Incorporation of superconducting MgB2 into plasma electrolytic oxidation coatings on aluminium

Journal Pre-proof Incorporation of superconducting MgB2 into plasma electrolytic oxidation coatings on aluminium S. Aliasghari, P. Skeldon, X. Zhou, G.B.G. Stenning, R. Valizadeh, T. Junginger, G. Burt PII:

S0257-8972(19)31082-5

DOI:

https://doi.org/10.1016/j.surfcoat.2019.125091

Reference:

SCT 125091

To appear in:

Surface & Coatings Technology

Received Date: 8 September 2019 Revised Date:

10 October 2019

Accepted Date: 22 October 2019

Please cite this article as: S. Aliasghari, P. Skeldon, X. Zhou, G.B.G. Stenning, R. Valizadeh, T. Junginger, G. Burt, Incorporation of superconducting MgB2 into plasma electrolytic oxidation coatings on aluminium, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125091. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Incorporation of superconducting MgB2 into plasma electrolytic oxidation coatings on aluminium S. Aliasgharia,b, P. Skeldonb, X. Zhoub, G.B.G. Stenningc R. Valizadeha, T. Jungingerd,e, , G. Burtf,g a b

ASTeC, STFC Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK. Corrosion and Protection Group, School of Materials, The University of Manchester,

Manchester M13 9PL, UK. c

ISIS, STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK

d

Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2,

Canada. e

TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3, Canada.

f

Engineering Building, Lancaster University, Lancaster, LA1 4YW, UK.

g

Cockcroft Institute, Keckwick Ln, Daresbury, Warrington, WA4 4AD, UK.

Abstract

MgB2 particles have been incorporated into a coating formed by AC plasma electrolytic oxidation (PEO) on aluminium under a constant current density. The incorporation of MgB2 into the coating was characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The additions of particles to the electrolyte led to a voltage decrease, possibly a result of the initiation of a “soft” sparking regime. The transition occurred at earlier times with increase of the MgB2 concentration and enhanced MgB2 incorporation into the coating. The coating containing particles was thicker than one in which MgB2 was absent, and had a nodular surface, with MgB2 accumulations in the valleys between the nodules. A superconducting quantum interference device was employed to determine the superconductivity behaviour below about 39 K, the critical temperature of MgB2, and revealed hysteresis in magnetic moment-field curves that typifies a type II superconductor.

Keywords: plasma electrolytic oxidation, coating, magnesium, boride, superconductivity

1

1. Introduction

Plasma electrolytic oxidation (PEO) is a process that creates ceramic coatings on metal surfaces. It is usually applied to metals that can be conventionally anodized, especially aluminium, magnesium and titanium, to improve surface properties, particularly corrosion resistance, wear resistance, or biocompatibility. A review of research on key aspects of the PEO process has recently been provided by Clyne and Troughton [1]. The coatings are usually formed at high voltages in aqueous electrolytes at room temperature, under DC, AC or pulsed conditions. Microdischarges on the coated substrates lead to deposition of the coating material with a composition dependent on the substrate and electrolyte and the applied electrical conditions. Owing to the locally high temperatures at the microdischarges, that may cause melting of the coating, crystalline phases not normally encountered in conventional anodizing may be formed.

The coating growth is accompanied by substantial evolution of oxygen, hydrogen and water vapour [1, 2]. Their escape from within the softened or molten coating leads to extensive porosity, ranging in size from the nanoscale to tens of microns [3, 4]. The porosity may benefit properties such as biocompatibility and thermal barrier protection but can be detrimental to corrosion protection and wear resistance. One approach to improve the properties is to incorporate nanoparticles, nanosheets or nanotubes into the coating. For example, Y2O3 [5], TiN [6], ZrO2 [7, 8], SiO2 [7, 8], ZnO [9], graphene [10], hallyosite [11], graphite [12] and hydroxyapatite [13] have been used to reduce porosity, and increase hardness, wear resistance, corrosion protection or bioactivity. Coatings exhibiting superconductivity have also been produced on niobium by incorporating MgB2 particles from a silicate-based electrolyte [14]. The effects of nanoparticles on PEO processes have been reviewed in [15].

