Hot-Corrosion Behavior of Gd₂O₃–Yb₂O₃ Co-Doped YSZ Thermal Barrier Coatings in the Presence of V₂O₅ Molten Salt

Round 1

Reviewer 1 Report

 

The manuscript "Hot-corrosion behaviour of Gd2O3–Yb2O3 co-doped YSZ thermal barrier coatings in the presence of V2O5 molten salt" deals with some interesting aspects of hot corrosion in a highly functional TBC coating. In this study, Thermal barrier coatings (TBC) consisting of 3.5 mol% Yb2O3-stabilized ZrO2 co-doped with 1 mol% Gd2O3 and 1 mol% Yb2O3 (referred to as GdYb-YSZ) were fabricated by means of air plasma spraying. The as-fabricated coatings exhibited a metastable tetragonal (t´) structure. The hot-corrosion behaviour of the GdYb–YSZ TBCs was investigated at 700, 800, 900, and 10 1000 °C for 10 h in the presence of V2O5 molten salt. Results showed that the GdYb–YSZ TBCs exhibited improved corrosion resistance to V2O5 molten salt when compared to YSZ TBCs, and the related mechanism is discussed in detail in this paper. I enjoyed reading this article and I do recommend it for publication. The manuscript, however, requires some mandatory revisions before being considered for publication.

1.     The paper is very rich in terms of data from the surface. But, when it comes to the cross-section, there is hardly any information given in this manuscript. This is very important to look at the cross-section of coating is such oxidation experiments. Please make sure you refer to the following articles as they contain useful information on the methodology of studying the cross-section of a coating. For instance, it is good to provide some data from the diffusion of elements towards the substrate.

-        Yang, J., Bai, S., Sun, J., Wu, H., Sun, S., Wang, S., Li, Y., Ma, W., Tang, X., Xu, D. (2023). Microstructural understanding of the oxidation and inter-diffusion behavior of Cr-coated Alloy 800H in supercritical water. Corrosion Science, 211, 110910. doi: https://doi.org/10.1016/j.corsci.2022.110910

-        Yang, J., Shang, L., Sun, J., Bai, S., Wang, S., Liu, J., Yun, D., Ma, D. (2023). Restraining the Cr-Zr interdiffusion of Cr-coated Zr alloys in high temperature environment: A Cr/CrN/Cr coating approach. Corrosion Science, 214, 111015. doi: https://doi.org/10.1016/j.corsci.2023.111015

 

2.     Please add more discussion and contents on the implication of secondary oxide particles for the adhesion and mechanical properties of the coating. Please consult the following manuscript:

-        Zhang, Z., Yang, Q., Yu, Z., Wang, H., & Zhang, T. (2022). Influence of Y2O3 addition on the microstructure of TiC reinforced Ti-based composite coating prepared by laser cladding. Materials characterization, 189. doi: 10.1016/j.matchar.2022.111962

 

3.     Please re-write the conclusion in bullet-points, with each part highlighting one of your findings.

4.     Did you perform your oxidation tests, based on a standard? If so, please specify that in your manuscript.

5.     If available, please provide data from variation of weight with time.

6.     Please report your data in Table 2 with 1 digit accuracy. Like 16.1 instead of 16.15 at.%.

7.     Please provide more information from the initial powders, like the average size, …

8.     Please mention the thickness of the coating.

 

 

Author Response

Dear Editors and reviewers

Thank you and the reviewers very much for your kind comments and patient corrections on our manuscript titled on " Hot-corrosion behaviour of Gd₂O₃–Yb₂O₃ co-doped YSZ thermal barrier coatings in the presence of V₂O₅ molten salt (coatings-239540)”.

According to the comments and suggestion of reviewers and editors, we have carefully revised our paper. The corrections of the manuscript are highlighted in red through the text of the manuscript.

The corrections and responses to the comments are given as follows:

Reviewer 1#

1. The paper is very rich in terms of data from the surface. But, when it comes to the cross-section, there is hardly any information given in this manuscript. This is very important to look at the cross-section of coating is such oxidation experiments. Please make sure you refer to the following articles as they contain useful information on the methodology of studying the cross-section of a coating. For instance, it is good to provide some data from the diffusion of elements towards the substrate.

