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Article

Effect of Micro-Arc Oxidation Voltage on the Surface Morphology and Properties of Ceramic Coatings on 7075 Aluminum Alloy

by
Zarina Satbayeva
,
Ainur Zhassulan
*,
Bauyrzhan Rakhadilov
,
Aibek Shynarbek
,
Kuanysh Ormanbekov
and
Aiym Leonidova
Engineering Center, Shakarim University NJSC, Glinka Street, 20A, Semey 071412, Kazakhstan
*
Author to whom correspondence should be addressed.
Metals 2025, 15(7), 746; https://doi.org/10.3390/met15070746
Submission received: 31 May 2025 / Revised: 25 June 2025 / Accepted: 30 June 2025 / Published: 2 July 2025
(This article belongs to the Special Issue Surface Modification and Characterization of Metals and Alloys)
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Versions Notes

Abstract

Ceramic oxide coatings were fabricated on 7075 aluminum alloy via micro-arc oxidation (MAO) in a silicate-phosphate electrolyte under voltages of 250 V, 300 V, and 350 V for 600 s. The effect of the applied voltage on the surface morphology, microstructure, phase composition, microhardness, roughness, coating thickness, and corrosion resistance was systematically studied. The coating obtained at 300 V demonstrated a dense structure with relatively low surface roughness (2.3 μm) and a thickness of approximately 70 μm. This sample also exhibited the most balanced performance, combining relatively high microhardness (~422 HV) and the lowest corrosion current density (6.1 × 10−7 A/cm2) in a 3.5 wt.% NaCl solution. X-ray diffraction patterns revealed the presence of both γ- and α-Al2O3 phases in all coated samples, with a relative increase in α-phase intensity observed at an intermediate voltage. The results demonstrate that the applied voltage plays a critical role in determining the coating structure and performance, offering insights into the surface treatment of high-strength aluminum alloys for engineering applications.

1. Introduction

Aluminum alloys are widely used in the aerospace, automotive, and marine industries due to their high strength-to-weight ratio, good formability, and excellent mechanical properties [1,2,3]. Among them, the 7075 aluminum alloy is particularly attractive because of its outstanding strength and hardness, which make it suitable for structural components subjected to high mechanical loads [3]. However, the use of 7075 Al is often limited by its relatively low wear and corrosion resistance, especially under harsh environmental conditions [4,5]. To overcome these limitations, various surface modification techniques have been developed to improve the durability and performance of aluminum components [6,7].
Among these techniques, micro-arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), has emerged as a promising method for producing hard, adherent, and chemically stable ceramic coatings on light alloys. MAO involves the formation of oxide layers through high-voltage electrochemical plasma discharges in alkaline electrolytes. Compared to conventional methods such as anodizing or sol-gel deposition, MAO provides a significantly enhanced mechanical and electrochemical performance due to its ability to form thick, strongly bonded oxide layers with high thermal and wear resistance [8,9,10,11,12,13,14]. In contrast, vacuum-based technologies such as plasma-enhanced chemical vapor deposition (PE-CVD) offer high coating uniformity but are more complex, costly, and limited in achievable coating thickness for aluminum alloys [15].
One of the key parameters affecting the MAO process is the applied voltage. It significantly influences the coating thickness, surface morphology, porosity, phase composition, and ultimately the functional properties of the ceramic layer. Although several studies have investigated the effects of process parameters on coatings [16,17,18,19,20,21,22,23] formed on pure aluminum or low-strength alloys, there is a noticeable lack of comprehensive research focused on the high-strength aluminum alloy 7075, despite its widespread use in aerospace and automotive industries due to its high strength-to-weight ratio [24]. Due to the complex alloying system, primarily consisting of zinc, magnesium, and copper, the behavior of the 7075 alloy during the MAO process can differ significantly from that of other aluminum alloys [25]. This complexity necessitates a detailed investigation of how individual MAO parameters affect the characteristics of the resulting coating. Among these parameters, the applied voltage plays a decisive role in determining the discharge behavior, coating thickness, phase composition, and surface morphology [26,27,28]. Previous studies have demonstrated that increasing the MAO voltage can enhance plasma discharges, resulting in thicker and harder coatings with varying degrees of porosity and roughness [29,30]. However, excessively high voltage may cause coating cracking or poor adhesion due to thermal stresses [31]. Although some attempts have been made to investigate composite coatings on 7075 alloy using additives such as graphene [22], boron carbide (B4C) [27], and molybdenum disulfide (MoS2) [30], most of these studies varied multiple parameters simultaneously or focused on hybrid treatments, making it difficult to isolate the effect of voltage alone. Moreover, few works provide a systematic correlation between structural evolution, phase transformation, and protective performance under controlled electrical conditions.
Therefore, this study aims to evaluate the effect of MAO voltage, as a single variable, on the surface morphology, hardness, corrosion resistance, and tribological performance of ceramic oxide coatings formed on 7075 aluminum alloy using a fixed electrolyte system. The results obtained will contribute to the optimization of MAO parameters for high-strength aluminum alloys and may serve as a reference for engineering applications in the aerospace, defense, and precision manufacturing sectors [32,33].
The objective of this study is to provide insights into the optimization of MAO parameters for the development of high-performance coatings on 7075 aluminum alloy.

