Thermal Barrier Coatings for High-Temperature Performance of Nickel-Based Superalloys: A Synthetic Review

Round 1

Reviewer 1 Report

The paper gives a synthetic review on thermal barrier coatings for high-temperature of nickel-based superalloys. The TBC are important for improving application temperatures for superalloys. The issue is proper for the magazine, and the logic, picture and description in this manuscript are well, some misleading or absence need to be added or revised as followings:

1.     The description on this sentence is not strict for a scientific paper: “the enhancement of about 100°C was observed for the modified technologies of nickel-based superalloy manufacturing.”

2.     In abstract, the author wants to build relationship between preparation method and its properties, while in the last paragraph, the author mentioned to “discuss the relationship between coatings and the high-temperature performance”. It misleads readers and one of them need to be revised to eliminate this mistaken.

3.     Where is the conclusion or prospect? A conclusion such as: different method makes different faults and different mechanical properties. A prospective that you summarized from your bibliography that can improved to make this issue better.

4.     Part 3, some figures are needed to make the paper more direct or friend to readers.

Author Response

Reviewer #1:

The paper gives a synthetic review on thermal barrier coatings for high-temperature of nickel-based superalloys. The TBC are important for improving application temperatures for superalloys. The issue is proper for the magazine, and the logic, picture and description in this manuscript are well, some misleading or absence need to be added or revised as followings:

1. The description on this sentence is not strict for a scientific paper: “the enhancement of about 100°C was observed for the modified technologies of nickel-based superalloy manufacturing.”

Response: The statement was changed to : “the increase of working temperature of about 100°C was observed for nickel-based super-alloys manufactured by using different technologies.”

2. In abstract, the author wants to build relationship between preparation method and its properties, while in the last paragraph, the author mentioned to “discuss the relationship between coatings and the high-temperature performance”. It misleads readers and one of them need to be revised to eliminate this mistaken.

Response: The authors have decided to remove the statement “The relationship between coating technology and substrate material was also analysed in terms of failure mechanisms that occurred during high-temperature performance.”

3. Where is the conclusion or prospect? A conclusion such as: different method makes different faults and different mechanical properties. A prospective that you summarized from your bibliography that can improved to make this issue better.

Response:  The subsection Conclusions and future trends was added as follows:

The characteristics of the most extensive materials used for thermal barrier coatings and deposition methods have been presented. The direction of future investigations is proposed with a special emphasis on novel coating materials to provide oxidation-resistant and highly durable components. Nowadays, the vast majority of high-temperature components are based on nickel alloys, due to their high durability and standardized technology. Nevertheless, further improvements, such as structure or surface modification or industrialization of 3D printing are mandatory to meet the requirements of the continuously expanding thermal barrier coatings market [95]. Based on the trends observed in the industry [96, 97], it is estimated that global thermal barrier coatings market valued at USD 15.97 billion in 2021 is expected to grow at a CAGR of 4.6% during the forecast period 2022 - 2030. It was reported [96], that the main damage mechanisms responsible for TBC failure are wear and corrosion. The deposition of hard-faced barrier coatings has been proposed as an useful approach to minimize the occurrence of such phenomena. Among conventionally used deposition methods, HVOF spraying was found as the one of the most effective. On the other hand, the flame spraying can also be applied to form WC-based barrier coatings on nickel substrates. One should mention the applicability of plasma-transfer arc (PTA) welding in the mining and oil & gas industry. The thick tungsten carbide overlaying barrier coatings obtained during such process improve the durability of protected components significantly. As reported in recent literature [98], deposition techniques with non-line-of-sight capabilities in combination with columnar structures (SPS, PS-PVD) appear to be attractive for the application of TBCs, with special dedication for complex shaped parts. On the other hand, double-/multilayer coatings made of novel thermally stable materials combined with tough YSZ used directly on the bond coat will probably be the most reliable approach for high-temperature applications [98].

4. Part 3, some figures are needed to make the paper more direct or friend to readers.

Response:  We have added more figures which mainly include microstructural images from our own studies.

Author Response File: Author Response.pdf

Reviewer 2 Report

Journal: Coatings (ISSN 2079-6412)

Manuscript ID: coatings-2334161

 The authors presented a review about “Thermal Barrier Coatings for High-Temperature Performance of Nickel-Based Superalloys: a synthetic review”. I think the review is well organized and appropriate for the “Coatings” journal, but the review will be ready for publication after major revision. The similarity rate is 24%. Comments are listed below.

