Microstructure and Cracking Behavior of a Four-Layer Thermal Barrier Coating After Thermal Cycle Test
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsGeneral comments
This manuscript investigates the microstructure evolution and cracking behavior of a four-layer zirconia-yttria thermal barrier coating (TBC) during thermal cycling. The thermal cycle test was conducted in the temperature range of room temperature to 1100 °C under atmospheric conditions. The study employs SEM, XRD, EDS, and bond strength measurements to analyze changes in sample characteristics after 500 thermal cycles. The authors identify crack formation and discuss its origins. The manuscript is well-written and logically structured, with a title that accurately reflects its content. The references are appropriate and up-to-date. The methodology is detailed, and the results are effectively presented through tables and figures. The interpretation of data and the conclusions are well-supported.
Specific comments
1) Abstract: While the abstract presents the study’s results, it does not clearly state the main conclusions. The authors should briefly state the main conclusions.
2) Introduction: The purpose of this study is not clearly defined. Stating the objective simply as an "investigation of failure mechanisms" is too broad. The authors should specify the primary research purpose more precisely.
3) Methodology: The bond strength measurement method is not described in the Methodology section. The authors should include details on the bond strength evaluation technique and testing conditions in the Methodology section. Was the bond strength measured as tensile strength?
4) The results of bond strength measurement are recommended to be shown in IS units, not American.
Author Response
This manuscript investigates the microstructure evolution and cracking behavior of a four-layer zirconia-yttria thermal barrier coating (TBC) during thermal cycling. The thermal cycle test was conducted in the temperature range of room temperature to 1100 °C under atmospheric conditions. The study employs SEM, XRD, EDS, and bond strength measurements to analyze changes in sample characteristics after 500 thermal cycles. The authors identify crack formation and discuss its origins. The manuscript is well-written and logically structured, with a title that accurately reflects its content. The references are appropriate and up-to-date. The methodology is detailed, and the results are effectively presented through tables and figures. The interpretation of data and the conclusions are well-supported.
Response: We thank the reviewer for the pertinent comments, these comments are instructive for revising and improving our paper. We believe that this research topic may be of interest to the readers of Coatings and hope this paper can be accepted. Again, we thank the reviewer for affirming our work, and it is our honor.
(1) Abstract: While the abstract presents the study’s results, it does not clearly state the main conclusions. The authors should briefly state the main conclusions.
Response: This comment is very instructive and valuable. According to the reviewer's suggestions, we have summarized the experimental results and the content has been added in Abstract: “Large horizontal cracks are likely to form at the TSL/TIL and TIL/TL interfaces, while vertical cracks tend to occur near the surface of the TSL and the propagation rate is relatively low. The propagation of horizontal cracks is the primary cause of failure in this four-layer structure.”
(2) Introduction: The purpose of this study is not clearly defined. Stating the objective simply as an "investigation of failure mechanisms" is too broad. The authors should specify the primary research purpose more precisely.
Response: Thank the reviewer for this comment, the primary research purpose of this article has been added to the final paragraph of the Introduction. The added content is as follows: “The DCL TBCs based on La2Zr2O7 (LZO)/YSZ and Gd2Zr2O7 (GZO)/YSZ have been extensively investigated [23,24]. However, TBC with double YSZ layers has been de-signed for 130MW gas turbine first stage blade [25], there is still a lack of studies on microstructure stability and failure mechanism. In this study, a thermal cycle test was conducted to simulate the start-stop processes of gas turbines, the microstructure change, mechanisms of crack initiation and growth in the TBC based on the double YSZ layers were investigated.”
(3) Methodology: The bond strength measurement method is not described in the Methodology section. The authors should include details on the bond strength evaluation technique and testing conditions in the Methodology section. Was the bond strength measured as tensile strength?
Response: Thank the reviewer for this comment, we have add the bond measurement method in the Methodology section. The added content is as follows: “To evaluate the bonding performance of TBC in this study, a bond strength test was performed using the ASTM C 633-13 method on coated GTD 111 samples. Figure 6 illustrates a schematic diagram of the bond strength test. The epoxy is FM 1000, Lot# 6349-0005. The cross-sectional areas of the samples are 5.03 cm² and 4.90 cm², respectively. The crosshead speed is 1.016 mm/min.”
