Plasma Electrolytic Oxidation Of Aluminum Research Articles

The Plasma Electrolytic Oxidation of Aluminum Using Microsecond-Range DC Pulsing

This manuscript presents the results of our recent work focused on the plasma electrolytic oxidation of aluminum in a sodium tungstate solution using a microsecond-range pulsed DC signal. DC pulses of 50, 300 and 900 μs were followed by 5- and 25-times longer pauses between the pulses, showing the effect of the pulse duration and duty cycle on the morphological, phase and chemical properties of formed oxide coatings. It is shown that all coatings are partially crystalline with gamma-alumina, WO3 and metallic W phases present in formed PEO coatings. A higher duty cycle value results in the higher crystallization of the obtained PEO coatings. Although the chemical composition of the obtained coatings is not very sensitive to processing parameters, their roughness and porosity change significantly, as well as their thickness. The photocatalytic activity and photoluminescence properties of the obtained coatings are dependent on their morphology and chemical composition, i.e., on the processing time. The highest photoactivity and photoluminescence intensity is observed for the coating formed with ton = 300 μs and toff = 25 ton. A comparable application potential is found for the sample processed with ton = 50 μs and toff = 25 ton, which requires considerably less energy for PEO processing.

Plasma electrolytic oxidation of aluminium in electrolytes containing various concentrations of carbon black nanoparticles

Incorporation of carbon-based nanoparticles into ceramic coatings during plasma electrolytic oxidation (PEO) is promising for the synthesis of new composite layers on lightweight metals. Specifically, the present study focuses on the incorporation of carbon black (CB) nanoparticles into PEO alumina layers. For this purpose, PEO of aluminium is performed in silicate-based electrolytes containing various concentrations of dispersed carbon black nanoparticles (from 0 to 6 g·L−1). The influence of this concentration on the microstructure of the achieved PEO coatings is investigated by combining complementary characterization techniques (scanning electron microscopy, X-ray diffraction and Raman spectroscopy). Results show that using concentrations up to 6 g·L−1 tend to limit the morphological inhomogeneity between the edges and the centre of the treated samples. Moreover, the addition of carbon black nanoparticles results in a sponge-like outermost sublayer covering larger areas of the surface with abilities to host a higher amount of these nanoparticles. It is also evidenced that CB nanoparticles do not suffer any further structural degradation during their incorporation. In addition, cross-checked results show that the presence of dispersed CB nanoparticles slightly affect the coating average growth rate. As for potential future applications, the electrical volume conductivity of grown carbon-alumina composite coatings is also measured.

How carbon black nanoparticles affect the occurrence of soft-sparking during plasma electrolytic oxidation

The effect of dispersed carbon black nanoparticles in the electrolyte on the plasma electrolytic oxidation of aluminium under soft sparking regime is investigated. Increasing the concentration of particles reduces the breakdown voltage. It also makes the transition to soft sparking to occur earlier, which can favour a more homogeneous coating. However, it is shown that a too high concentration of particles can be detrimental to the coating morphology despite an apparent soft sparking regime characterized by a decrease in anodic voltage.

Influence of carbon nanotubes on the plasma electrolytic oxidation process of aluminum under “soft” sparking conditions

Plasma electrolytic oxidation of aluminum (PEO) was performed with dispersed multi-walled carbon nanotubes (MWCNTs) into the electrolyte. The effect of the carbon nanotubes concentration, as well as that of the electrical processing conditions, on the PEO process was investigated. From the point of view of the electrical response of the process, results show that the “soft” sparking regime appears earlier in presence of MWCNTs. The process time at which the transition to the “soft” regime occurs decreases linearly with the increase in the MWCNTs concentration. Over a concentration threshold, depending on the applied pulse bipolar current waveform, the PEO process is inhibited. The dispersion of MWCNTs in the electrolyte results in thicker but more porous alumina layers. More specifically, the proportion of the α-alumina phase relative to the γ-alumina phase decreases with the increase in the MWCNTs concentration. Results are explained by considering the increased coating growth kinetics and lower heat accumulation from MWCNTs addition. Finally, the present study gives new insights into the right management of the PEO process under “soft” sparking regime with dispersed carbon-based nanoparticles in the electrolyte.