The present work investigates the potential for producing a superconducting coating containing MgB2 on aluminium. A silicate-based electrolyte was employed, similar to that used in an earlier work to incorporate ZrO2 nanoparticles [16]. The coatings differ significantly in morphology and composition to ones generated on the niobium substrate, which were silica-rich and highly porous [14]. The superconducting properties of MgB2 have been reviewed by Buzea and Yamashita [17]. MgB2 exhibits Type II superconductivity below the critical temperature of about 39 K. In contrast, aluminium is a type I superconductor 2

below about 1.2 K. A variety of surface coating technologies have been used previously to deposit MgB2 coatings, e.g. pulsed laser deposition (PLD) [18], low pressure chemical vapour deposition (LPCVD) [19], molecular beam epitaxy (MBE) [20], hybrid physical chemical vapour deposition (HPCVD) [21], reactive evaporation [22], electroless [23], electrochemical synthesis [24], electrophoretic [25], sol–gel [26], and ion beam [27]. Furthermore, MgB2 films have revealed high critical current densities [28]. MgB2 has also been used in powder form in powder-in-tube designs [29-31]. PEO offers an alternative comparatively simple coating process, not involving toxic chemical, vacuum systems or high temperatures, for production of MgB2-contining coatings. The present study contributes to the existing very limited knowledge base on the potential of PEO for production of superconducting coatings, showing that superconducting coatings can be produced on aluminium substrates by incorporating MgB2 particles. MgB2 coatings are of interest to the development of superconducting radio frequency cavities for accelerator applications [32], which are usually manufactured from niobium. Aluminium would provide a lower cost and more easily fabricated alternative [32].

2. Experimental

2.1. Material and PEO conditions

Specimens were cut from 0.35 mm thick 99.97% aluminium foil, degreased with ethanol, rinsed with deionized water, dried in air and coated in lacquer (Stopper 45 MacDermid) leaving a working area of ∼1.2 x 1.2 cm2. PEO was carried out for 2400 s at an rms current density of 550 mA cm-2 with a square waveform and a positive to negative current ratio (ip/in) of 1.3 using an ACS-FB power supply (ET System Electronic GmbH). The frequency was 50 Hz, with a duty cycle of 50%. The electrolyte comprised reagent grade sodium silicate (10.5 g l−1 specific gravity 1.5) and sodium hydroxide (2.8 g l−1) in deionized water. As required, additions were made of 5, 8 or 10 g l-1 of MgB2 powder (99% purity-Alfa Aesar, Al 166, Cr 35, Fe 348, Mo 39, Ni 25, W 57,Ca 11, Cu 10, Mn 13, Na 18, Ti 18, Zn 4 ppm), which was first dispersed ultrasonically in 100 cm3 of the electrolyte for 30 min. The electrolyte was then opaque, which prevented observation of microdischarges. A secondary electron micrograph of the powder (Fig. 1) shows particle sizes mainly in the range 0.1 to 2 µm. The larger particles consisted of several 3

crystallites resulting in irregular, angular shapes. A double-walled glass cell contained 1 dm3 of electrolyte, which was stirred using a magnetic stirrer. The electrolyte was kept at 25 °C by a flow of cold water through the cell wall. A 7.5 × 15 cm type 304 stainless steel plate was used as a counter electrode. Voltage–time responses were recorded employing LabView software with a sampling time of 20 ms.

2.2. Specimen examination

PEO-treated specimens were examined by scanning electron microscopy (SEM) using a Zeiss Ultra 55 instrument with energy dispersive X-ray spectroscopy (EDS). Cross-sections were prepared on successive grades of SiC paper and finished with 1 µm diamond paste. Phase composition was investigated by X-ray diffraction (XRD), using a Philips X’Pert-MPD (PW 3040) instrument with Cu Kα radiation, scanning from 5 to 85° in steps of 0.005°.

2.3 SQUID measurements

All deposited aluminium films were analyzed using a superconducting quantum interference device (SQUID), quantum design MPMS XL-7 to measure the magnetic momenttemperature relationship from 3 to 60 K. Measurements were made during cooling from 60 K under an applied magnetic field of 100 Oe. In addition, the magnetic hysteresis loops were measured in a dc magnetic field at 4.2 K. The sample size was 1 x 1 mm. All samples were oriented as close to parallel to the plane of the magnetic field as was possible to achieve. The error in the sample alignment with the magnetic field was 1 ° (17 mrad). The sensitivity of the magnetic property measurement systems is 10-7 emu; however, in the reported experiments the noise level of the magnetic moment measurements was observed to be 10-4 emu at zero fields.