-        Yang, J., Bai, S., Sun, J., Wu, H., Sun, S., Wang, S., Li, Y., Ma, W., Tang, X., Xu, D. (2023). Microstructural understanding of the oxidation and inter-diffusion behavior of Cr-coated Alloy 800H in supercritical water. Corrosion Science, 211, 110910. doi: https://doi.org/10.1016/j.corsci.2022.110910

-        Yang, J., Shang, L., Sun, J., Bai, S., Wang, S., Liu, J., Yun, D., Ma, D. (2023). Restraining the Cr-Zr interdiffusion of Cr-coated Zr alloys in high temperature environment: A Cr/CrN/Cr coating approach. Corrosion Science, 214, 111015. doi: https://doi.org/10.1016/j.corsci.2023.111015

Response: Thank you for your positive comments on our paper. According to the reviewer’s suggestion, we have referred to the above articles (the updated reference 27 and 28). They are very nice, and the authors have done excellent work. We have added these two articles in our revised text. In our study, we employ the m phase content resulting from V₂O₅ corrosion to determine the corrosion resistance of the coatings. Hence, the surface morphology observations are enough. Hope you could understand!

 

2. Pease add more discussion and contents on the implication of secondary oxide particles for the adhesion and mechanical properties of the coating. Please consult the following manuscript:

-        Zhang, Z., Yang, Q., Yu, Z., Wang, H., & Zhang, T. (2022). Influence of Y₂O₃ addition on the microstructure of TiC reinforced Ti-based composite coating prepared by laser cladding. Materials characterization, 189. doi: 10.1016/j.matchar.2022.111962

Response: By consulting the above article (the updated reference 39), we have refined our thinking, and this article has been added in our revised text. The main corrosion product influencing the properties of the coating is the m phase, the formation of which is accompanied with large volume expansion. Accordingly, large stress would be creased in the coating, which could cause coating spallation. We have added this discussion in the text. 

 

3. Please re-write the conclusion in bullet-points, with each part highlighting one of your findings.

Response: We have re-write the conclusion in bullet-points.

In this study, GdYb-YSZ TBCs fabricated through APS were examined for their hot-corrosion behavior when subjected to V₂O₅ molten salt at 700-1000 °C for 10 hours, and following conclusions could be obtained.

(1) GdYb-YSZ TBCs experienced some degree of attack by V₂O₅, but showed better corrosion resistance compared to YSZ TBCs. This improved resistance can be attributed to the higher rare-earth content and lower basicity of Yb₂O₃.

(2) The as-fabricated GdYb-YSZ TBCs contained t´ phase, and even after hot corrosion, the t´ phase was still detectable on the corroded surfaces, indicating that the coatings had a certain level of resistance to phase decomposition. As the temperature of corrosion increased, the quantity of t´ phase decreased, while the amount of m phase increased.

(3) In addition to the m phase, Yb- and Gd-doped YVO₄ were generated as corrosion products for the GdYb-YSZ TBCs in V₂O₅. Higher temperatures had no effect on the type of corrosion products but changed the morphologies of Yb- and Gd-doped YVO₄ crystals.

 

4. Did you perform your oxidation tests, based on a standard? If so, please specify that in your manuscript.

Response: We performed our oxidation tests based on literature reported by many researchers in this field. Please see the Refs 18-22.

 

5. If available, please provide data from variation of weight with time.

Response: The reviewer has proposed a professional comment. However, we have some considerations as followings. In this study, the main indicator to determine whether the GdYb-YSZ TBCs have a better corrosion resistance than YSZ TBCs is the m phase content resulting from V₂O₅ corrosion. Lower m phase content means better corrosion resistance. Hence, we calculated the m phase content, and the weight change with time was not necessary.

 

6. Please report your data in Table 2 with 1 digit accuracy. Like 16.1 instead of 16.15 at.%.

Response: Thanks for your kind suggestion. We have updated Table 2 as following.

