Micro-Nano Dual-Scale Coatings Prepared by Suspension Precursor Plasma Spraying for Resisting Molten Silicate Deposit

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Long title try to reduces it

Explain more about SPPS technology

No need to Table 6. Evolution of nanoparticles under various heat treatment durations , Explain within the text

Add more references

 

Comments on the Quality of English Language

Good

Author Response

1. Long title try to reduces it

Respond: Thank you for your suggestion. The title has been changed from “Micro-nano dual-scale thermal barrier coatings prepared by suspension precursor plasma spraying for repelling and resisting molten silicate deposit” to “Micro-nano dual-scale coatings prepared by suspension pre-cursor plasma spraying for resisting molten silicate deposit”. Relevant changes can be seen in the revised draft.

2. Explain more about SPPS technology

Respond: Thank you for your suggestion. SPPS technology directly uses solution precursors for spraying. The solution precursors undergo a series of processes such as solvent evaporation, droplet splitting, solute precipitation, high-temperature decomposition, sintering and melting in the plasma flame, and finally form a coating on the substrate. The technology breaks the limitation of powder particle size on the microstructure of coatings, with relatively low cost. In addition, it solves the flowability problem of small particle size powders and can prepare TBCs with nanostructures. Relevant changes can be seen in the revised draft.

3. No need to Table 6. Evolution of nanoparticles under various heat treatment durations, Explain within the text

Respond: Thank you for your suggestion. The contents of the table have been organized into the text. Relevant changes can be seen in the revised draft.

4. Add more references

Respond: Thank you for your suggestion. More references have been added.

Reviewer 2 Report

Comments and Suggestions for Authors

 I have reviewed the manuscript and find that the article presents scientifically valuable research, in developing thermal barrier coatings with improved resistance to molten silicate deposits. However, there are several areas where the manuscript could be enhanced to provide clearer justification, more comprehensive explanations, and a stronger contextual foundation. For further improvement, I suggest the following:

 

 

1. The introduction explains SPPS but does not provide enough context or background about the technique. How does SPPS differ from other thermal barrier coating methods, and why is it particularly relevant to this study?

2. The references provided are limited and do not sufficiently cover the breadth of research on CMAS infiltration and SPPS coatings. A broader literature review would strengthen the introduction.

3. The introduction assumes that readers are already familiar with certain terms and concepts such as CMAS, YSZ, etc., without providing adequate definitions or explanations, which could be confusing for a broader audience.

4. The introduction does not clearly articulate the novelty or unique contribution of this study compared to existing literature. Furthermore, the introduction does not clearly define the specific research gap that this study addresses. Focus on the existing research gap and how to address it by critically reviewing previous research.

5. The introduction would benefit from including some quantitative data or examples from the literature to illustrate the severity of CMAS infiltration or the effectiveness of SPPS coatings.

6. The introduction mentions Yb-doped YSZ but does not provide a strong justification for why this particular material was chosen over other potential candidates.

7. The introduction does not provide any hints or rationale for the experimental design or methods that will be used in the study, leaving the reader without a sense of how the research will be conducted.

8. Some statements, such as the effectiveness of YbYSZ in resisting CMAS infiltration, are made without sufficient citation to back up these claims.

9. Section 2.1 does not provide specific details about how the process parameters were controlled during the APS and SPPS techniques. For instance, were there any real-time monitoring mechanisms in place to ensure consistency in the coating application?

10. The pre-treatment process mentions sandblasting but lacks specific details such as the grit size, blasting pressure, and duration, which are crucial for ensuring reproducible surface conditions.

11. The ultrasonic cleaning process in anhydrous ethanol is mentioned but lacks critical details such as the frequency, duration, and temperature of the cleaning process, which can significantly affect surface cleanliness.

12. The choice of coating thicknesses is not justified. Why were these specific thicknesses chosen?

13. The text does not explain why SPPS process parameters were chosen.

14. Section 2.1 mentions the use of carbon steel substrates but does not provide information on the grade of steel, its thermal properties, or why it was chosen for this study.

