Preparation of High-Entropy Silicide Coating on Tantalum Substrate by Silicon Infiltration Method and Its Antioxidant Performance

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work investigates high-entropy silicide (MeSi2) coatings and their oxidation resistance. There are some major issues to be addressed:

1. It is not very clear what is the novelty and contribution of the present work to the existing knowledge in the field. The scientific significance of the work should be clearly articulated.
2. The information in the Introduction is too generic and does not represent the state of the art in the field. The statements like: “To improve oxidation resistance, researchers have done a lot of work.” should bе supported by particular details.   
3. What are the particular applications of the investigated films. Do the layers investigated in this work satisfy the requirements?
4. XRD results:

 – It is not clear how different XRD peaks, especially those belonging to BCC and (WMoNbTaCr)Si2 were recognized;

- For easy comparison, the XRD spectrum before oxidation should be given in Fig.7b and Fig.10b

5. Could you, please explain the change of the thickness of the coating with the oxidation time (Fig.9)?
6. The calculated mass gain rate of 002 mg/cm²·h is incorrect as it is not a constant value. (It is about 10X higher during the first 10 hours).
7. The text in lines 191 -201 is repeated.
8. There are missing or incorrect units, e.g. line 80; line 242. Please define BCC.

Author Response

This work investigates high-entropy silicide (MeSi2) coatings and their oxidation resistance. There are some major issues to be addressed:

It is not very clear what is the novelty and contribution of the present work to the existing knowledge in the field. The scientific significance of the work should be clearly articulated.

Reply:

Thank you for the advice. As reported, high entropy silicide can eliminate the ‘pesting’ phenomenon usually at 400~600 ℃ appeared for the single silicides coating. The high entropy silicide has great potential for application as a high-temperature protective coating. However, there are little literature about the high entropy silicide coating applied as protect coating for Tantalum alloys. Therefore, in this study, a (Mo_0.2Cr_0.2Ta_0.2Nb_0.2W_0.2)Si₂ high-entropy silicide coating on tantalum substrate was prepared using the slurry sintering method and halide embedding infiltration method, and the oxidation behavior of the coating at 600 ℃ and 1200 ℃ were evaluated. This study will provide experimental data for the application of high entropy silicides as high-temperature protective coatings.

The information in the Introduction is too generic and does not represent the state of the art in the field. The statements like: “To improve oxidation resistance, researchers have done a lot of work.” should bе supported by particular details.

Reply:

Thank you for your advice. According to the reviewer’s advice, we have added some research results of the literatures to illustrate the research progress on the protection of tantalum. All modified places are highlighted in red.

What are the particular applications of the investigated films. Do the layers investigated in this work satisfy the requirements?

Reply:

Thank you for your question. We did oxidation experiments at 100 °C and 1200 °C. Based on the characterization results for the samples after 100 hours of oxidation at 600 °C, it can be seen that high entropy silicide ceramics indeed overcome the pulverization phenomenon of traditional single silicide and have excellent oxidation resistance, which can effectively protect tantalum and tantalum alloy. However, the oxidation results at 1200 ℃ is not very ideal, and its duration is not long enough. Further modification is needed.

XRD results:

 – It is not clear how different XRD peaks, especially those belonging to BCC and (WMoNbTaCr)Si2 were recognized; For easy comparison, the XRD spectrum before oxidation should be given in Fig.7b and Fig.10b

Reply: Thak you for your question. From Fig. 3, we can see that the BCC phase, which is the high entropy alloy formed by the metals with BCC structure, still has the BCC structure. The diffraction peaks of the high entropy silicide obtained after silicon infiltration are completely different from those of the high entropy alloy. The diffraction patterns of high entropy alloys and high entropy silicides are reported in reference literature, such as references [1] and [2], which have also been supplemented in the paper. Regarding the significant difference between the diffraction peaks of the generated oxides and the original samples before oxidation, it is deemed unnecessary to include them in the following figures (Fig.7 and Fig.10). We hope the reviewer can understand.

[1] X. Hu, X.L.Liu, D.S.Yan, Z. M. Li, A high-density non-equiatomic WTaMoNbV high-entropy alloy: Alloying behavior, microstructure and mechanical properties, J. Alloys Compd. 894(2020) 162505

[2] L. Liu, L. Zhang, D. Liu, Complete elimination of pest oxidation by high entropy refractory metallic silicide (Mo_0.2W_0.2Cr_0.2Ta_0.2Nb_0.2)Si₂, Scr. Mater. 189 (2020) 25–29

Could you, please explain the change of the thickness of the coating with the oxidation time (Fig.9)?

