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The effect of Fe-containing colloid particles in electrolyte on the composition and magnetic characteristics of oxide layers on titanium formed using the method of plasma electrolytic oxidation
Abstract
The composition, structure, and magnetic characteristics of oxide layers on titanium formed in electrolytes containing colloid particles of iron hydroxo-compounds and their filtrates have been investigated. The obtained results corroborate that formation of Fe-containing crystallites in coating pores occurs due to ingress of negatively charged particles of hydroxo-compounds of transition metals from the electrolyte into breakdown channels and their transformation in local spaces of electric breakdowns. The presence of crystallites in pores is responsible for coatings ferromagnetic properties. Fe-containing crystallites were not found in pores of coatings formed in the electrolyte after filtering of iron hydroxides and hydroxo-salts, whereas coatings contained small concentrations of iron homogeneously distributed over the surface and manifested paramagnetic properties at room temperature.
... Therefore, this study discusses the influence mechanism of Fand Fe 3+ on micro-arc oxidation coating and their interaction. There is also a black micro-arc oxidation ceramic coating with Fe 3+ and Fon AZ40M magnesium alloy prepared by phosphorus-based electrolyte 18 in order to explore the change mechanism of Fe 3+ and Fas well as their effects on corrosion resistance and thermal control performance of coatings. ...
This study focuses on a black micro‐arc oxidation ceramic coating prepared on the surface of magnesium alloy by the technology of micro‐arc oxidation in the electrolyte containing F– and Fe³⁺ as well as its mechanism of F– and Fe³⁺. It needs coatings to experience detail analyses on their thickness, roughness, corrosion resistance, thermal control property, valence states of elements, phase composition, and morphology of coatings, respectively, through coating thickness gauge, roughness tester, electrochemical workstation, AE radiometer, X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), scanning electron microscope (SEM), and energy‐dispersive X‐ray spectroscopy (EDS). Results showed that with the help of F– in electrolyte, Fe³⁺ can be complexed and MgF2 can be obtained in the coating, which reduces the pores on the surface of micro‐arc oxidation coating. In addition, Fe³⁺ in the electrolyte contributes to the preparation of Fe2O3 and Fe3O4 in the coating, which can blacken the surface of the coating. Both F– and Fe³⁺ benefit to improve the corrosion resistance and thermal control performance of micro‐arc oxidation coating. There is higher iron oxide in the outer layer but higher fluoride in the inner layer of the coating.
... Rudnev et al. [506] synthesized PEO coatings containing iron using a phosphate-borate-tungstate electrolyte containing iron (III) oxalate. The negatively-charged hydroxo-compounds particles are drawn to the discharge sites, where the subsequent transformation leads to the formation of iron-containing crystals [507]. Magnetic properties of the coating are ascribed to the formation of agglomerates of reduced iron particles surrounded by hydration shells [506]. ...
The plasma electrolytic oxidation is an innovative method for the surface treatment of titanium and its alloys. This review provides an overview of the historical development of the process and summarizes the current state of the art. The chemical as well as the electro- and plasma-chemical basics of the layer forming mechanisms, which comprises the substrate/electrolyte interface before discharge initiation and the different types and stages of plasma electrolytic discharge phenomena are explained within the context of titanium-based materials. How these phenomena can be influenced by the use of suitable electrolytes and controlled by the electrical regime is described. Subsequently, the microstructures and composition of the layers are described in detail, and the properties for specific applications are then discussed. The resistance of a PEO coating to corrosive environments, tribological factors, and alternating mechanical stress is viewed critically, and the extensive functional properties such as physiological compatibility, photocatalytic activity, and decorative properties are revealed. Finally, examples of various practical applications in the medical engineering, aviation, automotive, and environmental technology fields, as well as other branches of industry, are presented.
... On the other hand, reduction of the friction coefficient of the obtained PEO coatings may be achieved using, e.g., the duplex technique, combining the PEO treatment with the ion implantation in plasma [49], by introducing into electrolyte the 150-550-nm polytetrafluoroethylene (PTFE) molecules and 150-nm carbon fibers [50] or by applying a layer of graphite as a solid lubricant [51]. It is also worth mentioning that the PEO coatings may be used to manufacture gas (H 2 and CO) and humidity sensors [52,53], as well as sensors for the electromagnetic radiation protection [54]. ...
