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Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasma-electrolytic oxidation
Journal of Alloys and Compounds xxx (xxxx) xxx
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Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasma-electrolytic oxidation V.S. Rudnev a, b, *, P.V. Kharitonskii c, **, A. Kosterov d, E.S. Sergienko d, E.V. Shevchenko d, I.V. Lukiyanchuk a, M.V. Adigamova a, V.P. Morozova a, I.A. Tkachenko a a
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok, Russia Far Eastern Federal University, Vladivostok, Russia Saint-Petersburg Electrotechnical University "LETI", St. Petersburg, Russia d Saint Petersburg State University, St. Petersburg, Russia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2019 Received in revised form 9 September 2019 Accepted 4 October 2019 Available online xxx
A comprehensive study of the magnetic behavior, morphology, and composition of Fe-containing oxide coatings on aluminum and titanium has been carried out to investigate the origin of their ferromagnetism. The coatings have been formed by the plasma electrolytic oxidation (PEO) technique in slurry electrolytes containing colloidal particles of iron(III) hydroxides. On the surface of coatings on Al, iron is distributed unevenly concentrating in defective areas with a large number of small pores, and near large pores. On the surface of coatings on Ti, iron and titanium are distributed in antiphase in areas of comparable size. Within the pores, iron concentration appears about 5e10 times higher and oxygen concentration 3e4 times lower than their average concentration over the surface. In both cases, localization of the areas with ferromagnetic properties follows the peculiarities of iron distribution on the surface. The magnetic fraction in the coatings on aluminum appears to be represented by iron-aluminum spinel Fe3-xAlxO4 with x > 0.06, likely cation-deficient. Elemental iron and traces of iron hydroxides are also possibly present. In the coatings on titanium, titanomagnetite (Fe3-xTixO4, where x ~ 0.2e0.3) or its oxidized analogue, titanomaghemite, appear to be present, and possibly also some FeeTi alloy particles. © 2019 Elsevier B.V. All rights reserved.
Keywords: Fe-containing oxide coatings Plasma electrolytic oxidation Morphology and composition Magnetic measurements Origin of ferromagnetism
1. Introduction Traditionally, plasma electrolytic oxidation (PEO) technique is used to obtain protective oxide layers on the surface of valve metals and alloys (Al, Ti, Mg and others) [1e3]. PEO is electrochemical oxidation of the surface of anodic or alternately anode-cathode polarized metals and alloys in electrolytes at voltages producing multiple spark or microarc electric discharges at the «electrolyte/ growing oxide » interface. Discharges produce conditions for introducing electrolyte components into the oxide layer, and cause high-temperature interactions and processes in its whole volume. PEO technology allows forming coatings of complex chemical composition, including those with a gradient or layered structure
* Corresponding author. Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok, Russia ** Corresponding author. E-mail addresses: [email protected] (V.S. Rudnev), [email protected] (P.V. Kharitonskii).
[4,5]. PEO is used to form multicomponent layers with various functional properties depending on the chemical composition, including optical [6], catalytic [7], biocompatible [8], biocidal [9] or sensory [10]. Studies have recently started on the use of PEO for obtaining dielectric oxide layers on paramagnetic aluminum and titanium having specific magnetic characteristics, e.g. absorbing microwave radiation [11], antiferromagnetic [12] and ferromagnetic [13e15]. Such layers may find applications as absorbers of electromagnetic radiation and materials for microwave waveguides, as well as in medicine and in the design of catalysts, separators, and microtransformers. The advantages of PEO technology include the following. The method allows applying coatings on products of complex geometric shapes and significant size. It does not require special equipment, creating a vacuum or a controlled atmosphere. In most cases, special pre-treatment of the metal surface before coating deposition is not required too. The process is carried out at room temperature of the electrolyte, within a short time (usually 5e30 min). When using this technology, a layer of oxide of the
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Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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metal treated is always formed between the metal substrate and the functional layer. This oxide layer provides high adhesion to the metal and has protective properties. PEO technology has good practical prospects for the production of multicomponent oxide protective coatings with special properties, including ferromagnetic ones. For obtaining oxide coatings with ferromagnetic properties by the PEO technique, we proposed [13] to use electrolyte sols, i.e. solutions spontaneously producing negatively charged colloidal particles of transition metal hydroxides as a result of hydrolysis. To achieve this, Fe(III) oxalate or/and Co(II) or Ni(II) acetate, which do not react chemically with aluminum, titanium and their oxides, were added to the alkaline electrolyte forming negatively charged colloidal particles of Fe (Co, Ni) hydroxides [16e20]. In the electric discharge zone, these latter are incorporated into a growing oxide coating, undergoing thermal transformations and engaging in high-temperature interactions with other components of the coating. Two features of this approach are noteworthy: (i) magnetic components are formed directly during the PEO process (in a onepot setting); (ii) simultaneously introducing salts of several transition metals into the electrolyte makes it possible, at least in theory, to incorporate all of them into the growing oxide layer opening up ways to control the magnetic characteristics of a forming structure. To date, the formation, composition, structure, and magnetic characteristics have been studied in most detail for PEO coatings grown on aluminum and titanium in the electrolytes with colloidal particles of iron hydroxides [16e22]. In them, iron tends to be distributed unevenly over the surface [17]. Its concentration reaches maximum, several times higher than in the bulk of the coating, in pores and in areas with many fine pores, i.e. in the areas exposed to electric discharges at the moment of the formation process completion [18]. In the pores open to the surface, iron is concentrated in the pore-lining layer and in the crystallites formed within the pores. Experimental data suggest that at least part of iron in pores might be in a reduced state. In some cases, presence of Fe0 has been confirmed by X-ray photoelectron spectroscopy [18] and Xray diffraction [19]. This led to a natural conclusion that iron contained in the pores is the main source of ferromagnetism in PEO coatings. In the case of Fe-containing coatings on an aluminum alloy, this hypothesis has been confirmed by comparing data on the surface morphology and distribution of ferromagnetic areas over the coatings surface determined using magnetic force microscopy [21]. Meanwhile, iron is also present in the bulk of the oxide layer [16e21]. Coatings magnetic properties may then be a superposition of contributions from various Fe-containing compounds present both in pores and in the bulk of coating layer. This work aims to expand our understanding of structure, composition, magnetic behavior, and of interrelationships between them, and to elucidate the causes of ferromagnetism of PEO coatings. Towards this end, Fe-containing oxide coatings formed by PEO on aluminum and titanium in electrolyte-sols with iron(III) hydroxides were investigated using magnetic force and scanning electron microscopies, X-ray diffraction and magnetometry. 2. Materials and methods 2.1. Materials The coatings were formed on flat samples of an aluminum alloy (4.8e5.8% Mg, 0.5e0.8% Mn, the rest Al) and technical titanium (99.2e99.7% Ti) 3 0.7 0.1 cm in size. Before oxidation, the samples were machined to remove burrs and sharp corners, and then chemically polished. A mixture of H3PO4:H2SO4:HNO3 ¼ 4:2:1
