Introduction
Zirconia (ZrO2) is a widely used material as it has high chemical stability at elevated temperatures. Therefore, ZrO2 is used in many applications such as batteries, thermal protective coatings, and capacitor dielectrics (1–3). Thin aluminum (Al) films are utilized to fabricate microchips, solar energy production, and mobile phones. Al thin film is used owing to its high conductivity, lightweight composition, and ease of deposition during manufacturing. Despite its advantages, Al thin film faces challenges like corrosion, especially in harsh environments with high water or chemical content (4). The occurrence of corrosion in Al thin films leads to the deterioration of the device’s performance. This can cause increased resistance towards conductivity, leading to device faults (5). Even though Al forms a thin layer of oxide over the surface, it doesn’t provide long-term corrosion protection (6–8). As a chemically stable ceramic material, ZrO2 is a potential coating material for enhancing corrosion resistance. Yang et al. (9) improved the corrosion resistance of Mg alloy through polydimethylsiloxane/ZrO2 nanoparticles coating. Here, polydimethylsiloxane acted as a polymer matrix where the ZrO2 nanoparticles improved the density of the matrix. In another study, Zhao et al. (10) improved the corrosion behavior of 316L stainless steel by coating ZrO2 through a cathode plasma electrolytic deposition technique. Owing to the dense coating of ZrO2, 316L stainless steel displayed enhanced anti-corrosive behavior in a 3.5 wt.% aqueous solution of NaCl. As a result, ZrO2 is an effective material for improving the corrosion resistance behavior of Al thin films (11). ZrO2 can work as a corrosion barrier, protecting Al from direct exposure to corrosive ions, and extending the substrate’s durability. Furthermore, due to its remarkable dielectric properties, ZrO2 is appropriate for usage in capacitors, sensors, and high-frequency devices (12). Especially, ZrO2 is an exceptional material for charge storage because of its high dielectric constant and low dielectric loss. Raj et al. (13) prepared ZrO2 nanoparticles, and its charge-storing capacity was evaluated as 246.98 Fg–1. The charge-storing efficiency was 98% even after 3000 cycles of charging and discharging. Likewise, Mansoor et al. (14) prepared ZrO2 solid-state sintering technique, and its electrochemical charge storage behavior was analyzed. The specific capacitance was found to be 546 Fg–1. Further, coating a thin film of ZrO2 poses challenges to material compatibility with the substrate, deposition techniques, uniformity, etc. Physical vapor deposition is a versatile technique where the thin film can be prepared with high quality, precise control, strong adhesion, etc., making it a better choice for depositing ZrO2.
In this study, ZrO2 thin film was sputter coated over Al-coated glass substrates. Even though ZrO2 was used as an anti-corrosive and charge storage material, its multi-functional studies have not been reported so far. Further, the influence of coating thickness on its dual functionality is focused on the present work. The electrochemical workstation was used to assess the dual functionality of the ZrO2 thin film as a corrosion-resistant and charge storage material. The results will provide insights into the potential applications of ZrO2 coating.
Methodology
Preparation of Al thin film
Al thin film was prepared by the sputtering route (Model: 12″MSPT, Planar Magnetron RF/DC sputtering unit, Hind Hivac, India) over a glass substrate. Before carrying out Al deposition, the glass substrate was cleaned using chromic acid and ultrasonicated in water. Wiped with acetone and dried. Al target of size 2.0 in. diameter was developed using 99.99% Al powder through a pelletizer by applying a 50 kN load. The sputter coating was performed using direct current (DC) mode, and the coating time was fixed as 10 min with an applied voltage of 0.4 V. Initially, the pressure of the chamber was reduced to 10–6 mbar, and then by admitting Ar as a plasma-producing gas, the pressure was raised to 10–3 mbar. While switching on the DC power, the plasma was produced, resulting in the sputtering of Al atoms from the target and simultaneous deposition of Al on the pre-cleaned glass substrate fixed opposite to the Al target.
Coating of ZrO2 thin film
For coating the ZrO2 thin film over the Al substrate, the Al target was replaced with the ZrO2 target. The procedure was repeated as in the case of Al sputtering. Here, the coating time was fixed as 20 min, 40 min, and 60 min. During the sputtering process, the ZrO2 thin film was formed over the Al film. The mass of the active material coated over Al thin film is recorded as 0.14 mg, 0.27 mg, and 0.46 mg for 20 min, 40 min, and 60 min coated samples, respectively. Further, the coated specimens were heated at 200 °C in an inert atmosphere to improve the adherence of ZrO2 over Al.
