Formation of Alpha-Al₂O₃ Coatings on Tungsten Substrate by Plasma Electrolytic Oxidation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is devoted to the plasma electrolytic oxidation (PEO) of tungsten substrate and will be of interest to readers. However, there are a number of questions and comments regarding the manuscript.
1) Section "Introduction". Why was it necessary to anodize the surface of metallic tungsten? Why is it necessary to replace the WO3 with aluminum oxide?
2) Section "Results". There is a lack of theoretical explanations of the occurring chemical processes. For example, it is worth explaining to readers why Na2O and P2O5 oxides are not deposited on the tungsten surface during the process.
3) Fig. 2a  is of poor quality and does not allow one to evaluate the porosity or tendency to cracking of the resulting coatings.
4) Tables 1 and 2. The EDS results are averaged from how many measurements?
5) Table 2. The column headings are shifted. This must be corrected.
6) List of references. 16 references may not be enough to form a literature review and publish in a journal.

Author Response

The manuscript is devoted to the plasma electrolytic oxidation (PEO) of tungsten substrate and will be of interest to readers. However, there are a number of questions and comments regarding the manuscript.

1) Section "Introduction". Why was it necessary to anodize the surface of metallic tungsten? Why is it necessary to replace the WO3 with aluminum oxide?

Answer: Anodized oxide layers on tungsten substrates, especially WO₃, are very interesting due to their numerous applications in photoelectrochemistry, photocatalysis, water splitting, hydrogen and ethanol sensors, lithium-ion batteries, supercapacitors and other areas. The preparation of WO₃ layers for photocatalysis was our original motivation for PEO research on tungsten. Our attempts to apply PEO on tungsten in the electrolytes widely used in this technology, including water solutions with potassium or sodium hydroxide, sodium silicate, sodium phosphate, sodium aluminate, sodium tungstate and some acidic solutions, failed because these electrolytes were not able to provide the high voltages required for micro-discharges. We used a water solution containing 2 g/L Na₃PO₄×12H₂O and varying concentrations of NaAlO₂ to achieve high voltages for micro-discharges. Al₂O₃ is the primary oxide layer formed in these electrolytes. We have added this in the revised text.

2) Section "Results". There is a lack of theoretical explanations of the occurring chemical processes. For example, it is worth explaining to readers why Na2O and P2O5 oxides are not deposited on the tungsten surface during the process.

Answer: We think that the formation of Na₂O in the PEO coatings was not to be expected. The EDS analysis also does not show the presence of Na in the formed coatings. P₂O₅ can probably be formed during the PEO, but we did not identify it in the XRD patterns. We have included the following test in the revised text:

It is also possible that PO₄^3- was ionized to P₂O₅ in the molten oxide [21]:

PO₄^3-  → 2P₂O₅ + 3O₂ + 12e^-                                     (9)

but P₂O₅ can be easily hydrolyzed to PO₄^3- [21]:

P₂O₅ + 3H₂O → PO₄^3- + 6H+                                        (10)

 

3) Fig. 2a is of poor quality and does not allow one to evaluate the porosity or tendency to cracking of the resulting coatings.

Answer: Figure 2a shows the typical morphologies of the formed coatings. It is very difficult to estimate the porosity of the formed coatings, as they are only slightly porous and show numerous molten areas. We have replaced Figure 2a with a new one showing the morphologies of the formed coatings at a lower magnification.

 

4) Tables 1 and 2. The EDS results are averaged from how many measurements?

Answer: The results of the EDS analysis of the surface in Figures 2a and 4a are shown in Tables 1 and 2 (the relative errors are less than 5%). Other places on the formed layers provide very similar results.

 

5) Table 2. The column headings are shifted. This must be corrected.

Answer: Corrected

 

6) List of references. 16 references may not be enough to form a literature review and publish in a journal.

