Characterization of the ceramic coating formed on 2024 Al alloy by scanning plasma electrolytic oxidation

doi:10.21203/rs.3.rs-5187214/v1

Download PDF

Article

Characterization of the ceramic coating formed on 2024 Al alloy by scanning plasma electrolytic oxidation

https://doi.org/10.21203/rs.3.rs-5187214/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

Scanning plasma electrolytic oxidation (SPEO) was used to prepare ceramic coating on 2024 Al alloy. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to examine the coating morphologies and phase compositions. Polarization curves were used to evaluate the anti-corrosion properties of the coated samples. The results revealed that a ceramic coating with the largest thickness of 30 μm was formed in a very short period of time, exhibiting high efficiency of coating formation. During the initial stage of the SPEO process under fixed-current mode, the anodic voltage rapidly increased to a steady value. In the stable stage, general anodization, spark oxidation, and micro-arc oxidation stages appeared at the same time in different places, which were generally found in different periods for the traditional plasma electrolytic oxidation (PEO) process. The coating was mainly composed of α-Al₂O₃ and γ-Al₂O₃. The treated sample exhibited better corrosion resistance than the uncoated substrate in the 3.5% NaCl solution.

Physical sciences/Engineering

Physical sciences/Materials science

Ceramic coating

Corrosion

Scanning plasma electrolytic oxidation

2024 Al alloy

Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), is a relatively new surface treatment technique that has garnered significant interest for fabricating ceramic coatings on valve metals such as aluminum, magnesium, and titanium alloys to enhance their functional properties.^1–3. The PEO technique is an electrochemical surface process in a plasma environment on these metals by spark micro-discharges, without using strong acid solutions, in contrast with conventional anodization^4–6. It is featured by high productivity, ecological friendliness, economic efficiency, good wear and corrosion resistance, high hardness, and excellent bonding strength with the substrate, due to the characterization of in-situ growth of the coating^7–9.

Despite these outstanding advantages, PEO process is usually difficult to be carried out on workpieces with large size, which strongly restricts its wide application in engineering^10,11. On the one hand, workpieces should be immersed in electrolyte in traditional PEO process, which leads to great difficulty to carry the large workpieces, high load for power supply and a large consumption of electrolyte. To solve this problem, partial cover by insulative layer on workpieces has been used on the surface area that does not need to be treated. However, this method is suitable for workpieces with simple structures but not for those with complex geometries. On another hand, usually, only some small partial areas of workpiece surface need to be treated, which are in harsh environment and perform some special functions, such as anti-wear and anti-corrosion. Therefore, treatment on unwanted area of workpiece is just a waste of electric energy. The growth of Cu in 2024 aluminum alloy into the coating makes the SPEO coating antimicrobial, which is expected to be applied in thoracic surgical instruments and microsurgical instruments for the repair of complicated structures.

In order to overcome these shortcomings for traditional PEO, some researches have been conducted to control treated area. Timoshenko et al¹². put forward a means to produce PEO coating for large and intricate workpieces, by immersing them gradually, which enables the PEO process to be performed under relatively low electric current. However, these ways do not break the limit that workpieces should be immersed in electrolyte. Therefore, in this study, we propose a local PEO treatment method suitable for large workpieces and characterize the coating.

2.1 Preparation of SPEO coating

Samples of 2024 Al alloy (0.50% Si plus Fe, 4.2% Cu, 0.50% Mn, 1.40% Mg, 0.30%Zn, balance Al) were used in these ex-periments. The samples were polished with #1000 SiC abra-sive paper, rinsed in distilled water, cleaned ultrasonically in alcohol and finally dried in air. Figure 1 shows the composition and working principle of SPEO system.

The SPEO process was carried out with a 10 kW homemade power supply equipped with a stirring system and a cooling system. A water pump was used to eject electrolyte to sample surface through a stainless steel tube with an inner diameter of 1.25 mm. The specimen was fixed on a three-axis motion platform controlled by a CNC system, which ensured the PEO process performed in any position. The distance between the tube and specimen was 2.0 mm. The scanning speed was fixed at 0.25 mm/s. The stainless steel tube and the specimen were used as cathodic electrode and anodic electrode, respectively. The current, frequency, duty ratio, overlapping factor and scanning speed were fixed at 1 A, 500 HZ, 50%, 0 and 0.25 mm/s. The electrolyte used during PEO process was composed of 1 g/L KOH and 8 g/L Na₂SiO₃. The temperature of the electrolytic solution was kept below 35°C.Based on these parameters, two kinds of SPEO coatings were prepared: line coating produced by rectilinear movement of tube and area coating by route movement depicted in Fig. 1(b), respectively.