2. Materials and Methods

The material under study was 7075 aluminum alloy, with its chemical composition presented in Table 1 [34]. The samples measured 20 mm × 20 mm × 30 mm and were preliminarily prepared by mechanical treatment: the surfaces were ground using 400-grit abrasive paper to remove irregularities and contaminants.
To improve coating adhesion and oxidation uniformity, the samples were additionally subjected to sandblasting. Afterwards, they were cleaned in an ultrasonic bath for 10 min, rinsed with distilled water, and dried at room temperature. The prepared samples were then used for MAO.
The MAO process was performed using the KP-HI-F-40A600V (Shenzhen Kairui Energy Saving Technology Co., Ltd., Shenzhen, China) unit, the schematic and appearance of which are shown in Figure 1. Oxidation was conducted in pulse mode under different conditions for the three samples, as specified in Table 2. The cooling system featured water recirculation to maintain a stable temperature. The treatment was carried out at an electrolyte temperature not exceeding 25 °C with continuous stirring. During the MAO process, the applied voltage (250 V, 300 V, and 350 V) was fixed for each sample, while the current was allowed to vary according to the plasma discharge regime. The instantaneous current values were recorded, and the corresponding current densities were calculated based on the exposed surface area of the sample (4 cm2), yielding values of approximately 9, 14, and 18 A/dm2 for the respective voltages.
An aqueous electrolyte solution containing sodium silicate (Na2SiO3)—10 g/dm3, sodium phosphate (Na3PO4)—2 g/dm3, and potassium hydroxide (KOH)—2 g/dm3 was used. The phase composition of the coatings was analyzed by X-ray diffraction (XRD) using an X’Pert PRO diffractometer (PANalytical, Bruker, Karlsruhe, Germany) over a 2θ range of 25–75°, with CuKα radiation (λ = 1.5406 Å). Surface morphology was examined using a JSM-6390LV low-vacuum scanning electron microscope (JEOL, Tokyo, Japan). Surface roughness was measured using an Anytester HY2300 profilometer (Hefei, China), and the values represent the average of ten measurements taken at different points on the surface. Microhardness was determined by a FISCHERSCOPE HM2000 S instrument (Helmut Fischer, Sindelfingen, Germany) using the same measurement strategy. Porosity was evaluated from five SEM micrographs per sample using the ImageJ software version:1.54d. Tribological tests were performed on an Anton Paar TRB3 tribometer (Tokyo, Japan) in a ball-on-disk configuration under the following conditions: load—2 N, sliding speed—2 cm/s, counterbody—100Cr6 steel ball (radius: 3 mm), and sliding distance—100 m. Corrosion resistance was evaluated by potentiodynamic polarization using a CS350M potentiostat-galvanostat (Corrtest Instruments, Wuhan, China) in combination with a flat-type corrosion cell model CS934, configured in a standard three-electrode setup. The working electrode was a coated or uncoated sample with an exposed area of 1 cm2. A saturated Ag/AgCl electrode was used as the reference electrode, and a platinum mesh served as the counter electrode. The tests were conducted in a 3.5 wt.% NaCl solution at 25 °C. Prior to polarization, the open-circuit potential (OCP) was stabilized for 30 min. The potential sweep was performed in the range of −0.25 V to 0.00 V versus Ag/AgCl, with a scan rate of 0.5 mV/s. The procedure was conducted in accordance with the ASTM G5-13 standard [35]. The CS Studio6 software version:6.3.1128.1 was used for automated data acquisition and processing. It enabled the determination of the corrosion potential (Ecorr), corrosion current density (Icorr), corrosion rate, and Tafel slopes (ba and bc), while also displaying the polarization curves in the logarithmic scale for enhanced interpretation.