Answer: We have extended the list of relevant references for this work.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript investigates the formation of alpha-Al₂O₃ coatings on tungsten substrates using plasma electrolytic oxidation (PEO) in a phosphate alkaline electrolyte (PAE) containing different concentrations of NaAlO₂. The study systematically examines the influence of NaAlO₂ concentration on the morphology, thickness, chemical composition, and crystallinity of the coatings. A combination of SEM/EDS and XRD analysis is used to characterize the coatings, demonstrating that an increase in NaAlO₂ concentration promotes the transformation of amorphous Al₂O₃ into gamma- and alpha-Al₂O₃ phases. The research highlights the formation of well-crystallized alpha-Al₂O₃ coatings in PAE with 4 g/L NaAlO₂, which enhances their suitability for high-temperature applications.

1. The choice of current density (400 mA/cm²) and oxidation time (10 min) lacks sufficient justification. Were different current densities tested to evaluate their effect on micro-discharge intensity and coating crystallinity? How was 10 min determined as the optimal oxidation time for forming alpha-Al₂O₃? Did longer oxidation times result in further phase transformations or excessive coating thickness?

2. The voltage-time curves are presented (Figure 1), but their interpretation is superficial. A more detailed discussion of breakdown voltage, micro-discharge evolution, and electrolyte conductivity effects on coating formation would enhance clarity. How do the breakdown voltages compare to previous studies on PEO of tungsten or aluminum?

3. The phase transformation sequence (amorphous → gamma → alpha-Al₂O₃) is described, but no quantitative data on phase fractions is provided. XRD peak intensities could be analyzed using Rietveld refinement to estimate the relative amounts of gamma- and alpha-Al₂O₃ in the coatings. Were any in-situ high-temperature XRD or DSC/TGA experiments performed to validate the temperature thresholds for phase transformation?

4. The study demonstrates successful coating formation, but no mechanical tests (e.g., microhardness, wear resistance, adhesion strength) were performed. Given the relevance of alpha-Al₂O₃ for high-temperature applications, a hardness or thermal stability analysis would significantly strengthen the manuscript. Has the thermal expansion mismatch between the Al₂O₃ coating and tungsten substrate been evaluated? This could affect coating durability under thermal cycling.

5. The conclusion states that these coatings could be used for high-temperature applications, but no direct comparison is made with existing high-temperature coatings (e.g., CVD or sol-gel-derived Al₂O₃ coatings). A discussion of how PEO-derived Al₂O₃ coatings compare in performance (e.g., thermal resistance, stability, adhesion) with other oxide coating techniques would improve the manuscript’s impact.

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Author Response

The manuscript investigates the formation of alpha-Al₂O₃ coatings on tungsten substrates using plasma electrolytic oxidation (PEO) in a phosphate alkaline electrolyte (PAE) containing different concentrations of NaAlO₂. The study systematically examines the influence of NaAlO₂ concentration on the morphology, thickness, chemical composition, and crystallinity of the coatings. A combination of SEM/EDS and XRD analysis is used to characterize the coatings, demonstrating that an increase in NaAlO₂ concentration promotes the transformation of amorphous Al₂O₃ into gamma- and alpha-Al₂O₃ phases. The research highlights the formation of well-crystallized alpha-Al₂O₃ coatings in PAE with 4 g/L NaAlO₂, which enhances their suitability for high-temperature applications.

1. The choice of current density (400 mA/cm²) and oxidation time (10 min) lacks sufficient justification. Were different current densities tested to evaluate their effect on micro-discharge intensity and coating crystallinity? How was 10 min determined as the optimal oxidation time for forming alpha-Al₂O₃? Did longer oxidation times result in further phase transformations or excessive coating thickness?

Answer: The current density of 400 mA/cm² was chosen because the micro-discharges takes place in a short time. Of course, we have also tried lower current densities, but it takes a very long time for a micro-discharge to occur, or it was generally not possible to achieve sufficiently high voltages to cause a micro-discharge. A PEO time of 10 minutes proved to be optimal. With a longer time, the PEO process is no efficient and due to the large thickness of the coatings micro-discharge begin to quench.