3. Results and discussion

3.1 Voltage-time response and coating development

The voltage-time curves (Fig. 2) of specimens following PEO for 2400 s in electrolytes containing 0, 5, 8 and 10 g l-1 MgB2 consisted of a rapid, approximately linear, increase in voltage to about 270 V, without sparking, when a barrier film is formed that increases in 4

thickness in proportion to the voltage, followed by a slow voltage rise to about 300 V that persisted to 1600 s accompanied by sparking, gas evolution and acoustic noise. The voltage up to 1600 s was not significantly affected by the presence of particles in the electrolyte. Thereafter, the behaviour depended on the concentration on MgB2 particles in the electrolyte. A steep drop in voltage to about 190 V was observed for additions of 8 and 10 g l-1 MgB2 after 2000 and 1600 s, respectively. The voltage then decreased more slowly to 160 and 180 V at the termination of the PEO treatment.

The transition, including the magnitudes of the voltages before and after the voltage drop, was very similar to that observed in a previous study of PEO of aluminium in a similar silicate electrolyte that contained ZrO2 nanoparticles [16]. The transition in that case coincided with the initiation of so-called “soft” sparking regime, characterized by a change in the intensity, size and colour of the microdischarges and a decrease in the acoustic emission [33]. Previous study showed that before “soft” sparking commenced strong optical emissions occurred from bound–bound transitions in atomic sodium, potassium and aluminium, as well as weaker emissions from hydrogen, ionized oxygen and OH radicals [34]. Under “soft” sparking, optical emission from sodium and a continuous emission due to bound-free electron transitions (collision–radiative recombination) and free–free transitions (bremsstrahlung radiation) dominated, while optical emission from aluminium was negligible [34]. The voltage during PEO has been shown to affect the electron temperature during discharges [35], and the transition to “soft” sparking results in generation of a relatively compact aluminabased coating of uniform thickness compared with the more porous coating formed prior to the voltage drop [33].

The measured values of MgB2 resistivity at room temperature can differ by up to several orders, dependent upon the MgB2 source [36]. (The resistivity of the present MgB2 particles is unknown). The lowest in the range of measured values (≈ 10 µΩ cm), determined for instance for some single crystals, is indicative of a good electrical conductor [36]. Hence, incorporated MgB2 might be anticipated to reduce the coating resistance and consequently the voltage during constant current PEO. In other work, silver nanoparticle incorporation led to a decrease in the voltage during DC PEO of magnesium [37], and carbon nanotube incorporation was associated with a higher current density at longer treatment times during pulsed voltage PEO of an aluminium alloy [38]. However, the voltage reduction in the former case occurred from the start of sparking, and the relatively higher current in the latter case 5

was due to an increased resistance during PEO in the nanotube-free electrolyte. Thus, these behaviours were different from the trends in the present voltage-time curves, suggesting that changes in coating resistance were of minor importance compared with the effects of the transition to “soft” sparking.

Optical images of the coated specimens (Fig. 3) show that a white coating was formed in the absence of MgB2. In contrast, in the presence of 10 g l-1 MgB2, the coating was dark grey, similar to the colour of the MgB2 powder. An addition of 5 g l-1 MgB2 led to white coating with a dark grey region around the periphery and in spots in the central area. The results indicate that obtaining a uniform grey appearance across all of the working area depends upon the presence and duration of “soft” sparking. Previous work has shown that “soft” sparking initiates around the edges of the working area and then spread to other parts of the coating surface [16]. For the present specimens, the spreading occurs by islands of “soft” sparking developing within the working area, resulting in the grey spots where relatively large amounts of MgB2 have been incorporated. These spots develop in number and size as the process proceeds. In view of the greater uniformity of particle coverage when using 10 g l-1 MgB2, the subsequent analysis and superconductivity measurements were focused on coatings formed in this electrolyte

3.2 Composition and morphology of coatings

A low magnification SEM image at a region of “soft” sparking of the surface of a specimen treated in the absence of MgB2 (Fig. 4 (a)) shows a porous coating, similar to that reported previously using a similar electrolyte and PEO condition [16]. At higher magnification (Fig. 4 (b)), pores are revealed with diameters from about 0.1 to 10 µm, although pores of larger size, to about 50 µm, can also be seen in Fig. 4 (a). The presence of 10 g l-1 MgB2 in the electrolyte led to a modified surface morphology, which comprised mainly nodules of about 20 to 50 µm diameter (Fig. 4 (c)). The nodules were partially covered by particles, but the valleys between the nodules were often filled with MgB2 particles (Fig. 4 d)). Nodular features containing relatively few particles compared to adjacent regions of the surface were also observed in coatings formed using a similar electrolyte but with the addition of 10 g l-1 ZrO2 nanoparticles [16]. The nodules revealed smaller pores, mainly of the order of a micron diameter or less, than those present on the coating surface produced without MgB2 (Fig. 4 6

(e)). An EDS map of magnesium and aluminium shows the magnesium-rich regions between the nodules (Fig. 4 (f)).