(4) The results of bond strength measurement are recommended to be shown in IS units, not American.
Response: The results of bond strength measurement have been shown in IS units.
Reviewer 2 Report
Comments and Suggestions for Authors-The similarity rate must be reduced; the overall similarity rate should be below 15%, and the similarity rate from a single source should be below 2%.
-In the first sentence of the abstract, the necessity of the study should be stated by highlighting its difference from the existing literature. Numerical critical results should be included in the abstract. The significance of the study in terms of its application area should be presented in the final sentence of the abstract.
-The introduction section must be completely rewritten. Please take similar articles published in high-impact factor journals as examples. The introduction should provide all the necessary literature information while demonstrating the necessity of the study. Each paragraph should complement the previous one, ultimately leading to the necessity of the study in the conclusion. The final paragraph should explicitly highlight the originality of the study. Examples from the literature should include who conducted the studies and what results were obtained.
-There are significant deficiencies in the experimental section of the study. It should provide detailed explanations on where the materials were sourced from and how the production processes were carried out. Additionally, a graphical workflow diagram should be included to illustrate the process.
-The hardness values of the obtained coatings should be determined in the study.
-The tests presented in this study seem insufficient for the Coatings journal. Additional tests, such as wear and corrosion, should be conducted to evaluate their effect on barrier protection.
-The discussion section of the study is significantly lacking. All obtained results should be analyzed and discussed based on the underlying mechanisms, supported by relevant literature.
-In the conclusion section, all results obtained throughout the study should be presented. Critical limitations should be highlighted, and future research directions should be mentioned.
-The references used in the study are quite outdated. More recent sources should be utilized to ensure the study reflects the latest developments in the field.
Author Response
(1) The similarity rate must be reduced; the overall similarity rate should be below 15%, and the similarity rate from a single source should be below 2%.
Response: Thank the reviewer for this comment, the similarity rate has been reduced.
(2) In the first sentence of the abstract, the necessity of the study should be stated by highlighting its difference from the existing literature. Numerical critical results should be included in the abstract. The significance of the study in terms of its application area should be presented in the final sentence of the abstract.
Response: Thank the reviewer for this comment. The DCL TBCs based on La2Zr2O7 (LZO)/YSZ and Gd2Zr2O7 (GZO)/YSZ have been extensively investigated. However, there is still a lack of studies on microstructure stability and failure mechanism of TBC with double YSZ layers. So, we have highlighted “thermal barrier coating (TBC) with double YSZ layers” in the first sentence of the abstract.
We have added two numerical results in the Abstract. “the thickness of the TGO layer increases with a relatively low increase rate during the test (where kp is about 0.17 μm2/h)” and “After the thermal cycle test, the porosity of the TSL decreased significantly, from 7.17% to 0.76%.”
The possible application area has been added to the final sentence of the abstract: “The results in this study may help optimize the design and preparation of TBCs with double YSZ layers.”
(3) The introduction section must be completely rewritten. Please take similar articles published in high-impact factor journals as examples. The introduction should provide all the necessary literature information while demonstrating the necessity of the study. Each paragraph should complement the previous one, ultimately leading to the necessity of the study in the conclusion. The final paragraph should explicitly highlight the originality of the study. Examples from the literature should include who conducted the studies and what results were obtained.
Response: Thank the reviewer for this comment，This comment is very instructive and valuable. The introduction section has been rewritten and the revised content has been highlighted in red in the Introduction section.
(4) There are significant deficiencies in the experimental section of the study. It should provide detailed explanations on where the materials were sourced from and how the production processes were carried out. Additionally, a graphical workflow diagram should be included to illustrate the process.
Response: Thank the reviewer for this comment. The source of the materials used in the experiment and the equipment for coating preparation have been specified in the Methodology section. Additionally, the schematic diagram of the thermal cycle test is shown in Figure 1.
(5) The hardness values of the obtained coatings should be determined in the study.
