Design and Processing of Hard and Self-Lubricating NiCr/hBN-cBN Composite Coatings by Laser Cladding: Investigation of Microstructure, Hardness, and Wear
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsWhat is the innovation of the manuscript?
Table 2 does not show the powder units.
How was the composition of the 3 samples chosen? Previous work?
In Figure 5(a), the diffraction peaks could be indexed for each type of powder, to identify the Ni and Cr powders.
Is Figure 7 of the cross-section of the substrates?
Figure 8 needs a change in the figure caption for better understanding. Was no mapping performed on the MT3 sample?
What is the influence of particle size on the hardness of the coatings? Since the cBN particle is much larger than the hBN particle. Why were the particle sizes not homogeneous?
In line 351, it says that area E is oxygen-rich and area D is oxygen-poor. However, the table shows the opposite.
Comments on the Quality of English LanguageEnglish needs to be increased.
Author Response
Dear Reviewer
Thank you for your valuable comments and guidance, which have greatly helped improve the quality of the article. We have tried to take all your comments into account as much as possible, and the desired changes have been made in the revised version. These changes are highlighted in red.
Best regards,
Authors
Comments 1: [What is the innovation of the manuscript?]
Response 1: [Thank you for your comment. Overall, the manuscript has presented an important innovation in improving the performance of gas turbine blades by introducing a new optimized coating with minimum wear rate and friction coefficient. Case in point:
(1) Development of new wear-resistant coatings: In this research, for the first time, the combination of three types of coatings (NiCr-2%hBN, NiCr-12%cBN, and NiCr-2%hBN-12%cBN) on GTD-111 superalloy has been investigated using laser cladding technology. (2) Simultaneous investigation of the effect of hBN and cBN on hardness and wear behavior: Although the effect of each of these compounds is known separately, this research has investigated for the first time the simultaneous effect of hBN (with low lubricating properties and hardness) and cBN (with high hardness), which has led to the creation of a coating with minimum wear rate and friction coefficient. (3) Identification of the mechanism of wear resistance improvement: The paper shows that in the hBN-cBN coating, the presence of stronger driving forces for heterogeneous nucleation in the melt leads to the formation of more equiaxed grains, which has a significant effect on increasing the wear resistance of the coating. (4) Detailed analysis of microstructure and wear behavior: The use of various analytical tools (SEM, hardness tester, pin-on-disk wear device, and 3D profiler) to systematically characterize these coatings has provided new data on the mechanical properties and wear performance of these compounds.]
Comments 2: Table 2 does not show the powder units.
Response 2: [Thank you for your attention, respected referee. The unit of Table 2 was wt%, which was added to Table 2.]
Comments 3: How was the composition of the 3 samples chosen? Previous work?
Response 3: [Thank you for your comment, esteemed referee. We have examined different compositions in the initial tests. The combination of the three samples occurred based on various reasons, which are mentioned below:
(1) The amount of 12% by volume of cBN was chosen because: cBN has a very high hardness, but in large amounts it may increase the brittleness of the coating. In the initial tests, a value higher than 12% led to an increase in residual stress cracks, which was due to the difference in the coefficient of thermal expansion between cBN and the NiCr matrix. Values ​​lower than 12% reduced the positive effect of cBN on hardness and wear resistance.
(2) The amount of 2% by volume of hBN was chosen for the following reasons: hBN has lubricating properties due to its layered structure and facilitates friction reduction. In amounts higher than 2%, hBN caused an excessive decrease in the hardness of the coating, which was determined in the microhardness tests. At levels below 2%, the lubricating effect of hBN on the friction coefficient decreased and its effect on wear reduction became less noticeable.] [Also, a summary of the mentioned items was added to the experiments section (page 3 lines 131-136).]
Comments 4: [In Figure 5(a), the diffraction peaks could be indexed for each type of powder, to identify the Ni and Cr powders.]
Response 4: [Thank you for your valuable comment. Under non-equilibrium cooling conditions (such as laser cladding in this study), the formation of a Ni-Cr solid solution with an FCC structure is possible. Therefore, distinct Ni and Cr peaks are not observed separately. These modifications have been incorporated into the manuscript (Page 6, Lines 194-197) and Figure 5(a).]
Comments 5: [Is Figure 7 of the cross-section of the substrates?]
Response 5: [Thank you for the comment, esteemed referee. Yes. As you mentioned, Figure 7a, b, and c are the cross-sectional areas of coatings MT1, MT2, and MT3, respectively, which simultaneously show the substrate and the coating. The other figures, which have indices (1 and 2), are only related to the coating with high magnifications.]
Comments 6: [Figure 8 needs a change in the figure caption for better understanding. Was no mapping performed on the MT3 sample?]