3. Results and Discussion

The X-ray diffraction (XRD) patterns of the untreated and MAO-coated samples are shown in Figure 2. The initial 7075 aluminum alloy exhibits only sharp Al peaks, indicating its high crystallinity and absence of oxide phases on the surface. After MAO treatment, new diffraction peaks corresponding to γ-Al2O3 and α-Al2O3 phases appear in all coated samples, confirming the formation of ceramic oxide layers typical for the MAO process in silicate-phosphate electrolytes [36]. With the increasing voltage from 250 V to 350 V, a gradual increase in the intensity and narrowing of the alumina peaks is observed. These changes suggest improved crystallinity and increased phase content. In particular, sample No. 3 demonstrates the most intense α-Al2O3 peak at ~43.4°, indicating the formation of a thermodynamically stable and mechanically robust phase. The simultaneous presence of broad γ-Al2O3 peaks at ~37.5° and ~67° suggests a mixed-phase structure with incomplete transformation to the α-phase. Although semi-quantitative phase identification is performed, Rietveld refinement is not applied due to the low thickness of the coatings and the significant overlap with substrate peaks. This also explains the visibility of Al peaks in all coated samples, confirming that the oxide layers remain within the X-ray penetration depth (~10 μm) and do not obscure the substrate signal. The full width at half maximum (FWHM) of the alumina peaks decreases slightly with increasing voltage, supporting the conclusion of increased crystallite size and enhanced phase stability [37]. Thus, the applied voltage directly influences both the intensity and type of oxide phases formed, with higher voltages favoring the development of stable α-Al2O3, which is beneficial for improving the mechanical and tribological performance of the coatings [38,39,40].
SEM images of the MAO-treated coatings at different voltages are presented in Figure 3. The images reveal the substantial evolution in surface morphology as the ap-plied voltage increases from 250 V to 350 V. At 250 V (Figure 3(a,a-1)), the coating exhibits a relatively smooth and compact surface, with a small number of shallow discharge pores and re-solidified regions. The pore size is minimal, and no signs of microcracking are observed. This morphology is consistent with low plasma discharge intensity, which corresponds to limited oxide growth and moderate phase development, as seen in the XRD analysis. At 300 V (Figure 3(b,b-1)), the surface becomes more textured and porous. Well-distributed discharge pores of varied sizes are visible, along with deeper microchannels. The increase in discharge activity results in a denser and more crystalline coating, as confirmed by the rise in both γ- and α-Al2O3 diffraction peak intensities. The morphology at this stage strikes a balance between porosity and uniformity, which contributes positively to both mechanical strength and corrosion resistance [41,42]. At 350 V (Figure 3(c,c-1,d,d-1)), the surface exhibits a pronounced increase in roughness and pore size. The coating shows a highly porous, heterogeneous structure with interconnected crater-like features and occasional microcracks. This morphology is a result of intensified plasma discharges and localized overheating, which, although promoting ceramic phase crystallization (Figure 2), also lead to excessive pore formation and structural discontinuities. Such features can reduce the protective integrity of the coating under wear and corrosion conditions. Higher magnification images (×3000) highlight the evolution of pore size and distribution across the samples. As voltage increases, the surface transforms from relatively closed pores (No. 1) to more open and interconnected structures (No. 3). These morphological changes have direct implications on the coating’s tribological and electro-chemical behavior, which are discussed in later sections. In summary, increasing the applied voltage intensifies the plasma discharge effects, enhancing coating crystallinity but also leading to greater surface porosity. The 300 V condition appears to offer the most favorable morphology for multifunctional performance, in agreement with microstructural and XRD data.
The cross-sectional SEM images of the MAO coatings obtained at different voltages are shown in Figure 4. The coatings exhibit a clear bilayer structure comprising a dense inner layer and a more porous outer layer, typical of MAO coatings. As the applied voltage increases from 250 V to 350 V, a significant increase in total coating thickness is observed. Sample No. 1 (250 V) shows a relatively thin coating with an average thickness of approximately 63 ± 5 µm, reflecting moderate plasma discharge activity and limited oxide growth. For sample No. 2 (300 V), the thickness increases to around 65 ± 4 µm, indicating enhanced oxide formation due to intensified plasma interactions. The most pronounced growth is observed in sample No. 3 (350 V), where the coating reaches 146 ± 3 µm, consistent with the stronger plasma discharges at high voltage. This trend confirms that applied voltage plays a dominant role in determining coating thickness by controlling discharge energy and plasma dynamics. However, excessive thickness at 350 V is accompanied by visible microstructural heterogeneities and interface roughening, which may affect the adhesion and long-term performance of the coatings [43,44].
Figure 5 illustrates the variation in microhardness and surface roughness (Ra) of the coatings as a function of applied voltage. The uncoated substrate exhibits a microhardness of approximately 161 ± 8.1 HV. After MAO treatment, the hardness significantly increases due to the formation of ceramic oxide phases. The improved hardness is at-tributed to the formation of α-Al2O3 and γ-Al2O3. Surface roughness measurements show an initial Ra of 3.04 ± 0.13 µm for the untreated alloy, caused by mechanical surface preparation. After MAO treatment, Ra decreases to 2.88 ± 0.11 µm at 250 V and reaches a minimum of 2.28 ± 0.09 µm at 300 V, indicating a more uniform and dense surface morphology. At 350 V, Ra increases to 3.42 ± 0.08 µm due to intensified discharge activity and the formation of larger molten zones and pores. These results demonstrate that the applied voltage has a pronounced effect on both surface hardness and texture, which are critical for improving the wear resistance and mechanical performance of the coatings [45,46,47].