2.2 Coating analysis

X-ray diffraction (XRD) was carried out using a D/max-rB to examine the composition of the SPEO coatings. The specimens were scanned using CuKα radiation. Diffraction data were obtained over scattering angle 2θ from 10° to 90°with a step of 0.04° and acquisition time of 1s/step. Scanning electron microscope (SEM, FEI, SIRION) was used to examine the surface and the cross-sectional morphologies of the SPEO coating. Electrochemical tests were performed on an electrochemical workstation (Reference 600+, Gamry) with a standard three-electrode system. After stabilization in 3.5 wt% NaCl for 30 min, kinetic potential polarization tests were performed at a scan rate of 1 mV/s from − 0.5 V to 0.5 V (vs. SCE). Corrosion current density (Icorr) and corrosion potential (Ecorr) were obtained using Tafel extrapolation.The hardness of the coatings was measured using a microhardness tester (EM-810, FUTURE-TECH, Japan) with an applied load of 4.9 N and a loading time of 15 s. A surface roughness tester (SJ-210, Mitutoyo, Japan) was used. Both hardness and roughness were measured in three different areas.SPSS 19.0 software was used for statistical analysis. Data were expressed as mean ± standard deviation (SD).

3.1. Voltage-time response

Figure 2 shows the voltage-time response during SPEO process. Generally, four stages are divided during PEO process: general anodization, sparking anodization, micro-arc oxida-tion and arc oxidation¹³. During general anodization stage, a passivation layer is formed on the surface of the Al alloy and the voltage increases abruptly. After the voltage reaches the breakdown voltage, sparking anodization stage occurs. In this stage, the voltage increases continually, but the voltage-time response slope decreases. As the oxidized process contin-ues, a steady-state sparking is established on metal surface and the voltage reaches a relatively stable value, which indi-cates the micro-arc oxidation stage, the main stage for coating formation^14–17. Usually, the fourth stage arc oxidation stage is not needed. However, the voltage-time response of SPEO does not conform to that of traditional PEO. In term of SPEO, two stages are divided. In the initial stage, the anodic voltage increased extremely fast and after the voltage reached about 520 V at about 7 seconds, the process turned into a stable stage, which was the main process of SPEO.

During the initial stage, as no ceramic coating was formed on the surface of substrate, the general anodization and sparking anodization for traditional PEO were dominant¹⁸. Since the major oxidizing area was very small (Fig. 1(b)), in which most of the power was consumed, the current density was as high as about 8149 A/dm² (EQ. (1)), which is hundreds of times higher than that of traditional PEO. Under such a high current density, ceramic coating was formed so fast that the initial stage lasted only about 7 s, which is usually several minutes for traditional PEO^19–21.

$$\:{\text{D}}_{\text{I}}\text{=}\frac{\text{4}\text{i}}{\text{π}{\text{d}}^{\text{2}}}\text{=}\frac{\text{4×1}}{\text{π}{\text{×}\left(\text{1.25×}{\text{10}}^{\text{-2}}\right)}^{\text{2}}}\text{=8149}\text{A}\text{/d}{\text{m}}^{\text{2}}$$

where D_I is the current density of in oxidizing area, i is the current during SPEO process, d is the diameter of the ejecting tube.

After the voltage reached about 520 V, the process turned into the second stage, stable stage. In this stage, all the first three stages for traditional PEO, general anodization, sparking anodization and micro-arc oxidation, existed at the same time in different place of oxidizing area and changed as the tube moved continuously. Since the area in the front of the water spot mainly consisted of uncoated substrate, general anodization and sparking anodization were dominant in this area. In contrast, the area in the back of the water spot was mainly composed of oxidized coating and thus, in this area, micro-arc oxidation was dominant. In sum, from the front to the back of the water spot, as the oxidizing time became longer, these three stages changed: micro-arc oxidation became more dominant; general anodization and sparking anodization became less dominant.

3.2. SPEO coating composition analysis

XRD pattern of the coated sample is shown in Fig. 3. Strong Al peaks in the pattern correspond to the metal substrate. According to the XRD analysis, The coating produced by SPEO is mainly composed of γ-Al₂O₃ and a relatively minor amount of α-Al₂O₃, which accords with traditional PEO coating.