Corrosion-resistant studies using Tafel analysis
3.5 wt% NaCl was dissolved in 100 ml of distilled water and used as a corrosive medium. The working electrode (ZrO2/Al), reference electrode (Ag/AgCl), and counter electrode (Pt) were immersed in the corrosive medium. Initially, open circuit potential was performed for 30 min to stabilize the working electrode in the electrolyte. Further, Tafel analysis was performed within the potential window of −1 V to 1 V where the corrosion potential (Ecorr) and pitting corrosion potential (Epit) values were elucidated (15). Further, corrosion protection efficiencies of the coating were evaluated using the formula,
where, icorr—current density of the coated specimen, —current density of uncoated specimen.
Charge storage behavior using CV measurements
The charge-storing performances of the prepared ZrO2/Al thin films were evaluated using CV measurements. Here, 3 electrode systems were used where 0.1 M sodium sulfate was used as an electrolyte, recorded in the potential window −0.3 V to 0.3 V at various scan rates from 5 mV to 200 mV. Further, the specific capacitance (Csp) of the prepared electrode was calculated using the formula (16),
Results and discussion
Figure 1 displays the XRD pattern of ZrO2/Al (60 min) specimen; the XRD pattern displayed a wide band at 2θ = 24°, indicating an amorphous nature, which was attributed to the amorphous glass substrate. The diffraction peaks at 2θ = 38°, 45°, 65°, and 78.54°, respectively, which verified the existence of Al crystalline phases through JCPDS card no.04-0787 (17). From the XRD pattern, it was clear that the peaks corresponding to the ZrO2 phase were not seen. This may be owing to the fact that the ZrO2 film thickness was very low, where the high-intensity Al peaks diminish the low-intensity ZrO2 peaks (18).
Figure 1. X-ray diffracted pattern of ZrO2/Al (60 min).
Figure 2 displays the SEM image of ZrO2/Al (60 min) where the image was recorded at an accelerating voltage of 20 kV and a working distance of 12.5 mm. Captured at a magnification of 50,000×, the SEM image revealed intricate details of the film. From the topographical image, it is clear that the ZrO2 was coated over the Al film in a particle aggregate manner where the particles are found to be near spherical-shaped structures with a scale range of 100–150 nm. Even though ZrO2 was found as particle aggregates, they were found uniformly all over the coated area.
Figure 2. SEM Analysis of ZrO2/Al.
The corrosion behavior of Al thin film sputter coated with ZrO2 for different time intervals like 20 min, 40 min, and 60 min was studied using Tafel analysis against a 3.5% NaCl corrosive medium (Figure 3). Here, the prepared samples, such as bare Al, ZrO2/Al (20 min), ZrO2/Al (40 min), and ZrO2/Al (60 min) were used as the working electrode in a 3-electrode system (19).
Figure 3. Tafel analysis of (a) bare Al, (b) ZrO2/Al (20 min), (c) ZrO2/Al (40 min), and (d) ZrO2/Al (60 min).
The uncoated Al displayed an Ecorr of −0.83 V and Epit of −0.53 V, substantiating high corrosion and pitting occurrence. From the figure, it was clear that the corrosion inhibition of Al improved with increased coating duration of ZrO2. An Ecorr of −0.626 V and Epit of −0.52 V were obtained for the sample after a 20 min ZrO2 deposition. After a 40 min ZrO2 deposition, there was a further improvement in anti-corrosion behavior, with the Ecorr of −0.597 V and the Epit of −0.426 V. The sample with a ZrO2 deposition of 60 min showed an enhanced anti-corrosive nature where the Ecorr was −0.5 V and Epit was −0.41 V, demonstrating a significant increase in inhibition against corrosion. These results demonstrate how ZrO2 coatings hinder the corrosion behavior of Al. As the coating duration increases, ZrO2 thickness increases, thereby effectively restricting the corrosive ions from reaching the underlying Al substrate (20).