The appearance of the nodules suggests that they might have developed due to the pressure of gas generated and trapped within the coating. The addition of nanoparticles to the electrolyte has been reported to reduce porosity and cracking of PEO coatings [7, 8]. A similar influence of MgB2 would provide fewer paths for escape of gas resulting in higher pressures than when MgB2 particles were absent. The pressure may be relieved by expansion of the gas volume in the softened or molten coating material. Cross-sections of specimens treated in the absence and presence of 10 g l-1 MgB2 showed coatings of respective thickness 48 and 71 µm, representing an increase in thickness by 48% with MgB2 (Figs. 5 (a,b)). Previous work revealed a thickness increase of 37% with addition of 10 g l-1 ZrO2 nanoparticles to a similar silicate electrolyte [16]. The general appearance of the coatings is similar to PEO coatings on aluminium and magnesium formed under a “soft” sparking regime [16, 33]. Such coatings in cross-section show a comparatively uniform appearance free-of large pores, contrasting with coatings before “soft” sparking has initiated, which contain large internal pores [33]. The outer 30 to 40% of the coating thickness appeared to more porous than the inner region, with higher porosity in the coating containing MgB2. Although not observable at the magnification in Fig. 5, a barrier layer of several hundred nanometres thickness that support a large fraction of the voltage is present at the base of the coatings [16].

Particles could be observed within pores in the outer region of the coating formed in the presence of MgB2. EDS mapping disclosed magnesium-rich spots within coating material that contained aluminium, oxygen and silicon species (Fig. 6); sodium (map not shown) was also detected, distributed similarly to the silicon. EDS analyses were made in rectangular areas of length 30 µm and height 3 µm at the six locations marked in the cross-section in Fig. 5 to determine the through-thickness distribution of the coating constituents. Boron could not be analysed owing to the low energy of the characteristic X-rays. The results are shown in Fig 7. The porous coating region contained relatively large amounts of silicon (≈ 8 at.%), sodium (≈ 10 at.%), aluminium ( ≈ 19 at.%) and oxygen (≈ 58 at.%). The amounts of silicon and sodium diminished significantly with depth and were very low (< 1 at.%) in the more compact inner region. In contrast, aluminium increased with depth and the inner region was 7

composed essentially of alumina. Magnesium was mainly confined to the porous region of the coating, with a concentration of ≈ 5 at. %. XRD patterns showed peaks for α-, γ-, and θAl2O3 for the coatings formed both in the absence and presence of MgB2 (Fig. 8). The latter coating also contained MgB2. There was also a broad, shallow rise in intensity between 25 and 45° consistent with an amorphous phase. A small peak at 43° coating is possibly due to MgO [39, 40], formed by thermal oxidation of the MgB2, which can become significant above 973 K [41]. No peaks were detected from silicon- and sodium-containing compounds, indicating their presence elements in amorphous phases, possibly as units of SiO2 and Na2O. The incorporation of MgB2 particles into the coating may first occur by their migration to the coating surface under the applied electric field. Measurements made on MgB2 nanosheets revealed a negative zeta potential in the pH range 3 to 11 [42], suggesting that the MgB2 particles may be drawn to the surface during the anodic stage of the current cycle, and adhere to the coating due to van de Waals or electrostatic forces, mechanical interlocking or chemical bonding. They may then be progressively buried by subsequently deposited siliconand sodium-enriched material that forms at the coating surface. Local boiling of the electrolyte may also deposit particles on the coating surface and within the porosity.

The introduction of MgB2 into the coating primarily during “soft” sparking indicates the differing influences of the discharge properties prior to and following the sparking transition. The reason for enhanced incorporation in the “soft” sparking stage is unclear from the present results. A range of factors may be operative, including the discharge location within the coating thickness, the magnitude of the local current density, the discharge size and duration, the temperature and pressure in the discharge channel, the gas evolution, and the mechanism of coating growth. The present work also indicated that the transition to “soft” sparking occurred sooner with as the concentration of MgB2 particles in the electrolyte was increased. Other work has shown that the onset of “soft” sparking can be promoted by preforming a conventional nanoporous anodic film, with the time of the transition decreasing with increasing thickness of the pre-formed oxide [43]. This suggests a role of the oxide thickness or the pore size, which may possibly be affected in the present coatings by the MgB2 particles.