1.      A comprehensive literature review is required in the introduction. However, in this study, it is seen that the literature is not sufficiently utilized.

2.      The purpose of examining scientific documents is not specified in the article. Based on the title, it can be assumed that this will be a comparison of the performance of nickel-based superalloys at high temperatures. But I think the authors should put more effort instead listing these issues as a summary.

3.      The main problem addressed in the article in the last paragraph of the introduction is not adequately explained. In this paragraph, the study's primary purpose and its difference from other studies should be clearly stated.

4.      The section "2. Application methods for TBC layers" does not match the content. The general properties of thermal barrier coatings are mentioned in the content. Only in the last paragraph, thermal barrier coating methods are briefly mentioned.

5.      The most important problem in thermal barrier coatings is that the coating has a fragile structure. The authors should discuss this issue extensively.

6.      Please fix the typographical and eventual language problems in paper.

7.      The review should be rearranged by taking into account the journal writing rules and citation rules.

*** Authors must consider them properly before submitting the revised review. A point-by-point reply is required when the revised files are submitted.

Author Response

Reviewer #2

Journal: Coatings (ISSN 2079-6412)

Manuscript ID: coatings-2334161

 The authors presented a review about “Thermal Barrier Coatings for High-Temperature Performance of Nickel-Based Superalloys: a synthetic review”. I think the review is well organized and appropriate for the “Coatings” journal, but the review will be ready for publication after major revision. The similarity rate is 24%. Comments are listed below.

1. A comprehensive literature review is required in the introduction. However, in this study, it is seen that the literature is not sufficiently utilized.

Response: A systematic literature review was provided in the following subsections. The introduction presented by the authors aims to provide a general information about materials and TBC, but the most importantly highlights the motivation of the paper: “Although there are a number of review papers found on the TBC deposited on nickel alloys, the effect of coating technology and coating itself on the mechanical response of the substrate material at high temperature and their corrosion resistance was insufficiently discussed. Therefore, the main aim of this paper was to discuss the relationship between coatings and the high-temperature performance (mechanical behaviour and corrosion resistance) of nickel-based alloys.”

Based on such motivation, following sections were build to provide more detailed information about the current state of art.

2. The purpose of examining scientific documents is not specified in the article. Based on the title, it can be assumed that this will be a comparison of the performance of nickel-based superalloys at high temperatures. But I think the authors should put more effort instead listing these issues as a summary.

Response: A new subsection Review methodology was added as follows:

The literature review involved the detailed investigation of thermal barrier coatings applied on nickel-based superalloys with particular emphasis on their high-temperature performance (mechanical response and oxidation resistance). The review was conducted in the PubMed and Web of Science databases with the following keywords used in various combinations: “thermal barrier coatings, nickel alloys, high temperature performance, fatigue, creep, wear, coatings, oxidation, turbine blades, deposition”. A number of 120 scientific papers and conference proceedings were found through the electronic databases. They were subsequently considered by independent reviewers to assess their eligibility. The assessment includes screening the title, the abstract and summary of each paper using the pre-specified inclusion and exclusion criteria. In the next step, the papers not fitting the inclusion criteria were excluded. A total number of 98 papers were found as relevant and thus were used in this review.

3. The main problem addressed in the article in the last paragraph of the introduction is not adequately explained. In this paragraph, the study's primary purpose and its difference from other studies should be clearly stated.

Response: This issue is directly related to the first reviewers concern, thus the answer is similar to that already presented above.

4. The section "2. Application methods for TBC layers" does not match the content. The general properties of thermal barrier coatings are mentioned in the content. Only in the last paragraph, thermal barrier coating methods are briefly mentioned.

Response: The sections title has been changed to “Deposition of TBC layers”.

5. The most important problem in thermal barrier coatings is that the coating has a fragile structure. The authors should discuss this issue extensively.

