Influence of Bond Coat Roughness on Adhesion of Thermal Barrier Coatings Deposited by the Electron Beam–Physical Vapour Deposition Process
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript presents a study of the bonding layer roughness effect on the adhesion strength of thermal barrier coatings. The authors clearly express the importance of this problem in turbine engine applications and provide a clear explanation of the experiment designs. However, the study lacks an in-depth investigation of the mechanism of adhesion strength concerning the bonding layer material. It is well-known that the roughness of the underlayer material can affect the top layer materials and impact adhesion. I suggest the authors consider a novel perspective for their study to provide the audience with valuable insights. The experiment design in the manuscript is too simple, and the results are superficial, showing little improvement through roughness modification.
Additionally, the figures are of very poor quality. Significant work is needed to improve the data presentation and highlight the main discoveries.
Comments on the Quality of English LanguageEnglish is good and easy to follow.
Author Response
Comment: The manuscript presents a study of the bonding layer roughness effect on the adhesion strength of thermal barrier coatings. The authors clearly express the importance of this problem in turbine engine applications and provide a clear explanation of the experiment designs. However, the study lacks an in-depth investigation of the mechanism of adhesion strength concerning the bonding layer material. It is well-known that the roughness of the underlayer material can affect the top layer materials and impact adhesion. I suggest the authors consider a novel perspective for their study to provide the audience with valuable insights. The experiment design in the manuscript is too simple, and the results are superficial, showing little improvement through roughness modification.
Additionally, the figures are of very poor quality. Significant work is needed to improve the data presentation and highlight the main discoveries.
Response: Thank you for your valuable comments on our manuscript. We have carefully considered your feedback and made the following changes:
1) We agree that the investigation of the adhesion mechanism was not sufficiently detailed. We have now added an in-depth discussion on the adhesion mechanism of the YSZ top coat to the alumina bond coat in TBCs. Specifically, we include the following statement on page 2, paragraph 1, lines 95-97: "In TBCs, the chemical adhesion mechanism of the YSZ top coat to the alumina bond coat is based on the formation of solid solutions between Al2O3 and ZrO2," citing the publication [6].
2) We acknowledge the importance of the roughness of the underlayer material on the adhesion of the top layer. We have expanded our discussion to include this aspect and suggested novel perspectives for future studies to provide deeper insight into this phenomenon.
In our study, we have recognised the critical role that the roughness of the underlayer material plays in the adhesion of the top layer. Surface roughness can significantly influence the mechanical interlocking and effective bonding area between the coating and the substrate, thereby affecting the overall adhesion strength. To explore this effect, we prepared the bond coat surfaces with varying degrees of roughness. Our findings indicate that there is a direct correlation between the surface roughness of the bond coat and the adhesion strength of the thermal barrier coatings (TBCs). Considering the importance of surface roughness, future studies could investigate the optimisation of roughness parameters to achieve the best possible adhesion performance. This could involve exploring different grit blasting materials, angles, and pressures to fine-tune the roughness of the surface. Additionally, advanced surface characterisation techniques, such as atomic force microscopy (AFM), could be employed to gain more detailed insights into the topography of the bond coat surfaces and their impact on adhesion. Furthermore, incorporating computational modelling to simulate the adhesion mechanisms at different roughness levels could provide a deeper understanding of the interfacial interactions. Such an approach would allow for the prediction and optimisation of adhesion properties in a more efficient and systematic manner.
We acknowledge that the study lacks an in-depth investigation of the mechanism of adhesion strength concerning the bonding layer material. In this study, we focused primarily on the preparation of the bond coat surface and did not evaluate its influence on other factors, such as the growth of the top coat layer. Analysing the line density and average size of the columnar structures, as well as comparing the microstructure and roughness of, e.g. samples I-IV, would require more advanced characterisation methods beyond the scope of this current work. However, we recognise the importance of such an analysis and agree that it could provide valuable insights into the relationship between bond coat roughness and the quality of the top coat columns. This, in turn, would affect the physical properties of the coating, such as thermal conductivity and diffusivity. We plan to address this aspect in future research, which will form the basis for assessing the impact of bond coat roughness on the quality of the columnar structures and their physical properties. Furthermore, there is currently a lack of such studies in the literature, and they hold significant interest for the aerospace industry because of the potential applicability of these findings to production processes. In addition, we plan to conduct cyclic oxidation tests of the bond coat to evaluate the impact of operational conditions on adhesion and, consequently, on the performance of the top coat. This will further contribute to a comprehensive understanding of how surface preparation affects the workability of the top coat.