The tribological behavior of the coated and uncoated samples is assessed through ball-on-disk wear testing, and the corresponding coefficient of friction (COF) profiles are shown in Figure 6. Quantitative wear parameters are summarized in Table 3. The uncoated 7075 aluminum alloy exhibits the highest COF (0.907 µ), with significant fluctuations throughout the test, indicative of unstable sliding and pronounced adhesive wear. In contrast, all MAO-coated samples show significantly lower COF values and improved sliding stability. Among them, sample No. 2 demonstrates the lowest average COF (0.489 µ), suggesting the formation of a smooth, hard, and compact oxide layer that minimizes frictional resistance. Sample No. 1 also exhibits a reduced COF (0.548 µ) compared to the bare alloy, although slightly higher than that of No. 2 due to the lower coating thickness and less-developed oxide structure. For sample No. 3, the COF (0.511 µ) is similar to that of No. 2, but with a slightly higher fluctuation, which can be attributed to increased surface roughness and porosity caused by intensified discharges at a high voltage. Overall, the reduction in COF and enhancement in tribological stability across all coated samples confirm the beneficial effect of MAO treatment. The optimal performance of sample No. 2 correlates with its denser morphology and balanced phase composition, making it favorable for wear-critical applications [48].
The porosity of the ceramic coatings is quantified using the ImageJ software [49], based on the analysis of five representative SEM micrographs. A clear trend of increasing porosity is observed with the rising applied voltage: from 3 ± 0.23% at 250 V, to 5 ± 0.18% at 300 V, and 11 ± 0.9% at 350 V. This behavior is attributed to the intensification of plasma discharge events at higher voltages, which results in the formation of deeper and more numerous discharge craters on the coating surface. The morphology observed in Figure 3 supports these measurements: Sample No. 1 exhibits relatively shallow and isolated pores, while sample No. 3 presents a highly porous network with interconnected channels. Such high porosity is typically associated with local overheating and discharge coalescence at elevated voltages. Although a certain degree of porosity may increase the surface area and could be beneficial in biomedical applications (e.g., for cell attachment), excessive porosity compromises barrier properties by facilitating electrolyte penetration and accelerating corrosive degradation [50]. Therefore, sample No. 2, which exhibits moderate porosity and a more compact morphology, offers the most favorable balance between structural integrity and functional performance.
Figure 7 presents the anodic polarization curves of the untreated aluminum alloy and the MAO-coated samples obtained at 250 V, 300 V, and 350 V. The corresponding electrochemical parameters are summarized in Table 3. The untreated specimen exhibits the highest anodic activity, characterized by a high current density, indicating poor corrosion resistance due to the absence of a protective oxide layer. Following MAO treatment, a substantial decrease in current density is observed, particularly for samples No. 1 and No. 2, suggesting a significant enhancement in corrosion resistance. Samples No. 2 (300 V) and No. 3 (350 V) both exhibit similarly favorable corrosion behavior, with sample No. 3 showing the lowest current density. However, sample No. 2 demonstrates a more positive corrosion potential and denser microstructure, contributing to better overall protection. These differences should be considered in the context of both electrochemical and morphological features. These results indicate that the MAO process at 300 V leads to the formation of a dense and homogeneous oxide layer, effectively improving the barrier properties of the surface and offering optimal protection against electrochemical degradation.
Despite sample No. 3 showing lower corrosion rate values than sample No. 2, its more negative corrosion potential and significantly higher porosity suggest that the long-term barrier performance may be compromised. Therefore, while sample No. 3 exhibits promising electrochemical parameters, sample No. 2 offers a better combination of compact structure and corrosion resistance.
No. 3, despite exhibiting a similarly low current density (Icorr, A/cm2), demonstrates a more negative corrosion potential, a highly porous and irregular surface morphology (Figure 4(c,c-1)), and a higher porosity level, as measured and presented in Table 3. The increased number and size of the pores facilitate localized corrosion processes and compromise the protective efficiency of the oxide layer [51,52]. These findings suggest that excessive porosity can significantly impair corrosion resistance, even when ceramic phases such as α- and γ-Al2O3 are present within the coating.
Therefore, the corrosion performance of MAO coatings is closely associated with their microstructural integrity and porosity: a dense and homogeneous oxide layer formed at an intermediate voltage (300 V) offers the most effective barrier by limiting electrolyte penetration to the substrate. Furthermore, a comparison of the tribological data reveals a strong correlation between improvements in corrosion resistance and tribological performance. The enhanced corrosion protection is likely attributed to the compaction and structural uniformity of the surface, which simultaneously contributes to a lower and more stable friction coefficient during wear.
All MAO-treated samples exhibit a noticeable passivation region in the potential range of approximately −0.18 V to −0.12 V vs. Ag/AgCl, where the current density remains relatively stable. This behavior confirms the protective effect of the oxide layer and indicates the formation of a passive film that limits further metal dissolution. In contrast, the untreated sample does not show a clear passivation plateau, reflecting its poor corrosion resistance. The shape of the polarization curves is also influenced by the porosity of the coatings. The sample with the highest porosity (No. 3) shows a steeper increase in anodic current density beyond the passivation region, indicating easier access of the electrolyte to the substrate through interconnected pores. Conversely, sample No. 2, with its moderate porosity, exhibits the lowest and most stable current, confirming the beneficial effect of a dense and compact oxide structure.