Response: A fragile structure of the coating and its behaviour was discussed as highlighted in blue in the manuscript. Further description based on the authors research was added as follows:

However, due to the poor mechanical bonding between SPS top coat and a smooth bond coat surface, spallation frequently occurs during metallography preparation for microscopic observations. Such a low adhesion also affects the formation of spallation during the isothermal heat treatment test at 1100°C. Selected examples of images showing cracked coating observed after preparation of the metallographic specimen was presented in Figure 9a. One should highlight, that the TBC layers have fragile nature, and thus, they are cracking easily, when subjected to excessive loading. However, if the optimized parameters of deposition process are selected, the coating is well adhered to the substrate. Figure 9b shown the fracture area of the Inconel 740 with NiAl coating subjected to cyclic loading at room temperature. It should be noticed, that even the surface cracks appear, the coating remained on the substrate which may confirmed the effectiveness of the process parameters applied.

Figure 9. The morphology of Inconel 740 with NiAl coating obtained during CVD process after preparation of metallographic specimen (a), top view of the NiAl coating after fracture of the specimen subjected to cyclic loading (b).

6. Please fix the typographical and eventual language problems in paper.

Response: The paper was proof-readed before submission.

7. The review should be rearranged by taking into account the journal writing rules and citation rules.

*** Authors must consider them properly before submitting the revised review. A point-by-point reply is required when the revised files are submitted.

Response: We hope that the improved version of the manuscript will meet reviewers requirements. If there will be anything more we could do to improve the quality of work submitted, we will be more than than happy to provide more comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper reviewed the coating technologies with special emphasis on plasma-sprayed thermal barrier coatings (PS), and those produced by physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods. The paper has some interesting aspects.

Author Response

We would like to thank the reviewer for the positive feedback.

Reviewer 4 Report

The authors have reviewed the deposition methods of TBCs and their mechanical and corrosion resistance properties for Ni-based alloys. The content is good, flow is nice, but some re-arrangement of the text is needed as pointed out below. Some minor grammatical mistakes are there, please check carefully. 

1. If the schematics of the deposition processes (Figures 2, 3, 4, and 5) are not re-drawn by the authors and copied from the online sources, please include the sources.

2. Can authors categorize the corrosion resistance properties based on the deposition methods? Currently, section 3.1 is very long and not convenient for readers to keep track of the difference in performance based on the deposition methods. The literature showing the difference between methods (APS, PVD, etc) can be placed at the end of the section.

3. line 328: the phrase should be '5 fold per 100C' or '5 fold for every 100C'.

4. Same comment as for section 3.1. Can authors rearrange the section 3.2 either based on the mechanical properties or the deposition methods?

5. There is no mention of Table 1 in the manuscript text, authors should include it. 

6. Authors should include a section at the end indicating the future trend/ focus of these TBCs.  

Author Response

Reviewer #4:

The authors have reviewed the deposition methods of TBCs and their mechanical and corrosion resistance properties for Ni-based alloys. The content is good, flow is nice, but some re-arrangement of the text is needed as pointed out below. Some minor grammatical mistakes are there, please check carefully. 

1. If the schematics of the deposition processes (Figures 2, 3, 4, and 5) are not re-drawn by the authors and copied from the online sources, please include the sources.

Response: We would like to express our gratitude for the positive feedback. The comments on each suggestion were added and implemented into manuscript. Figures 2-5 were prepared by the authors thus no source is needed.

2. Can authors categorize the corrosion resistance properties based on the deposition methods? Currently, section 3.1 is very long and not convenient for readers to keep track of the difference in performance based on the deposition methods. The literature showing the difference between methods (APS, PVD, etc) can be placed at the end of the section.

Response: The additional table was provided in Table 1. The microstructural studies performed by the authors and not published before were used to clearly explain some phenomena occurring during high temperature oxidation.

Table 1. Summary of the oxidation performance depending on the deposition method applied.

3. line 328: the phrase should be '5 fold per 100C' or '5 fold for every 100C'.

Response: The phrase was corrected to “5 fold for every 100°C.”

4. Same comment as for section 3.1. Can authors rearrange the section 3.2 either based on the mechanical properties or the deposition methods?

Response: Table 2 was provided to provide essential information from section 3.2.

Table 2. Summary of the improved mechanical properties depending on the coating, deposition method and substrate applied.

5. There is no mention of Table 1 in the manuscript text, authors should include it. 

Response: The content of Table 1 was slightly changed and it was referred in the manuscript text.