(by volume) and a mixture of HF:HNO3 ¼ 1:3 (by volume) were used for treating aluminum alloy and titanium, respectively. In both cases, the samples were polished at a temperature of 60e80 C with an exposure of 2e5 s several times with intermediate washing with water until a mirror-quality surface was formed. Then the polished samples were washed with distilled water and dried at 70 C in air. 2.2. Electrolyte preparation and PEO coatings fabrication As in Refs. [18,19], an alkaline electrolyte containing (mol/L) 0.066 Na3PO4 þ 0.034 Na2B4O7 þ 0.006 Na2WO4 (PBW electrolyte thereafter) was used as the base electrolyte. Fe-containing PEO coatings on titanium were formed in the PBW electrolyte þ0.04 mol/L Fe2(C2O4)3, those on the aluminum alloy in the PBW electrolyte þ 0.015 mol/L Fe2(C2O4)3. Commercial reagents of analytical grade were used. Each component was preliminarily dissolved in distilled water and then the solutions were mixed and stirred for at least 1 h. The electrochemical cell for PEO consisted of a 1 dm3 glass vessel, a cathode in the form of coiled hollow pipe of nickel alloy, and the sample processed as an anode. Cold tap water was passed through the coil pipe to cool the electrolyte. Coatings were formed galvanostatically on anodically polarized samples at an effective current density of 0.1 A/cm2 for 10 min. The electrolyte was agitated with a magnetic stirrer. The electrolyte temperature during the PEO process did not exceed 30 C. A computer-controlled reverse thyristor aggregate TER4-100/460 N (Russia) operating in a unipolar mode was used as a current source. 2.3. Morphology and composition Element composition of the coatings, averaged for five ~250 250 mm surface areas, was determined using an electron probe micro-analyzer JXA 8100 (Japan) with an INCA energy spectrum analyzer (United Kingdom). Pores in the coatings were investigated using a high-resolution Hitachi S5500 (Japan) electron scanning microscope with a Thermo Scientific (USA) attachment for energy-dispersive analysis. To determine the element composition of individual areas of the pores, the probe beam was focused on areas ~200 200 nm in size. Distribution maps of the elements in the surface layer of the coatings were obtained for 60 40 mm areas using an electron scanning microscope Hitachi Se3400 N (Japan) with a WDS-INCA 500 (United Kingdom) energy dispersive attachment. The X-ray diffraction (XRD) patterns of the coated samples were recorded on a D8 ADVANCE X-ray diffractometer (Bruker, Germany) in CuKa-radiation. The EVA search program with the PDF-2 database (Powder Diffraction File; Kabekkodu, 2007) was used for X-ray patterns processing. 2.4. Magnetic measurements Studies of magnetic properties aimed at identifying a composition of ferromagnetic phases and its magnetic state included both low-temperature measurements (in the temperature range 2e300 K) and measurements of magnetic characteristics after heatings up to a high temperature. Since it was expected that concentration of magnetic phase(s) with respect to the total sample volume would be fairly low, of the order of that observed in typical rocks, experimental methods based on the analysis of remanent magnetization, as developed in the field of rock magnetism, were employed. Magnetic hysteresis (maximum field 7 T, at 295 K) and lowtemperature magnetization measurements were performed using an MPMS 3 instrument (Quantum Design, USA) in a vibrating
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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sample magnetometer mode. The saturation isothermal remanent magnetization (SIRM) created in a field of 5 T at 2 K after cooling in zero field (Zero field cooling, ZFC) and the respective thermoremanent magnetization in a strong (5 T, Field cooling, FC) magnetic field respectively were then measured during warming in zero field. SIRM (5 T) acquired at 300 K was measured during the cooling-warming cycle between 300 and 2 K, also in zero field. To avoid confusion, we note that our remanence-based ZFC-FC procedure closely follows that introduced in Ref. [22] and is different from the common ZFC-FC measurement of induced magnetization in relatively weak (1000 Oe) field commonly used to characterize superparamagnetic behavior in nanoparticle ensembles [23]. High-temperature magnetic measurements were performed using an equipment for paleomagnetic analysis. A SQUID SRM-755 magnetometer (2G Enterprises, USA) was employed to measure remanent magnetization, and a TD-48-SC furnace (ASC Scientific, USA) for thermal demagnetization. A three-component isothermal remanent magnetization was created at room temperature so that the components along the three orthogonal axes 1, 2, 3 of the sample were acquired in fields 0.1, 0.3 and 1 T, respectively. Samples were then heated in zero field to progressively higher temperatures, 650 and 775 C for the samples on aluminum and titanium substrates, respectively. Remanent magnetization vector was measured after each heating step. In paleomagnetic practice, this experiment is referred to as Lowrie test [24] and used to identify various magnetic phases in a sample by unblocking temperatures of the respective remanence components. Distribution of stray magnetic fields over the surface of the sample was visualized by magnetic force microscopy using an atomic force microscope INTEGRA AURA (NT-MDT, Russia) with a CoCr magnetic probe.
3. Results and discussion 3.1. Morphology and composition of coatings Fig. 1 shows SEM images of the surface of coatings on aluminum
3
and titanium. In the case of aluminum, the surface relief (Fig. 1a) consists of rather large pores distributed over the surface, and of elevations surrounding the pores. On the surface of coatings on titanium (Fig. 1c), three main components of the relief can be recognized: large pores, extended smoothed elevations between pores, and depressions ("valleys") between large pores with numerous small pores. From a comparison of images obtained in a phase contrast mode (Fig. 1b, d), phase composition of the coating surface on aluminum appears more uniform than on titanium. Light spots, indicating the difference between the phase composition of the material in these areas and that of the coating bulk, are visible only around pores. For coatings on titanium, the phase composition of extended elevations between large pores differs from that of depressions. In addition to the images collected in a phase contrast mode, distribution maps of the main elements over the surface are presented in Figs. 2 and 3. In the case of Al samples, aluminum and magnesium, which are part of the alloy, and oxygen are concentrated in the same surface areas (Fig. 2). These areas occupy most of the coating surface. We suppose that they consist mainly of aluminum oxides with an admixture of magnesium oxides or an aluminum-magnesium spinel. Areas with high iron content are distributed over the surface in a highly localized fashion. Iron is concentrated in depressions, defective areas, and along the pores’ perimeter. However, iron is also present in other areas of the surface, but in lower concentration. In contrast to the coatings on Al, the distribution of the main elements over the surface of the coatings on Ti is more complex (Fig. 3). Titanium and iron are distributed over the surface in an antiphase manner. Where there is a high amount of titanium, there is little iron and vice versa. Iron-rich areas are located between large pores being comparable in size with areas enriched with titanium. On the other hand, oxygen is distributed over the surface more evenly than either iron or titanium, so that the areas enriched with titanium and iron oxides appear to alternate. Element compositions of the surface part of the coatings and that of the pores are listed in the Table 1. Surface compositions were
Fig. 1. Surface morphology of the coatings on aluminum alloy (a, b) and titanium (c, d). Amplitude (a, c) and phase representation (b, d).