Reply: During the oxidation process, the silicon element from the silicide in the outer layer will combine with oxygen to form silicon oxide. With the consumption of silicon, the disilicides will transform into low silicides, so the thickness of the disilicide layer gradually decreases, while the thickness of the low silicide layer gradually increases.

The calculated mass gain rate of 002 mg/cm²·h is incorrect as it is not a constant value. (It is about 10X higher during the first 10 hours).

Reply:

Thank you for your good question. Indeed, the mass gain rate is different in different stage. At 600 ℃, the mass gain rate is bigger than that of in the later stage. At 1200 ℃, the oxidation rate is first fast, then slow, and then fast again. Regardless of oxidation at 600 ℃ or 1200 ℃, silicon dioxide is gradually generated in the initial stage, resulting in a significant increase in oxidation weight. With the complete coverage of silicon dioxide on the surface, silicon dioxide can further block the invasion of oxygen, especially the isolation of oxygen at 600 ℃, leading to the weight gain curve remain nearly horizontal. But at higher temperature of 1200 ℃, metal oxides are generated, so in the later stage, accompanied by the volatilization of some metal oxides, there is a decreasing trend in the weight gain cure.

The text in lines 191 -201 is repeated.

Reply: Thank you for the careful reminder. We have checked the manuscript carefully to avoid these small mistakes.

 There are missing or incorrect units, e.g. line 80; line 242. Please define BCC.

Reply:

Thank you for the careful reminder. We have read and checked the manuscript carefully. The BCC phase has also been explained. All modified places are highlighted in red.

Reviewer 2 Report

Comments and Suggestions for Authors

The article discusses the results of using high-entropy coatings as protective materials used to protect against high-temperature oxidation and degradation, and describes the main oxidation mechanisms that occur during exposure to high temperatures. In general, the presented results are quite interesting and relevant, and the article itself can be accepted for publication after the authors answer a number of reviewer questions that arose during its reading.
1. Is the formation of pores (see data in Figure 11) associated with oxidation processes or deformation cracking during high-temperature exposure for a long time?
2. According to the data provided, the oxidation process has a clearly non-uniform rate, which is clearly visible from the side chips of the coatings, what is this associated with?
3. According to the element distribution data, all elements are distributed equally probable, but the authors should provide their numerical values ​​(weight contributions) since these elements have different masses, which can lead to differences in their contribution to the composition of the coatings.
4. How accurate is the formula used to calculate oxide compounds, does it take into account the embrittlement effect? ​​With what accuracy were the mass changes measured?
5. The authors state that the observed silicides are oxidized to oxides during thermal exposure, with subsequent destruction of the coatings. In this regard, the authors should indicate whether the oxidation processes (formation of oxides) are accompanied by amorphization or whether the destruction occurs due to deformation effects.

Author Response

Reviewer 2

The article discusses the results of using high-entropy coatings as protective materials used to protect against high-temperature oxidation and degradation, and describes the main oxidation mechanisms that occur during exposure to high temperatures. In general, the presented results are quite interesting and relevant, and the article itself can be accepted for publication after the authors answer a number of reviewer questions that arose during its reading.
1. Is the formation of pores (see data in Figure 11) associated with oxidation processes or deformation cracking during high-temperature exposure for a long time?

Reply：Thank you for your question. The pores also present in the outermost layer of the coating before oxidation due to the incomplete densification of the powder during the sintering process. This can be seen from Fig. 6. Of course, as oxidation progresses, especially the generation of metal oxides caused volume expansion, which can also result in pores and cracks in the coating.
2. According to the data provided, the oxidation process has a clearly non-uniform rate, which is clearly visible from the side chips of the coatings, what is this associated with?

Reply:

Thank you for your question. Regardless of oxidation at 600 ℃ or 1200 ℃, silicon dioxide is gradually generated in the initial stage, resulting in a significant increase in oxidation weight. With the complete coverage of silicon dioxide on the surface, silicon dioxide can further block the invasion of oxygen, especially the isolation of oxygen at 600 ℃, leading to the weight gain curve remain nearly horizontal. But at higher temperature of 1200 ℃, metal oxides are generated, so in the later stage, accompanied by the volatilization of some metal oxides, there is a decreasing trend in the weight gain cure.
3. According to the element distribution data, all elements are distributed equally probable, but the authors should provide their numerical values (weight contributions) since these elements have different masses, which can lead to differences in their contribution to the composition of the coatings.