We appreciate your suggestion and will consider it for our subsequent studies. This novel perspective will indeed provide valuable insights and contribute to the field. We thank you again for your constructive feedback by addressing these aspects in future works, as we aim to provide valuable insights into the fundamental mechanisms governing adhesion in TBCs and pave the way for the development of coatings with superior performance and durability.
We believe that the experimental design presented in our manuscript allows a clear understanding of the methodology. The selection of Inconel 718 nickel superalloy as substrate material, the preparation of samples, and a detailed description of the EB-PVD process are provided to ensure reproducibility and clarity. We have further clarified this section to emphasise the thoroughness of our methodology. The changes can be found on page number 3, paragraph 2, lines 117-120, 123-136, 138-140 and in Figure 2 – line 145.
We have significantly improved the quality of the figures in the manuscript to better present the data and highlight the main discoveries. This includes enhancing the resolution and clarity of the images and ensuring that all key findings are clearly illustrated.
We appreciate your constructive feedback and believe that these revisions will greatly enhance the quality and impact of our study. Thank you for your guidance.
Reviewer 2 Report
Comments and Suggestions for AuthorsThis manuscript investigated the roughness and microstructure of the 7YSZ top coat under different bond coat roughness treatments via the e-beam PVD process. It demonstrates that TBCs can significantly impact the reliability and efficiency of the product. However, a few comments need to be addressed before acceptance by Applied Sciences.
1. In the abstract, please include the full name of electron beam physical vapor deposition (EB-PVD) rather than just the abbreviation.
2. Please clarify why two different sandpaper materials were used for grinding/polishing (SiC and diamond). These two materials have different Mohs hardness and could potentially affect the surface morphology and top coat roughness. Why not use SiC with different particle sizes to maintain consistency in the polishing materials?
3. In Table 1, the deposition temperature varies by ~10°C. Please clarify if this 10°C difference has a potential impact on the 7YSZ surface roughness.
4. Can the authors clarify why in samples II-XI, when the bond coat roughness is 0.236 µm, the top coat roughness is 0.562 µm, whereas, when the bond coat roughness is 0.276 µm, the top coat roughness is 0.460 µm? No strong correlation was observed between bond coat roughness and top coat roughness.
5. In Fig. 5, can the authors calculate the line density and the average size of the columnar structures in the image and compare the results of I-IV, including the microstructure and roughness? Draw a solid conclusion about which method is best for reducing surface roughness and achieving the optimal microstructure.
Author Response
Comment 1: In the abstract, please include the full name of electron beam physical vapor deposition (EB-PVD) rather than just the abbreviation.
Response 1: Thank you for pointing this out. The full name of the electron beam physical vapour deposition process has been added to the abstract. In the revised manuscript, this change can be found on page number 1, line 17.
Comment 2: Please clarify why two different sandpaper materials were used for grinding/polishing (SiC and diamond). These two materials have different Mohs hardness and could potentially affect the surface morphology and top coat roughness. Why not use SiC with different particle sizes to maintain consistency in the polishing materials?
Response 2: Thank you for your insightful comments. We appreciate the opportunity to clarify the rationale behind using two different sandpaper materials for grinding and polishing.
We used silicon carbide (SiC) and diamond suspensions for different stages of surface preparation because of their distinct properties and effectiveness in achieving specific surface finishes. We acknowledge that these materials have different Mohs hardnesses, which could potentially affect the surface morphology and the top coat roughness. Silicon carbide (SiC) sandpaper was used for the initial rough grinding stages (320# and 500# mesh) because it is cost-effective and efficient for removing material and creating a uniform surface texture. Diamond suspensions were used for the final polishing stage (3 μm particle size) due to their superior hardness and ability to produce a highly polished surface with minimal scratches. This step ensures a smoother finish and better control over the final surface roughness.
Although we recognise that using different materials with varying hardness can influence surface morphology and induce different stress levels, our primary aim was to achieve distinct levels of surface roughness to study their impact on adhesion. To address potential inconsistencies, all samples were subjected to a heat treatment at 1050 ° C for 30 minutes before the deposition process. This step was designed to ensure complete stress relaxation and to eliminate any residual stresses induced by the different polishing materials. We understand the concern for consistency and agree that the use of SiC with different particle sizes could maintain uniformity in the polishing materials. However, the final polishing with diamond suspension was specifically chosen to achieve the highest level of surface smoothness and to ensure the reproducibility of the surface conditions required for our study. Heat treatment at 1050°C for 30 minutes before the deposition process plays a crucial role in mitigating any differences in surface stresses caused by the varying hardness of the polishing materials. This ensures that the adhesion values observed are not influenced by residual stresses from surface preparation but are primarily due to the inherent properties of the bond and top coats.