Alumina Coatings Containing Niobium Pentoxide Polymorphs Prepared by Plasma Electrolytic Oxidation of Aluminum

This study explores the experimental parameters related to the production of Nb2O5 over alumina coatings produced by plasma electrolytic oxidation (PEO) and evaluates its use in photocatalytic applications. The influence of niobium precursor concentration and the electrolyte temperature on the electrochemical, morphological, and compositional responses are investigated using chemometric tools. The experimental design provides a geometrical representation of experimental parameters and shows the effect of the anodizing conditions on the pore density and the anodization curves. The samples are prepared at a constant current (10 mA cm−2) under the PEO regime. The coatings reveal a porous structure and crystalline phases of Nb2O5 and γ‐Al2O3. A preliminary photodegradation test under UV radiation and utilizing Rhodamine B (RhB) solution shows the best photocatalytic performance for the sample produced by the highest Nb salt concentration (16.0 g L−1) and highest temperature (18.7 °C). These results are attributed to a higher niobium content in this sample and crystalline phases of Nb2O5 over the Al substrate.

Phase transitions in alumina films during post-sparking anodising of Al alloys

Formation of the high-temperature α-Al2O3 phase during Plasma Electrolytic Oxidation of aluminium at ambient bulk temperatures has been previously attributed to local microdischarge events providing multiple melting-solidification cycles in micro-volumes of the surface oxide layer. In this work, it is demonstrated that the α phase can be formed even if the microdischarge is fully suppressed under specific processing conditions. Oxide layers produced in the post-sparking anodising mode were studied by FIB, TEM, EBSD, EDS and GDOES techniques to reveal microstructural and chemical evolutions that accompany the γ to α alumina transition. Our results provide strong evidence that the α phase can form spontaneously in regions of oxide with the appropriate temperature, grain size and impurity distributions in the γ-Al2O3 matrix that allow sufficient mobility of α/γ grain boundaries. Ionic migration within the oxide and hydrothermal dissolution/precipitation in the associated microporous network that facilitate species mobility at the grain boundaries allow the critical temperature for activation of γ→α transition to be reduced. Overall, it is suggested that oxide layer growth can be considered in terms of a relatively simple Plug Flow Reactor model. This can help predict the phase transition kinetics depending on key processing parameters such as current density and frequency of pulse polarisation, thus enabling optimum control of coating microstructure for specific application requirements.

Windmill discharge migration pattern during plasma electrolytic oxidation of aluminium

Plasma electrolytic oxidation process is a surface modification technique using plasma-assisted oxidation to prepare the protective layers that improve the surface performances of valve metals such as aluminium, magnesium, and titanium. However, there is poor understanding and ability to control the process, and many uncertainties. The main objective of this paper is to present the discharge behavior with windmill structure, stable group movement and long-term stability. All processes were carried out on 6061 aluminium alloy by applying a bipolar pulsed current with a frequency of 200 Hz, positive and negative pulse durations of 2 ms, and equal pulse pauses. Utilizing current pulses having equal amplitudes between the positive and negative periods resulted in destructive discharges in the late stage. By adjusting the average positive current at a certain time, destructive discharges were eliminated. Meanwhile, various windmill discharge migration patterns were established, in which collective discharges aggregated into several clusters and rotated as a whole on the sample surface. The number of the clusters was correlated to the average positive current. A complete plasma electrolytic oxidation process was proposed to produce a series of coatings, in which the average positive current was reduced at a rate of 0.1 A·min−1 from 95 min to 100 min, and the windmill discharge migration pattern gradually emerged after 100 min. The proposed method increased the treatment duration from ~95 min to ~180 min compared to the process with no current adjustment. In addition, the thickness, relative content of α- and γ-Al2O3 phases, and Vickers hardness of the coatings were significantly enhanced, allowing for the preparation of high-quality coatings.