4. Conclusions

Ceramic oxide coatings were formed on 7075 aluminum alloy by micro-arc oxidation (MAO) at applied voltages from 250 V to 350 V. A comprehensive analysis showed that the applied voltage directly affects the surface morphology, microstructure, phase composition, mechanical properties, and corrosion resistance of the coatings.
-
The coating obtained at 300 V showed the most balanced characteristics, including moderate porosity, microhardness (422 ± 21.2 HV), low surface roughness (2.28 ± 0.09 μm), and excellent corrosion resistance. Although sample No. 3 exhibited a slightly lower corrosion current density, its higher porosity and less favorable Ecorr made sample No. 2 more suitable for long-term protection.
-
The coating thickness increased with the voltage, reaching from 56 to 149 μm. However, excessive voltage led to structural defects and high porosity, which negatively affected the protective properties.
-
The X-ray diffraction analysis revealed the presence of both γ- and α-Al2O3 phases in all coatings, with increased intensity and crystallinity of the α-phase at higher voltages.
-
Sample No. 2 demonstrated the lowest and most stable friction coefficient (0.489), confirming the excellent wear resistance and adhesion of the coating under sliding conditions.
In summary, the MAO process at 300 V provides the most favorable balance of structural integrity, mechanical performance, and electrochemical protection, making it an optimal condition for enhancing the surface functionality of 7075 aluminum alloy under severe wear and corrosion conditions.

