Effects of Matrix Silicon Content on the Plasma Electrolytic Oxidation of Al-Si Alloys Using Different Power Modes
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The plasma electrolytic oxidation (PEO) of pure Al and Al alloys containing 4, 9, 12, or 15 wt.% Si were investigated under pulsed bipolar current and pulsed bipolar voltage modes, respectively. It was determined that the discharge sparks preferentially occurred on the SiO2 relative to the Al2O3 during the initial stage of PEO processing regardless of the power mode. Following 30 min of PEO treatment under the two modes, the thicknesses of the layers decreased, whereas their specific energy consumption increased with increasing Si content in the matrix. The presence of primary Si in the alloy with 15 wt.% Si had a significantly negative effect on the PEO process in the pulsed bipolar current mode: The layer thickness decreased by 45%, and its specific energy consumption increased by 52%, compared with those on pure Al. However, in the pulsed bipolar voltage mode, the layer thickness on the evaluated samples only decreased slightly, and it became much more similar after treatment.Keywords:
Plasma electrolytic oxidation
Matrix (chemical analysis)
The plasma electrolytic oxidation (PEO) procedure is utilized in order to amend the surface properties of Mg and its alloys. This procedure creates a ceramic coating on the surface applying high-voltage. The presence of deep pores and porosities in the surface that affect the corrosion resistance of the coatings is one of the PEO procedure limitations. One of the useful methods to decrease porosities of coating and improve its final properties is changing electrolyte conditions based on the presence of micro- and nanoparticles. The present paper reviews the mechanisms of particle adsorption and composition in PEO thin films in addition to the effect of particle addition on the microstructure, composition and corrosion behavior of coatings that were applied on magnesium alloys.
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The ceramic coating on aluminum alloy was prepared in sodium metasilicate electrolyte by plasma electrolytic oxidation (PEO).The eect of PEO treating time on surface layer was investigated.The morphology and phase composition of the ceramic coatings were characterized by scanning electron microscopy (SEM) and X-ray diractometer (XRD).The eect of the electrolyte contents on the growth mechanism, element distribution and properties of oxide layers were studied.Oxide coatings morphology is strongly dependent on PEO process time.The microdischarges characteristics were studied as well, and it is shown that size of microdischarges becomes larger with increasing time of PEO.XRD analysis showed that Plasma Electrolytic Oxidation coating has hard, dominantly Al2O3 phase.
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Plasma electrolytic oxidation (PEO), a promising surface treatment method to improve the corrosion and wear resistance of magnesium and its alloys, operates at high voltages, resulting in a relatively high energy cost. To make the PEO process more economically viable, its energy efficiency needs to be improved. This study investigates the growth behaviour and microstructural characteristics of low-energy PEO coatings on an AM50 magnesium alloy in a concentrated electrolyte containing sodium tetraborate. The surface morphology of the coatings was different from typical PEO coating morphologies and a large voltage oscillation was observed during treatment. Using different characterisation techniques, and based on a micro-discharge model, a correlation was made between the voltage-time behaviour, micro-discharge characteristics and the composition and microstructure of the coated samples. The results suggest electrolyte chemistry can somewhat control discharge behaviour, which plays an important role in PEO coating growth.
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In this work, the composition of an electrolyte was selected and optimized to induce the formation of hydroxyapatite during Plasma electrolytic oxidation (PEO) treatment on an AZ31 alloy for application in bioabsorbable implants. In detail, the PEO process, called PEO-BIO (Plasma Electrolytic Oxidation-Biocompatible), was performed using a silicate-phosphate-based electrolyte with the addition of calcium oxide in direct-current mode using high current densities and short treatment times. For comparison, a known PEO process for producing anticorrosive coatings, called standard, was applied on the same alloy. The coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and XPS analyses. The corrosion performance was evaluated in simulated body fluid (SBF) at 37 °C. The coating produced on the PEO-BIO sample was porous and thicker than the standard PEO one, with zones enriched in Ca and P. The XRD analysis showed the formation of hydroxyapatite and calcium oxides in addition to magnesium-silicon oxide and magnesium oxide in the PEO-BIO sample. The corrosion resistance of PEO-BIO sample was comparable with that of a traditional PEO treated sample, and higher than that of the untreated alloy.
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