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 2. The distribution of elements in the surface layer of the coating on aluminum.
measurements in different pores. The elemental composition of the outer part of the coatings surface (analysis depth 2e5 mm) and that of the inner part of the pores are clearly different (Table 1). In pores, the iron concentration is about 5e10 times higher and the oxygen concentration is 3e4 times lower than their average concentrations over the surface. The deficit of oxygen in relation to metals may indicate that in the pores reduced metals, as well as particles of a complex structure, for example, iron surrounded by an oxide layer, or other heterophase systems, could be present [20]. We also note that tungsten (a metal from the electrolyte) tends to concentrate in the pores of the coatings on aluminum. The latter behavior is however not typical for coatings on titanium. Possible reasons for concentrating and reducing iron in the pores of coatings formed in electrolyte sols were previously discussed in Refs. [18,19,25]. According to XRD, coatings on titanium contain TiO2 as both rutile and anatase, and those on aluminum contain g-Al2O3 (Fig. 5). In summary, the distribution of iron over the surface of Al and Ti samples is different, although average iron contents on the surface in both cases are similar and equal to 4.2 and 6.6 at. %, respectively. In both samples, concentration of iron is increased and that of oxygen decreased in the pores, compared with the average concentration on the surface. Furthermore, reduced iron may be present in the pores. This is confirmed by XPS data: upon etching of similar coatings on titanium with argon to a depth of 3 nm, a signal of the reduced iron was recorded [18]. XRD data obtained with signal accumulation showed the presence of metallic iron in the composition of PEO coatings on aluminum [19]. According to SEM studies of the cross section, the internal structure of a coating on aluminum is heterogeneous; there are cracks and pores, including those with dispersed particles. Iron concentration in the outer part of the coating is higher than in the depth. At the coating outer boundary, the Fe-rich areas follow the outlines of the pore boundaries or concentrate near the pores [25].
3.2. Room temperature magnetic properties
Fig. 3. The general view of the surface (a) and the distribution of elements in the surface layer of the coating on titanium (bed).
estimated by averaging of the results of scanning five 250-mm2 areas. Element composition of the pores was determined using an electron scanning microscope attachment for energy dispersive analysis focusing the probe beam on ~200 200 nm areas in the pore bottoms or on crystallites contained in the pores, as shown in Fig. 4. Average results are given for at least five separate
We investigated two PEO coated titanium samples (hereinafter referred to as Ti1 and Ti2) and three PEO coated aluminum samples (hereinafter referred to as Al1, Al2 and Al3). All samples manifest ferromagnetic properties. The hysteresis loops of the samples measured at room temperature are shown in Figs. 6 and 7. For Al and Ti samples, the values of hysteresis parameters demonstrate no significant difference and only slightly depend on the substrate material. For example, for samples Ti1 and Ti2 (Fig. 6b, d), coercive force Hc is 97 and 35 Oe, and coercivity of remanence Hcr is 301 and 227 Oe, respectively. For samples Al1, Al2 and Al3 (Fig. 7b, d, f), Hc is 54, 89, 123 Oe, and Hcr is 394, 598, 710 Oe, respectively. Fig. 8 shows the observations made by magnetic force microscopy. The images on the left are the surface reliefs, those on the right present the distribution of the ferromagnetic areas over the surface. The distribution of ferromagnetic areas for PEO coatings on titanium (Fig. 8a and b) and aluminum (Fig. 8cee) appears quite
Table 1 The average element compositions of the coating surface layers and pores. Sample
Ti Al
Element composition (at.%)
Surface Pore Surface Pore
C
O
Na
P
Fe
W
Mg
Me
10.1 4.6 12.4 4.8
67.0 18.1 56.0 14.9
1.3 e 0.3 e
6.1 5.6 1.4 0.6
6.6 40.8 4.2 51.6
1.5 1.7 0.3 15.9
e e 1.2 0.2
7.4 29.2 24.2 12.0
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 4. Pores on the surface of coatings on Al (a) and Ti (b). The images show the areas for which the elemental composition was determined.
Fig. 5. XRD patterns of samples on aluminum (a) and titanium (b).
different. For the coated aluminum samples, ferromagnetic areas are associated with concentrating iron in the pores. In addition, for the pores in Fig. 8c and d ferromagnetic areas have circular shape, which may be due to the contribution of the compounds lining the pore walls. Weak ferromagnetic response in the remaining part of the coating is apparently related to concentrating iron in cracks, small pores, and depressions of the coating relief. Thus, iron present in the bulk of the coating on Al samples does not exhibit ferromagnetic properties. Based on the conditions of PEO treatment and electrolyte formula, it can be incorporated in the composition of non-ferromagnetic oxides, phosphates, borates [26]. For the coated titanium samples, the distribution of ferromagnetic areas is more complex (Fig. 8a and b). Areas with maximum ferromagnetic response (light areas in Fig. 8b) are also associated with the relief depressions. Since such areas are generally of circular shape, one can suggest that they are associated with
numerous small pores in the depressions. However, as follows from magnetic force microscopy data, the bulk of the coating on titanium shows ferromagnetic properties. Apparently, under electrical discharges during PEO, i.e. in the conditions of elevated temperatures and significant pressure drops, ferromagnetic compounds are formed in the bulk of oxide coating on titanium. Given the electrolyte composition and formation conditions, the bulk of the coating may contain oxides, titanates, phosphates, and/or borates of iron [4,27]. In summary, the distribution of ferromagnetic areas over the surface of coated Al and Ti samples is different. Whereas for Al samples the ferromagnetic areas are associated with pores in the coating, for Ti samples ferromagnetic compounds or particles occur not only in the pores, but also in the bulk of the coating. In both cases, though, the distribution of ferromagnetic areas correlates well with iron distribution over the surface.
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 6. Curves of temperature dependence of the magnetic moment for coated titanium samples (a, c) and central part of hysteresis loops of these samples at 295 K (b, d). The arrows indicate the direction of temperature change.
