measured by glow discharge optical emission spectroscopy (GDOES). It was found that the mass gain of ZrO2/Cr-coated Zr was about 22–45% of that of bare Zr after 3600 s steam oxidation at 900–1100 °C, and the PEO and FCVAD composite treatments significantly improved the steam oxidation resistance of Zr. After steam oxidation, the phosphorous element in the outer layer of PEO film from phosphate electrolyte diffused into the residual Cr layer, however, most of the phosphorous in the interior of PEO layer was remained although the phosphorus diffusion in the region was accelerated with the increase of steam temperature. The ZrO2 to Zr3O phase transformation in PEO intermediate layer occurred in high temperature steam. Furthermore, the PEO zirconia interlayer suppressed hydrogen permeation into the Zr substrate during steam oxidation at 900–1100 °C.

Effects of Li, B and H elements on corrosion property of oxide films on ZIRLO alloy in 300 °C/14 MPa lithium borate buffer solutions

Bare and plasma electrolytic oxidation (PEO)-coated ZIRLO alloys were exposed in 300 °C/14 MPa lithium borate solutions, then the distributions of Li, B and H elements in the oxide films were measured by glow discharge optical emission spectroscopy (GDOES). The PEO film can protect the ZIRLO alloy from the corrosion attack of Li＋. Addition of H3BO3 into the LiOH solution suppressed the diffusion of Li element into oxide film to mitigate the LiOH corrosion for ZIRLO alloy, while LiOH could also inhibit the entrance of B element. The enrichment of H near the oxide film/Zr interface was detected.

Bioactive element coatings on nano-mesh formed Ti-6Al-4V alloy surface using plasma electrolytic oxidation

Bioactive element coatings on nano-mesh formed Ti-6Al-4V alloy surface using plasma electrolytic oxidation were studied. Ti-6Al-4V alloy was used for nano-mesh formation, which was carried out in 1 M H3PO4 + 0.8 wt% NaF at 30 V for 1 h. For micro-pore formation, plasma electrolytic oxidation (PEO) treatment was performed in a solution containing bioactive ions at 280 V for 3 min. After that, surface and biological characteristics were investigated using FE-SEM, EDS, AFM, Scratch test, XRD, wettability test, and MC3T3–1 cell culture test.The nanotubes and nano-mesh were well formed in the α-phase compared to β-phase. The many micro-sized pores on PEO(5ion) samples were formed with circular and irregularly shaped pores and the circular precipitate with oxide thickness of 2.0– 3.0 μm. PEO(5ion) and PEO(Ca + P) samples consisted of anatase and HA phases, and the surface roughness of the PEO samples was higher than those of nanotube and nano-mesh formed surface, whereas, the contact angle of the PEO samples was lower than those of nanotube and nano-mesh formed surface. The adhesive strength of the PEO(5ion) films showed higher compared to PEO(Ca + P) sample. The number of cells on the modified surface and oxide layers increased with the surface roughness. The cell growth and proliferation on the PEO(5ion) films were higher than those of polished surface, the adhesion of cells is greatly improved on the nano-mesh surface.

Characterization of Microstructure and Composition of Plasma Electrolytic Oxide Film Formed on AZ31 Mg Alloy

Magnesium alloy has been widely investigated as a biodegradable implant material owing to its unique properties to degrade spontaneously in human body fluid without causing toxicity. However, the degradation rate needs to be controlled. An effective way to lower down the degradation rate of Mg alloy is by coating with plasma electrolytic oxidation (PEO) technique. In this research, the microstructure and mechanical hardness of the PEO film formed on AZ31 were investigated. The film was prepared under a constant current of 400 A/m2 in the Na3PO4 solution at 30°C. The voltage-time curve showed an immediate increase of current during the first 25 s before reaching a steady-state voltage of 150 V. The spark discharge revealed as white micro discharges. The film formed for 3 min exhibited a high surface roughness with a large variety of thickness in the range of 1-20 µm. The film contained pores and cracks. The big pores with diameter size 10-20 µm were formed as a result of gas entrapment, while the small pores with a radius of 1-3 µm were associated with the discharge tunnel during the PEO process. The X-ray diffraction pattern indicated that the film composed of crystalline Mg3(PO4)2.

In-situ electrochemical study of plasma electrolytic oxidation treated Zr3Al based alloy in 300°C/14 MPa lithium borate buffer solution

Plasma electrolytic oxidation (PEO) film was fabricated on Zr3Al based alloy. In-situ 768 h electrochemical impedance spectroscopy tests of bare and PEO coated Zr3Al based alloys were performed in 300°C/14 MPa lithium borate buffer solution. The results indicated that the impedance of PEO film was higher than that of bare Zr3Al based alloy in high temperature and high pressure lithium borate solution. Due to the partial chemical dissolution of outer layer, the impedance of outer layer of PEO film decreased while the impedance of inner layer increased until the end of 768 h exposure time. The uniform compact inner layer played a key role in improving the corrosion resistance of Zr3Al based alloy. Mott-Schottky analysis revealed that both passive film and PEO film on bare Zr3Al based alloy had n-type semiconductor feature, corresponding to oxygen vacancies or zirconium/aluminum interstitials. The high defect density in film reduced impedance after a critical exposure time. In addition, the PEO film with high flat-band potential had lower electron transfer capability, which was also beneficial to improving the corrosion resistance of Zr3Al based alloy.