 

2. The voltage-time curves are presented (Figure 1), but their interpretation is superficial. A more detailed discussion of breakdown voltage, micro-discharge evolution, and electrolyte conductivity effects on coating formation would enhance clarity. How do the breakdown voltages compare to previous studies on PEO of tungsten or aluminum?

Answer: Thank you for this helpful comment. In the revised paper we have expanded the discussion. We have added the text:

The increasing tendency of the potential during anodization is related to the current density distribution. The total current density is the sum of the ion and electron current densities [17]. At the beginning of anodization, the electric field strength remains constant at a certain current density, while the ion current is two to three orders of magnitude greater than the electronic component. In order to keep a constant electric field strength, the anodizing voltage must increase linearly with increasing layer thickness. In addition, during anodization, electrons are injected into the conduction band of the anodic oxide and accelerated by the electric field, which leads to avalanches via an impact ionization mechanism [17]. When the electronic current of the avalanche reaches a critical level, breakdown occurs [18]. Thereafter, a low voltage is required to maintain the same total current density, since the electron current density is independent of the thickness of the anodic oxide layer. Finally, the electron current density component dominates the total current density. The total current density is almost independent of the thickness of the anodic oxide layer, and the voltage-time slope is almost zero.

 

It is very difficult to compare the breakdown voltages of different metals. This is because the breakdown voltage depends on the type of metal, the electrolyte used, the anodizing conditions, etc. For the PEO process on tungsten, there is so far only one paper that we have recently published (Stojadinović, S.; Nelson, P. Plasma electrolytic oxidation of tungsten. Mater. Lett. 2024, 365, 136427), using an electrolyte of 2 g/L Na₃PO₄×2H₂O + 2 g/L NaAlO₂ and a current density of 350 mA/cm². There is a lot of work on PEO on aluminum and it is difficult to compare the breakdown voltages. We are not aware of any paper on the PEO process for aluminum in the electrolytes used in the work.

 

3. The phase transformation sequence (amorphous → gamma → alpha-Al₂O₃) is described, but no quantitative data on phase fractions is provided. XRD peak intensities could be analyzed using Rietveld refinement to estimate the relative amounts of gamma- and alpha-Al₂O₃ in the coatings. Were any in-situ high-temperature XRD or DSC/TGA experiments performed to validate the temperature thresholds for phase transformation?

Answer: We have no experimental possibilities to do in situ high-temperature XRD or DSC/TGA experiments. We believe that these investigations go far beyond the scope of this work. We have added the text:

Sharp and intense XRD peaks of alpha-Al₂O₃ show the good crystallinity of the formed coating in PAE + 4 g/L NaAlO₂.The phase ratio of alpha and gamma Al₂O3 in the coatings formed in PAE with the addition of 4 g/L NaAlO₂ is about 87:13, based on the comparison of the integrated intensities of alpha and gamma diffraction peaks.

 

4. The study demonstrates successful coating formation, but no mechanical tests (e.g., microhardness, wear resistance, adhesion strength) were performed. Given the relevance of alpha-Al₂O₃ for high-temperature applications, a hardness or thermal stability analysis would significantly strengthen the manuscript. Has the thermal expansion mismatch between the Al₂O₃ coating and tungsten substrate been evaluated? This could affect coating durability under thermal cycling.

Answer: This is a very good commentary. Unfortunately, we are not in a position to perform a thermal stability test. We think that the results presented are a good basis for other research groups working on the PEO process and investigating the mechanical and thermal properties.

 

5. The conclusion states that these coatings could be used for high-temperature applications, but no direct comparison is made with existing high-temperature coatings (e.g., CVD or sol-gel-derived Al₂O₃ coatings). A discussion of how PEO-derived Al₂O₃ coatings compare in performance (e.g., thermal resistance, stability, adhesion) with other oxide coating techniques would improve the manuscript’s impact.