3.3. Temperature dependences of magnetic parameters 3.3.1. Measurements at cryogenic temperatures For all three Al samples, SIRM acquired at 2 K is not particularly stable. From 50 to 75% of the SIRM is demagnetized below 50 K, and then the SIRM decrease rate is significantly slowed down (Fig. 7). FC and ZFC curves converge at ~70 K, indicating that iron hydroxides, for example, goethite a-FeOOH, even if present, occur in a very small amount and in the form of very small particles, which would be superparamagnetic at room temperature [28]. Alternatively, it can be other hydroxides, for example, lepidocrocite g-FeOOH with a el temperature of 50e60 K [29]. The irreversibility of SIRM cycles Ne (300-2-300 K) indicates, by analogy with titanomagnetites [30,31], the possible presence of a spinel phase (Al-substituted magnetite). If this is so, the absence of the Verwey transition indicates that the Al content in Fe3-xAlxO4 spinel is quite noticeable (x>0.06 [32]). For PEO-coated titanium samples, SIRM acquired at 2 K is even less stable than for aluminum samples (Fig. 6). On reaching 300 K, the non-demagnetized residue is no more than 10% of the original value. An interesting behavior of the ZFC and FC curves is observed at temperatures below 50 K. After the initial (<5 K) sharp decline, the non-demagnetized part of thermoremanent magnetization acquired by FC is noticeably larger than that of the isothermal remanent magnetization acquired at 2 K after ZFC, and the corresponding curves then converge only at T ~ 50 K. A broadly similar behavior was observed for Fe2TiO5 pseudobrookite [33], which may
indicate its presence in the samples. The Verwey transition in both samples is not observed thus excluding the presence of Fe3O4 magnetite phase. At the same time, the shape of SIRM(300-2-300 K) curves may indicate that some amount of titanomagnetite or titanomaghemite with a rather high titanium content (x ~ 0.2e0.3 in Fe3-xTixO4 [30,31]) is present. 3.3.2. High-temperature measurements We investigated two samples: PEO-coated titanium (Ti2) and PEO-coated aluminum (Al3) (Fig. 9). In both cases, at room temperature the component of isothermal remanent magnetization (IRM) acquired between 0.1 and 0.3 T is the largest. The component acquired in a field of 0.1 T, although it is ~70% of IRM (0.3 T), is significantly less thermally stable: its significant part is demagnetized after heating to 200e300 C. The presence of a noticeable IRM(1 T) component which is ~30e40% of IRM(0.3 T) is also noteworthy. Its temperature stability is the same as that of IRM(0.3 T). Such a ratio between the components of magnetization acquired in different fields is not typical of magnetite, including that with up to moderate cation substitution, e.g. titanomagnetite, for which one would expect IRM(0.1 T) > IRM(0.3 T) > (>>) IRM(1 T) [34]. Accordingly, it can be hypothesized that the main magnetic phase, which responsible for the magnetization at room temperature, is metallic iron. In general, the data obtained for Al3 sample do not contradict this, but it could not be heated above 650 C in order to avoid destruction due to melting the aluminum alloy. At the
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 7. Curves of temperature dependence of magnetic moment for coated aluminum samples (a, c, e) and central part of hysteresis loops of these samples at 295 K (b, d, f). The arrows indicate the direction of temperature change.
same time, in the Ti2 sample, the main part of the magnetization is erased already at 450 C, which is much lower than the Curie temperature of iron. It can be suggested (while hypothetically) that during the formation of the coating an iron-titanium alloy was formed making a noticeable contribution to the magnetization along with titanomagnetite or titanomaghemite.
4. Conclusions Summarizing the data obtained from a comprehensive study of magnetic behavior, morphology, phase and element composition of Fe-containing oxide coatings formed by PEO on aluminum and titanium in slurry electrolytes with iron(III) hydroxides, it can be concluded that their magnetic properties are strongly dependent
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 8. Surface relief (a, c, e) and distribution of magnetic fields on the surface of the coatings (b, d, f) on titanium (a, b) and on aluminum (c, d, e, f).
Fig. 9. Temperature behavior of the residual magnetization of the samples in the Lowrie test. PEO-coated aluminum (a) and titanium (b) samples were magnetized in a threecomponent field with a strength of 0.1 (1), 0.3 (2) and 1 T (3).
on the nature of the metal substrate. The main feature, characteristic of all studied samples, is that in the pores, iron concentration is about 5e10 times higher and the oxygen concentration is 3e4 times lower than their average concentrations in the bulk of the coating layer. The deficit of oxygen in relation to metals may indicate the presence in the pores of both oxidized and reduced metals, as well as of particles with complex structure, for example, iron surrounded by an oxide layer or vice versa. The highly uneven distribution of iron over the surface is typical for the coatings on aluminum. Iron-containing particles and compounds are concentrated in cracks, pores, depressions of the surface relief. The areas with ferromagnetic properties are distributed over the surface in a similar way. Based on the room temperature
magnetic properties and its variation at high temperatures, it may be proposed that the magnetic phase primarily responsible for the magnetization at and above room temperature is metallic iron. The presence of a spinel phase (Al-substituted magnetite) is also possible as indicated by magnetic measurements at cryogenic temperatures. These measurements further imply that a small amount of iron hydroxides (goethite or lepidocrocite) in the form of ultrafine particles, which are superparamagnetic at room temperature, is also present. For PEO-coated titanium samples, areas with maximum magnetic response are also associated with pores and iron concentrating therein. However, the bulk of the coating also has pronounced ferromagnetic properties. Based on the analysis of the temperature behavior of the magnetization curves, the