ZrO2 coatings are hard and can create a homogenous and inert thin film on the Al surface. This layer acts as a barrier that hinders the Al surface from corrosive ions. The ZrO2 coating inhibits the electrochemical corrosion process by inhibiting corrosive ions from contacting the Al surface. ZrO2 has good resistance to chemicals owing to its stability in acidic and mildly alkaline conditions. This characteristic allows the coating to remain intact and does not deteriorate when subjected to a corrosive environment; hence, it can preserve its protective efficacy over time.
Further, the corrosion protection efficiency (η%) of the prepared specimens was evaluated (21). η% were evaluated as 89%, 89%, and 92% for ZrO2/Al (20 min), ZrO2/Al (40 min), and ZrO2/Al (60 min), respectively. These results also support the fact that the ZrO2/Al (60 min) displayed better anti-corrosive behavior.
The supercapacitive behavior of ZrO2 coated over Al film at various coating intervals was examined employing CV measurements utilizing CHI660new software. Figure 4 displays the CV graphs of the prepared samples and their respective Csp in F/g against 0.1 M Na2SO4 solution. Among various samples, the ZrO2/Al (20 min) had higher Csp, reaching 1223 F/g at a scan rate of 25 mV/s. The significant charge-storage capability demonstrated via the elevated Csp was probably caused by the synergistic effect of ZrO2 topography along with the large surface area that the 20 min ZrO2 deposition created. Also, it is known that the reduction in the thickness of the dielectric material increases the Csp. By comparison, ZrO2/Al samples with 40 min and 60 min deposition time resulted in lower Csp of 336 F/g and 174 F/g at 25 mV/s, respectively. As time increases, the ZrO2 thickness increases, leading to low Csp. The thicker and denser ZrO2 dielectric layer in these samples may have blocked ion movement and reduced the available area for charge storage, which could explain the lower performance. This fact implies that the 20 min coating offers the highest combination regarding surface area, ionic availability, and electrolytic operation, which makes it the most promising candidate for high-performance charge storage electrodes. The Csp declined as the rate of scanning increased, which is expected since a high scan rate restricts the number of ions that may diffuse into an electrode’s substance (22). Since the ZrO2 was coated using a physical vapor deposition route, which is well known for its coating stability and adherence. ZrO2 coatings often demonstrate superior adherence to the Al substrate, guaranteeing the integrity of the coating despite mechanical stress or temperature variations due to the application of the physical vapor deposition method. This adherence reduces the likelihood of coating delamination, improving the overall performance of ZrO2/Al thin films. Further, in settings characterized by mechanical wear, the exceptional hardness and abrasion resistance of ZrO2 guarantee the efficacy of the protective layer. If the ZrO2 coating is stable, then there will be a high cycling ability having a high retention rate.
Figure 4. CV and Csp vs. scan rate plots of (a, d) ZrO2/Al (20 min), (b, e) ZrO2/Al (40 min), and (c, f) ZrO2/Al (60 min).
Conclusion
The present study highlights the dual functioning of ZrO2/Al thin films, exhibiting anticorrosive and supercapacitive characteristics. The results substantiated that the deposition time influences the performance, where the ZrO2/Al (60 min) coating displayed improved anticorrosion efficacy and the ZrO2/Al (20 min) coating exhibited the highest specific capacitance (1223 F/g). The findings indicate significant industrial potential for ZrO2/Al coatings, especially in aerospace, automotive, and renewable energy sectors, where lightweight materials with integrated corrosion resistance and energy-storage capabilities are essential. Future studies should concentrate on enhancing deposition techniques, such as roll-to-roll sputtering, to facilitate industrial manufacturing, while also assessing the long-term durability of these coatings under extreme environmental conditions, including elevated temperatures, salt, and fluctuating pH levels. This research was an attempt to tackle global issues in energy storage and material durability in advanced engineering fields.
Acknowledgments
I would like to sincerely thank the principal of AAA College of Engineering and Technology for his continual support and encouragement. Additionally, I wish to express my sincere gratitude to the institution’s management for establishing a supportive atmosphere for research. We particularly want to acknowledge the Head of the Departments (HoDs) of Science and Humanities as well as Electrical and Electronics Engineering for their invaluable advice and knowledge during this research. I am also grateful to my co-authors for their support and encouragement.
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