15. Why was 1300°C chosen as the test temperature, and how does it relate to the operational temperatures of TBCs in real applications?

16. Section 2.2 does not describe the characteristics of the control samples. Were the control samples untreated or treated differently? How were they prepared?

17. The paragraph describing SEM does not address potential artifacts such as charging effects or sample preparation-induced damage that could affect the accuracy of the results.

18. The method for testing CMAS wetting performance is mentioned without any details on the conditions of the test.

19. There is no mention of statistical analysis or error bars in the data presented. For a robust conclusion, the reproducibility of EDS, XRD, and SEM data should be statistically validated.

20. The structural stability section relies heavily on visual interpretation of SEM images without providing supporting quantitative data, such as porosity measurements or particle size distribution analysis.

21. The discussion of sintering lacks depth. The authors mention "sintering" and "fusion," but there is no detailed explanation of the underlying mechanisms specific to the micro-nano dual-scale layers.

22. Section 3 mention comparisons with previous studies or standard YSZ coatings, but these comparisons are not sufficiently detailed. Direct comparisons of performance metrics such as thermal stability/phase stability would strengthen the discussion.

23. Terms like "non-equilibrium tetragonal phase," "sheet-like structures," "hollow-sphere structures", etc., are used without prior definition or explanation.

24. The discussion on the interaction with CMAS is superficial. Present this discussion in more depth.

25. The heat treatment effects are discussed without considering possible variations in temperature distribution within the furnace or the effect of different heating rates, which could influence sintering behavior and phase transitions.

26. In the conclusion section, highlight the significance of the results and their contribution to the existing body of knowledge. Also, address any limitations or potential areas for future research.

Comments on the Quality of English Language

Minor editing of English language required.

Author Response

1. The introduction explains SPPS but does not provide enough context or background about the technique. How does SPPS differ from other thermal barrier coating methods, and why is it particularly relevant to this study?

Respond: Thank you for your suggestion. SPPS technology directly uses solution precursors for spraying. The solution precursors undergo a series of processes such as solvent evaporation, droplet splitting, solute precipitation, high-temperature decomposition, sintering and melting in the plasma flame, and finally form a coating on the substrate. The technology breaks the limitation of powder particle size on the microstructure of coatings, with relatively low cost. In addition, it solves the flowability problem of small particle size powders and can prepare TBCs with nanostructures. Compared to the common preparation processes currently used in industrial applications, including atmospheric plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD), these processes can melt powder particles to form coatings, but cannot form nanostructures on the surface, which is beneficial for hydrophobicity. Relevant changes can be seen in the revised draft.

2. The references provided are limited and do not sufficiently cover the breadth of research on CMAS infiltration and SPPS coatings. A broader literature review would strengthen the introduction.

Respond: Thank you for your suggestion. Research related to CMAS and SPPS has been added. Relevant changes can be seen in the revised draft.

3. The introduction assumes that readers are already familiar with certain terms and concepts such as CMAS, YSZ, etc., without providing adequate definitions or explanations, which could be confusing for a broader audience.

Respond: Thank you for your suggestion. CMAS mainly comes from small particles such as dust, sand, or volcanic ash in the air (composed of CaO, MgO, Al₂O₃, and SiO₂, abbreviated as CMAS), which are carried by the airflow and reach the surface of the hot-end components in engine. With the improvement of engine performance and efficiency, the service temperature exceeds the melting point of CMAS, causing them to melt and penetrate on the surface of the components.

8 wt% yttria stabilized zirconia (8YSZ) and other low thermal conductivity materials are successfully employed upon the hot-section components to protect the superalloy from hot gases. Relevant changes can be seen in the revised draft.

4. The introduction does not clearly articulate the novelty or unique contribution of this study compared to existing literature. Furthermore, the introduction does not clearly define the specific research gap that this study addresses. Focus on the existing research gap and how to address it by critically reviewing previous research.