Response: Nano-indentation tests have been performed on the cross-sections in the samples of as-sprayed state and after thermal cycle test. The related content is added to Section 3.6.
(6) The tests presented in this study seem insufficient for the Coatings journal. Additional tests, such as wear and corrosion, should be conducted to evaluate their effect on barrier protection.
Response: As the reviewer mentioned, in addition to thermal cycling performance, corrosion resistance is another very important performance indicator. In fact, we have conducted a 3000-hour thermal exposure test on the thermal barrier coatings to study their corrosion resistance. Due to the limited time for revising this article, we regret that we cannot include this part in the current paper.
(7) The discussion section of the study is significantly lacking. All obtained results should be analyzed and discussed based on the underlying mechanisms, supported by relevant literature.
Response: Thank the reviewer for this comment. We have added analysis of the experimental results and comparisons with relevant literature in the Results and Discussion section. The related content has been highlighted in red in the paper.
(8) In the conclusion section, all results obtained throughout the study should be presented. Critical limitations should be highlighted, and future research directions should be mentioned.
Response: Thank the reviewer for this comment. We have added several important conclusions in the Conclusion section.
(9) The references used in the study are quite outdated. More recent sources should be utilized to ensure the study reflects the latest developments in the field.
Response: Thank the reviewer for this comment. To improve the quality of the article, we have cited recently published papers in the Introduction and Results and Discussion sections.
Reviewer 3 Report
Comments and Suggestions for Authors1. Specify powder fractions.
2. Decode HVOF. Include this method in the Introduction section. Include this abbreviation also in the Abbreviations section.
3. How was the temperature controlled in the termal cycle tests? Cooling was performed up to which temperature?
4. Indicate the cross-sectional areas also in mm2. (Line 143). It is not clear if the specimens were either rectangular shape or circular one for the bond strength tests?
5. Describe the technique for determining bond strength in the Methodology section.
6. Give the accuracy of the bond strength measurement to one decimal place. (Line 144, 235).
7. What was material for parent metal ? And how was its tensile strength obtained after testing?
8. Also indicate the values in SI system in Table 2.
9. What epoxy adhesive was used for the tests? How were the test specimens prepared?
10. In formula (1), instead of the ^ character, use the upper index to indicate degree.
11. Which finite element analysis is mentioned in the paper (Line 207)?
12. In Table 2, change Tensile Strength to Bond Strength.
13. The bond strength increased or decreased after thermal cycling tests?
Author Response
(1) Specify powder fractions.
Response: The BL, TL, TIL and TSL were sprayed using pure Amdry 9954 powder, Amdry 962 powder, Amdry 204NS-1 powder and Amdry 204F powder, respectively.
(2) Decode HVOF. Include this method in the Introduction section. Include this abbreviation also in the Abbreviations section.
Response: Thank the reviewer for this comment. The relevant content has been added in Methodology section:“In addition to the APS, high velocity oxygen fuel spray (HVOF) is also a common method for preparing the BC.”
(3) How was the temperature controlled in the termal cycle tests? Cooling was performed up to which temperature?
Response: The temperature was controlled by a K-type thermocouple. The TBC surface temperature fan-cooled down to below 200 °C with 10 minutes during the thermal cycle test. The relevant content has been added in Methodology section.
(4) Indicate the cross-sectional areas also in mm2. (Line 143). It is not clear if the specimens were either rectangular shape or circular one for the bond strength tests?
Response: The cross-sectional areas have been indicated in mm2. The specimens are rectangular shape for the bond strength tests. The relevant content has been added in Methodology section.
(5) Describe the technique for determining bond strength in the Methodology section.
Response: The technique for determining bond strength has been described in Methodology section.
(6) Give the accuracy of the bond strength measurement to one decimal place. (Line 144, 235).
Response: The bond strength is already given to one decimal place.
(7) What was material for parent metal ? And how was its tensile strength obtained after testing?
Response: The material for parent metal is also GTD 111 alloy. The tensile strength was performed using the ASTM C 633-13 method.
(8) Also indicate the values in SI system in Table 2.
Response: The values in Table 2 have been indicated in SI system.