Coatings enriched with zinc and copper as well as calcium or magnesium, fabricated on titanium substrate by Plasma Electrolytic Oxidation (PEO) under AC conditions (two cathodic voltages, i.e., −35 or −135 V, and anodic voltage of +400 V), were investigated. In all experiments, the electrolytes were based on concentrated orthophosphoric acid (85 wt%) and zinc, copper, calcium and/or magnesium nitrates. It was found that the introduced calcium and magnesium were in the ranges 5.0–5.4 at% and 5.6–6.5 at%, respectively, while the zinc and copper amounts were in the range of 0.3–0.6 at%. Additionally, it was noted that the metals of the block S (Ca and Mg) could be incorporated into the structure about 13 times more than metals of the transition group (Zn and Cu). The incorporated metals (from the electrolyte) into the top-layer of PEO phosphate coatings were on their first (Cu+) or second (Cu2+, Ca2+ and Mg2+) oxidation states. The crystalline phases (TiO and Ti3O) were detected only in coatings fabricated at cathodic voltage of −135 V. It has also been pointed that fabricated porous calcium–phosphate coatings enriched with biocompatible magnesium as well as with antibacterial zinc and copper are dedicated mainly to medical applications. However, their use for other applications (e.g., catalysis and photocatalysis) after additional functionalizations is not excluded.
... According to [13], including the iron into the coating pores is a result of the capture of iron hydroxide particles from the electrolyte by electric discharges. Therefore, another possible reason for the absence of ferromagnetic properties in many elevations is that not all discharges capture and involve into coatings the dispersed particles of iron hydroxides from the electrolyte. ...
Oxide coatings formed on aluminum alloy in electrolyte with colloidal particles of iron hydroxides show ferromagnetic properties. Iron distribution have been studied using electron scanning microscopy and magnetic force microscopy. It is found that the iron localized in the areas exposed to spark and microarc electric discharges makes the main contribution to the ferromagnetic properties of coatings.
... The formation of the crystallites in the coating pores is associated with involving the colloidal particles of transition metal hydroxides into the coating material from the slurry electrolytes during discharge phenomena [25]. At the same time, the coatings formed in electrolytes simultaneously containing colloid particles of hydroxides of different nature are poorly studied. ...
Ferromagnetic oxide coatings were formed on aluminum alloy by the plasma-electrolytic oxidation technique in an electrolyte with colloidal particles of iron and cobalt hydroxides. Iron and cobalt are concentrated in the coating pores as a part of nanosized crystallites. The size of individual crystallites in the pores was ∼50–100 nm. The deficit of oxygen to form oxides in crystallites shows that the metals in the crystallites are predominantly in a reduced state. It is also possible that the metal or oxide nuclei are surrounded by oxide-hydroxide shells. The coatings obtained within 5 min have a high coercive force Hc = 1300 Oe. A theoretical analysis of the magnetic properties of Fe-, Co-containing coatings has been performed using the model of clusters consisting of magnetostatically interacting particles. The theoretical value of the saturation magnetization and the experimental values of the coercive force can be explained with the presence of two phases in the nanoparticles: a large antiferromagnetic or ferromagnetic (hydroxides and/or oxides of iron and cobalt) and a small superparamagnetic (iron, cobalt, magnetite, maghemite).
Fe- and/or Co-containing coatings have been formed on titanium by plasma electrolytic oxidation (PEO) in electrolytes-sols with colloidal particles of Fe(III) and/or Co(II) compounds. The influence of the electrolyte formula has been studied on the coatings' composition, surface morphology, and magnetic properties. It has been established that the total concentration of iron, cobalt, and titanium in the coating composition is 13.1–22.0 at. %, while their thickness varies from 22 to 37 μm, and the porosity ranges from 10 to 26 %. Micro- and nanosized crystal-like particles have been found both on the surface (in valleys, around pore mouths, and in caps covering pores) and inside pores (at the bottom and walls). In the particles and the layer lining the bottom of the pores, the concentration of iron, cobalt, and titanium is higher than over the surface (49.7–59.1 at. %). Depending on electrolyte formula, the crystallites located in the pores contain (at. %) 28.4–39.3 O; 3.5–9.5 P; 20.3–29.4 Ti; 2.1–7.5 W; up to 22 Fe and up to 38.8 Co. The values of the coercive force measured at 300 K vary from 6366 to 27,056 A/m, and those at 3 K range from 3581 to 17,428 A/m. The ferromagnetic properties of the coated samples correlate with the elemental composition of the particles found in the pores and on the surface.