Reply:

Thank you for your suggestion. After being oxidized at 600 ℃, we conducted distribution of elements on the surface. From Fig. 8, the distribution of elements such as W, Mo, Ta, Nb, Si, O is relatively uniform, and deduced only a thin SiO₂ adhesion layer was formed on the surface combined with the XRD results. At the same time, there is no MoO₃ or Nb₂O₅ enrichment phase generated. During oxidation at 1200 ℃, a surface distribution scan was also performed on the oxidized sample, and the specific elemental composition is listed in Table 2. By comparing the changes in elements, possible phases were analyzed. But EDS only qualitatively analyzes the content of each element, and the composition of phases should also be determined by XRD results.

4. How accurate is the formula used to calculate oxide compounds; does it take into account the embrittlement effect? With what accuracy were the mass changes measured?

Reply: Thank you for your suggestion. Firstly, during the experiment, the sample was placed on an open ceramic boat and weighed together with the boat. In the later stage of oxidation at 1200 ℃, metal oxides produced, and some oxides may evaporate, such as the generated MoO₃. Therefore, this is also the reason why the weight gain curve shows a downward trend in the late stage of oxidation. The XRD results can determine which oxides were generated, and from EDS results, we can analyze the trend of changes in element content and can infer possible oxides based on the XRD results. The contents of generated oxides need further characterization.

5. The authors state that the observed silicides are oxidized to oxides during thermal exposure, with subsequent destruction of the coatings. In this regard, the authors should indicate whether the oxidation processes (formation of oxides) are accompanied by amorphization or whether the destruction occurs due to deformation effects.

Reply: We did not characterize the oxides by TEM. But according to the literature reports the oxides generated for the silicides during the oxidation process did not undergo amorphous transformation. Secondly, it can also be seen from the XRD pattern that there is no amorphous phase. The ultimate failure of the coating is due to the generation of metal oxides, which causes volume expansion and big cracks in the coating, then oxygen enters more quickly to accelerate oxidation, further causing coating failure. This can also be proved from the pulverization of the sample after oxidation.

Reviewer 3 Report

Comments and Suggestions for Authors

What is the main question addressed by the research? Do you consider the topic original or relevant to the field? Does it address a specific gap in the field? Please also explain why this is/ is not the case.

The work carried out is original and relevant to current literature as well as to the journal. The
manuscript outlines important coating procedures and optimization of MeSi₂. Hence, it is interesting for the researchers of oxide coatings.

- What does it add to the subject area compared with other published material?

The processing of targetted material (Mo0.2Cr0.2Ta0.2Nb0.2W0.2)Si2, its phase, composition, and formation procedures are mentioned in detail.

-What specific improvements should the authors consider regarding the methodology?

The authors should compare the MeSi₂ coating with other high-entropy silicide coatings or conventional coatings to highlight the importance of current work.

- Are the conclusions consistent with the evidence and arguments presented, and do they address the main question posed? Please also explain why this is/is not the case.

Conclusions are consistent with the results and analysis.

- Are the references appropriate?

Yes. The references are appropriate. Since a comparison of literature can be more useful, authors should consider citing relevant literature.

- Any additional comments on the tables and figures.
In the caption of Fig. 10, it should explain sections (a) and (b). (a) and (b) of Fig. (6) are missing.

Author Response

Reviewer 3

What is the main question addressed by the research? Do you consider the topic original or relevant to the field? Does it address a specific gap in the field? Please also explain why this is/ is not the case.

The work carried out is original and relevant to current literature as well as to the journal. The manuscript outlines important coating procedures and optimization of MeSi₂. Hence, it is interesting for the researchers of oxide coatings.

- What does it add to the subject area compared with other published material?

The processing of targeted material (Mo0.2Cr0.2Ta0.2Nb0.2W0.2)Si2, its phase, composition, and formation procedures are mentioned in detail.

-What specific improvements should the authors consider regarding the methodology?

The authors should compare the MeSi₂ coating with other high-entropy silicide coatings or conventional coatings to highlight the importance of current work.

Reply: The main preparation methods for high entropy silicide coatings now include the same method as in this article, silicon infiltration, and the slurry method we previously reported using silicides as materials. The literature on high entropy silica coatings is rare. Compared with the traditional single silicide coatings, the high entropy silicide coating can eliminate the ‘pesting’ phenomenon at low temperature and has better antioxidant effect due to the high entropy effect. These contents have been stated in the “Introduction” and “results and discussion” parts.

-Are the conclusions consistent with the evidence and arguments presented, and do they address the main question posed? Please also explain why this is/is not the case.