6. Authors should include a section at the end indicating the future trend/ focus of these TBCs.  

Response: A new section “Conclusions and future trends” was added as follows:

The characteristics of the most extensive materials used for thermal barrier coatings and deposition methods have been presented. The direction of future investigations is proposed with a special emphasis on novel coating materials to provide oxidation-resistant and highly durable components. Nowadays, the vast majority of high-temperature components are based on nickel alloys, due to their high durability and standardized technology. Nevertheless, further improvements, such as structure or surface modification or industrialization of 3D printing are mandatory to meet the requirements of the continuously expanding thermal barrier coatings market [95]. Based on the trends observed in the industry [96, 97], it is estimated that global thermal barrier coatings market valued at USD 15.97 billion in 2021 is expected to grow at a CAGR of 4.6% during the forecast period 2022 - 2030. It was reported [96], that the main damage mechanisms responsible for TBC failure are wear and corrosion. The deposition of hard-faced barrier coatings has been proposed as an useful approach to minimize the occurrence of such phenomena. Among conventionally used deposition methods, HVOF spraying was found as the one of the most effective. On the other hand, the flame spraying can also be applied to form WC-based barrier coatings on nickel substrates. One should mention the applicability of plasma-transfer arc (PTA) welding in the mining and oil & gas industry. The thick tungsten carbide overlaying barrier coatings obtained during such process improve the durability of protected components significantly. As reported in recent literature [98], deposition techniques with non-line-of-sight capabilities in combination with columnar structures (SPS, PS-PVD) appear to be attractive for the application of TBCs, with special dedication for complex shaped parts. On the other hand, double-/multilayer coatings made of novel thermally stable materials combined with tough YSZ used directly on the bond coat will probably be the most reliable approach for high-temperature applications [98].

Author Response File: Author Response.pdf

Reviewer 5 Report

Paper requires correction.

1. The abstract is representative of the content, but has no goal.  Keywords cannot duplicate the words from the title of the paper.

2.  A review article is not a bibliographic review. The task of the authors is to show the level of knowledge of the scientific problem, if possible, to give a critical assessment of the works published on the problem under consideration and draw conclusions based on these works. The authors need to objectively consider the degree of study of a particular problem (it is not sufficiently indicated), give a reasonable assessment of the published works of other researchers, and draw logical conclusions from the study. Specify - what problem your review is dedicated to solving.

3. The bibliographic review is insufficient. I would suggest that the literature review be more systematic in comparing predecessor papers. The authors should carefully choose which points they want to highlight from previous studies, including any disadvantages they plan to address. 

4. In the conclusions, it is necessary to describe in more detail the recommendations/suggestions for future industry research to find solutions to the problem described by the authors

Overall, I believe that addressing these points will significantly improve the quality of your manuscript and increase its chances of acceptance for publication.

Author Response

Reviewer #5

1. The abstract is representative of the content, but has no goal.  Keywords cannot duplicate the words from the title of the paper.

Response: The authors have specified the goal of the paper in the middle section of the abstract: “Thus, this paper aims to review coating technologies with special emphasis on plasma-sprayed thermal barrier coatings (PS), and those produced by physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods. Each technology was assessed in terms of its effectiveness to enhance the mechanical response and oxidation resistance of nickel-based parts working at high temperature.”

The keywords have been changed to “high temperature corrosion; fatigue; creep; aggressive environment; turbine engine”.

2. A review article is not a bibliographic review. The task of the authors is to show the level of knowledge of the scientific problem, if possible, to give a critical assessment of the works published on the problem under consideration and draw conclusions based on these works. The authors need to objectively consider the degree of study of a particular problem (it is not sufficiently indicated), give a reasonable assessment of the published works of other researchers, and draw logical conclusions from the study. Specify - what problem your review is dedicated to solving.