Respond: Thank you for your suggestion. Many scholars have conducted research on the anti-CMAS wetting performance. Dai et al. conducted a surface treatment on Gd₂Zr₂O₇ coatings using picosecond laser surface texture, and introduced nanostructure on the surface, resulting in an increase in the wetting angle of coating from 16.0° to 66.3°[5]. Moreover, Zhang et al. introduced bionic structural design based on the lotus leaf structure into TBCs under different processes to explore the universality of the structure[6]. Al₂O₃ modification was introduced to prepare a dense Al₂O₃ coating with nano/micro sized grains on the surface of TBCs. The results showed that in terms of wettability, the contact angle of Al₂O₃-modified APS 7YSZ TBCs (34.2°) increased by 129.5% compared to the contact angle of APS 7YSZ TBCs (14.9°); the contact angle of Al₂O₃ modified EB-PVD 7YSZ TBCs (11.8°) increased by 56.8% compared to the contact angle of EB-PVD 7YSZ TBCs (18.5°); the contact angle (22.0°) of Al₂O₃ modified PS-PVD 7YSZ TBCs increased by 87.3% compared to the contact angle (41.2°) of PS-PVD 7YSZ TBCs. The Al₂O₃ overlay with a lotus leaf like structure on the surface can effectively improve the wetting performance of coatings, regardless of various processes. The above research proves that introducing micro-nano structures can effectively increase the wetting performance of coatings. At present, surface treatment on the prepared coating to form this structure is not only complex in process, but also costly. The use of Solution precursor plasma spraying (SPPS) can more conveniently form coatings with micro-nano structure in one step. Relevant changes can be seen in the revised draft.

5. The introduction would benefit from including some quantitative data or examples from the literature to illustrate the severity of CMAS infiltration or the effectiveness of SPPS coatings.

Respond: Thank you for your suggestion. The introduction section has introduced some quantitative data and examples to illustrate the severity of CMAS penetration and the effectiveness of micro-nano dual-scale coatings. Relevant changes can be seen in the revised draft.

6. The introduction mentions Yb-doped YSZ but does not provide a strong justification for why this particular material was chosen over other potential candidates.

Respond: Thank you for your suggestion. In order to improve the high-temperature sintering resistance of YSZ materials, Liu et al.[18] subjected YSZ and Yb-doped YSZ (YbYSZ) to a heat treatment at 1300 ℃ × 32 h, indicating that YbYSZ materials had significantly improved sintering resistance due to their larger atomic mass. In addition, Fang et al. subjected YSZ and YbYSZ samples to 1500 ℃ × 600h heat treatment, suggesting that YSZ underwent phase transition, while YbYSZ did not undergo phase transition and exhibited good phase stability. Relevant changes can be seen in the revised draft.

7. The introduction does not provide any hints or rationale for the experimental design or methods that will be used in the study, leaving the reader without a sense of how the research will be conducted.

Respond: Thank you for your suggestion. Free-standing YbYSZ-SPPS coating was subjected to heat treatment at 1300 ℃ for 0.5h, 1h, 2h and 8h, followed by sessile drop experiments to investigate its wettability. Relevant changes can be seen in the revised draft.

8. Some statements, such as the effectiveness of YbYSZ in resisting CMAS infiltration, are made without sufficient citation to back up these claims.

Respond: Thank you for your suggestion. Given that Yb-doped YSZ (YbYSZ) has also shown promise in resisting CMAS infiltration[18, 20], the degradation depth of YbYSZ was improved by about 30% compared to YSZ samples under the conditions of 1300 ℃ × 2h and 12h. Relevant changes can be seen in the revised draft.

9. Section 2.1 does not provide specific details about how the process parameters were controlled during the APS and SPPS techniques. For instance, were there any real-time monitoring mechanisms in place to ensure consistency in the coating application?

Respond: Thank you for your suggestion. In APS and SPPS processes, these parameters can be controlled through the APS and SPPS digital systems. The solution components used for SPPS coating are all commercial regents from Sinopharm Chemical Reagent Co., Ltd, (Shanghai, China). Relevant changes can be seen in the revised draft.

10. The pre-treatment process mentions sandblasting but lacks specific details such as the grit size, blasting pressure, and duration, which are crucial for ensuring reproducible surface conditions.