As the tubular cathode moved continuously, the average oxidizing time for individual point was only less than 10 seconds in the major SPEO area (Eq. (2)), much shorter than that of traditional PEO, which usually lasts between 3 to 60 minutes^21,22. Therefore, the growth of SPEO coatings in such a short time is mainly due to the extremely high current density.

$$\:{\text{t}}_{\text{O}}\text{=}\raisebox{1ex}{${\text{d}}_{\text{s}}$}\!\left/\:\!\raisebox{-1ex}{$\text{v}$}\right.\text{=}\raisebox{1ex}{$\text{2}\sqrt{{\text{r}}^{\text{2}}\text{-}{\text{d}}_{\text{c}}^{\text{2}}}$}\!\left/\:\!\raisebox{-1ex}{$\text{v}$}\right.$$

where d_C is the distance from the central line, d_S is the distance that the main SPEO area passes, at certain point d_C away from the central line, r is the radius of the main SPEO area, v is scanning speed of ejecting tube, t_O is oxidizing time, at certain point, d_C away from the central line.

3.3. Surface morphology

Figure 4 shows image of line coating and area coating pre-pared on 2024Al alloy by SPEO. From this picture, a relatively large area of ceramic coating was formed by SPEO, which consists of many line coatings. Moreover, due to the flexibility of the CNC system, any area that needs to be coated can be reached. SPEO, therefore, is proved to be an applicable way to produce or repair ceramic coating, extremely fit for large work-pieces.

Figure 5 shows SEM images of the surface of SPEO coating prepared on 2024 Al alloy. The surface of ceramic coating produced by SPEO is porous and loose, which is consistent with traditional PEO coating[27].Large numbers of lamellar spherical pieces like volcano top are distributed randomly on the surface, attributed to discharging activity and gas evolution during the PEO process[28]. Also, some micro-cracks are found on the coating surface, caused by the thermal stress, on account of the rapid solidification of the alumina melted in the discharge tunnel [28].

As is the basic factor of various area coating, line coating is mainly investigated in this work. Largest roughness appears near the central line of scanning route of water spot. As the distance away from central line increases, the roughness decreases from Fig. 5(b) to (e). This result is owing to the decrease of oxidizing time, with the increase of the distance away from central line in major SPEO area (EQ. 3). After the distance sur-passes the boundary of major SPEO area, ceramic coating becomes not obvious, which was generated under very low current density and very short time in minor SPEO area (Fig. 5(f)). In this area, extremely smooth surface and small holes are found, indicating that general anodization and sparking anodization are dominant. Moreover, along the scanning route, coating morphology appears consistent, due to the same oxidizing condition, except for the oxidizing area in initial stage. For area coating, the roughness stays the same in the main SPEO area but becomes higher in the boundary of the major SPEO area and minor SPEO area, which was oxidized twice.

Figure 6 shows cross-sectional images of SPEO coating pre-pared on 2024 Al alloy at different distances away from the central line of scanning route. Porous layer and compact layer are identified in these images (Fig. 6(a) to (d)), in conformity with traditional PEO coating. The largest thickness appears in the central line of scanning route, which is as large as 25 µm, due to the longest oxidizing time. As distance from central line increases, the thickness decreases, as a result of the de-crease of oxidizing time. After surpassing the boundary line of major SPEO area, coating is extremely thin, since the current density was very small and oxidizing time was very short. During SPEO process, general anodization and sparking anodization were dominant in this area and thus only thin oxidized layer, not typical structure of PEO coating, was formed (Fig. 6(e)).

3.4. The corrosion properties test

The corrosion properties of the 2024 alloy and the SPEO coating produced in sodium silicate aqueous solution were evaluated using potential dynamic polarization tests in a 3.5 wt.% NaCl solution. The results are shown in Fig. 7. The parameters related to the corrosion properties, the corrosion potential (Ecorr) and corrosion current density (icorr) were meas-ured from the potential dynamic polarization curves[29].

As shown in Fig. 7, the untreated aluminum alloy has a low corrosion potential (Ecorr − 0.59 V) and a high corrosion current density (icorr 7×10^− 7 A/cm²). By contrast, the Ecorr of aluminum alloy treated by SPEO is -0.47 V and its icorris 4.0×10^− 8 A/cm². The increase of corrosion potential of the treated sample by about 120 mV indicates the thermodynamic tendency of the corrosion decreases considerably. Moreover, the corrosion current density of the SPEO coating decreases to about one-sixth that of aluminum alloy substrate, which demonstrates that the corrosion rate decreases markedly. To sum up, potential dynamic polarization tests meant that SPEO treatment improved corrosion resistance significantly.