Corrosion protection of Ti6Al4V by a composite coating with a plasma electrolytic oxidation layer and sol-gel layer filled with graphene oxide

Seawater is regarded as the most corrosive natural environment, and can lead to serious corrosion, friction, and their collaborative effects. In this work, composite films were fabricated by combining a graphene oxide (GO)-modified hybrid sol-gel (sol-gel/GO) layer and plasma electrolytic oxidation (PEO) layer for corrosion protection. XRD, TFIR, XPS, and Raman analyses were used to prove the covalent functionalisation between the GO and silane coupling agents. The results showed that the micropores in the PEO film were filled with the GO/sol-gel layer. Electrochemical tests indicated that the addition of GO at different concentrations could significantly improve the corrosion performance of the substrates through the good barrier property of the GO nanosheets. Moreover, the enhanced wear resistance of the GO/sol-gel layer during tribocorrosion tests could be attributed to the solid lubrication property of the GO nanosheets.

Morphology, abrasion and corrosion resistance study of the plasma electrolytic oxidation films formed on pure titanium by adding silane in the electrolyte

Surface modification of pure Ti is performed with the addition of aminopropyl trimethoxy-silane (APS) in the anodic electrolyte to obtain plasma electrolytic oxidation (PEO) ceramic films at a median voltage of 200 V. The addition of silane in Na2SiO3 solution does not change the phase compositions of the PEO films while promoting the faster growth of both rutile and anatase, meanwhile smoother and thicker ceramic films with more regular pores on the outer layers are obtained as reflected by the X-ray diffraction spectra (XRD) and scanning electron microscopy (SEM) results, respectively. The wear resistance in dry friction condition of the PEO films with silane addition is improved as seen from their friction coefficient and abrasive track image; the increase in corrosion potential (Ecorr) and decrease in corrosion current density (Icorr) suggest the improvement of their resisting ability to corrosive media, attributing to the formation of compacter inner and outer layer films as supported by the electrochemical impedance spectroscopy. The results of attenuated total reflection-Fourier transform infrared spectra (ATR-FTIR) and X-ray photoelectron spectra (XPS) reveal that APS exists on the surfaces and among the defects of ceramic films by adsorption, self-condensation and/or chemical linkage (Ti–O–Si) as various states, therefore their existence and filling effects result in the improvement in their abrasive and corrosive resistances.

Functional element coatings on Ti-alloys for biomaterials by plasma electrolytic oxidation

In this study, functional element coatings on Ti-alloys for biomaterials by plasma electrolytic oxidation (PEO) were studied using different experimental techniques. Si- and Mg-doped hydroxyapatite films were formed on the Ti–6Al–4V alloy via anodization at an applied DC voltage of 280 V for 3 min. The chemical composition of the surface layers was examined using energy-dispersive X-ray spectroscopy mapping and line-profile analysis. The morphology of the surface was observed using field-emission scanning electron microscopy. The phase composition was determined using X-ray diffraction. The size of the micro-pore formed on the Ti–6Al–4V alloy decreased, and the area occupied by the micro-pores decreased with increasing Mg concentration. With the increase in Mg concentration, the number of small micro-pores on the PEO films formed on Ti–6Al–4V alloys increased, whereas the number of large micro-pores decreased. The number of circular particles formed in the pores and on the exterior surface increased with increasing Mg concentration. The thickness of the PEO film was approximately 1.94–2.03 μm and the coating elements Ca, P, Si, and Mg were uniformly distributed throughout the PEO film. The anatase and hydroxyapatite(HA) peaks shifted slightly from the original anatase and pure HA peaks as the Mg concentration increased.

Anodizing Aluminum and Its Alloys in Etidronic Acid to Enhance Their Corrosion Resistance in a Sodium Chloride Solution

The formation behaviors of the anodic oxide film that forms on various commercially available aluminum plates were investigated by galvanostatic anodizing in etidronic acid, and their corrosion resistances were examined in a 3.5 wt% NaCl solution. High-purity aluminum and A1050, A5052, and A7075 aluminum alloys were galvanostatically anodized in an etidronic acid solution. An amorphous porous oxide film formed on the pure aluminum, the A1050 alloy, and the A5052 alloy, whereas a plasma electrolytic oxidation (PEO) film formed on the A7075 alloy. The nanopores formed in the porous oxide film could be sealed with hydroxides by immersion in boiling water. Corrosion-resistant pure aluminum with a measured corrosion current density of icorr = 10−9 Am−2 could be formed by anodizing and subsequent pore-sealing. Although the corrosion current increased due to the presence of alloying elements in the matrix, the fabrication of the corrosion-resistant A1050 and A5052 alloys with icorr = 10−6 Am−2 could also be achieved by anodizing. Alternately, the corrosion resistance of the A7075 alloy covered with the PEO film was measured to be icorr = 10−4 Am−2, which is not as small as the icorr of the A1050 and A5052 alloys due to its open microporous structure.