Answer: We fully agree with the reviewer. However, as we have already explained in the previous reply, we are not in a position to carry out the above research. Our further research in this area will be related to doping alpha-Al₂O₃ coatings with ions of rare earths and transition metals for application in high temperature luminescence.

Reviewer 3 Report

Comments and Suggestions for Authors

This paper investigated the effects of different NaAlO2 concentration on the alpha-Al2O3 coatings formation on a tungsten substrate by a PEO process. The technological and coating structure prperties had been analyzed. The research ideas and results presented herein are of high reference value for subsequent studies on such materials.

Specific suggestions are as follows:

- The last paragraph of the introduction is probably out of place and should be before the purpose of the study.
- The peaks in Figure 3 should be indexed.
- The images in Figure 4 lack size. In their current state, it is difficult to see anything on them. It is worthwhile to arrange them differently and make them larger.
- The images representing the surface in Figure 4 look rather uninformative. At the same time, the roughness of the formed coating, and the dependence of roughness on the parameters of coating formation would be more representative. Similar data could be presented for characterisation of the initial substrate.

Author Response

This paper investigated the effects of different NaAlO2 concentration on the alpha-Al2O3 coatings formation on a tungsten substrate by a PEO process. The technological and coating structure properties had been analyzed. The research ideas and results presented herein are of high reference value for subsequent studies on such materials.

Specific suggestions are as follows:

- The last paragraph of the introduction is probably out of place and should be before the purpose of the study.

Answer: Thank you for this comment. We have corrected the text according to the reviewer's suggestion.

- The peaks in Figure 3 should be indexed.

Answer: We have identified all the most intense diffraction peaks. We were not able to associate some low intensity diffraction peaks with crystalline phases that can be obtained during PEO.

- The images in Figure 4 lack size. In their current state, it is difficult to see anything on them. It is worthwhile to arrange them differently and make them larger.

Answer: SEM images of surfaces formed in a short time are very smooth and it is difficult to recognize anything on them. We had a dilemma whether to include Figure 4a, but we included it in the manuscript anyway because of the EDS analysis.

 

- The images representing the surface in Figure 4 look rather uninformative. At the same time, the roughness of the formed coating, and the dependence of roughness on the parameters of coating formation would be more representative. Similar data could be presented for characterisation of the initial substrate.

Answer: We are not able to assess the surface roughness from the SEM images. We could probably obtain this data from AFM images. At the moment we are not in a position to provide AFM measurements, but if the reviewer considers this data necessary, we will try to find a suitable instrument.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

There are still some questions about the practical application of such a material. The authors should more clearly present the answer to the question of where it is possible to use a W rod with an aluminum oxide coating.

Author Response

There are still some questions about the practical application of such a material. The authors should more clearly present the answer to the question of where it is possible to use a W rod with an aluminum oxide coating.

Answer: We have added the following text in the revised manuscript:

Al₂O₃ coatings formed by PEO on aluminum and its alloys substrates exhibit excellent mechanical and thermal properties, depending on the substrate and electrolyte used [27-31]. This indicates that alpha-Al₂O₃ coatings on tungsten substrate can be used wherever a high degree of hardness, thermal stability and wear resistance is required. These coatings can also serve as protective coatings for tungsten, as tungsten has poor oxidation resistance at high temperatures (especially above 800 ^oC) [32]. Alpha-Al₂O₃ coatings on tungsten are promising candidates for high-temperature luminescence thermometry due to their high refractive index and high transparency from the ultraviolet to the near infrared region, which makes them suitable hosts for rare earths and transition metal ions [33, 34].