(9) What epoxy adhesive was used for the tests? How were the test specimens prepared?
Response: The epoxy is FM 1000, Lot# 6349-0005. The process and structure for the TBC on the test specimens are the same as that for the thermal cycle test specimens.
(10) In formula (1), instead of the ^ character, use the upper index to indicate degree.
Response: The formula (1) has been expressed as h2=kp∙t.
(11) Which finite element analysis is mentioned in the paper (Line 207)?
Response: The relevant reference has been added.
(12) In Table 2, change Tensile Strength to Bond Strength.
Response: “Tensile Strength” has been changed to “Bond Strength” in Table 2.
(13) The bond strength increased or decreased after thermal cycling tests?
Response: This comment is very instructive and valuable. We apologize for not being able to conduct the bond strength test after thermal cycling test. In the following experiments, we will conduct this test if there is an opportunity.
Round 2
Reviewer 2 Report
Comments and Suggestions for Authorsthanks for satisfactory revisions
Author Response
Comments and Suggestions for Authors: thanks for satisfactory revisions. Response: Thank you very much for the reviewers' affirmation.Reviewer 3 Report
Comments and Suggestions for Authors1. Why did the mention of Fig. 6 appeared in Section 2.Methodology? Following the order in Section 2.Methodology, the reference to Fig. 6 should have the number 2 (i.e. Fig. 6 should be replaced by Fig. 2). And this figure should be after Section 2.Methodology.
2. ASTM C 633-13 uses 25 mm diameter cylindrical specimens, not rectangular specimens!
3. It is not clear what strength is meant for parent metal. In paper [Pauzi, A.A., Ghaffar, M.A., Chang, S.Y., Ng, G.P. and Husin, S., 2017, October. A study on tensile deformation at room temperature and 650° C in the directional solidified Ni-base superalloy GTD-111. In Journal of Physics: Conference Series (Vol. 914, No. 1, p. 012021). IOP Publishing.] for Ni-base superalloy GTD-111 indicated yield strength has a value of 702.72 MPa. However, in the peer-reviewed paper it is mentioned that the parent metal (uperalloy GTD-111) has yielding at a value of 84.9 MPa. The values given in the paper do not correspond to the literature data. In addition, it is not clear how the bond strength of the coating is obtained for the parent metal.
4. The strength of the adhesive (FM-1000) is less than 73 MPa. How can the bond strength of a coating of 73.7 MPa be measured with using this adhesive?
5. In Line 170 the value of 84.9 MPa is given for parent metal. And in Table 2, this value is given for bond strength.
Author Response
Dear Reviewer,
On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript. We hope that our responses could meet your requirements. For clearness, the corrections made to the manuscript were highlighted in the red color. The corrections in the paper and the responses to your comments are as follows:
- Why did the mention of Fig. 6 appeared in Section 2.Methodology? Following the order in Section 2.Methodology, the reference to Fig. 6 should have the number 2 (i.e. Fig. 6 should be replaced by Fig. 2). And this figure should be after Section 2.Methodology.
Response: This comment is very beneficial to this article. According to the reviewer's suggestions, we have moved Figure 6 into Methodology and replaced Figure 6 by Figure 2.
- ASTM C 633-13 uses 25 mm diameter cylindrical specimens, not rectangular specimens!
Response: Thank you very much for this comment. We checked the test report once again and the specimens really are cylindrical shape. The corresponding content has been revised in the article.
- It is not clear what strength is meant for parent metal. In paper [Pauzi, A.A., Ghaffar, M.A., Chang, S.Y., Ng, G.P. and Husin, S., 2017, October. A study on tensile deformation at room temperature and 650° C in the directional solidified Ni-base superalloy GTD-111. In Journal of Physics: Conference Series (Vol. 914, No. 1, p. 012021). IOP Publishing.] for Ni-base superalloy GTD-111 indicated yield strength has a value of 702.72 MPa. However, in the peer-reviewed paper it is mentioned that the parent metal (uperalloy GTD-111) has yielding at a value of 84.9 MPa. The values given in the paper do not correspond to the literature data. In addition, it is not clear how the bond strength of the coating is obtained for the parent metal.