Black micro‐arc oxidation (MAO) coatings were fabricated on ZK61M Mg alloy in alkaline electrolyte containing sodium phosphate, ferric citrate, and potassium glucose. The formation process of Fe³⁺ complexes in the electrolyte and the phase composition and coloration mechanisms of the MAO coatings were investigated. The results showed that Fe³⁺ complexes move to the anode and are liable to thermal decomposition continuously in the location of spark discharges, resulting in the formation of iron oxide and magnetite incorporated in the black ceramic coatings.
Fe-containing oxide coatings fabricated by the plasma-electrolytic oxidation (PEO) method have been studied by means of scanning electron and magnetic force microscopy. The data comparison has demonstrated that ferromagnetic properties of the coatings are mainly associated with concentrating of iron in specific pores. A scheme has been suggested that would explain iron distribution in the course of the coating growth.
The effect of iron sulfate and citrate addition into the base alkaline phosphate-borate-tungstate electrolyte on the peculiarities of plasma-electrolytic formation of coatings on titanium, their thickness, surface morphology, composition, and magnetic characteristics has been investigated. In the first electrolyte, dispersed particles of iron hydroxides and hydroxo salts are formed, whereas the second one comprises a true solution. Numerous Fe-containing crystallites of a size of ~50 nm united into agglomerates have been found in the suspension electrolyte with FeSO4. Such coatings manifest ferromagnetic properties: coercive force Hc of the samples is 62 and 148 Oe at 300 and 2 K, respectively. In pores of the coatings obtained in the electrolyte with FeC6H5O7 (true solution), the presence of crystallites is less clearly expressed, while crystallites themselves are larger and molten to a higher degree. At room temperature, such coatings are paramagnetic; at 2 K, they manifest ferromagnetic behavior with the Нс value of up to 200 Oe. The available data enable one to associate ferromagnetic properties of the formed coatings with metals concentrated in pores.
Oxide coatings formed on titanium by plasma-electrolytic oxidation in a Na3PO4 + Na2B4O7 + Na2WO4 + Fe2(C2O4)3 electrolyte-suspension at different current densities and different durations of treatment are shown to have ferromagnetic properties. The coercive force of the specimens reaches maximum values of 124 and 380 Oe at 300 and 10 K, respectively, when the thickness of coatings is about 3–5 mm. Crystallites with a mean size of ∼50 nm are found to be present in pores of the coatings. Based on the experimental data, combined with the results of theoretical modeling carried out previously, crystallites are concluded to be iron particles surrounded with a shell composed of oxides and/or hydroxides. The existence of crystallites and their spatial sizes determine the ferromagnetic properties of the coatings.
The effect of the nature of the supporting electrolyte in the composition of electrolytic suspensions containing dispersed particles of Fe(III) and Co(II) hydroxides, and of anodic and bipolar anodic-cathodic polarization on features of the formation, composition, and magnetic characteristics of oxide coatings is studied. In all cases, iron and cobalt are incorporated into the coatings and are concentrated predominantly in pores. The pores of the coatings include particles consisting of the reduced metals, presumably surrounded by oxide or hydroxide shells. The electrolyte composition affects the concentration and ratio of the metals in the particles. A correlation is observed between the ferro- or ferrimagnetism of the coatings and the content and ratio of cobalt and iron in the pores.
The coatings manifesting ferromagnetic characteristics have been formed on titanium and aluminum by plasma electrolytic oxidation in alkaline electrolytes additionally containing iron oxalate and cobalt or nickel acetate. The metals of iron subgroup are found be concentrated in pores of PEO coatings, as a rule, in form of crystallites. In a number of cases the relation between crystallite compositions and magnetic properties of the coatings has been established.