Reviewer 2 Report

Comments and Suggestions for Authors

The revised manuscript presents significant improvements in the discussion of the electrochemical process, particularly regarding the justification of the chosen current density and oxidation time, as well as the interpretation of the voltage-time curves. The explanation provided for the selection of a current density of 400 mA/cm², based on the rapid onset of micro-discharges, is reasonable and justified by experimental observations. The discussion on the influence of oxidation time is also improved, with a clear explanation of how prolonged exposure leads to a decrease in efficiency due to the quenching of micro-discharges caused by excessive coating thickness.

Additionally, the interpretation of the voltage-time curves has been expanded to include a more detailed discussion on the role of current density distribution, impact ionization, and the transition from ion current dominance to electron current dominance. The clarification regarding breakdown voltage is appreciated, though it is acknowledged that direct comparisons with other metals are complex due to variations in electrolyte composition and anodizing conditions.

One notable improvement is the inclusion of quantitative data on the phase composition of the coatings. The estimated alpha-to-gamma Al₂O₃ ratio of 87:13, based on the integrated intensities of XRD peaks, provides a useful insight into the phase transformation process. However, the absence of a more rigorous Rietveld refinement analysis limits the precision of this estimation. While the authors state that in-situ high-temperature XRD or DSC/TGA experiments are beyond the scope of this study, such data would provide valuable confirmation of the transformation temperatures of amorphous alumina into gamma and alpha phases. A more in-depth discussion referencing relevant literature on this topic could strengthen the manuscript.

Despite these improvements, certain critical aspects remain insufficiently addressed. The manuscript lacks experimental data on the mechanical properties of the coatings, such as hardness, wear resistance, and adhesion strength. Given the proposed high-temperature applications, these characteristics are essential for evaluating the coating’s long-term durability and performance. The authors acknowledge the importance of such tests but indicate that they are unable to conduct them. However, at the very least, a discussion incorporating relevant studies on the mechanical and thermal properties of similar PEO coatings should be included.

Furthermore, the manuscript does not provide a comparative analysis of PEO-derived Al₂O₃ coatings with coatings produced using alternative techniques such as chemical vapor deposition (CVD) or sol-gel methods. While the authors indicate that they cannot perform direct experimental comparisons, a discussion on the advantages and limitations of PEO relative to other deposition techniques would significantly enhance the impact of the study.

Overall, while the revised manuscript demonstrates progress in addressing several key points raised by the reviewer, the absence of experimental validation of mechanical and thermal properties, as well as the lack of comparative discussion with other coating technologies, still represent major limitations. To further improve the manuscript, the authors should strengthen the discussion on the mechanical and thermal performance of PEO coatings, referencing existing literature where experimental data is unavailable. Additionally, a more detailed comparison with other Al₂O₃ deposition techniques would provide valuable context for evaluating the potential advantages of PEO in high-temperature applications. Addressing these aspects would enhance the completeness of the study and its suitability for publication.

Author Response

The revised manuscript presents significant improvements in the discussion of the electrochemical process, particularly regarding the justification of the chosen current density and oxidation time, as well as the interpretation of the voltage-time curves. The explanation provided for the selection of a current density of 400 mA/cm², based on the rapid onset of micro-discharges, is reasonable and justified by experimental observations. The discussion on the influence of oxidation time is also improved, with a clear explanation of how prolonged exposure leads to a decrease in efficiency due to the quenching of micro-discharges caused by excessive coating thickness.

Additionally, the interpretation of the voltage-time curves has been expanded to include a more detailed discussion on the role of current density distribution, impact ionization, and the transition from ion current dominance to electron current dominance. The clarification regarding breakdown voltage is appreciated, though it is acknowledged that direct comparisons with other metals are complex due to variations in electrolyte composition and anodizing conditions.

One notable improvement is the inclusion of quantitative data on the phase composition of the coatings. The estimated alpha-to-gamma Al₂O₃ ratio of 87:13, based on the integrated intensities of XRD peaks, provides a useful insight into the phase transformation process. However, the absence of a more rigorous Rietveld refinement analysis limits the precision of this estimation. While the authors state that in-situ high-temperature XRD or DSC/TGA experiments are beyond the scope of this study, such data would provide valuable confirmation of the transformation temperatures of amorphous alumina into gamma and alpha phases. A more in-depth discussion referencing relevant literature on this topic could strengthen the manuscript.