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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ferromagnetism of the samples is likely due to the presence of a mixed iron-titanium oxide e titanomagnetite Fe3-xTixO4 with x~0.2e0.3 or its oxidized analogue titanomaghemite. We also cannot exclude the presence of particles of an iron-titanium alloy, although this requires further study. Acknowledgements Measurements were carried out using the equipment of the Science Park of the St. Petersburg State University at the Nanophotonics Center, Center for Diagnostics of Functional Materials for Medicine, Pharmacology and Nanoelectronics, Resource Center for Geo-Environmental Research and Modeling (GEOMODEL), and Resource Center of Microscopy and Microanalysis. This research was performed within the framework of the Institute of Chemistry FEB RAS State Order (project no. 265-2019-0001). References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73e93. [2] B.V. Vladimirov, B.L. Krit, V.B. Lyudin, N.V. Morozova, A.D. Rossiiskaya, I.V. Suminov, A.V. Epel’feld, Microarc discharge oxidizing of magnesium alloys: a review, Surf. Eng. Appl. Electrochem. 50 (3) (2014) 195e232. [3] B.L. Jiang, Y.M. Wang, Plasma electrolytic oxidation treatment of aluminum and titanium alloys, in: H. Dong (Ed.), Surface Engineering of Light Alloys: Aluminum, Magnesium and Titanium Alloys, Woodhead Publishing, 2010, pp. 110e153. [4] P.S. Gordienko, Formation of Coatings on the Anode-Polarized Electrodes in Aqueous Electrolytes at Sparking and Breakdown Potentials, Dal’nauka, Vladivostok, 1996 (in Russian). [5] V.S. Rudnev, I.V. Lukiyanchuk, M.S. Vasilyeva, M.A. Medkov, M.V. Adigamova, V.I. Sergienko, Aluminum- and titanium-supported plasma electrolytic multicomponent coatings with magnetic, catalytic, biocide or biocompatible properties, Surf. Coat. Technol. 307 (2016) 1219e1235. [6] S. Stojadinovic, N. Radic, B. Grbic, S. Maletic, P. Stefanov, A. Pacevski, R. Vasilic, Structural, photoluminescent and photocatalytic properties of TiO2:Eu3þ coatings formed by plasma electrolytic oxidation, Appl. Surf. Sci. 370 (2016) 218e228. [7] F. Patcas, W. Krysmann, Efficient catalysts with controlled porous structure obtained by anodic oxidation under spark-discharge, Appl. Catal. Gen. 316 (2) (2007) 240e249. [8] O.P. Terleeva, YuP. Sharkeev, A.I. Slonova, I.V. Mironov, E.V. Legostaeva, I.A. Khlusov, E. Matykina, P. Skeldon, G.E. Thompson, Effect of microplasma modes and electrolyte composition on micro-arc oxidation coatings on titanium for medical applications, Surf. Coat. Technol. 205 (6) (2010) 1723e1729. [9] W.H. Song, H.S. Ryu, S.H. Hong, Antibacterial properties of Ag (or Pt)containing calcium phosphate coating formed by micro-arc oxidation, J. Biomed. Mater. Res. A 88 (1) (2009) 246e254. [10] G.I. Marinina, M.S. Vasilyeva, A.S. Lapina, A.Yu Ustinov, V.S. Rudnev, Electroanalytical properties of metal-oxide electrodes formed by plasma electrolytic oxidation, J. Electroanal. Chem. 689 (2013) 262e268. [11] F.Y. Jin, H.H. Tong, J. Li, L.R. Shen, P.K. Chu, Structure and microwave absorbing properties of Fe-particle containing alumina prepared by micro-arc discharge oxidation, Surf. Coat. Technol. 201 (1e2) (2006) 292e295. [12] A. Jagminas, R. Ragalevicius, K. Mazeika, J. Reklaitis, V. Jasulaitiene, A. Selskis, D. Baltrunas, A new strategy for fabrication Fe2O3/SiO2 composite coatings on the Ti substrate, J. Solid State Electrochem. 14 (2) (2010) 271e277. [13] V.S. Rudnev, A.Yu Ustinov, I.V. Lukiyanchuk, P.V. Kharitonskii, A.M. Frolov, V.P. Morozova, I.A. Tkachenko, V.I. Sergienko, Magnetic properties of plasma electrolytic iron-containing oxide coatings on aluminum, Dokl. Phys. Chem.
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Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasma-electrolytic oxidation V.S. Rudnev a, b, *, P.V. Kharitonskii c, **, A. Kosterov d, E.S. Sergienko d, E.V. Shevchenko d, I.V. Lukiyanchuk a, M.V. Adigamova a, V.P. Morozova a, I.A. Tkachenko a a
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok, Russia Far Eastern Federal University, Vladivostok, Russia Saint-Petersburg Electrotechnical University "LETI", St. Petersburg, Russia d Saint Petersburg State University, St. Petersburg, Russia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2019 Received in revised form 9 September 2019 Accepted 4 October 2019 Available online xxx
A comprehensive study of the magnetic behavior, morphology, and composition of Fe-containing oxide coatings on aluminum and titanium has been carried out to investigate the origin of their ferromagnetism. The coatings have been formed by the plasma electrolytic oxidation (PEO) technique in slurry electrolytes containing colloidal particles of iron(III) hydroxides. On the surface of coatings on Al, iron is distributed unevenly concentrating in defective areas with a large number of small pores, and near large pores. On the surface of coatings on Ti, iron and titanium are distributed in antiphase in areas of comparable size. Within the pores, iron concentration appears about 5e10 times higher and oxygen concentration 3e4 times lower than their average concentration over the surface. In both cases, localization of the areas with ferromagnetic properties follows the peculiarities of iron distribution on the surface. The magnetic fraction in the coatings on aluminum appears to be represented by iron-aluminum spinel Fe3-xAlxO4 with x > 0.06, likely cation-deficient. Elemental iron and traces of iron hydroxides are also possibly present. In the coatings on titanium, titanomagnetite (Fe3-xTixO4, where x ~ 0.2e0.3) or its oxidized analogue, titanomaghemite, appear to be present, and possibly also some FeeTi alloy particles. © 2019 Elsevier B.V. All rights reserved.
Keywords: Fe-containing oxide coatings Plasma electrolytic oxidation Morphology and composition Magnetic measurements Origin of ferromagnetism
1. Introduction Traditionally, plasma electrolytic oxidation (PEO) technique is used to obtain protective oxide layers on the surface of valve metals and alloys (Al, Ti, Mg and others) [1e3]. PEO is electrochemical oxidation of the surface of anodic or alternately anode-cathode polarized metals and alloys in electrolytes at voltages producing multiple spark or microarc electric discharges at the «electrolyte/ growing oxide » interface. Discharges produce conditions for introducing electrolyte components into the oxide layer, and cause high-temperature interactions and processes in its whole volume. PEO technology allows forming coatings of complex chemical composition, including those with a gradient or layered structure
* Corresponding author. Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok, Russia ** Corresponding author. E-mail addresses: [email protected] (V.S. Rudnev), [email protected] (P.V. Kharitonskii).