Author Contributions

Conceptualization, A.Z., Z.S. and B.R.; methodology, A.Z., A.S. and K.O.; validation, K.O., A.L. and A.Z.; formal analysis, A.S., K.O. and A.L.; investigation, A.Z. and A.L.; resources, Z.S. and B.R.; data curation, A.L. and K.O.; writing—original draft preparation, A.Z. and A.L.; writing—review and editing, Z.S. and B.R.; visualization, K.O. and A.S.; supervision, Z.S. and B.R.; project administration, Z.S. and B.R.; funding acquisition, B.R. and Z.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research has been funded by the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. BR24992870).

Data Availability Statement

The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Wahid, M.A.; Siddiquee, A.N.; Khan, Z.A. Aluminum alloys in marine construction: Characteristics, application, and problems from a fabrication viewpoint. Mar. Syst. Ocean Technol. 2020, 15, 70–80. [Google Scholar] [CrossRef]
  2. Yadav, R.; Dwivedi, V.K.; Dwivedi, S.P. Analysis of Mechanical Properties and Manufacturing Process of Aluminum Series. J. Inst. Eng. (India) Ser. D 2024, 1–13. [Google Scholar] [CrossRef]
  3. Khalid, M.Y.; Umer, R.; Khan, K.A. Review of recent trends and developments in aluminium 7075 alloy and its metal matrix composites (MMCs) for aircraft applications. Results Eng. 2023, 20, 101372. [Google Scholar] [CrossRef]
  4. Qi, X.; Li, J.; He, Y.; Liu, Y.; Liu, R.; Song, R. Study on the wear resistance and corrosion behaviour of self-sealed MAO/ZrO2 coatings prepared on 7075 aluminium alloy. J. Alloys Compd. 2023, 969, 172436. [Google Scholar] [CrossRef]
  5. Imran, M.; Khan, A.R.A. Characterization of Al-7075 metal matrix composites: A review. J. Mater. Res. Technol. 2019, 8, 3347–3356. [Google Scholar] [CrossRef]
  6. Rao, Y.; Wang, Q.; Oka, D.; Ramachandran, C.S. On the PEO treatment of cold sprayed 7075 aluminum alloy and its effects on mechanical, corrosion and dry sliding wear performances thereof. Surf. Coat. Technol. 2020, 383, 125271. [Google Scholar] [CrossRef]
  7. Jayappa, K.; Anil, K.C.; Khan, Z.A. Enhancing wear resistance in Al-7075 composites through conventional mixing and casting techniques. J. Mater. Res. Technol. 2023, 27, 7935–7945. [Google Scholar] [CrossRef]
  8. Zhao, C.; Wang, X.; Yu, B.; Cai, M.; Yu, Q.; Zhou, F. Research progress on the wear and corrosion resistant plasma electrolytic oxidation composite coatings on magnesium and its alloys. Coatings 2023, 13, 1189. [Google Scholar] [CrossRef]
  9. Sikdar, S.; Menezes, P.V.; Maccione, R.; Jacob, T.; Menezes, P.L. Plasma electrolytic oxidation (PEO) process—Processing, properties, and applications. Nanomaterials 2021, 11, 1375. [Google Scholar] [CrossRef]
  10. Fernández-López, P.; Alves, S.A.; San-Jose, J.T.; Gutierrez-Berasategui, E.; Bayón, R. Plasma electrolytic oxidation (PEO) as a promising technology for the development of high-performance coatings on cast Al-Si alloys: A review. Coatings 2024, 14, 217. [Google Scholar] [CrossRef]
  11. Wierzbicka, E.; Vaghefinazari, B.; Mohedano, M.; Visser, P.; Posner, R.; Blawert, C.; Zheludkevich, M.; Lamaka, S.; Matykina, E.; Arrabal, R. Chromate-free corrosion protection strategies for magnesium alloys—A review: Part II—PEO and anodizing. Materials 2022, 15, 8515. [Google Scholar] [CrossRef] [PubMed]
  12. Nie, X.; Cai, R.; Zhao, C.; Sun, J.; Zhang, J.; Matthews, D.T. Advancement of plasma electrolytic oxidation towards non-valve metals. Surf. Coat. Technol. 2022, 442, 128403. [Google Scholar] [CrossRef]
  13. Jadhav, P.; Bongale, A.; Kumar, S. A review of process characteristics of plasma electrolytic oxidation of aluminium alloy. J. Phys. Conf. Ser. 2021, 1854, 012030. [Google Scholar] [CrossRef]
  14. Fattah-alhosseini, A.; Fardosi, A.; Karbasi, M.; Kaseem, M. Advancements in enhancing corrosion protection of Mg alloys: A comprehensive review on the synergistic effects of combining inhibitors with PEO coating. J. Magnes. Alloys 2024, 12, 465–489. [Google Scholar] [CrossRef]
  15. Varghese, J.; Vieth, P.; Xie, X.; Grundmeier, G. Enhanced corrosion resistance of epoxy-films on ultra-thin SiOx PECVD film coated laser surface melted Al-alloys. SN Appl. Sci. 2023, 5, 29. [Google Scholar] [CrossRef]
  16. Jiang, B.; Wen, Z.; Wang, P.; Huang, X.; Yang, X.; Yuan, M.; Xi, J. Study on the Optimization of the Preparation Process of ZM5 Magnesium Alloy Micro-Arc Oxidation Hard Ceramic Coatings and Coatings Properties. Metals 2024, 14, 594. [Google Scholar] [CrossRef]
  17. Li, G.; Ma, F.; Liu, P.; Qi, S.; Li, W.; Zhang, K.; Chen, X. Review of micro-arc oxidation of titanium alloys: Mechanism, properties and applications. J. Alloys Compd. 2023, 948, 169773. [Google Scholar] [CrossRef]
  18. Rakhadilov, B.; Zhassulan, A.; Baizhan, D.; Shynarbek, A.; Ormanbekov, K.; Aldabergenova, T. The effect of the electrolyte composition on the microstructure and properties of coatings formed on a titanium substrate by microarc oxidation. AIMS Mater. Sci. 2024, 11, 547–564. [Google Scholar] [CrossRef]
  19. Rakhadilov, B.; Zhassulan, A.; Ormanbekov, K.; Shynarbek, A.; Baizhan, D.; Aldabergenova, T. Surface Modification and Tribological Performance of Calcium Phosphate Coatings with TiO2 Nanoparticles on VT1-0 Titanium by Micro-Arc Oxidation. Crystals 2024, 14, 945. [Google Scholar] [CrossRef]
  20. Bayatanova, L.B.; Zhassulankyzy, A.Z.; Magazov, N.M.; Rakhadilov, B.K.; Muktanova, N.; Uazyrkhanova, G.K. Effect of plasma-electrolytic oxidation on mechanical properties of titanium coatings. Bull. Karaganda Univ. Phys. Ser. 2023, 111, 65–74. [Google Scholar] [CrossRef]
  21. Sobolev, A.; Peretz, T.; Borodianskiy, K. Micro-Arc Oxidation Coating of 7075 Aluminum Alloy in Molten Salt. Coatings 2020, 10, 993. [Google Scholar] [CrossRef]