Respond: Thank you for your suggestion. The substrate is pre-treated by sandblasting with 0.7 MPa pressure and 80^# brown-corundum particles for 2 min. Relevant changes can be seen in the revised draft.

11. The ultrasonic cleaning process in anhydrous ethanol is mentioned but lacks critical details such as the frequency, duration, and temperature of the cleaning process, which can significantly affect surface cleanliness.

Respond: Thank you for your suggestion. Then, the substrate was cleaned with ultrasonic instrument with 40 KHz at room temperature for 15 min in anhydrous ethanol medium to ensure optimal surface adhesion. Relevant changes can be seen in the revised draft.

12. The choice of coating thicknesses is not justified. Why were these specific thicknesses chosen?

Respond: Thank you for your suggestion. According to the research of Padture et.al, the thickness of the bond coat is generally between 75-150 μm and the thickness of the APS coating is generally around 300 μm. In the paper, the total thickness of the ceramic layer consists of a 250 μm layer by APS and a 50 μm layer by SPPS. The thickness of SPPS layer is thinner that of APS layer, because the overall structure of SPPS lauyer is looser compared to APS layer.

Padture, P. N., Thermal barrier coatings for gas-turbine engine applications, Science 296(5566) (2002) 280-284.

13. The text does not explain why SPPS process parameters were chosen.

Respond: Thank you for your suggestion. The paper only explains the long-term stability of micro-nano dual-scale coatings prepared by SPPS parameters, and the influence of different process parameters of SPPS on the performance of coatings is not the focus of discussion. We will explore this in the subsequent optimization of coatings.

14. Section 2.1 mentions the use of carbon steel substrates but does not provide information on the grade of steel, its thermal properties, or why it was chosen for this study.

Respond: Thank you for your suggestion. The use of carbon steel is mainly for the convenience of obtaining free-standing coating under the corrosion of hydrochloric acid. Relevant changes can be seen in the revised draft.

15. Why was 1300°C chosen as the test temperature, and how does it relate to the operational temperatures of TBCs in real applications?

Respond: Thank you for your suggestion. The temperature of TBC for gas turbine (e.g., Fclass land-based gas turbine) is ~1000 ℃, and the lifetime is often 16000-24000 h. To simulate this condition, accelerating test method has to be used at a higher temperature for a relatively shorter duration. Among the studies in the literature, the most widely reported temperature is in the range of 1200-1400 ℃. In this study, 1300 ℃ was adopted for the high-temperature exposure [1].

[1] G.-J. Yang, Z.-L. Chen, C.-X. Li, C.-J. Li, Microstructural and Mechanical Property Evolutions of Plasma-Sprayed YSZ Coating During High-Temperature Exposure: Comparison Study Between 8YSZ and 20YSZ, J. Therm. Spray Technol. 22(8) (2013) 1294-1302.

16. Section 2.2 does not describe the characteristics of the control samples. Were the control samples untreated or treated differently? How were they prepared?

Respond: Thank you for your suggestion. Set the prepared YSZ sample as a control, and the specific settings are shown in table 2. YSZ samples are untreated. Relevant changes can be seen in the revised draft.

17. The paragraph describing SEM does not address potential artifacts such as charging effects or sample preparation-induced damage that could affect the accuracy of the results.

Respond: Thank you for your suggestion. The performance of YSZ ceramics is relatively stable and the conductivity is poor, so SEM has almost no potential impact on this sample.

18. The method for testing CMAS wetting performance is mentioned without any details on the conditions of the test.

Respond: Thank you for your suggestion. Subsequently, the micro-nano dual-scale coating was subjected to heat treatment at 1300 ℃ for different durations to investigate the long-term stability of micro-nano structure. Additionally, in order to study the wetting behavior of micro-nano dual-scale coatings, the sessile drop method was used for detection. The CMAS powders, prepared in the laboratory, were uniaxially pressed into CMAS cylinders of φ 5 × 4 mm and then placed on the free-standing coatings surfaces. Finally, the samples were heated to the target temperature of 1300 °C for 180 s, 300 s, 420 s and 600 s. Relevant changes can be seen in the revised draft.