[4,5]. PEO is used to form multicomponent layers with various functional properties depending on the chemical composition, including optical [6], catalytic [7], biocompatible [8], biocidal [9] or sensory [10]. Studies have recently started on the use of PEO for obtaining dielectric oxide layers on paramagnetic aluminum and titanium having specific magnetic characteristics, e.g. absorbing microwave radiation [11], antiferromagnetic [12] and ferromagnetic [13e15]. Such layers may find applications as absorbers of electromagnetic radiation and materials for microwave waveguides, as well as in medicine and in the design of catalysts, separators, and microtransformers. The advantages of PEO technology include the following. The method allows applying coatings on products of complex geometric shapes and significant size. It does not require special equipment, creating a vacuum or a controlled atmosphere. In most cases, special pre-treatment of the metal surface before coating deposition is not required too. The process is carried out at room temperature of the electrolyte, within a short time (usually 5e30 min). When using this technology, a layer of oxide of the
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Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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metal treated is always formed between the metal substrate and the functional layer. This oxide layer provides high adhesion to the metal and has protective properties. PEO technology has good practical prospects for the production of multicomponent oxide protective coatings with special properties, including ferromagnetic ones. For obtaining oxide coatings with ferromagnetic properties by the PEO technique, we proposed [13] to use electrolyte sols, i.e. solutions spontaneously producing negatively charged colloidal particles of transition metal hydroxides as a result of hydrolysis. To achieve this, Fe(III) oxalate or/and Co(II) or Ni(II) acetate, which do not react chemically with aluminum, titanium and their oxides, were added to the alkaline electrolyte forming negatively charged colloidal particles of Fe (Co, Ni) hydroxides [16e20]. In the electric discharge zone, these latter are incorporated into a growing oxide coating, undergoing thermal transformations and engaging in high-temperature interactions with other components of the coating. Two features of this approach are noteworthy: (i) magnetic components are formed directly during the PEO process (in a onepot setting); (ii) simultaneously introducing salts of several transition metals into the electrolyte makes it possible, at least in theory, to incorporate all of them into the growing oxide layer opening up ways to control the magnetic characteristics of a forming structure. To date, the formation, composition, structure, and magnetic characteristics have been studied in most detail for PEO coatings grown on aluminum and titanium in the electrolytes with colloidal particles of iron hydroxides [16e22]. In them, iron tends to be distributed unevenly over the surface [17]. Its concentration reaches maximum, several times higher than in the bulk of the coating, in pores and in areas with many fine pores, i.e. in the areas exposed to electric discharges at the moment of the formation process completion [18]. In the pores open to the surface, iron is concentrated in the pore-lining layer and in the crystallites formed within the pores. Experimental data suggest that at least part of iron in pores might be in a reduced state. In some cases, presence of Fe0 has been confirmed by X-ray photoelectron spectroscopy [18] and Xray diffraction [19]. This led to a natural conclusion that iron contained in the pores is the main source of ferromagnetism in PEO coatings. In the case of Fe-containing coatings on an aluminum alloy, this hypothesis has been confirmed by comparing data on the surface morphology and distribution of ferromagnetic areas over the coatings surface determined using magnetic force microscopy [21]. Meanwhile, iron is also present in the bulk of the oxide layer [16e21]. Coatings magnetic properties may then be a superposition of contributions from various Fe-containing compounds present both in pores and in the bulk of coating layer. This work aims to expand our understanding of structure, composition, magnetic behavior, and of interrelationships between them, and to elucidate the causes of ferromagnetism of PEO coatings. Towards this end, Fe-containing oxide coatings formed by PEO on aluminum and titanium in electrolyte-sols with iron(III) hydroxides were investigated using magnetic force and scanning electron microscopies, X-ray diffraction and magnetometry. 2. Materials and methods 2.1. Materials The coatings were formed on flat samples of an aluminum alloy (4.8e5.8% Mg, 0.5e0.8% Mn, the rest Al) and technical titanium (99.2e99.7% Ti) 3 0.7 0.1 cm in size. Before oxidation, the samples were machined to remove burrs and sharp corners, and then chemically polished. A mixture of H3PO4:H2SO4:HNO3 ¼ 4:2:1
(by volume) and a mixture of HF:HNO3 ¼ 1:3 (by volume) were used for treating aluminum alloy and titanium, respectively. In both cases, the samples were polished at a temperature of 60e80 C with an exposure of 2e5 s several times with intermediate washing with water until a mirror-quality surface was formed. Then the polished samples were washed with distilled water and dried at 70 C in air. 2.2. Electrolyte preparation and PEO coatings fabrication As in Refs. [18,19], an alkaline electrolyte containing (mol/L) 0.066 Na3PO4 þ 0.034 Na2B4O7 þ 0.006 Na2WO4 (PBW electrolyte thereafter) was used as the base electrolyte. Fe-containing PEO coatings on titanium were formed in the PBW electrolyte þ0.04 mol/L Fe2(C2O4)3, those on the aluminum alloy in the PBW electrolyte þ 0.015 mol/L Fe2(C2O4)3. Commercial reagents of analytical grade were used. Each component was preliminarily dissolved in distilled water and then the solutions were mixed and stirred for at least 1 h. The electrochemical cell for PEO consisted of a 1 dm3 glass vessel, a cathode in the form of coiled hollow pipe of nickel alloy, and the sample processed as an anode. Cold tap water was passed through the coil pipe to cool the electrolyte. Coatings were formed galvanostatically on anodically polarized samples at an effective current density of 0.1 A/cm2 for 10 min. The electrolyte was agitated with a magnetic stirrer. The electrolyte temperature during the PEO process did not exceed 30 C. A computer-controlled reverse thyristor aggregate TER4-100/460 N (Russia) operating in a unipolar mode was used as a current source. 2.3. Morphology and composition Element composition of the coatings, averaged for five ~250 250 mm surface areas, was determined using an electron probe micro-analyzer JXA 8100 (Japan) with an INCA energy spectrum analyzer (United Kingdom). Pores in the coatings were investigated using a high-resolution Hitachi S5500 (Japan) electron scanning microscope with a Thermo Scientific (USA) attachment for energy-dispersive analysis. To determine the element composition of individual areas of the pores, the probe beam was focused on areas ~200 200 nm in size. Distribution maps of the elements in the surface layer of the coatings were obtained for 60 40 mm areas using an electron scanning microscope Hitachi Se3400 N (Japan) with a WDS-INCA 500 (United Kingdom) energy dispersive attachment. The X-ray diffraction (XRD) patterns of the coated samples were recorded on a D8 ADVANCE X-ray diffractometer (Bruker, Germany) in CuKa-radiation. The EVA search program with the PDF-2 database (Powder Diffraction File; Kabekkodu, 2007) was used for X-ray patterns processing. 2.4. Magnetic measurements Studies of magnetic properties aimed at identifying a composition of ferromagnetic phases and its magnetic state included both low-temperature measurements (in the temperature range 2e300 K) and measurements of magnetic characteristics after heatings up to a high temperature. Since it was expected that concentration of magnetic phase(s) with respect to the total sample volume would be fairly low, of the order of that observed in typical rocks, experimental methods based on the analysis of remanent magnetization, as developed in the field of rock magnetism, were employed. Magnetic hysteresis (maximum field 7 T, at 295 K) and lowtemperature magnetization measurements were performed using an MPMS 3 instrument (Quantum Design, USA) in a vibrating
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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sample magnetometer mode. The saturation isothermal remanent magnetization (SIRM) created in a field of 5 T at 2 K after cooling in zero field (Zero field cooling, ZFC) and the respective thermoremanent magnetization in a strong (5 T, Field cooling, FC) magnetic field respectively were then measured during warming in zero field. SIRM (5 T) acquired at 300 K was measured during the cooling-warming cycle between 300 and 2 K, also in zero field. To avoid confusion, we note that our remanence-based ZFC-FC procedure closely follows that introduced in Ref. [22] and is different from the common ZFC-FC measurement of induced magnetization in relatively weak (1000 Oe) field commonly used to characterize superparamagnetic behavior in nanoparticle ensembles [23]. High-temperature magnetic measurements were performed using an equipment for paleomagnetic analysis. A SQUID SRM-755 magnetometer (2G Enterprises, USA) was employed to measure remanent magnetization, and a TD-48-SC furnace (ASC Scientific, USA) for thermal demagnetization. A three-component isothermal remanent magnetization was created at room temperature so that the components along the three orthogonal axes 1, 2, 3 of the sample were acquired in fields 0.1, 0.3 and 1 T, respectively. Samples were then heated in zero field to progressively higher temperatures, 650 and 775 C for the samples on aluminum and titanium substrates, respectively. Remanent magnetization vector was measured after each heating step. In paleomagnetic practice, this experiment is referred to as Lowrie test [24] and used to identify various magnetic phases in a sample by unblocking temperatures of the respective remanence components. Distribution of stray magnetic fields over the surface of the sample was visualized by magnetic force microscopy using an atomic force microscope INTEGRA AURA (NT-MDT, Russia) with a CoCr magnetic probe.