  22. Chen, X.; Tang, S.; Zhang, D.; Yan, J.; Xie, W.; Ran, Q. Influence of graphene additions on the corrosion resistance of micro-arc oxidation coatings formed on 7075 aluminum alloy. Int. J. Appl. Ceram. Technol. 2024, 21, 3516–3525. [Google Scholar] [CrossRef]
  23. Barati, N.; Jiang, J.; Meletis, E. Microstructural Evolution of Ceramic Nanocomposites on 7075 Al Alloy by Plasma Electrolytic Oxidation. Adv. Eng. Mater. 2021, 437, 128345. [Google Scholar] [CrossRef]
  24. Sun, H.O.; Dela Pena, E.M.B. Effects of pulse unipolar voltage on microstructure and corrosion behavior of MAO coatings on AA7075. Coatings 2024, 14, 1498. [Google Scholar] [CrossRef]
  25. Li, Y.H.; Zhao, Y.; Li, B.Y. Corrosion Resistance of Al2O3-ZrO2 Composite Coatings on Aluminum Alloy by MAO. Adv. Mater. Res. 2011, 189, 672–675. [Google Scholar] [CrossRef]
  26. Thunaipragasam, S.; Kolekar, A.B.; Chandrashekhar, K.G.; Pandey, R.; Shahid, M.; Rajesh, K.; Singh, B. Investigation of mechanical behavior and surface characteristics of cold spray metallized B4C/AA7075 composites coated by AZ64 alloy through plasma electrolytic oxidation. J. Nanomater. 2023, 2023, 7267093. [Google Scholar] [CrossRef]
  27. Su, K.; Zhang, J.; Wu, S.; Guan, J.; Li, H.; Ji, D.; Xie, H. Corrosion and corrosion-fatigue properties of composite ceramic coated aluminum alloy formed by combining surface nanocrystallization pre-treatment, micro-arc oxidation, and sealing post-treatment. Int. J. Fatigue 2025, 190, 108661. [Google Scholar] [CrossRef]
  28. Wu, X.; Li, H.; Lu, J.; Li, Y.; Yang, C.; Cen, Y.; Song, R. MoS2 additive to the MAO Al2O3 composite coatings with enhanced mechanical performances. Mater. Res. Express 2019, 6, 016543. [Google Scholar] [CrossRef]
  29. Krishtal, M.M.; Ivashin, P.V.; Yasnikov, I.S.; Polunin, A.V. Effect of nanosize SiO2 particles added into electrolyte on the composition and morphology of oxide layers formed in alloy AK6M2 under microarc oxidizing. Met. Sci. Heat Treat. 2015, 57, 428–435. [Google Scholar] [CrossRef]
  30. Pesode, P.; Barve, S.; Dayane, S. Antibacterial coating on magnesium alloys by MAO for biomedical applications. Res. Biomed. Eng. 2024, 40, 409–433. [Google Scholar] [CrossRef]
  31. Hashemzadeh, M.; Raeissi, K.; Ashrafizadeh, F.; Hakimizad, A.; Santamaria, M. Incorporation mechanism of colloidal TiO2 nanoparticles and their effect on properties of coatings grown on 7075 Al alloy from silicate-based solution using plasma electrolytic oxidation. Trans. Nonferrous Met. Soc. China 2024, 31, 3659–3676. [Google Scholar] [CrossRef]
  32. Zhu, M.; Song, Y.; Liu, Z.; Xu, D.; Dong, K.; Han, E.H. Optimization of thermal control and corrosion resistance of PEO coatings on 7075 aluminum alloy by frequency alteration. Surf. Coat. Technol. 2022, 446, 128797. [Google Scholar] [CrossRef]
  33. Muhaffel, F.; Baydogan, M.; Cimenoglu, H. A study to enhance the mechanical durability of the MAO coating fabricated on the 7075 Al alloy for wear-related high temperature applications. Surf. Coat. Technol. 2021, 409, 126843. [Google Scholar] [CrossRef]
  34. Shew-E Steel. Aluminum Alloy 7075. SHEW-E Steel Pipe Co., Ltd. Available online: https://ru.shew-esteelpipe.com/aluminum-alloys/aluminum-alloy-7075.html (accessed on 9 May 2025).
  35. ASTM G5-13; Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements. ASTM International: West Conshohocken, PA, USA, 2013.
  36. Cao, G.P.; Song, R.G. Microstructure and properties of ceramic coatings prepared by micro-arc oxidation on 7075 aluminum alloy. Mater. Res. Express 2018, 5, 026407. [Google Scholar] [CrossRef]
  37. Kushwaha, A.K.; Maccione, R.; John, M.; Lanka, S.; Misra, M.; Menezes, P.L. Influence of cryomilling on crystallite size of aluminum powder and spark plasma sintered component. Nanomaterials 2022, 12, 551. [Google Scholar] [CrossRef]
  38. Rodriguez, L.; Paris, J.Y.; Denape, J.; Delbé, K. Micro-arcs oxidation layer formation on aluminium and coatings tribological properties—A review. Coatings 2023, 13, 373. [Google Scholar] [CrossRef]
  39. Szkodo, M.; Stanisławska, A.; Komarov, A.; Bolewski, Ł. Effect of MAO coatings on cavitation erosion and tribological properties of 5056 and 7075 aluminum alloys. Wear 2021, 474, 203709. [Google Scholar] [CrossRef]
  40. Wang, C.; Chen, J.; He, J.; Jiang, J.; Zhang, Q. Effect of electrolyte concentration on the tribological performance of MAO coatings on aluminum alloys. Front. Chem. Sci. Eng. 2020, 14, 1065–1071. [Google Scholar] [CrossRef]
  41. Wang, S.; Gu, Y.; Geng, Y.; Liang, J.; Zhao, J.; Kang, J. Investigating local corrosion behavior and mechanism of MAO coated 7075 aluminum alloy. J. Alloys Compd. 2020, 826, 153976. [Google Scholar] [CrossRef]
  42. Gong, Y.; Geng, J.; Huang, J.; Chen, Z.; Wang, M.; Chen, D.; Wang, H. Self-healing performance and corrosion resistance of novel CeO2-sealed MAO film on aluminum alloy. Surf. Coat. Technol. 2021, 417, 127208. [Google Scholar] [CrossRef]
  43. Dai, W.; Zhang, C.; Yue, H.; Li, Q.; Guo, C.; Zhang, J.; Yang, X. A review on the fatigue performance of micro-arc oxidation coated Al alloys with micro-defects and residual stress. J. Mater. Res. Technol. 2023, 25, 4554–4581. [Google Scholar] [CrossRef]
  44. Liu, X.; Wang, C.; Lei, J.; Song, P.; Huang, T.; Zhang, X.; Ji, V. Micro-Arc Oxidation of Aluminum Alloys: Mechanism, Defects, and Corrosion Resistance. Adv. Eng. Mater. 2025, 27, 2402748. [Google Scholar] [CrossRef]
  45. Paidar, M.; Vignesh, R.V.; Khorram, A.; Ojo, O.O.; Rasoulpouraghdam, A.; Pustokhina, I. Dissimilar modified friction stir clinch- ing of AA2024-AA6061 aluminum alloys: Effects of materials positioning. J. Mater. Res. Technol. 2020, 9, 6037–6047. [Google Scholar] [CrossRef]
  46. Li, H.; Huang, Y.; Meng, Q.; Wang, X. Effect of electrical parameters on the structure and properties of aluminum foil microarc oxidation film. Metals 2024, 14, 1108. [Google Scholar] [CrossRef]