Table 2. Chemical compositions of regions A-J in Figures 4-7 (in at.%)

	Y	Yb	Gd	Zr	V	O
A	16.3	1.1	1.2	—	21.1	60.3
B	2.0	—	0.6	46.6	—	51.8
C	2.3	1.1	0.8	46.4	—	49.4
D	16.4	1.1	1.4	—	20.7	60.4
E	1.9	—	0.6	47.9	—	49.6
F	2.5	0.9	0.8	45.4	—	50.4
G	16.0	0.9	1.0	—	19.8	62.3
H	2.1	—	0.7	49.3	—	47.9
I	17.1	1.4	2.0	—	22.6	56.9
J	1.9	—	0.8	48.4	—	48.9

 

 

7. Please provide more information from the initial powders, like the average size, …

Response: The initial powders for APS have a spherical shape with sizes ranging from 20 to 40 μm. We have provided this in the revised text.

8. Please mention the thickness of the coating.

Response: It is our carelessness. The GdYb-YSZ coating for corrosion tests is about 200 μm.

Sincerely yours,

Kai Liao, Ph. D

Hunan Applied Technology University

Reviewer 2 Report

The comments can be found in the attached file. 

Comments for author File: Comments.pdf

Author Response

Dear Editors and reviewers

Thank you and the reviewers very much for your kind comments and patient corrections on our manuscript titled on " Hot-corrosion behaviour of Gd₂O₃–Yb₂O₃ co-doped YSZ thermal barrier coatings in the presence of V₂O₅ molten salt (coatings-239540)”.

According to the comments and suggestion of reviewers and editors, we have carefully revised our paper. The corrections of the manuscript are highlighted in red through the text of the manuscript.

Reviewer 2#

The quality of the manuscript is enough for publication, but the following comments must be considered completely, at the first step, to guarantee its process:

Response: Thank you for your positive comments on our paper. We believe that the paper can be largely improved based on your guidance.

 

1. Please explore that why the obtained GdYb–YSZ powder is not suitable for direct thermal spraying?

Response: Our GdYb–YSZ powder was fabricated by a chemical co-precipitation and calcination method, which has small particle sizes, usually in nano level. Such small powder can not be delivered effectively during APS process. Hence, they are agglomerated into microscopic particles before thermal spray.

 

2. Please support the following comment’’ the relative intensity of the m phase increases with higher temperatures, signifying an increase in the formation of the m phase’’ in the revised version.

Response: The as-fabricated GdYb–YSZ TBCs are composed of t´ phase. After V₂O₅ corrosion, the coatings undergo phase transformation from t´ to m phases, which could be confirmed by XRD analysis. The content of m phase can be qualitatively determined by the relative intensity of the m phase, and a stronger peak intensity means a higher content. The quantitative analysis can be conducted by employing Equations (1) and (2), and the results are provided in Fig. 3, which clearly shows that the m phase content increases with corrosion temperature. The detailed discussion has been provided in the revised version.

 

3. Based on the outcomes of Fig.2, what subjects can be remarked?

Response: XRD is a useful technique to characterize the phase structure of a material. From Fig.2, it can be obtained the phase compositions of samples subjected to V₂O₅ corrosion at different temperature. Then, the m phase content can be calculated, by which it could judge that GdYb–YSZ TBCs are better resistant to V₂O₅ corrosion than YSZ TBCs.

 

4. For what reason, the higher corrosion temperatures cause an increase in m phase content and a decrease in t´ phase content?

Response: The reviewer has proposed a professional question. The formation mechanism could be understood according to Equation (3). At high temperatures, V₂O₅ reacts with GdYb–YSZ TBCs, causing the coating deficient in stabilizers. As a result, phase transformation from t´ to m phases occurs. The reaction is accelerated with temperature. Hence, the higher corrosion temperatures cause an increase in m phase content and a decrease in t´ phase content.

 

5. Since this study is performed in the thermal environment, the following approaches https://doi.org/10.1016/j.compstruct.2021.114557, https://doi.org/10.1016/j.ast.2017.06.008 should be also investigated.

Response: Thank you for providing the useful references, and we have added these in our revised text. Please see the updated References 27 and 28.

 Thank you and reviewers again. We are very happy to have improved our manuscript with your helps.