3. Results and discussion 3.1. Morphology and composition of coatings Fig. 1 shows SEM images of the surface of coatings on aluminum
3
and titanium. In the case of aluminum, the surface relief (Fig. 1a) consists of rather large pores distributed over the surface, and of elevations surrounding the pores. On the surface of coatings on titanium (Fig. 1c), three main components of the relief can be recognized: large pores, extended smoothed elevations between pores, and depressions ("valleys") between large pores with numerous small pores. From a comparison of images obtained in a phase contrast mode (Fig. 1b, d), phase composition of the coating surface on aluminum appears more uniform than on titanium. Light spots, indicating the difference between the phase composition of the material in these areas and that of the coating bulk, are visible only around pores. For coatings on titanium, the phase composition of extended elevations between large pores differs from that of depressions. In addition to the images collected in a phase contrast mode, distribution maps of the main elements over the surface are presented in Figs. 2 and 3. In the case of Al samples, aluminum and magnesium, which are part of the alloy, and oxygen are concentrated in the same surface areas (Fig. 2). These areas occupy most of the coating surface. We suppose that they consist mainly of aluminum oxides with an admixture of magnesium oxides or an aluminum-magnesium spinel. Areas with high iron content are distributed over the surface in a highly localized fashion. Iron is concentrated in depressions, defective areas, and along the pores’ perimeter. However, iron is also present in other areas of the surface, but in lower concentration. In contrast to the coatings on Al, the distribution of the main elements over the surface of the coatings on Ti is more complex (Fig. 3). Titanium and iron are distributed over the surface in an antiphase manner. Where there is a high amount of titanium, there is little iron and vice versa. Iron-rich areas are located between large pores being comparable in size with areas enriched with titanium. On the other hand, oxygen is distributed over the surface more evenly than either iron or titanium, so that the areas enriched with titanium and iron oxides appear to alternate. Element compositions of the surface part of the coatings and that of the pores are listed in the Table 1. Surface compositions were
Fig. 1. Surface morphology of the coatings on aluminum alloy (a, b) and titanium (c, d). Amplitude (a, c) and phase representation (b, d).
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 2. The distribution of elements in the surface layer of the coating on aluminum.
measurements in different pores. The elemental composition of the outer part of the coatings surface (analysis depth 2e5 mm) and that of the inner part of the pores are clearly different (Table 1). In pores, the iron concentration is about 5e10 times higher and the oxygen concentration is 3e4 times lower than their average concentrations over the surface. The deficit of oxygen in relation to metals may indicate that in the pores reduced metals, as well as particles of a complex structure, for example, iron surrounded by an oxide layer, or other heterophase systems, could be present [20]. We also note that tungsten (a metal from the electrolyte) tends to concentrate in the pores of the coatings on aluminum. The latter behavior is however not typical for coatings on titanium. Possible reasons for concentrating and reducing iron in the pores of coatings formed in electrolyte sols were previously discussed in Refs. [18,19,25]. According to XRD, coatings on titanium contain TiO2 as both rutile and anatase, and those on aluminum contain g-Al2O3 (Fig. 5). In summary, the distribution of iron over the surface of Al and Ti samples is different, although average iron contents on the surface in both cases are similar and equal to 4.2 and 6.6 at. %, respectively. In both samples, concentration of iron is increased and that of oxygen decreased in the pores, compared with the average concentration on the surface. Furthermore, reduced iron may be present in the pores. This is confirmed by XPS data: upon etching of similar coatings on titanium with argon to a depth of 3 nm, a signal of the reduced iron was recorded [18]. XRD data obtained with signal accumulation showed the presence of metallic iron in the composition of PEO coatings on aluminum [19]. According to SEM studies of the cross section, the internal structure of a coating on aluminum is heterogeneous; there are cracks and pores, including those with dispersed particles. Iron concentration in the outer part of the coating is higher than in the depth. At the coating outer boundary, the Fe-rich areas follow the outlines of the pore boundaries or concentrate near the pores [25].
3.2. Room temperature magnetic properties
Fig. 3. The general view of the surface (a) and the distribution of elements in the surface layer of the coating on titanium (bed).
estimated by averaging of the results of scanning five 250-mm2 areas. Element composition of the pores was determined using an electron scanning microscope attachment for energy dispersive analysis focusing the probe beam on ~200 200 nm areas in the pore bottoms or on crystallites contained in the pores, as shown in Fig. 4. Average results are given for at least five separate
We investigated two PEO coated titanium samples (hereinafter referred to as Ti1 and Ti2) and three PEO coated aluminum samples (hereinafter referred to as Al1, Al2 and Al3). All samples manifest ferromagnetic properties. The hysteresis loops of the samples measured at room temperature are shown in Figs. 6 and 7. For Al and Ti samples, the values of hysteresis parameters demonstrate no significant difference and only slightly depend on the substrate material. For example, for samples Ti1 and Ti2 (Fig. 6b, d), coercive force Hc is 97 and 35 Oe, and coercivity of remanence Hcr is 301 and 227 Oe, respectively. For samples Al1, Al2 and Al3 (Fig. 7b, d, f), Hc is 54, 89, 123 Oe, and Hcr is 394, 598, 710 Oe, respectively. Fig. 8 shows the observations made by magnetic force microscopy. The images on the left are the surface reliefs, those on the right present the distribution of the ferromagnetic areas over the surface. The distribution of ferromagnetic areas for PEO coatings on titanium (Fig. 8a and b) and aluminum (Fig. 8cee) appears quite
Table 1 The average element compositions of the coating surface layers and pores. Sample
Ti Al
Element composition (at.%)
Surface Pore Surface Pore
C
O
Na
P
Fe
W
Mg
Me
10.1 4.6 12.4 4.8
67.0 18.1 56.0 14.9
1.3 e 0.3 e
6.1 5.6 1.4 0.6
6.6 40.8 4.2 51.6
1.5 1.7 0.3 15.9
e e 1.2 0.2
7.4 29.2 24.2 12.0
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Fig. 4. Pores on the surface of coatings on Al (a) and Ti (b). The images show the areas for which the elemental composition was determined.