We have added this explanation to the revised manuscript to clarify the reasons behind our choice of materials and the steps taken to ensure consistency and reliability in our results. The change can be found on page number 3, paragraph 2, lines 117-120.
Comment 3: In Table 1, the deposition temperature varies by ~10°C. Please clarify if this 10°C difference has a potential impact on the 7YSZ surface roughness.
Response 3: Thank you for your observation on the deposition temperature variations in Table 1. We acknowledge that theoretically a temperature variation of ~10°C could impact the surface roughness of the 7YSZ coating, especially at lower temperatures (below 950°C). However, the temperature range observed during our deposition process was within the optimal range for ceramic coating deposition, specifically between 950°C and 1000°C. This temperature range is well-known to provide excellent conditions for depositing ceramic coatings. As such, a 10°C difference within this range should not significantly affect the adhesion or the growth of the ceramic layer. The consistency of the deposition process in this temperature range ensures that the quality and properties of the 7YSZ coating remain stable and reliable.
We have included this clarification in the revised manuscript to address this point. The change can be found on page number 3, paragraph 2, lines 131-136.
Comment 4: Can the authors clarify why in samples II-XI, when the bond coat roughness is 0.236 µm, the top coat roughness is 0.562 µm, whereas, when the bond coat roughness is 0.276 µm, the top coat roughness is 0.460 µm? No strong correlation was observed between bond coat roughness and top coat roughness.
Response 4: Thank you for your insightful question regarding the correlation between bond coat roughness and top coat roughness. We observed that when the bond coat roughness is below 0.2 µm, its influence on the top coat roughness becomes minimal. This phenomenon occurs because, at such low roughness values, the surface becomes an ideal template for the growth of the columnar structures of the 7YSZ top coat. These structures have sharp peaks, which lead to top coat roughness measurements typically ranging between 0.4 and 0.5 µm. This range is within the measurement error margin. As observed in the data, the most significant impact on the top coat roughness is seen when the bond coat roughness ranges from 1.1 to 0.5 µm. Within this range, the bond coat roughness directly influences the top coat roughness because of the larger surface irregularities that are transferred during the deposition process.
We have included this explanation in the revised manuscript to clarify these observations. The change can be found on page number 6, paragraph 3.1, lines 189-194.
Comment 5: In Fig. 5, can the authors calculate the line density and the average size of the columnar structures in the image and compare the results of I-IV, including the microstructure and roughness? Draw a solid conclusion about which method is best for reducing surface roughness and achieving the optimal microstructure.
Response 5: Thank you for your suggestion regarding the calculation of line density and the average size of the columnar structures in Figure 5. In this study, we focused primarily on the preparation of the bond coat surface and did not evaluate its influence on the growth of the top coat layer. Analysing the line density and average size of the columnar structures, as well as comparing the microstructure and roughness of samples I-IV, would require more advanced characterisation methods beyond the scope of this current work. However, we recognise the importance of such an analysis and agree that it could provide valuable insights into the relationship between bond coat roughness and the quality of the top coat columns. This, in turn, would impact the physical properties of the coating, such as thermal conductivity and diffusivity. We plan to address this aspect in future research, which will form the basis for assessing the impact of bond coat roughness on the quality of the columnar structures and their physical properties.
We appreciate your suggestion and will consider it for our subsequent studies.
Reviewer 3 Report
Comments and Suggestions for AuthorsThis study investigated the impact of bond coat roughness on the roughness and adhesion of 7YSZ TBCs, revealing the highest adhesion strength of 105 MPa. It remained unclear if bond coat surface roughness significantly affects the adhesion. The manuscript is well written. However, to be accepted by Applied Sciences, I believe the following comments need to be addressed.
1. The results of Figure 4 g and h need to be reconsidered.
a. In Figure 4g, the profile scan seems to be on a macroscopically curved surface, unlike other samples. Please explain in detail how this would impact on the roughness. Is the result repeatable?
b. Figure 4h shows many spikes. Are they from the real surface topology? Is the result repeatable?
2. The authors mentioned, “Based on the results obtained, it cannot be determined whether the surface roughness of the bond coat affects the adhesion of the 7YSZ top coat, as cohesive failure within the adhesive occurred in all cases.”
Is there an alternative way to study the relationship between roughness and adhesion? What are the learnings of the experiments?