Best Wishes & Regards,

Sincerely yours,

Kai Liao, Ph. D

Hunan Applied Technology University

Reviewer 3 Report

Regarding the presented article Hot-corrosion behavior of Gd2O3–Yb2O3 co-doped YSZ thermal barrier coatings in the presence of V2O5 molten salt, I have the following comments:

 

1. Line 68 - why the temperatures from 700 to 1000 °C were chosen. why didn't you use a temperature of at least 1200 °C, since it degrades the formed phases from the point of view of volume expansion?

2. Line 87 - add the chemical composition of NiCoCrAlY superalloy

3. Line 109 - fig. 1- put a larger scale in both images

Fig.1a consider keeping the picture, the picture only tells about the size of the agglomerate parts.

Fig. 1b I recommend removing the image because the authors use the image only to determine the roughness. For this purpose, it would be better to use an AFM microscope or a Nanoindentor. or insert arrows with some description into the image.

4. I recommend adding a photo or schematic image of the hot-corrosion test to the post.

5. Aká bola hrúbka povlaku

 

Comments:

1. incorporate (images) of the resulting layer on the superalloy into the article (macrophoto)

2. write the conclusion in specific points 1 2 3 .... in which you clearly state what your experiences were.

3 I recommend expanding the article by at least 2 pages.

4. The main thing is to supplement the measurements with higher temperatures up to 1200 °C.

5. The work lacks an overall in-depth discussion of the obtained results, which are compared with other authors.

 

The article offers an interesting insight into the issue, but it is necessary to improve it according to the comments.

Author Response

Dear Editors and reviewers

Thank you and the reviewers very much for your kind comments and patient corrections on our manuscript titled on " Hot-corrosion behaviour of Gd₂O₃–Yb₂O₃ co-doped YSZ thermal barrier coatings in the presence of V₂O₅ molten salt (coatings-239540)”.

According to the comments and suggestion of reviewers and editors, we have carefully revised our paper. The corrections of the manuscript are highlighted in red through the text of the manuscript.

Reviewer 3#

Regarding the presented article Hot-corrosion behavior of Gd2O3–Yb2O3 co-doped YSZ thermal barrier coatings in the presence of V2O5 molten salt, I have the following comments:

1. Line 68 - why the temperatures from 700 to 1000 °C were chosen. why didn't you use a temperature of at least 1200 °C, since it degrades the formed phases from the point of view of volume expansion?

Response: Thank you for proposing professional comments. As the reviewer’s comment, higher temperature indeed causes more phase transformation from t´ to m phases, giving rise to larger volume expansion. However, this study focuses on phase transformation and volume expansion resulting from hot corrosion. At temperature above 1000 °C, such as at least 1200 °C, the molten salt would decompose, which leads to the hot corrosion not being a main factor causing coating phase transformation. Hence, we chose the corrosion temperature from 700 to 1000 °C. Meanwhile, this corrosion temperature is chosen by many researchers for TBCs subjected to V₂O₅ molten salt, please see references 24, 25, 29, 39, 32-35.    

2. Line 87 - add the chemical composition of NiCoCrAlY superalloy

Response:

The chemical composition of NiCoCrAlY superalloy is provided in Table 1 in the revised text.

Table 1. The chemical composition of NiCoCrAlY bond coat

Element	Ni	Co	Cr	Al	Y
Content	Bal.	20-22	24	20	1.5

3. Line 109 - fig. 1- put a larger scale in both images

Fig.1a consider keeping the picture, the picture only tells about the size of the agglomerate parts.

Fig. 1b I recommend removing the image because the authors use the image only to determine the roughness. For this purpose, it would be better to use an AFM microscope or a Nanoindentor. or insert arrows with some description into the image.

Response: We have put a larger scale of Fig. 1a, and removed Fig. 1b in the revised text.

4. I recommend adding a photo or schematic image of the hot-corrosion test to the post.

Response: We have described the corrosion tests clearly in the text, and consider that it is not necessary to add a photo or schematic image of the hot-corrosion test. The detailed experimental procedure is as followings: V₂O₅ powder was put on GdYb–YSZ TBCs with a content of 10 mg/cm², followed by put to a furnace heating to 700, 800, 900, and 1000 °C for a duration of 10 hours.