Response: Thank you very much for this comment. We apologize for not clearly explaining this test in the article. This test actually act as a reference test. In the reference test, there was only epoxy between the pulling fixture and parent metal (superalloy), and the result demonstrated the strength of the adhesive (FM1000). The corresponding content has been added in the article.
- The strength of the adhesive (FM-1000) is less than 73 MPa. How can the bond strength of a coating of 73.7 MPa be measured with using this adhesive?
Response: In this article, the Bond strength of TBC was tested by a professional third party testing body (Metal lmprovement Company TechnologyService (suzhou) Co., Ltd.). In paper [Lima, R.S. Porous APS YSZ TBC Manufactured at High Powder Feed Rate (100 g/min) and Deposition Efficiency (70%): Microstructure, Bond Strength and Thermal Gradients. J. Therm. Spray. Tech. 2022, 31, 396–414], the strength of the adhesive (FM-1000) is 77 MPa. Part of the test report in this article is shown in Figure 1.
Figure 1 results of the Bond strength.
- In Line 170 the value of 84.9 MPa is given for parent metal. And in Table 2, this value is given for bond strength.
Response: Thank you very much for this comment. We sincerely apologize for the error. Actually, the strength of the adhesive (FM1000) is 84.9 MPa. The corresponding content has been revised in the article.
Round 3
Reviewer 3 Report
Comments and Suggestions for Authors1. The description of the tensile testing machine used for tensile testing according to ASTM C 633 in section 2 Methodology is missing.
2. If the specimens are cylindrical, their diameter should be indicated. And delete the cross-sectional areas of these specimens from the text (Line 100, Line 101).
3. The information from the sentence ‘In addition, the strength of the adhesive (FM1000) is 84.9 MPa.’ should be added to Table 2. And this sentence should be deleted from the text of the paper.
4. A column should be added to Table 2 specifying the diameter of the samples. And the column ‘Cross Sectional Area (mm2)’ should be deleted from the table, as it duplicates the information that can be obtained after knowing the diameter of cylindrical specimens. Or explain the necessity for the reader to know the cross sectional area.
5. ASTM C 633 standard requires the use of 5 specimens. Only two specimens are used in this paper. Therefore, it is not correct to determine the average bond strength after testing only two specimens.
Author Response
1. The description of the tensile testing machine used for tensile testing according to ASTM C 633 in section 2 Methodology is missing.
Response: This comment is beneficial to this article. The bonding strength test was conducted by Metal lmprovement Company Technology Service (suzhou) Co., Ltd., the company is unwilling to provide information about the equipment. So, we apologize for being unable to provide information of the tensile testing machine.
2. If the specimens are cylindrical, their diameter should be indicated. And delete the cross-sectional areas of these specimens from the text (Line 100, Line 101).
Response: Thank you very much for this comment. The diameters of the specimens have been indicated. The corresponding content has been added at Line 100: “The diameters of the samples are 25.30 mm and 24.97 mm, respectively.”
3. The information from the sentence ‘In addition, the strength of the adhesive (FM1000) is 84.9 MPa.’ should be added to Table 2. And this sentence should be deleted from the text of the paper.
Response: Thank you very much for this comment. The strength of the adhesive (FM1000) has been added to Table 2 and the corresponding sentence has been deleted from the text of the paper.
4. A column should be added to Table 2 specifying the diameter of the samples. And the column ‘Cross Sectional Area (mm2)’ should be deleted from the table, as it duplicates the information that can be obtained after knowing the diameter of cylindrical specimens. Or explain the necessity for the reader to know the cross sectional area.
Response: Thank you very much for this comment. A column “diameter” has been added to Table 2 and the column “Cross Sectional Area (mm2)” has been deleted.
5. ASTM C 633 standard requires the use of 5 specimens. Only two specimens are used in this paper. Therefore, it is not correct to determine the average bond strength after testing only two specimens.
Response: This comment is very beneficial to this article. The corresponding content has been revised in Section 3.3: “The bond strengths of the samples are 76.7 MPa and 70.8 MPa, respectively.”