Fig. 5. XRD patterns of samples on aluminum (a) and titanium (b).
different. For the coated aluminum samples, ferromagnetic areas are associated with concentrating iron in the pores. In addition, for the pores in Fig. 8c and d ferromagnetic areas have circular shape, which may be due to the contribution of the compounds lining the pore walls. Weak ferromagnetic response in the remaining part of the coating is apparently related to concentrating iron in cracks, small pores, and depressions of the coating relief. Thus, iron present in the bulk of the coating on Al samples does not exhibit ferromagnetic properties. Based on the conditions of PEO treatment and electrolyte formula, it can be incorporated in the composition of non-ferromagnetic oxides, phosphates, borates [26]. For the coated titanium samples, the distribution of ferromagnetic areas is more complex (Fig. 8a and b). Areas with maximum ferromagnetic response (light areas in Fig. 8b) are also associated with the relief depressions. Since such areas are generally of circular shape, one can suggest that they are associated with
numerous small pores in the depressions. However, as follows from magnetic force microscopy data, the bulk of the coating on titanium shows ferromagnetic properties. Apparently, under electrical discharges during PEO, i.e. in the conditions of elevated temperatures and significant pressure drops, ferromagnetic compounds are formed in the bulk of oxide coating on titanium. Given the electrolyte composition and formation conditions, the bulk of the coating may contain oxides, titanates, phosphates, and/or borates of iron [4,27]. In summary, the distribution of ferromagnetic areas over the surface of coated Al and Ti samples is different. Whereas for Al samples the ferromagnetic areas are associated with pores in the coating, for Ti samples ferromagnetic compounds or particles occur not only in the pores, but also in the bulk of the coating. In both cases, though, the distribution of ferromagnetic areas correlates well with iron distribution over the surface.
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 6. Curves of temperature dependence of the magnetic moment for coated titanium samples (a, c) and central part of hysteresis loops of these samples at 295 K (b, d). The arrows indicate the direction of temperature change.
3.3. Temperature dependences of magnetic parameters 3.3.1. Measurements at cryogenic temperatures For all three Al samples, SIRM acquired at 2 K is not particularly stable. From 50 to 75% of the SIRM is demagnetized below 50 K, and then the SIRM decrease rate is significantly slowed down (Fig. 7). FC and ZFC curves converge at ~70 K, indicating that iron hydroxides, for example, goethite a-FeOOH, even if present, occur in a very small amount and in the form of very small particles, which would be superparamagnetic at room temperature [28]. Alternatively, it can be other hydroxides, for example, lepidocrocite g-FeOOH with a el temperature of 50e60 K [29]. The irreversibility of SIRM cycles Ne (300-2-300 K) indicates, by analogy with titanomagnetites [30,31], the possible presence of a spinel phase (Al-substituted magnetite). If this is so, the absence of the Verwey transition indicates that the Al content in Fe3-xAlxO4 spinel is quite noticeable (x>0.06 [32]). For PEO-coated titanium samples, SIRM acquired at 2 K is even less stable than for aluminum samples (Fig. 6). On reaching 300 K, the non-demagnetized residue is no more than 10% of the original value. An interesting behavior of the ZFC and FC curves is observed at temperatures below 50 K. After the initial (<5 K) sharp decline, the non-demagnetized part of thermoremanent magnetization acquired by FC is noticeably larger than that of the isothermal remanent magnetization acquired at 2 K after ZFC, and the corresponding curves then converge only at T ~ 50 K. A broadly similar behavior was observed for Fe2TiO5 pseudobrookite [33], which may
indicate its presence in the samples. The Verwey transition in both samples is not observed thus excluding the presence of Fe3O4 magnetite phase. At the same time, the shape of SIRM(300-2-300 K) curves may indicate that some amount of titanomagnetite or titanomaghemite with a rather high titanium content (x ~ 0.2e0.3 in Fe3-xTixO4 [30,31]) is present. 3.3.2. High-temperature measurements We investigated two samples: PEO-coated titanium (Ti2) and PEO-coated aluminum (Al3) (Fig. 9). In both cases, at room temperature the component of isothermal remanent magnetization (IRM) acquired between 0.1 and 0.3 T is the largest. The component acquired in a field of 0.1 T, although it is ~70% of IRM (0.3 T), is significantly less thermally stable: its significant part is demagnetized after heating to 200e300 C. The presence of a noticeable IRM(1 T) component which is ~30e40% of IRM(0.3 T) is also noteworthy. Its temperature stability is the same as that of IRM(0.3 T). Such a ratio between the components of magnetization acquired in different fields is not typical of magnetite, including that with up to moderate cation substitution, e.g. titanomagnetite, for which one would expect IRM(0.1 T) > IRM(0.3 T) > (>>) IRM(1 T) [34]. Accordingly, it can be hypothesized that the main magnetic phase, which responsible for the magnetization at room temperature, is metallic iron. In general, the data obtained for Al3 sample do not contradict this, but it could not be heated above 650 C in order to avoid destruction due to melting the aluminum alloy. At the
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 7. Curves of temperature dependence of magnetic moment for coated aluminum samples (a, c, e) and central part of hysteresis loops of these samples at 295 K (b, d, f). The arrows indicate the direction of temperature change.
same time, in the Ti2 sample, the main part of the magnetization is erased already at 450 C, which is much lower than the Curie temperature of iron. It can be suggested (while hypothetically) that during the formation of the coating an iron-titanium alloy was formed making a noticeable contribution to the magnetization along with titanomagnetite or titanomaghemite.
4. Conclusions Summarizing the data obtained from a comprehensive study of magnetic behavior, morphology, phase and element composition of Fe-containing oxide coatings formed by PEO on aluminum and titanium in slurry electrolytes with iron(III) hydroxides, it can be concluded that their magnetic properties are strongly dependent
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Fig. 8. Surface relief (a, c, e) and distribution of magnetic fields on the surface of the coatings (b, d, f) on titanium (a, b) and on aluminum (c, d, e, f).
Fig. 9. Temperature behavior of the residual magnetization of the samples in the Lowrie test. PEO-coated aluminum (a) and titanium (b) samples were magnetized in a threecomponent field with a strength of 0.1 (1), 0.3 (2) and 1 T (3).
on the nature of the metal substrate. The main feature, characteristic of all studied samples, is that in the pores, iron concentration is about 5e10 times higher and the oxygen concentration is 3e4 times lower than their average concentrations in the bulk of the coating layer. The deficit of oxygen in relation to metals may indicate the presence in the pores of both oxidized and reduced metals, as well as of particles with complex structure, for example, iron surrounded by an oxide layer or vice versa. The highly uneven distribution of iron over the surface is typical for the coatings on aluminum. Iron-containing particles and compounds are concentrated in cracks, pores, depressions of the surface relief. The areas with ferromagnetic properties are distributed over the surface in a similar way. Based on the room temperature
magnetic properties and its variation at high temperatures, it may be proposed that the magnetic phase primarily responsible for the magnetization at and above room temperature is metallic iron. The presence of a spinel phase (Al-substituted magnetite) is also possible as indicated by magnetic measurements at cryogenic temperatures. These measurements further imply that a small amount of iron hydroxides (goethite or lepidocrocite) in the form of ultrafine particles, which are superparamagnetic at room temperature, is also present. For PEO-coated titanium samples, areas with maximum magnetic response are also associated with pores and iron concentrating therein. However, the bulk of the coating also has pronounced ferromagnetic properties. Based on the analysis of the temperature behavior of the magnetization curves, the
Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579
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Please cite this article as: V.S. Rudnev et al., Magnetism of Fe-doped Al2O3 and TiO2 layers formed on aluminum and titanium by plasmaelectrolytic oxidation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152579