Answer: The main result of this work is that alpha-Al₂O₃ coatings can be formed on a tungsten substrate by a simple, cheap and environmentally friendly PEO process in a short time. It was shown that the concentration of NaAlO₂ in PAE and the duration of the process are factors that influence the phase composition of the obtained coatings. During PEO, high temperatures develop locally at the micro-discharge sites, so it is questionable whether in-situ high-temperature XRD or DSC/TGA experiments would provide useful information.

 Despite these improvements, certain critical aspects remain insufficiently addressed. The manuscript lacks experimental data on the mechanical properties of the coatings, such as hardness, wear resistance, and adhesion strength. Given the proposed high-temperature applications, these characteristics are essential for evaluating the coating’s long-term durability and performance. The authors acknowledge the importance of such tests but indicate that they are unable to conduct them. However, at the very least, a discussion incorporating relevant studies on the mechanical and thermal properties of similar PEO coatings should be included.

Answer: Thank you for this comment. We have added the following text in the revised manuscript:

Al₂O₃ coatings formed by PEO on aluminum and its alloys substrates exhibit excellent mechanical and thermal properties, depending on the substrate and electrolyte used [27-31]. This indicates that alpha-Al₂O₃ coatings on tungsten substrate can be used wherever a high degree of hardness, thermal stability and wear resistance is required. These coatings can also serve as protective coatings for tungsten, as tungsten has poor oxidation resistance at high temperatures (especially above 800 ^oC) [32]. Alpha-Al₂O₃ coatings on tungsten are promising candidates for high-temperature luminescence thermometry due to their high refractive index and high transparency from the ultraviolet to the near infrared region, which makes them suitable hosts for rare earths and transition metal ions [33, 34].

 Furthermore the manuscript does not provide a comparative analysis of PEO-derived Al₂O₃ coatings with coatings produced using alternative techniques such as chemical vapor deposition (CVD) or sol-gel methods. While the authors indicate that they cannot perform direct experimental comparisons, a discussion on the advantages and limitations of PEO relative to other deposition techniques would significantly enhance the impact of the study.

Answer: We have added following text in revised manuscript:

The formation of Al₂O₃ films or coatings on a tungsten substrate has not been extensively studied in the literature [11, 26], but these films or coatings can most likely be formed using various techniques such as sol-gel, atomic layer deposition (ALD), chemical vapour deposition (CVD), etc. The PEO process offers a number of advantages over alternative methods of creating alpha-Al₂O₃ films/coatings on a tungsten substrate. PEO forms alpha-Al₂O₃ coatings on a tungsten substrate in a short time, skipping the process of annealing at high temperatures required to convert amorphous Al₂O₃ into a crystalline phase, as is the case with other techniques. The process is also simple, cost-effective and environmentally friendly.

 Overall, while the revised manuscript demonstrates progress in addressing several key points raised by the reviewer, the absence of experimental validation of mechanical and thermal properties, as well as the lack of comparative discussion with other coating technologies, still represent major limitations. To further improve the manuscript, the authors should strengthen the discussion on the mechanical and thermal performance of PEO coatings, referencing existing literature where experimental data is unavailable. Additionally, a more detailed comparison with other Al₂O₃ deposition techniques would provide valuable context for evaluating the potential advantages of PEO in high-temperature applications. Addressing these aspects would enhance the completeness of the study and its suitability for publication.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors responded adequately to the review requests and improved the manuscript with significant additions in critical sections. Although the lack of experimental data on mechanical properties remains a limitation, the updated discussion provides sufficient context to support the study's conclusions. The manuscript is now ready for publication.