3.3 Superconducting behaviour

8

The magnetic moment vs temperature curve measured in the field cooled condition and a magnetic field of 100 Oe is shown in Fig. 9 for a coating formed in the electrolyte containing 10 g l-1 MgB2. Other coatings produced similar trends. Above 39 K, the magnetic moment was positive, with a value of 2 x 10-5 emu, due to the paramagnetism of aluminium and ferromagnetism of MgB2 in the normal conducting state [14, 44]. At about 39 K, the magnetic moment decreased steeply to negative values owing to an increasing diamagnetic contribution to the magnetic moment following the transition to the superconducting state of MgB2 [45]. The rate of decrease slowed as the temperature was reduced from 39 to 3 K. The diamagnetic contribution to the magnetic moment was 4.4 x 10-5 emu at the lowest temperature. It was previously proposed that the decreasing slope is due to MgB2 clusters transitioning to the normal state at different temperatures owing to local differences in the critical field at individual MgB2 clusters [14]. The field of first penetration of individual clusters in the absence of an energy barrier is set by the lower critical field, Hc1, which increases linearly as the temperature decreases.

The cyclic magnetic moment-magnetic field curves measured between -5000 and 5000 Oe at 4.2 K are shown in Fig. 10 for coatings formed using 5, 8 and 10 g l-1 MgB2. The curves were symmetrical about the field axis, exhibiting hysteresis due to flux pinning. The widths of the hysteresis loops increased as the concentration of MgB2 increased, consistent with the increasing MgB2 content in the respective coatings. The initial slope at 10 g l-1 was linear up to a field of 475 Oe and a magnetic moment of -3.2 x 10-4 emu. Thereafter, the slope was less steep due to flux penetration. A lower critical field of 475 Oe (47 mT) indicated by the extent of the linear region is consistent with experimental values determined in other studies [17]. Assuming a susceptibility of ≈ 1 in the linear region, the volume of MgB2 of 6.6 x 10-12 m3, which for the 1 x 1 mm samples used for the measurements, indicates an effective MgB2 thickness of 6.6 µm. The maximum negative magnetic moment (-4.2 x 10-4 emu) was reached at 1300 Oe. At higher fields, the magnetic moment become less negative as the applied field exceeded the local values of the upper critical field of the individual MgB2 clusters. On reversing the direction field scan, there was a small negative magnetic moment of ≈ -0.7 x 104

emu, with the magnetic moment falling to zero at a field of 590 Oe. The width of the

hysteresis loop at zero field was 3.2 x 10-4 emu. The width reduced to 8.2 x 10-5 and 2.5 x 10-5 for additions of 5 and 8 g l-1 MgB2, respectively, owing to the lower concentrations and less uniform distributions of MgB2 in the coatings. The critical current density at zero field will 9

depend upon the distribution, connectivity, size shape and orientation of the MgB2 particles that requires investigation be other measurements, such as DC resistivity.

4. Conclusions

PEO coatings containing α-, γ-, and θ-Al2O3 and MgB2 were formed on aluminium in a silicate electrolyte containing MgB2 particles. Particle incorporation was promoted by “soft” sparking, which occurred earlier with increasing concentration of MgB2 in the electrolyte. The particles were contained mainly in the more porous outer 30% of the coating thickness. The coatings had a nodular surface with a particle network concentrated in the valleys between nodules, which contrasted with the more porous surface formed in the absence of MgB2. The coatings displayed superconductivity below the critical temperature of MgB2 and hysteresis characteristic due to flux pinning of a type II superconductor. The demonstration of superconductivity in PEO coatings on aluminium is of potential interest for production of SRF cavities for accelerator applications.

Acknowledgements

The authors acknowledge funding from the European Union’s Horizon 2020 Research and Innovation programme under the Marie Skłodowska-Curie grant agreement No. 665593 awarded to UKRI Science and Technology Facilities Council (STFC). They also are grateful to the Material Characterisation Laboratory at ISIS, STFC Rutherford Appleton Laboratory for superconductivity measurements.

References

[1] T.W. Clyne, S.C. Troughton, A review of recent work on discharge characteristics during plasma

electrolytic

oxidation

of

various

metals,

Internat.

Mater.

Rev.,

DOI:

10.1080/09506608.2018.1466492 (2018). [2] L.O. Snizhko, A.L. Yerokhin, N.L. Gurevina, V.A. Patalakha, A. Matthews, Excessive oxygen evolution during plasma electrolytic oxidation of aluminium, Thin Solid Films 56 (2007) 460-464.