Conclusions are consistent with the results and analysis.

- Are the references appropriate? Yes. The references are appropriate. Since a comparison of literature can be more useful, authors should consider citing relevant literature.

Reply: Yes, the author has cited the relevant literature.

-Any additional comments on the tables and figures.

In the caption of Fig. 10, it should explain sections (a) and (b). (a) and (b) of Fig. (6) are missing.

Reply: Thank you for your advice, we have added expiations on Fig. 6 and Fig. 10.

Reviewer 4 Report

Comments and Suggestions for Authors

Dear authors,
You submitted the manuscript of purely applied focus. The literature review covers the subject area sufficiently. The paper  material is well structured, there are no gaps.
I would like to add the following as comments.
As far as I understand, this type of coatings based on silicides of high-entropy materials can be used in the aerospace industry. In this regard, the choice of temperature values ​​is unclear to me. Why did you choose only 2 points at 600 and 1200 degrees?
On page 7, you give a set of chemical reactions, but do not indicate the values ​​of thermodynamic functions. It is difficult to judge the mechanism of reactions and their order.
The results of scanning electron microscopy for determining the local chemical composition should be rounded to tenths.
Good luck

Author Response

Reviewer 4

Dear authors,
You submitted the manuscript of purely applied focus. The literature review covers the subject area sufficiently. The paper material is well structured, there are no gaps. I would like to add the following as comments.
1. As far as I understand, this type of coatings based on silicides of high-entropy materials can be used in the aerospace industry. In this regard, the choice of temperature values is unclear to me. Why did you choose only 2 points at 600 and 1200 degrees?

Reply: Thank you for your comments. This is a good question. 600 ℃ was chosen to investigate whether high-entropy ceramics can overcome the pesting effect at low temperatures. Given the current conditions in our laboratory, where the muffle furnace can only reach a maximum heating temperature of 1200 ℃, so we opted for this temperature. However, in practical applications for tantalum, 1200 ℃ is far from sufficient, and we plan to conduct oxidation tests at even higher temperatures in the future.

2. On page 7, you give a set of chemical reactions, but do not indicate the values of thermodynamic functions. It is difficult to judge the mechanism of reactions and their order.

Reply：The Gibbs free energy of these reactions has been mentioned in our previous publications, so it was not included here. We have also made appropriate citations in the text.

3.The results of scanning electron microscopy for determining the local chemical composition should be rounded to tenths.

Reply：Thanks for the thoughtful advice. We have changed the data according to the reviewer’s advice. As shown in Table 1 and Table 2.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

I could not see any revision of the text. The changes should be carefully marked. I see no highlighted in red text as claimed in your answer.

Author Response

Sorry, only one file can be uploaded in the system, so I only uploaded the unmarked version. This time, I will upload the marked version and hope you can provide valuable suggestions.

Reviewer 2 Report

Comments and Suggestions for Authors

Accept.

Author Response

Thank you very much.

Reviewer 4 Report

Comments and Suggestions for Authors

Dear authors,
You answered my questions. Your мanuscript looks more attractive.
Good luck

Author Response

Thank you for your suggestion. We further revised the manuscript.

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have answered to some extend my questions and remarks. However, there are still issues (coming from the revised version), e.g.:

1. Sentence “The reaction between Si and O always have low Gibbs free energy between 0 k-2000 k, and the reaction can occur spontaneously, which is much lower than that of other metal elements reacting with oxygen [18].” is very confusing. Could you explain this “k” unit? This is not the SI unit for Gibbs free energy. The wide range (three orders of magnitude) of Gibbs free energy for Si-O reaction is confusing and should be explained and referenced. The cited ref [18] has nothing to do with the statement.
2. Caption of Fig.10a – is it “cross section”?

Author Response

Comment 1: Sentence “The reaction between Si and O always have low Gibbs free energy between 0 k-2000 k, and the reaction can occur spontaneously, which is much lower than that of other metal elements reacting with oxygen [18].” is very confusing. Could you explain this “k” unit? This is not the SI unit for Gibbs free energy. The wide range (three orders of magnitude) of Gibbs free energy for Si-O reaction is confusing and should be explained and referenced. The cited ref [18] has nothing to do with the statement.

Response: Thank you very much for your attention. The K here should be capitalized, representing temperature. Moreover, the reference is 19. In our previous published work, we calculated the Gibbs free energy of the reaction between silicon and oxygen, so we did not calculate it again here.

Comment 2: Caption of Fig.10a – is it “cross section”?
Response: Thank you for your reminder. It should be more reasonable to call it surface.