19. There is no mention of statistical analysis or error bars in the data presented. For a robust conclusion, the reproducibility of EDS, XRD, and SEM data should be statistically validated.

Respond: Thank you for your suggestion. EDS is used to obtain the overall elemental composition of the samples, XRD is used to obtain the phase composition of the coating, and SEM is used to characterize the microstructure of the coating, with almost no error bars.

 

20. The structural stability section relies heavily on visual interpretation of SEM images without providing supporting quantitative data, such as porosity measurements or particle size distribution analysis.

Respond: Thank you for your suggestion. Quantitative analysis of structural stability in micro-nano dual-scale layer has always been our goal, but subjective factors such as porosity or particle size are significant. Therefore, currently using SEM image method is a relatively intuitive and accurate approach. We will continue to search for methods to conduct quantitative analysis and provide accurate supporting evidence in the future.

21. The discussion of sintering lacks depth. The authors mention "sintering" and "fusion," but there is no detailed explanation of the underlying mechanisms specific to the micro-nano dual-scale layers.

Respond: Thank you for your suggestion. In the last paragraph of section 3.2 in the paper, it is explained in detail that sintering is essentially the transfer of matter from high-energy state and low-energy state. Sintering is the condition applied, while fusion is the external manifestation.

22. Section 3 mention comparisons with previous studies or standard YSZ coatings, but these comparisons are not sufficiently detailed. Direct comparisons of performance metrics such as thermal stability/phase stability would strengthen the discussion.

Respond: Thank you for your suggestion. The point has been confirmed in the paper of Fang et al., where Yb element can better ensure the thermal stability of ZrO₂ at 1500 ℃ × 600 h. Relevant changes can be seen in the revised draft.

23. Terms like "non-equilibrium tetragonal phase," "sheet-like structures," "hollow-sphere structures", etc., are used without prior definition or explanation.

Respond: Thank you for your suggestion. Non-equilibrium tetragonal phase refers to the rapid cooling rate of molten droplets during plasma spraying, making it difficult to achieve complete phase equilibrium and eutectoid phase transition. Some tetragonal phases still exist in the form of small particles and are easily transformed into t and c phases at high temperatures. The structured descriptions of “sheet like structures” and “hollow sphere structures” can be subjectively understood by comparing them with figures, and there is no official standard definition. Relevant changes can be seen in the revised draft.

24. The discussion on the interaction with CMAS is superficial. Present this discussion in more depth.

Respond: Thank you for your suggestion. The protuberances on the micro-nano dual-scale coating grow under sintering, reducing the complex nanostructure and becoming a factor in reducing the contact angle. According to research of Guo et al.[36], the lotus-leaf- like dual-scale microstructure coating prepared by PS-PVD technology also had many micro-nano protrusions on the surface, forming discontinuous three-phase (solid, liquid, gas) contact lines on the coating surface, reducing the contact cotton between CMAS and the coating, and helping to further reduce their adhesion. Based on the above description, it can be inferred that the coating can still maintain the three-phase contact line after sintering, but the reduction of nano protrusions on the surface leads to an increase in the contact area. Relevant changes can be seen in the revised draft.

25. The heat treatment effects are discussed without considering possible variations in temperature distribution within the furnace or the effect of different heating rates, which could influence sintering behavior and phase transitions.

Respond: Thank you for your suggestion. To avoid experimental errors, all heating rates were set at 10 ℃/min. Relevant changes can be seen in the revised draft.

26. In the conclusion section, highlight the significance of the results and their contribution to the existing body of knowledge. Also, address any limitations or potential areas for future research.

Respond: Thank you for your suggestion. The study has guiding significance for exploring the anti-CMAS corrosion performance of nanoscale coatings, and the wetting behavior of micro-nano dual-scale coatings provides a new solution for solving CMAS corrosion. Relevant changes can be seen in the revised draft.