Formation and Properties of Oxide Coatings with Immobilized Zeolites Obtained by Plasma Electrolytic Oxidation of Aluminum

In this paper, we employed plasma electrolytic oxidation (PEO) of aluminum in a water solution of sodium tungstate (Na2WO4∙2H2O) with the addition of the pure and Ce-loaded zeolites clinoptilolite and 13 X for the preparation of oxide coatings. The obtained coatings were characterized with respect to their morphologies and chemical and phase compositions using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and X-ray diffraction. The prepared coatings contained γ-alumina, WO3, and metallic tungsten. The surface morphologies of the obtained coatings strongly depended on the PEO processing time; the roughness of all coatings increased with PEO time, while porosity decreased with PEO processing time as a result of microdischarge coalescence and growth. All coatings contained elements originating from the substrate and from the electrolytes. Coatings containing zeolites with Ce showed higher photoactivity than those with immobilized pure zeolites. The highest photocatalytic activity levels were observed for coatings containing immobilized Ce-exchanged clinoptilolite processed for 10 min. It was observed that both clinoptilolite and 13X zeolites improved the features of the PEO coatings in a similar manner, making natural and abundant clinoptilolite an excellent candidate for various applications.

Conversion layers by plasma-electrolytic oxidation of aluminum in acrylate and benzoate electrolytes

Within this work, aluminum is oxidized via plasma-electrolytic oxidation (PEO) in the presence of organic substances, including reactive monomers. The aim of this approach is to generate polymers and simultaneously bind them to the surface of the freshly generated oxide layers. For this purpose, sheets of aluminum were immersed into electrolytes that comprised either 4 % of sodium acrylate or 10 % of sodium benzoate. The aluminum sheets were oxidized by anodic pulse current at 0.25 A/cm2 for 30–90 s. By this process, predominantly oxidic conversion layers were produced. The presence of the monomers in the electrolyte influenced the passivation and discharge behavior and finally the microstructure of the layers. It further gave rise to organic material in the layer. In particular, infrared attenuated total reflection spectroscopy (ATR-IR) and X-ray photoemission spectroscopy (XPS) show that layers which were generated in the presence of acrylate or benzoate comprised -C-C-, -C=C-, -C-O, and C=O bonds.

INFLUENCE OF PLASMA-ELECTROLYTIC OXIDATION PROCESS TECHNOLOGICAL PARAMETERS OF ALUMINUM ON COATING GROWTH RATE

There has been carried out an analysis of methods of oxide covering formation productivity increasing during plasma electrolytic oxidation of aluminum in electrolyte. There has been developed a technology of blank manufacturing and part strengthening by plasma electrolytic oxidation in the electrolyte, as well as the workbench has been modernized. There has been studied the process of oxidoceramic coating synthesis for the D16T aluminum deformed alloy of during plasma electrolytic oxidation in the electrolyte for different process parameters. It is established that the growth rate of oxidoceramic coating can be significantly increased by electrolyte component concentration involved in aluminum oxidation and rational choice of process electrical parameters. Hydrogen peroxide addition leads to obtained oxoceramic coating thickness increasing due to O, O2, OH, OH– concentration increasing in the electrolyte. It is established that the optimal concentration of H2O2 ranges from 5 g/l to 7 g/l. A further increase of peroxide concentration leads to a decrease in peroxide effect on oxoceramic coating growth rate on the D16T aluminum deformed alloy due to pH changes of the electrolyte and the deterioration of the oxide coating.