10

[3] J.A. Curran, T.W. Clyne, Porosity in plasma electrolytic oxidation coatings, Acta Mater. 54 (2006) 1985-1993. [4] X. Zhang, S. Aliasghari, A. Nemcova, T.L. Burnett, I. Kubena, M. Smid, G.E. Thompson, P. Skeldon, P.J. Withers, X-ray computed tomographic investigation of the porosity and morphology of plasma electrolytic oxidation coatings, ACS Appl. Mater. Inter. 8 (2016) 8801-8810. [5] Baojun Han, Yang Yang, Hao Deng, Yaowu Chen, Chubin Yang, Plasma-electrolyticoxidation coating containing Y2O3 nanoparticles on AZ91 magnesium alloy, Int. J. Electrochem. Sci, 13 (2018) 5681-5697. [6] D.V. Mashtalyar, S.V. Gnedenkov, S.L. Sinebryukhov, I.M. Imshinetskiy, A.V. Puz, Plasma electrolytic oxidation of the magnesium alloy MA8 in electrolytes containing TiN nanoparticles, J. Mater. Sci. Technol. 33 (2017) 461-468. [7] S.V. Gnedenkov, S.L. Sinebryukhov, D.V. Mashtalyar, I.M. Imshinetskiy, A.V. Samokhin, Yu. V. Tsvetkov, Fabrication of coatings on the surface of magnesium alloy by plasma electrolytic oxidation using ZrO2 and SiO2 nanoparticles, J. Nanomater. 2015 (2015) Article ID 154298, 12 pages. [8] S. Fatimah, M.P. Kamil, J.H. Kwon, M. Kaseem, Y.G. Ko, Dual incorporation of SiO2 and ZrO2 nanoparticles into the oxide layer on 6061 Al alloy via plasma electrolytic oxidation: Coating structure and corrosion properties, J. Alloy Compd. 707 (2017) 358-364. [9] Aidin Bordbar-Khiabani, Benyamin Yarmand, Masoud Mozafari, Enhanced corrosion resistance and in-vitro biodegradation of plasma electrolytic oxidation coatings, Surf. Coat. Technol. 360 (2019) 153-171. [10] Wanying Liu, Carsten Blawert, Mikhail L. Zheludkevich, Yuanhua Lin, Mohd Talha, Yunsheng Shi, Long Chen, Effects of graphene nanosheets on the ceramic coatings formed on Ti6Al4V alloy drill pipe by plasma electrolytic oxidation, J. Alloy Compd. 789 (2019) 996-1007. [11] Beatriz Mingo, Yue Guo, Aneta Němcová, Ali Gholinia, Marta Mohedano, Ming Sun, Allan Matthews, AlekseyYerokhin, Incorporation of halloysite nanotubes into forsterite surface layer during plasma electrolytic oxidation of AM50 Mg alloy, Electrochim. Acta 299 (2019) 772-788. [12] S. Arun, S. Hariprasad, A. Saikiran, B. Ravisankar, E.V. Parfenov, V.R. Mukaeva, N. Rameshbabu, The effect of graphite particle size on the corrosion and wear behaviour of the PEO-EPD coating fabricated on commercially pure zirconium, Surf. Coat. Technol. 363 (2019) 301-313. 11

[13] S. Lederer, S. Sankaran, T. Smith, W. Fürbeth, Formation of bioactive hydroxyapatitecontaining titania coatings on CP-Ti 4+ alloy generated by plasma electrolytic oxidation, Surf. Coat. Technol. 363 (2019) 66-74. [14] S. Aliasghari, P. Skeldon, X. Zhou, R. Valizadeh, T. Junginger, G.B.G. Stenning, G. Burt, Communication - Formation of a superconducting MgB2-containing coating on niobium by plasma electrolytic oxidation, ECS J. Solid State Sci. Technol. 8 (2019) N39N41. [15] Xiaopeng Lu, Marta Mohedano, Carsten Blawert, Endzhe Matykina, Raul Arrabal, Karl Ulrich Kainer, Mikhail L. Zheludkevich, Plasma electrolytic oxidation coatings with particle additions- A Review, Surf. Coat. Technol. 307 (2016) 1165-1182. [16] E. Matykina, R. Arrabal, P. Skeldon, G.E. Thompson, Investigation of the growth processes of coatings formed by AC plasma electrolytic oxidation of aluminium, Electrochim. Acta 54 (2009) 6767-6778. [17] Cristina Buzea, Tsutomu Yamashita, Review of superconducting properties of MgB2, Supercond. Sci. Technol. 14 (2001) R115-R146. [18] D. Mijatovic, A. Brinkman, J.W.M. Hilgenkamp, H. Rogalla, A.J.H.M. Rijnders, D.H.A. Blank, Pulsed-laser deposition of MgB2 and B thin films, Appl. Phys. A: Mater. Sci. Proc. 79 (2004) 1243-1246. [19] S. Zhang, C.-Y. Deng, X. Wang, Y.-P. Wu, Y. Fu, and X.-H. Fu, Superconducting MgB2 film prepared by chemical vapor deposition at atmospheric pressure of N2, Thin Solid Films 584 (2015) 300-304. [20] K. Ueda, M. Naito, As-grown superconducting MgB2 thin films prepared by molecular beam epitaxy, Appl. Phys. Lett. 79 (2001) 2046-2048. [21] X.X. Xi, A.V. Pogrebnyakov, S.Y. Xu, K. Chen, Y. Cui, E.C. Maertz, C.G. Zhuang, Qi Li, D.R. Lamborn, Joan Marie Redwing, Zi-kui Liu, A. Soukiassian, D.G. Schlom, X.J. Weng, E.C. Dickey, Y.B. Chen, W. Tian, X.Q. Pan, S.A. Cybart, R.C. Dynes, MgB2 thin films by hybrid physical-chemical vapor deposition, Physica C: Superconductivity 456 (2007) 22-37. [22] B.H. Moeckly, W.S. Ruby, Growth of high-quality large-area MgB2 thin films by reactive evaporation, Supercond. Sci. Technol. 19 (2006) L21-L24. [23] K.S. Vijayaragavan, S.K. Putatunda, A. Dixit, G. Lawes, Electroless deposition of superconducting MgB2 films on various substrates, Thin Solid Films 51 (2010) 658-661.