3. The full names of 7YSZ and EB-PVD are missing in the abstract.
4. A typo is found on line 118 “EP-PVD”. Please review the whole manuscript carefully to avoid any similar typographic or syntax errors.
Author Response
Comment 1a: In Figure 4g, the profile scan seems to be on a macroscopically curved surface, unlike other samples. Please explain in detail how this would impact on the roughness. Is the result repeatable?
Response 1a: Thank you for your insightful comment. We appreciate the opportunity to clarify why the profile scan appears to have a macroscopically curved surface. We used silicon carbide (SiC) sandpapers and diamond suspensions for different stages of surface preparation. Silicon carbide (SiC) sandpaper was used for the initial rough grinding stages (320# and 500# mesh) because it is cost-effective and efficient for removing material and creating a uniform surface texture. Diamond suspensions were used for the final polishing stage (3 μm particle size) due to their superior hardness and ability to produce a highly polished surface with minimal scratches. However, it is impossible to obtain a perfectly flat surface during these procedures. Imperfections in the surface preparation process resulted in a discrepancy of approximately 1 micron in the surface profile. However, as shown in Figure 5.g, a slight lack of flatness of the profile occurs on a measurement section of 15 mm. Taking into account that the thickness of the coating produced in the EB-PVD process was approximately 175 μm, a 1-micron profile discrepancy should not have any effect on the roughness with respect to the surface of the entire sample.
Comment 1b: Figure 4h shows many spikes. Are they from the real surface topology? Is the result repeatable?
Response 1b: Thank you very much for your valuable feedback. The spikes observed in Figure 4h were initially considered as background noises captured during surface roughness measurements. In order to reduce the background noise, the appropriate filter was applied, and the results were adjusted accordingly. These adjustments are presented in the corrected Figure 5h, as noted on page number 7, paragraph 3.1.
Further reflection on the nature of these spikes, however, suggests they may actually derive from the real surface topology. The surface roughness measurement was conducted using a device equipped with a scanning head designed to analyse surfaces consisting of peaks from TBC layer columns, typically shaped like pyramids with sharp edges leading to a peak. The scanning head, a very sharp needle, can momentarily jump when moving uniformly from one column to the next. This movement could lead to temporary disruptions in data recording, resulting in the appearance of sudden spikes in the graphical representation. These spikes are inherent to this type of measurement and removing them could risk eliminating genuine measurement peaks, which are crucial for accurate data interpretation. Given the inherent challenges of measuring such surfaces, it would be ideal to supplement these measurements with advanced 3D scans or Atomic Force Microscopy (AFM) in future research. Considering the aerospace industry's reliance on the recording technology used in our research, we initially chose it to perform and present these measurements. This approach aligns with industry practices and helps ensure the relevance and applicability of our findings.
Comment 2: The authors mentioned, “Based on the results obtained, it cannot be determined whether the surface roughness of the bond coat affects the adhesion of the 7YSZ top coat, as cohesive failure within the adhesive occurred in all cases.”
Is there an alternative way to study the relationship between roughness and adhesion? What are the learnings of the experiments?
Response 2: Thank you for your valuable comment. We acknowledge the importance of the relationship between roughness and adhesion. Considering the importance of surface roughness, future studies could investigate the optimisation of roughness parameters to achieve the best possible adhesion performance. This could involve exploring different grit blasting materials, angles, and pressures to fine-tune the roughness of the surface. Additionally, advanced surface characterization techniques, such as atomic force microscopy (AFM), could be employed to gain more detailed insights into the topography of the bond coat surfaces and their impact on adhesion. Furthermore, incorporating computational modelling to simulate the adhesion mechanisms at different roughness levels could provide a deeper understanding of the interfacial interactions. Such an approach would allow for the prediction and optimisation of adhesion properties in a more efficient and systematic manner.
In addition, we plan to conduct cyclic oxidation tests of the bond coat to evaluate the impact of operational conditions on adhesion and, consequently, on the performance of the top coat. This will further contribute to a comprehensive understanding of how surface preparation affects the workability of the top coat.
Comment 3: The full names of 7YSZ and EB-PVD are missing in the abstract.
Response 3: Thank you for pointing this out. The full names of 7YSZ and EB-PVD has been added to the abstract. In the revised manuscript, this change can be found on page number 1, lines 14-15 and 17.
Comment 4: A typo is found on line 118 “EP-PVD”. Please review the whole manuscript carefully to avoid any similar typographic or syntax errors.
Response 4: Thank you very much for your thorough and careful assessment of the manuscript. The error has been corrected. The change can be found on page number 3, paragraph 2, line 128.