In-situ control of microdischarge characteristics in unipolar pulsed plasma electrolytic oxidation of aluminum

Microdischarges occurring during plasma electrolytic oxidation are the main mechanism promoting oxide growth compared to classical anodization. When the dissipated energy by microdischarges during the coating process gets too large, high-intensity discharges might occur, which are detrimental to the oxide layer. In bipolar pulsed plasma electrolytic oxidation a so called ‘soft-sparking’ mode limits microdischarge growth. This method is not available for unipolar pulsing and for all material combinations. In this work, the authors provide a method to control the size- and intensity distributions of microdischarges by utilizing a multivariable closed-loop control. In-situ detection of microdischarge properties by CCD-camera measurements and fast image processing algorithms are deployed. The visible size of microdischarges is controlled by adjusting the duty cycle in a closed-loop feedback scheme, utilizing a PI-controller. Uncontrolled measurements are compared to controlled cases. The microdischarge sizes are controlled to a mean value of and , respectively. Results for controlled cases show, that size and intensity distributions remain constant over the processing time of 35 minutes. Larger, high-intensity discharges can be effectively prevented. Optical emission spectra reveal, that certain spectral lines can be influenced or controlled with this method. Calculated black body radiation fits with very good agreement to measured continuum emission spectra (). Variance of microdischarge size, emission intensity and continuum radiation between consecutive measurements is reduced to a large extent, promoting uniform microdischarge and oxide layer properties. A reduced variance in surface defects can be seen in SEM measurements, after coating for 35 minutes, for controlled cases. Surface defect study shows increased number density of microdischarge impact regions, while at the same time reducing pancake diameters, implying reduced microdischarge energies compared to uncontrolled cases.

Characterization of metal oxide micro/nanoparticles elaborated by plasma electrolytic oxidation of aluminium and zirconium alloys

The formation of fine particles during the Plasma Electrolytic Oxidation process (PEO) was investigated. Particles were produced by carrying out PEO treatments on two different grade of aluminium alloy (Al1050, Al2024) and a low-alloyed zirconium grade (Zr-M5). They were characterized at different scales mainly using transmission electron microscopy. Whatever the processed material, results show that particles exhibit a hollow spherical shape with outer diameter ranging from 100 nm to 10 μm. They consist of polycrystalline alumina (η- and α-Al2O3) for the PEO of aluminium alloys and zirconia (m-ZrO2) for the zirconium alloy. A correlation is drawn between the microstructure of the produced particles and the “pancake”-like structure that usually develops over PEO coatings. On this basis, a descriptive mechanism of the formation of these metal oxide particles is proposed. The ignition of a micro-discharge inside the “pancake” cavity induces the propagation of a pressure shockwave that pushes the molten oxide through the narrow orifice of the “pancake” structure. Like a rubber balloon, the molten oxide forms a hollow sphere and is rapidly quenched by the surrounding electrolyte and ejected into the electrolyte.

Highly-increased photoluminescence of Pr3+ due to the Eu2+→ Pr3+ energy transfer in Al2O3 coatings formed by plasma electrolytic oxidation

In this paper, photoluminescence (PL) properties of Pr3+ single doped and Pr3+/Eu2+ co-doped Al2O3 coatings formed by plasma electrolytic oxidation of aluminum were investigated. The PL spectra of Al2O3:Pr3+ coatings feature bands ascribed to 4f-4f transitions of Pr3+. PL excitation spectra contain three broad bands in the violet and blue regions which correspond to the direct excitation of the Pr3+ ground state 3H4 into higher levels 3PJ (J = 0, 1, 2), while PL emission spectra contain several bands attributed to transitions of Pr3+ from excited levels 3P0 and 3P1 to the lower multiplets 3HJ (J = 4, 5, 6) and 3FJ (J = 2, 3, 4). Among these transitions, 3P0→3F2 transition in the red region (644 nm) is the most intense. PL emission intensity of Pr3+ increases by co-doping Eu2+ into Al2O3:Pr3+ coatings due to the energy transfer from Eu2+ to Pr3+, occurring due to the overlap between PL emission spectra of Eu2+, excited by middle ultraviolet radiation, and PL excitation spectra of Pr3+ in Al2O3 host. Under middle ultraviolet excitation, 3P0→3F2 transition of Pr3+ in Al2O3:Pr3+/Eu2+ coatings is 62 times more intense than without co-doping and 16 times if comparison is made with Al2O3:Pr3+ excited by 446 nm. The total luminescence intensity of Al2O3 co-doped with Pr3+/Eu2+ and doped with Pr3+ is 3.6 times more intense of the former if both samples are excited by 446 nm. The gains are linearly increasing with Eu2+ doping concentration. Al2O3:Pr3+/Eu2+ poses as an alternative phosphor to YAG:Ce3+ for the creation of neutral white LEDs by excitation with InGaN semiconductors.