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[24] A.B. Jadhav, S.H. Pawar, Electrochemical synthesis of superconducting magnesium diboride films: a novel potential technique, Supercond. Sci. Technol. 16 (2003) 752-759. [25] M. Ochsenkühn-Petropoulou, .L. Mendrinos, A. Altzoumailis, and R. Argyropoulou, Production and characterization of MgB2 coatings on various substrates by electrophoretic deposition, J. Mater. Processing Technol. 161 (2005) 16-21. [26] M. Nath, B.A. Parkinson, A simple sol-gel synthesis of superconducting MgB₂ nanowires, Adv. Mater. 18 (2006) 1865-1868. [27] N. Peng, G. Shao, C. Jeynes, R.P. Webb, R.M. Gwilliam, G. Boudreault, D.M. Astill, W.Y. Liang, Ion beam synthesis of superconducting MgB2 thin films, Appl. Phys. Lett. 82 (2003) 236-238. [28] C.B. Eom, M.K. Lee, J.H. Choi, L.J. Belenky, X. Song, L.D. Cooley, M.T. Naus, S. Patnaik, J. Jiang, M. Rikel, A. Polyanskii, A. Gurevich, X.Y. Cai, S.D. Bu, S.E. Babcock, E.E. Hellstrom, D.C. Larbalestier, N. Rogado, K.A. Regan, M.A. Hayward, T. He , J.S. Slusky, K. Inumaru, M.K. Haas, R.J. Cava, High critical current density and enhanced irreversibility in superconducting MgB2 thin films, Nature 411 (2001) 558-560. [29] B.A. Glowacki, M. Majoros, M. Vickers, J.E. Evetts, Y. Shi, I. McDougall, Superconductivity of powder-in-tube MgB2 wires, Supercond. Sci. Technol. 14 (2001) 193199. [30] Y. Akyüz, A. Biçer, M. Gürü, Synthesis and processing of Al sheathed MgB2 tapes by powder in tube method and determination of superconducting and mechanical properties, Mater. Des. 28 (2007) 2500-2504. [31] H. Kumakura, A. Matsumoto, H. Kitaguchi, H. Hatakeyama, Critical current densities of powder-in-tube (PIT)-processed MgB2 tapes, Mater. Trans. 45 (2004) 3056-3059. [32] Anne-Marie Valente-Feliciano, Superconducting RF materials other than bulk niobium: a review. Supercond. Sci. Technol. 29 (2016) 113002. [33] F. Jaspard-Mécuson, T. Czerwiec, G. Henrion, T. Belmonte, L. Dujardin, A. Viola, J. Beauvir. Tailored aluminium oxide layers by bipolar current adjustment in the plasma electrolytic oxidation (PEO) process, Surf. Coat. Technol. 201 (2007) 8677–8682. [34] E. Matykina, R. Arrabal, P. Skeldon, G.E. Thompson, P. Belenguer, AC PEO of aluminium with porous alumina precursor films, Surf. Coat. Technol. 205 (2010) 1668-1678. [35] R.O. Hussein, D.O. Northwood, X. Nie, Coating growth behavior during the plasma electrolytic oxidation process, J. Vac. Sci. Technol. A 28 (2010) 766-773. [36] John M. Rowell, The widely variable resistivity of MgB2 samples, Supercond. Sci. Technol. 16 (2003) R17–R27. 13