3.5 Microhardness and Roughness

The microhardness and roughness of the coatings are shown in Fig. 8, and the microhardness of the SPEO coatings is 648.8 HV, and the roughness(Ra) is 3.485 µm. The hardness of the coatings is mainly determined by the voltage, and the high voltage of 520 V makes the heat of the micro-arc discharges higher, which promotes the generation of the α-Al₂O₃ hard phases in the coatings, thus increasing the hardness of the coatings. The maximum roughness of the line coating surface occurs near the centerline of the scanning trajectory in the discharge region. As the distance from the centerline increases, the surface roughness of the coating gradually decreases. The area coating is formed by connecting micro-arc oxidized ceramic layers through line scanning, resulting in a relatively uniform surface roughness across the entire coating. The roughness of the coating in the transition area between the trajectories is slightly reduced.

In this research, an applicable way for partial PEO process was proposed and proved to be an effective way to repair partial damaged PEO coating or produce coating on partial area of large workpieces. According to results and discussions above, it is found that there are many differences between SPEO and traditional PEO.

(1)During SPEO process, two stages were divided: initial stage, which lasts only 7 seconds, and stable stage, the main process stage. In initial stage, general anodization and spark oxidation are dominant and the voltage increased extremely fast to a stable value. In stable stage, with the tubular cathode moving, three different stages for traditional PEO, general anodization, spark oxidation and micro-arc oxidation, existed at the same time in different places.

(2)As the treated area was very small, the current density was extremely large, which lead to the fast voltage-time response and the fast formation of coating.

(3)As a result of the different current density and oxidizing time in different places of water spot, the coating was formed into a peak shape: the thickness is largest around the central line of moving route and decreases with the increase of distance from the central line.

However, the SPEO coating is fundamentally similar to the traditional one. The surface of the prepared coating is porous, and two distinct layers—a porous layer and a compact layer—are identified. SPEO coating mainly consists of γ-Al₂O₃ and α-Al₂O₃.SPEO coating performs better corrosion resistance than uncoated Al substrate in 3.5%NaCl solution.

Competing interest

The author(s) declare no competing interests.

Funding

This work was supported by the "Dalian University Discipline Construction—Interdisciplinary Project (DLUXK-2023-ZD-003)" for the innovative research and development of minimally invasive thoracic surgical instruments (2023–2024).

Author Contribution

X.X. and P.L. drafted the main manuscript text. W.J., T.Y., and Q.D. prepared Figures 1-3. Y.M. and Z.W. contributed to the experimental setup and data collection. Z.Z. provided technical guidance and analysis. P.L. supervised the project and reviewed the manuscript. All authors reviewed and approved the final manuscript.

Acknowledgement

The authors would like to thank the Materials Heat Treatment Laboratory, School of Mechanical Engineering, Dalian University, for helpful discussions on topics related to this work. The authors also gratefully acknowledge the support of the "Dalian University Discipline Construction—Interdisciplinary Project (DLUXK-2023-ZD-003)" for the innovative research and development of minimally invasive thoracic surgical instruments (2023-2024).

Data Availability

All data generated or analysed during this study are included in this published article.