Mechanisms of plasma electrolytic oxidation of aluminum at the multi-hour timescales

AbstractWe present certain novel insights into the processes that occur during the prolonged (several hours) plasma electrolytic oxidation (PEO) of aluminum. Scanning electron microscopy (SEM) equipped with the energy-dispersive spectroscopy option (EDS) was used to study morphologies of the PEO-formed ceramic coating/substrate (C/S) surfaces on aluminum. The morphologies of the oxide layers' cross-sections have also been examined. The plasma electron temperature was calculated from the spectral lines measured by the optical emission spectroscopy (OES). The obtained results demonstrate that during the one single discharge event, a completely new inner oxide layer is being formed at the bottom of the discharge channel. This layer is positioned between the metal substrate and the previous Al2O3 layers. It appears that this new oxide layer, together with the gas trapped in the channel becomes a barrier for the dielectric breakdown that is higher than the original coating layer itself.

Enhanced ultraviolet light driven photocatalytic activity of ZnO particles incorporated by plasma electrolytic oxidation into Al2O3 coatings co-doped with Ce3+

This paper presents a new method for improvement of ultraviolet light driven photocatalytic activity of ZnO by immobilizing ZnO particles into Al2O3:Ce3+ coatings which emit intense radiation in the near ultraviolet region when excited by middle ultraviolet radiation. When Al2O3:Ce3+/ZnO coatings are illuminated by ultraviolet light in photocatalytic reactor, emitted ultraviolet radiation originating from Ce3+ ions synergistically participates in photocatalytic reactions on ZnO. Al2O3:Ce3+/ZnO coatings were formed by plasma electrolytic oxidation of aluminum in electrolyte containing 2 g/L of ZnO particles and up to 8 g/L of CeO2 particles. Structural, optical, photoluminescent, and photocatalytic properties of formed surface coatings were examined. All formed coatings feature virtually the same thickness, morphology, phase composition, and absorption properties. Concentration of incorporated Ce3+ ions into Al2O3 host and concentration of immobilized ZnO particles are the only parameters by which the coatings differ from each other resulting in difference in photocatalytic activity. Al2O3:Ce3+/ZnO coatings show better photodegradation of methyl orange with respect to Al2O3/ZnO coatings. The best photocatalytic activity is observed for the coating formed in electrolyte containing 4 g/L of CeO2 particles.

Ce3+/Eu2+ Doped Al2O3 Coatings Formed by Plasma Electrolytic Oxidation of Aluminum: Photoluminescence Enhancement by Ce3+→Eu2+ Energy Transfer