[37] B.S. Necula, L. E. Fratila-Apachitei, A. Berkani, I. Apachitei, J. Duszczyk, Enrichment of anodic MgO layers with Ag nanoparticles for biomedical applications, J. Mater. Sci.: Mater. Med. 20 (2009) 20:339–345 [38] Yakup Yürektürk, Faiz Muhaffel, Murat Baydoğan, Characterization of micro arc oxidized 6082 aluminum alloy in an electrolyte containing carbon nanotubes, Surf. Coat. Technol. 269 (2015) 83-90.

[39] Se Young O, Chan-Gyu Lee, Alexander J. Shapiro, William F. Egelhoff Jr., Mark D. Vaudin, Jennifer L. Ruglovsky, Jonathan Mallett, Philip W. T. Pong, X-ray diffraction study of the optimization of MgO growth conditions for magnetic tunnel junctions, J. Appl. Phys 103 (2008) 07A920. [40] Rajesh Kumar, Ashwani Sharma, Nawal Kishore, Preparation and characterization of MgO nanoparticles by co-precipitation method, Int. J. Eng, Appl. Man. Sci. Parad. 07 (2013) 66-70. [41] Delin Yang, Hongwei Sun, Hongxia Lu, Yiqun Guo, Xinjian Li and Xing Hu, Experimental study on the oxidation of MgB2 in air at high temperature, Supercond. Sci. Technol. 16 (2003) 576-581. [42] Saroj Kumar Das, Amita Bedar, Aadithya Kannan, Kabeer Jasuja, Aqueous dispersions of fewlayer-thick chemically modified magnesium diboride nanosheets by ultrasonication assisted exfoliation, Sci. Rep. 5 (2015) 10522. [43] E. Matykina, R. Arrabal, P. Skeldon, G.E. Thompson, P. Belenguer, AC PEO of aluminium with porous alumina precursor films, Surf. Coat. Technol. 205 (2010) 1668–1678. [44] S. Reich, G. Leitus, I. Felner, On the magnetism of the normal state in MgB2, J. Supercond. 15 (2002) 109-111. [45] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Superconductivity at 39 K in magnesium diboride, Nature 410 (2001) 63-64.

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Figure captions

Figure 1. SEM micrograph (secondary electrons) of the as-received MgB2 powder. Figure 2. Voltage-time curves for PEO of aluminium for 2400 s in silicate-electrolyte containing 0, 5, 8 and 10 g l-1 MgB2. Figure 3. Photographs of aluminium following PEO for 2400 s in silicate-electrolyte containing (a) 0, (b) 5, (c) 8 and (d) 10 g l-1 MgB2. Figure 4. SEM micrographs (secondary electrons) of the coating surface formed following PEO for 2400 s in silicate electrolyte containing (a, b) 0 and (c-e) 10 g l-1 MgB2. (d) EDS map of magnesium and aluminium on the coating surface formed using 10 g l-1 MgB2. Figure 5. SEM micrographs (secondary electrons) of cross-sections of coatings formed following PEO for 2400 s in silicate electrolyte containing (a) 0 and (b) 10 g l-1 MgB2 Figure 6. (a) SEM micrograph (secondary electrons) and EDS maps of (b) Al, (c) Si, (d) O and (e) Mg of the near surface of a coating formed for 2400 s in silicate electrolyte containing 10 g l-1 MgB2. Figure 7. Results of EDS analyses at the six locations labelled on the cross-section of the coating shown in Fig. 5 (b).

Figure 8. XRD patterns for aluminium before PEO and after PEO for 2400 s in silicate electrolyte containing 0 and 10 g l-1 MgB2. Figure 9. Magnetic moment-temperature relationship for aluminium following PEO for 2400 s in silicate-electrolyte containing 5, 8 and 10 g l-1 MgB2. Figure 10. Magnetic moment as a function applied field, measured at 4.2 K, for aluminium following PEO for 2400 s in silicate-electrolyte containing 5 (shown in inset), 8 and 10 g l-1 MgB2.

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Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

• • • • •

PEO coatings containing MgB2 were formed on aluminium in silicate electrolyte. Particles were mainly distributed in the outer 40% of the coating thickness. Coatings exhibited superconductivity below the critical temperature of MgB2 (39 K). MgB2 was incorporated primarily during “soft” sparking period of PEO. The transition to “soft” sparking was promoted by increase of MgB2 in electrolyte.

Authors have no declaration of interest.