 

5. Aká bola hrúbka povlaku

 Response: The coating thickness is about 200 μm.

Comments:

1. incorporate (images) of the resulting layer on the superalloy into the article (macrophoto)

Response: Thanks for your kind suggestion. As the response to Reviewer 3#, we consider that it is not necessary to add a photo or schematic image of the hot-corrosion test. The detailed experimental procedure is as followings: V₂O₅ powder was put on GdYb–YSZ TBCs with a content of 10 mg/cm², followed by put to a furnace heating to 700, 800, 900, and 1000 °C for a duration of 10 hours.

2. write the conclusion in specific points 1 2 3 .... in which you clearly state what your experiences were.

Response: We have re-write the conclusion in bullet-points.

In this study, GdYb-YSZ TBCs fabricated through APS were examined for their hot-corrosion behavior when subjected to V₂O₅ molten salt at 700-1000 °C for 10 hours, and following conclusions could be obtained.

(1) GdYb-YSZ TBCs experienced some degree of attack by V₂O₅, but showed better corrosion resistance compared to YSZ TBCs. This improved resistance can be attributed to the higher rare-earth content and lower basicity of Yb₂O₃.

(2) The as-fabricated GdYb-YSZ TBCs contained t´ phase, and even after hot corrosion, the t´ phase was still detectable on the corroded surfaces, indicating that the coatings had a certain level of resistance to phase decomposition. As the temperature of corrosion increased, the quantity of t´ phase decreased, while the amount of m phase increased.

(3) In addition to the m phase, Yb- and Gd-doped YVO₄ were generated as corrosion products for the GdYb-YSZ TBCs in V₂O₅. Higher temperatures had no effect on the type of corrosion products but changed the morphologies of Yb- and Gd-doped YVO₄ crystals.

3 I recommend expanding the article by at least 2 pages.

Response: Thank you for your kind suggestion. We have expanded the article. Compared to long length, we consider that the significance is more important for an article. Previous study revealed that GdYb-YSZ is a promising candidate for TBC applications due to its several excellent properties, however, there is no report on the properties of GdYb-YSZ TBCs. Hence, we conduced this study, and it is of great significance.

4. The main thing is to supplement the measurements with higher temperatures up to 1200 °C.

Response: Please see the response to Reviewer 3#, in which we consider that it is not necessary to increase the corrosion temperature up to 1200 °C. This study focuses on phase transformation and volume expansion resulting from hot corrosion. At temperature above 1000 °C, such as at least 1200 °C, the molten salt would decompose, which leads to the hot corrosion not being a main factor causing coating phase transformation. Hence, we chose the corrosion temperature from 700 to 1000 °C. Meanwhile, this corrosion temperature is chosen by many researchers for TBCs subjected to V₂O₅ molten salt, please see references 24, 25, 29, 39, 32-35.    

 

5. The work lacks an overall in-depth discussion of the obtained results, which are compared with other authors.

Response: In this study, the emphasis is put on investigating the V₂O₅ hot corrosion behavior of GdYb-YSZ TBCs, to determine whether they have improved corrosion resistance compared to YSZ TBCs. Previous study revealed that GdYb-YSZ is a promising candidate for TBC applications due to its low thermal conductivity, good phase stability, and high thermal expansion coefficient, and good hot corrosion resistance. However, these desirable properties belong to GdYb-YSZ pellets, rather than its coating counterpart. For potential applications, it is important to determine whether GdYb-YSZ TBCs have these desirable properties. Hence, we investigate the V₂O₅ hot corrosion behavior of GdYb-YSZ TBCs. Although the hot corrosion mechanism for the coating and pellet are same, the study is still of great significance.

 Thank you and reviewers again. We are very happy to have improved our manuscript with your helps.

 

Best Wishes & Regards,

Sincerely yours,

Kai Liao

Hunan Applied Technology University

 

Round 2

Reviewer 1 Report

Authors have done a great revision. This revised version is acceptable. 

Reviewer 3 Report

I thank the authors for incorporating my comments into the article as well as for answering my questions.