Reviewer 3 Report

Comments and Suggestions for Authors

Micro-nano dual-scale thermal barrier are an interesting solution used in various sectors of the economy. Different methods of application allow for achieving various effects, important from the point of view of application.

However, the introduction should be expanded in order to present, e.g. in the form of a table, currently used coating solutions and the effects achieved thanks to the applied solutions.

Methodology - information should be supplemented with the accuracy/error of measurements and conducted analyses, the type of measuring devices used, the source of reagents and their purity.

Discussion of results - the units in the table and in the discussion are not the same, the formulas used in the conversion should be provided. The authors should also compare the research results with the results of other researchers, and not only refer to the processes that occur during the experiment.

The authors should also compare the research results with the results of other researchers, and not only refer to the processes that take place during the experiment. Information that is in the tables and is presented in graphs should also be removed from the description - there is no need to duplicate information.

Author Response

Micro-nano dual-scale thermal barrier are an interesting solution used in various sectors of the economy. Different methods of application allow for achieving various effects, important from the point of view of application.

However, the introduction should be expanded in order to present, e.g. in the form of a table, currently used coating solutions and the effects achieved thanks to the applied solutions.

Methodology - information should be supplemented with the accuracy/error of measurements and conducted analyses, the type of measuring devices used, the source of reagents and their purity.

Discussion of results - the units in the table and in the discussion are not the same, the formulas used in the conversion should be provided. The authors should also compare the research results with the results of other researchers, and not only refer to the processes that occur during the experiment.

The authors should also compare the research results with the results of other researchers, and not only refer to the processes that take place during the experiment. Information that is in the tables and is presented in graphs should also be removed from the description - there is no need to duplicate information.

Respond: Thank you for your suggestion. coatings solution, achieve effects and application in the table has been provided. The corresponding sources of reagents and accuracy errors have been provided or explained. The research comparison of other researchers has also been added to the paper. The units and significant figures of the entire article have been checked and modified, and the table regarding morphology has been deleted. Relevant changes can be seen in the revised draft.

Table1 The solution, achieved effects thanks to the applied solutions and practical cases of micro-nano dual-scale coatings.

Solution	Achieved effects	Practical cases
Anti-CMAS attack	Reduce CMAS penetration and improve coating life	High temperature protection of aircraft engine turbine blades
Enhance thermal isolation	Reduce thermal conductivity and improve insulation performance	Thermal insulation coating in gas turbines
Improve mechanical performance	Improve strain tolerance and crack resistance, stabilize mechanical performance	Application of surface coating for high-temperature industrial equipment

Reviewer 4 Report

Comments and Suggestions for Authors

Authors talk about the development of thermal resistance coating using plasma treatment method. 

1. In section 2.1 authors can also consider adding the type of nozzle used for the plasma treatment.

2. Authors can consider moving table 2 on the same page for reader's convenience. 

3. Graph quality in figure 8, 9 and 10 can be improved.

4. Authors can consider addition pictures of commercial coatings used for similar application and show comparison to better asses the performance of overally coating. 

Author Response

Authors talk about the development of thermal resistance coating using plasma treatment method.

1. In section 2.1 authors can also consider adding the type of nozzle used for the plasma treatment.

Respond: Thank you for your suggestion. In APS technology, the distance between the plasma flame and the powder nozzle with a diameter of 1.8mm is 6 mm. In SPPS technology, the angle between injector with a diameter of 1mm and plasma flame is 60°. Relevant changes can be seen in the revised draft.

2. Authors can consider moving table 2 on the same page for reader's convenience.

Respond: Thank you for your advice. All tables have been adjusted to the same page for the convenience of readers. Relevant changes can be seen in the revised draft.

3. Graph quality in figure 8, 9 and 10 can be improved.

Respond: Thank you for your suggestion. The graphs have been remade. Relevant changes can be seen in the revised draft.

4. Authors can consider addition pictures of commercial coatings used for similar application and show comparison to better asses the performance of overally coating.

Respond: Thank you for your suggestion. A micro-nano dual-scale biomimetic structure picture has been added. Relevant changes can be seen in the revised draft.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Accept