  47. Li, H.; Kong, S.; Liu, Z.; Wang, Z.; Geng, Y. Hardness distribution and growth behavior of micro-arc oxide ceramic film with positive and negative pulse coordination. Nanomaterials 2024, 14, 842. [Google Scholar] [CrossRef]
  48. Wang, J.; Xu, G.; Jia, E.; Zhao, Z.; Wang, Z.; Lu, H. Preparation of wear-resistant ceramic coating by dynamically controlling the resistance during MAO. Int. J. Appl. Ceram. Technol. 2024, 21, 876–887. [Google Scholar] [CrossRef]
  49. Astuti, N.H.; Wibowo, N.A.; Ayub, M. The porosity calculation of various types of paper using image analysis. J. Pendidik. Fis. Indones. 2018, 14, 46–51. [Google Scholar] [CrossRef]
  50. Fernández-Hernán, J.P.; Campos, G.G.; García, A.G.; Rodríguez, J.R.; Mendoza, A.A.; Martínez, R.P. Anticorrosion and cyto- compatibility assessment of graphene-doped hybrid silica and plasma electrolytic oxidation coatings for biomedical applications. ACS Biomater. Sci. Eng. 2021, 7, 5861–5877. [Google Scholar] [CrossRef]
  51. Venugopal, A.; Panda, R.; Manwatkar, S.; Sreekumar, K.; Krishna, L.R.; Sundararajan, G. Effect of micro arc oxidation treatment on localized corrosion behaviour of AA7075 aluminum alloy in 3.5% NaCl solution. Trans. Nonferrous Met. Soc. China 2012, 22, 700–710. [Google Scholar] [CrossRef]
  52. Zhou, S.; Tong, R.; Li, H.; Tao, X.; Chen, J. Effects of Current Output Modes on Corrosion Resistance of Micro-Arc Oxidation Black Coatings on Aluminum Alloy. Materials 2025, 18, 2949. [Google Scholar] [CrossRef]
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Figure 1. Micro-arc oxidation system: (a)—installation diagram, (b)—electrolytic bath, (c)—power source.
Figure 1. Micro-arc oxidation system: (a)—installation diagram, (b)—electrolytic bath, (c)—power source.
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Figure 2. XRD patterns of untreated 7075 aluminum alloy and ceramic coatings formed by micro-arc oxidation at different voltages.
Figure 2. XRD patterns of untreated 7075 aluminum alloy and ceramic coatings formed by micro-arc oxidation at different voltages.
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Figure 3. SEM images of the surface morphology of the 7075 aluminum alloy: (a,a-1) initial uncoated sample; (b,b-1) sample No. 1 (250 V); (c,c-1) sample No. 2 (300 V); (d,d-1) sample No. 3 (350 V) at different magnifications. Images (a–d): ×1000; (a-1–d-1): ×3000.
Figure 3. SEM images of the surface morphology of the 7075 aluminum alloy: (a,a-1) initial uncoated sample; (b,b-1) sample No. 1 (250 V); (c,c-1) sample No. 2 (300 V); (d,d-1) sample No. 3 (350 V) at different magnifications. Images (a–d): ×1000; (a-1–d-1): ×3000.
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Figure 4. Surface morphology of MAO coating at different voltages ×500: (a) No. 1; (b) No. 2; (c) No. 3, and cross-section: (a-1) No. 1; (b-1) No. 2; (c-1) No. 3.
Figure 4. Surface morphology of MAO coating at different voltages ×500: (a) No. 1; (b) No. 2; (c) No. 3, and cross-section: (a-1) No. 1; (b-1) No. 2; (c-1) No. 3.
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Figure 5. Microhardness and surface roughness (Ra) of the coatings formed at different MAO voltages on 7075 aluminum alloy.
Figure 5. Microhardness and surface roughness (Ra) of the coatings formed at different MAO voltages on 7075 aluminum alloy.
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Figure 6. Tribological test results.
Figure 6. Tribological test results.
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Figure 7. Polarization curves of samples.
Figure 7. Polarization curves of samples.
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Table 1. Chemical composition of aluminum 7075. Adapted from Ref. [34].
Table 1. Chemical composition of aluminum 7075. Adapted from Ref. [34].
CrCuFeMgMnSiTiZnOther ElementsAl
0.18–0.281.2–2.0<0.502.10–2.90<0.30<0.40<0.205.1–6.1<0.15balance
Table 2. MAO process parameters.
Table 2. MAO process parameters.
SampleVoltage, VFrequency, HzDuty Cycle, %Time, sPulse Duration, µsCurrent Density, A/dm2
No. 1250500306001009
No. 23005003060010014
No. 33505003060010018
Table 3. Electrochemical data of samples before and after MAO.
Table 3. Electrochemical data of samples before and after MAO.
Sample−Ecorr (mV)Icorr (A/cm2)Vcorr (mm/a)Tafel Slopes (mV)
babc
Initial149.796.495 × 10−60.055186121.512704.8
No. 1171.021.1356 × 10−70.00096493321.761.856
No. 2151.029.186 × 10−0.80.000780511435.759.534
No. 3138.058.6833 × 10−0.80.0007378104.2756.784
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Satbayeva, Z.; Zhassulan, A.; Rakhadilov, B.; Shynarbek, A.; Ormanbekov, K.; Leonidova, A. Effect of Micro-Arc Oxidation Voltage on the Surface Morphology and Properties of Ceramic Coatings on 7075 Aluminum Alloy. Metals 2025, 15, 746. https://doi.org/10.3390/met15070746

AMA Style

Satbayeva Z, Zhassulan A, Rakhadilov B, Shynarbek A, Ormanbekov K, Leonidova A. Effect of Micro-Arc Oxidation Voltage on the Surface Morphology and Properties of Ceramic Coatings on 7075 Aluminum Alloy. Metals. 2025; 15(7):746. https://doi.org/10.3390/met15070746

Chicago/Turabian Style

Satbayeva, Zarina, Ainur Zhassulan, Bauyrzhan Rakhadilov, Aibek Shynarbek, Kuanysh Ormanbekov, and Aiym Leonidova. 2025. "Effect of Micro-Arc Oxidation Voltage on the Surface Morphology and Properties of Ceramic Coatings on 7075 Aluminum Alloy" Metals 15, no. 7: 746. https://doi.org/10.3390/met15070746

APA Style

Satbayeva, Z., Zhassulan, A., Rakhadilov, B., Shynarbek, A., Ormanbekov, K., & Leonidova, A. (2025). Effect of Micro-Arc Oxidation Voltage on the Surface Morphology and Properties of Ceramic Coatings on 7075 Aluminum Alloy. Metals, 15(7), 746. https://doi.org/10.3390/met15070746

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