Wu, T., Blawert, C., Serdechnova, M. & Zheludkevich, M. L. Dissimilar metal joints on macro-and micro scales: Impact on PEO processing-A review. Journal of Materials Science & Technology (2024).
Walsh, F. C. et al. Plasma electrolytic oxidation (PEO) for production of anodised coatings on lightweight metal (Al, Mg, Ti) alloys. Transactions of the IMF 87, 122–135 (2009).
Jadhav, P., Bongale, A. & Kumar, S. A review of process characteristics of plasma electrolytic oxidation of aluminium alloy. J. Phys.: Conf. Ser. 1854, 012030 (2021).
Dudareva, N. Y., Kruglov, A. & Gallyamova, R. Structure and Thermophysical Properties of Coatings Formed by the Method of Microarc Oxidation on an Aluminum Alloy AK4-1. Solid State Phenomena 284, 1235–1241 (2018).
Markov, M. et al. Study of the microarc oxidation of aluminum modified with silicon carbide particles. Russian Journal of Applied Chemistry 91, 543–549 (2018).
Fattah-Alhosseini, A., Keshavarz, M. K., Molaei, M. & Gashti, S. O. Plasma electrolytic oxidation (PEO) process on commercially pure Ti surface: effects of electrolyte on the microstructure and corrosion behavior of coatings. Metallurgical and Materials Transactions A 49, 4966–4979 (2018).
Nikoomanzari, E. et al. Impressive strides in antibacterial performance amelioration of Ti-based implants via plasma electrolytic oxidation (PEO): A review of the recent advancements. Chemical Engineering Journal 441, 136003 (2022).
Yao, W. et al. Recent advances in protective coatings and surface modifications for corrosion protection of Mg alloys. Journal of Materials Research and Technology (2024).
Yan, H. et al. Effects of Micro-arc Oxidation Process Parameters on Micro-structure and Properties of Al2O3 Coatings Prepared on Sintered 2024 Aluminum Alloy. Journal of Materials Engineering and Performance 1–12 (2023).
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 14, 217 (2024).
Sikdar, S., Menezes, P. V., Maccione, R., Jacob, T. & Menezes, P. L. Plasma electrolytic oxidation (PEO) process—processing, properties, and applications. Nanomaterials 11, 1375 (2021).
Timoshenko, A. V. & Rakoch, A. G. Method for microplasma electrolytic processing of surfaces of electroconductive materials. (2001).
Dai, W. et al. Influence of duty cycle on fatigue life of AA2024 with thin coating fabricated by micro-arc oxidation. Surface and Coatings Technology 360, 347–357 (2019).
Hao, Y. et al. Construction and growth of black PEO coatings on aluminum alloys for enhanced wear and impact resistance. Ceramics International 49, 30782–30793 (2023).
Sieber, M. et al. Wear-resistant coatings on aluminium produced by plasma anodising—A correlation of wear properties, microstructure, phase composition and distribution. Surface and Coatings Technology 240, 96–102 (2014).
Nominé, A., Martin, J., Henrion, G. & Belmonte, T. Effect of cathodic micro-discharges on oxide growth during plasma electrolytic oxidation (PEO). Surface and Coatings Technology 269, 131–137 (2015).
Kasalica, B. et al. The mechanism of evolution of microdischarges at the beginning of the PEO process on aluminum. Surface and Coatings Technology 298, 24–32 (2016).
Dmitruk, A. et al. PEO-coated aluminum alloys with good thermal conductivity for TES applications. in Advanced Ceramic Coatings for Energy Applications 213–247 (Elsevier, 2024).
Sobolev, A., Kossenko, A., Zinigrad, M. & Borodianskiy, K. Comparison of plasma electrolytic oxidation coatings on Al alloy created in aqueous solution and molten salt electrolytes. Surface and Coatings Technology 344, 590–595 (2018).
Tillous, K., Toll-Duchanoy, T., Bauer-Grosse, E., Hericher, L. & Geandier, G. Microstructure and phase composition of microarc oxidation surface layers formed on aluminium and its alloys 2214-T6 and 7050-T74. Surface and Coatings Technology 203, 2969–2973 (2009).
Hussein, R. O., Northwood, D. O. & Nie, X. The effect of processing parameters and substrate composition on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloys. Surface and Coatings Technology 237, 357–368 (2013).
Hussein, R. O., Northwood, D. O. & Nie, X. Coating growth behavior during the plasma electrolytic oxidation process. Journal of Vacuum Science & Technology A28, 766–773 (2010).

No competing interests reported.

Download PDF

Reviews received at journal
30 Oct, 2024
Reviewers agreed at journal
30 Oct, 2024
Reviewers invited by journal
30 Oct, 2024
Editor assigned by journal
30 Oct, 2024
Editor invited by journal
30 Oct, 2024
Submission checks completed at journal
28 Oct, 2024
First submitted to journal
01 Oct, 2024

You are reading this latest preprint version

Characterization of the ceramic coating formed on 2024 Al alloy by scanning plasma electrolytic oxidation

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and Methods

2.1 Preparation of SPEO coating

2.2 Coating analysis

3. Results and Discussion

3.1. Voltage-time response

3.2. SPEO coating composition analysis

3.3. Surface morphology

3.4. The corrosion properties test

3.5 Microhardness and Roughness

4. Conclusion

Declarations

Competing interest

Funding

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Status:

Version 1