Response: Thanks for the very good comment. As the authors stated in the last paragraph of the Introduction: “Although there are a number of review papers found on the TBC deposited on nickel alloys, the effect of coating technology and coating itself on the mechanical response of the substrate material at high temperature and their corrosion resistance was insufficiently discussed. Therefore, the main aim of this paper was to discuss the relationship between coatings and the high-temperature performance (mechanical behaviour and corrosion resistance) of nickel-based alloys.” Based on this problem, the authors formulated conclusions:

The characteristics of the most extensive materials used for thermal barrier coatings and deposition methods have been presented. It could be observed, that most of the papers considered in this review are focused on the improvement of the particular property in-cluding oxidation resistance, hardness, and mechanical behaviour. It should be empha-sized, however, that the components protected with thermal barrier coatings operate un-der extreme conditions and in aggressive environments, thus they are subjected to the combination of excessive loadings and oxidation. Hence, the coatings deposited on the substrate material, ideally whole components, should be subjected to the service loads under working conditions to simulate their real mechanical response in order to increase the reliability of the study performed. One should mention, that the specific tests, either corrosion or mechanical one, should not be performed on the coating only since its inter-action with the substrate material should be investigated as well. The coating itself may have outstanding wear properties or be extremely resistant to oxidation, however, if it will not be adhered enough to the substrate, then its application is not possible. Thus, it is recommended to perform more studies which will focus on the actual interaction between the coating and substrate material subjected to loading under high temperature to assess the applicability of new methods or materials.

3. The bibliographic review is insufficient. I would suggest that the literature review be more systematic in comparing predecessor papers. The authors should carefully choose which points they want to highlight from previous studies, including any disadvantages they plan to address. 

Response: As the authors stated, the effect of coating technology and coating itself on the mechanical response of the substrate material at high temperature and their corrosion resistance is discussed in the literature at limited level. The most studies are focused on microstructural changes occurring on the coating/substrate material border or on the properties of the coating only. The mechanical behaviour of TB coated nickel-based alloys subjected to high temperature is rarely discussed in the literature as presented in Table 2. The authors put much efforts to review the existing literature in order to highlight studies which are purely focused on the mechanical response of the coated material at high temperature and their corrosion resistance.  The disadvantages of the review performed were discussed in the Conclusion subsection.

4. In the conclusions, it is necessary to describe in more detail the recommendations/suggestions for future industry research to find solutions to the problem described by the authors

Overall, I believe that addressing these points will significantly improve the quality of your manuscript and increase its chances of acceptance for publication.

Response: A new section “Conclusions and future trends” was added as follows:

The direction of future investigations is pro-posed with a special emphasis on novel coating materials to provide oxidation-resistant and highly durable components. Nowadays, the vast majority of high-temperature components are based on nickel alloys, due to their high durability and standardized technology. Nevertheless, further improvements, such as structure or sur-face modification or industrialization of 3D printing are mandatory to meet the requirements of the continuously expanding thermal barrier coatings market [95]. Based on the trends observed in the industry [96, 97], it is estimated that global thermal barrier coatings market valued at USD 15.97 billion in 2021 is expected to grow at a CAGR of 4.6% during the forecast period 2022 - 2030. It was reported [96], that the main damage mechanisms responsible for TBC failure are wear and corrosion. The deposition of hard-faced barrier coatings has been proposed as an useful approach to minimize the occurrence of such phenomena. Among conventionally used deposition methods, HVOF spraying was found as the one of the most effective. On the other hand, the flame spraying can also be applied to form WC-based barrier coatings on nickel substrates. One should mention the applicability of plasma-transfer arc (PTA) welding in the mining and oil & gas industry. The thick tungsten carbide overlaying barrier coatings obtained during such process improve the durability of protected components significantly. As reported in recent literature [98], deposition techniques with non-line-of-sight capabilities in combination with columnar structures (SPS, PS-PVD) appear to be attractive for the application of TBCs, with special dedication for complex shaped parts. On the other hand, double-/multilayer coatings made of novel thermally stable materials combined with tough YSZ used directly on the bond coat will probably be the most reliable approach for high-temperature applications [98].

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Journal: Coatings (ISSN 2079-6412)

Manuscript ID: coatings-2334161

Review #R1

The authors completed the requested corrections. In my opinion this compilation is acceptable for final publication in the "Coatings" journal.

Comments for author File: Comments.docx

Author Response

We would like to thank for useful comments that improved our paper.

Reviewer 4 Report

The authors have addressed the comments, implemented the changes, and therefore recommend publication. 

Author Response

We would like to thank for useful comments that improved our paper.

Reviewer 5 Report

Overall, I find the edited paper's reasoning and approach quite promising. I recommend to accept it.

Author Response

We would like to thank for useful comments that improved our paper.