Plasma electrolytic oxidation (PEO) of aluminum in electrolytes containing CeO2 and Eu2O3 powders in various concentrations was used for creating Al2O3 coatings doped with Ce3+ and Eu2+ ions. Phase and chemical composition, surface morphology, photoluminescence (PL) properties and energy transfer from Ce3+ to Eu2+ were investigated. When excited by middle ultraviolet radiation, Al2O3:Ce3+/Eu2+ coatings exhibited intense and broad emission PL bands in the ultraviolet/visible spectral range, attributed to the characteristic electric dipole 4f05d1→4f1 transition of Ce3+ (centered at about 345 nm) and 4f65d1→4f7 transition of Eu2+ (centered at about 405 and 500 nm). Due to the overlap between the PL emission of Al2O3:Ce3+ and the PL excitation of Al2O3:Eu2+, energy transfer from Ce3+ sensitizer to the Eu2+ activator occurs. The energy transfer is identified as an electric dipole–dipole interaction. The critical distance between Eu2+ and Ce3+ ions in Al2O3 was estimated to be 8.6 Å by the spectral overlap method.

Evaluation of the thermochemical reactions in the micro-discharges during plasma electrolytic oxidation of aluminium

Evaluation of the thermochemical reactions in the micro-discharges during plasma electrolytic oxidation of aluminium

One‐Step Synthesis of Antibacterial Coatings by Plasma Electrolytic Oxidation of Aluminum

Aluminum oxide films containing small quantities of silver are produced by plasma electrolytic oxidation of aluminum (97%) in different electrolytes aiming the synthesis of bactericidal coatings over aluminum for food packaging applications. Sodium citrate, citric acid, and sodium silicate, containing Ag nanoparticles (Ag‐NPs) or Ag+ ions (AgNO3) are tested as electrolyte. The galvanostatic anodization curves are used as diagnostic to evaluate the production of Ag‐alumina coatings in a one‐step procedure. Citric acid and silicate media provide more stability and reproducibility in coatings production; however, only coatings produced in silicate solution exhibit antimicrobial effect against Bacillus subtilis bacteria. The oxide films are characterized by scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDS). An irregular porous structure and a silver content of 0.1–1.9 wt.% in the oxide coatings are observed.

Formation of a metastable nanostructured mullite during Plasma Electrolytic Oxidation of aluminium in “soft” regime condition

This paper demonstrates the possibility of producing a lamellar ceramic nanocomposite at the top-most surface of oxide coatings grown with the Plasma Electrolytic Oxidation process (PEO). PEO was conducted on aluminium in a silicate-rich electrolyte under the so-called “soft” regime. Nanoscale characterisation showed that the transition from the “arcs” to the “soft” regime was concomitant with the gradual formation of a 1:1 mullite/alumina lamellar nanocomposite (≈120 nm thick) that filled the cavity of the PEO “pancake” structure. Combined with plasma diagnostic techniques, a three-step growth mechanism was proposed: (i) local melting of alumina under the PEO micro-discharges (≈3200 K at high heating rate ≈3 × 108 K·s−1); (ii) progressive silicon enrichment of the melt coming from the electrolyte; and (iii) quenching of the melt at a cooling rate of ≈3.3 × 107 K·s−1 as the micro-discharge extinguishes. Under such severe cooling conditions, the solidification process was non-equilibrium as predicted by the metastable SiO2 – Al2O3 binary phase diagram. This resulted in phase separation where pure alumina lamellae alternate periodically with 1:1 mullite lamellae.

Highly thermally conductive dielectric coatings produced by plasma electrolytic oxidation of aluminum

Dielectric materials with high thermal conductivity are of great interest in applications as heat sinks for microelectronics. In this work, an experimentally simple and economic viable technique, the Plasma Electrolytic Oxidation, has been applied to produce dielectric coatings with high thermal conductivity on aluminum substrates. The samples have been prepared in sodium silicate solutions using pulsed DC voltage. Scanning electron microscopy with X-ray energy dispersive spectroscopy, X-ray diffraction, ultraviolet-visible reflectance and impedance spectroscopies, and photoacoustic spectroscopy have been applied to characterize the produced materials. Under certain conditions, samples with electric resistivities as high as 4.6 × 1012 Ω.cm and thermal diffusivity 900% higher than that of Al2O3 or AlN have been grown at room temperature.