The magnetic properties of the coatings formed by plasma electrolytic oxidation on titanium and modified by nanoparticles of cobalt were studied. The coercitivity of the obtained magnetoactive layer were equal to 514 Oe at room temperature and to 1024 Oe at 2 K. The high coercitivity is the result of the nanosize effects of particles embedded in the coating. The structure of the nanoparticles, which consist of a Co ferromagnetic core and a CoO antiferromagnetic shell, determines the magnetic properties of the coatings on the whole.
Two kinds of iron-containing coatings, specifically those obtained from electrolytes containing polyphosphate iron complexes
(no. 1) and from electrolyte suspensions (no. 2), are formed and studied. According to the microprobe analysis, the iron content
in the coatings is 6–7 at %. Coatings of type 1 are paramagnetic, while coatings of type 2 are ferromagnetic. The distribution
of elements over the depth of coatings is heterogeneous and the typical components of the surfaces have different compositions.
Iron and titanium are concentrated at the bottom and walls of pores. Upon annealing in air, iron and titanium phosphates crystallize
in coatings no. 1, while maghemite is formed in coatings no. 2. Based on the results obtained, the supposition is made that
the ferromagnetic properties of type 2 coatings are determined by the presence of fine-dispersed magnetite and/or maghemite
particles in them, as well as titanium-magnetite and/or titanium-maghemite grains.
Alumina films containing Fe particles were produced on aluminum alloy substrates by micro-arc oxidation with additive Fe particles in the electrolyte. The permittivity–frequency and permeability–frequency properties were determined in the microwave frequency regime of 6.5–18 GHz by vector network analysis. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF) were employed to investigate the microstructures and compositions. Our results show that about 16.16 wt.% Fe particles are embedded in the alumina films and the acquired complex permittivity and permeability values match the microwave frequency. When the matching thickness is 2 mm, the minimum RL values are −10.5 dB at 9.6 GHz and −9.1 dB at 16.3 GHz. The results suggest a new design of microwave absorbers based on electromagnetic wave-absorbing materials that eliminate the use of a mixture of agglutinants in the preparation.
We report here on the synthesis of homogenous, well-adherent composite film of Fe2O3/SiO2, up to 7 μm thick, on the titanium substrate by anodic treatment optimized for an aqueous suspension of K2O·SiO2 and Fe2O3 powder under galvanostatic conditions. The end products were characterized by scanning electron microscopy, energy-dispersive
X-ray, X-ray powder diffractometry, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy, concluding that the formation
of composite coating at the SiO2 to Fe2O3 ratio of approximately 1:1 proceeds just after formation of a thin TiO2 layer with Fe2O3 particle inclusions without transformations via an electrophoresis deposition of negatively charged Fe2O3 species enveloped by silica ions.
The present study provides a detailed discussion of mechanisms underlying anomalous gas evolution and changes in electrolyte pH during Plasma Electrolytic Oxidation (PEO) of Al in alkaline solutions. This is important for process optimisation and control in deposition of wear and corrosion resistant PEO coatings, which currently attract significant interest. The oxidation was performed under galvanostatic conditions in the range of current densities from 400 to 1500 A m − 2 , with concentration of aqueous alkaline electrolyte varied from 0.5 to 2 g l − 1 KOH. Electrolyte pH was evaluated before and after each PEO treatment. The volume of anodic gas evolved during the process was also measured, with the composition of the gaseous products being subsequently analysed using a quadrupole mass spectrometer. It was proposed that, within the studied ranges of experimental parameters, the evolution excessive oxygen is primarily caused by peroxide decomposition due to the interaction with both hydroxyl ions and radicals at the discharge–electrolyte interface. The interaction with the ions also results in electrolyte acidification, although this process is likely to be contributed by water decomposition due to the ion bombardment from discharge.
Applications of the plasma-electrolytic oxidation technique for the formation of magnetically active oxide coatings on aluminum and titanium are reviewed. Specimens of aluminum-, iron-, and tungsten-containing oxide layers on aluminum substrates with ferro- and ferrimagnetic properties are experimentally produced and studied, as well as specimens that can be remagnetized at certain external magnetic field intensities and specimens the magnetization of which is opposite to the external field. The existence of nano- and microscale crystallites, in which aluminum and metals from the electrolyte are accumulated, are found in pores of the coatings. The crystallites supposedly determine the magnetic properties of the specimens. A correlation between the Fe/Σ(W, Al) atomic ratio in crystallites and the magnetic properties of the systems studied is discovered.
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