Response 6: [Thank you for your comment. Mapping analysis was conducted specifically for samples MT1 and MT2 to illustrate the distribution and decomposition behavior of hBN and cBN particles separately. Since MT3 contains both types of particles, an additional mapping was not necessary for achieving the study's objective. However, we will revise the figure caption to clarify this point for better understanding]
Comments 7: [What is the influence of particle size on the hardness of the coatings? Since the cBN particle is much larger than the hBN particle. Why were the particle sizes not homogeneous?]
Response 7: [Thank you for your insightful question. The particle size of cBN and hBN was not homogeneous because they inherently have different physical and structural properties that affect their synthesis and availability in commercial powders. cBN particles are typically larger due to their high hardness and cubic crystalline structure, which makes them more resistant to size reduction during the manufacturing process. In contrast, hBN, with its layered and lubricating structure, is naturally available in much finer particle sizes.] [The influence of particle size on the hardness of coatings is primarily related to the distribution, dissolution, and interaction of particles within the molten pool during laser cladding. Larger cBN particles provide stronger nucleation sites for dendrite refinement and enhance hardness by forming a more rigid structure. However, hBN, despite its smaller particle size, decomposes during processing, leading to the formation of Cr-rich borides, which also contribute to hardness enhancement. The combination of these effects resulted in the observed hardness variations among the coatings.] [A summary of the above was added to page 11 lines 303-307.]
Comments 8: In line 351, it says that area E is oxygen-rich and area D is oxygen-poor. However, the table shows the opposite.
Response 8: [Thank you for pointing this out. You are correct—the labels for areas E and D were mistakenly swapped. The Map image in Figure 15 also confirms this. This has now been corrected in the manuscript and can be seen on Page 16, Lines 408-410.]
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript is well-written and provides valuable insights into the development of wear-resistant coatings. My suggestion on this manuscript is a minor revision. By addressing the following points, the clarity and impact of the paper can be further improved. For the abstract section, it is suggested to further clarify the key findings and how they contribute to advancing the field. A more concise statement of results would strengthen its impact.
- The introduction provides a solid overview for understanding the importance of wear resistance in turbine blades and the rationale for using laser cladding. However, a more explicit definition of the key research gaps that this study addresses would be helpful.
- The authors mentioned the benefits of hBN and cBN but could enhance the argument by comparing these materials with other emerging alternatives for similar applications.
- Consider adding more detail about why the specific laser cladding parameters were chosen, especially with regard to the scanning speed and pulse frequency.
- It is needed to explicitly connect the microstructural findings to the observed tribological behavior in each of the samples. How do the different morphologies of the coatings correlate with the wear behavior?
- Some Figures could be improved by better labeling, particularly in complex diagrams like Figure 7. Consider adding more descriptive captions to ensure that each figure can be fully understood independently of the text.
- If possible, it is suggested to compare your findings to those of similar studies in the literature. A more direct comparison would strengthen the conclusion about the unique contributions of your study.
- The wear mechanism analysis can be expanded by providing further details on how these findings compare with other known tribological behaviors of coatings in similar contexts.
And referring to the following papers would be helpful to strength your analysis on wear mechanism: https://doi.org/10.1016/j.matchar.2024.114250 , https://doi.org/10.1007/s11665-023-09101-y , https://doi.org/10.1016/j.wear.2024.205609, https://doi.org/10.1016/j.triboint.2024.110189
Comments on the Quality of English LanguageIt is suggested to polish the English writing
Author Response
Dear Reviewer
Thank you for your valuable comments and guidance, which have greatly helped improve the quality of the article. We have tried to take all your comments into account as much as possible, and the desired changes have been made in the revised version. These changes are highlighted in red.
Best regards,
Authors
The manuscript is well-written and provides valuable insights into the development of wear-resistant coatings. My suggestion on this manuscript is a minor revision. By addressing the following points, the clarity and impact of the paper can be further improved. For the abstract section, it is suggested to further clarify the key findings and how they contribute to advancing the field. A more concise statement of results would strengthen its impact.
Thank you for the reviewer’s valuable comment. We have tried to include only the key points in the abstract to make it more understandable for the readers, as per the reviewer’s suggestion.
Comments 1: [The introduction provides a solid overview for understanding the importance of wear resistance in turbine blades and the rationale for using laser cladding. However, a more explicit definition of the key research gaps that this study addresses would be helpful.]
Response 1: [Thank you for your valuable feedback. As you noted, the introduction of the paper provides a general overview of the importance of wear resistance in turbine blades and the role of laser cladding, but does not specifically address the research gaps. To improve this section, we will revise the final section of the introduction to clearly state that previous research has mainly investigated the effect of hBN and cBN separately, but the simultaneous effect of these two materials on improving wear resistance has been less studied. We will also further emphasize the importance of investigating this combination in the context of GTD-111 superalloy. The revised text of the final section of the introduction has been included in the revised version of the paper (page 2-3, lines 96-108) to clearly indicate which gaps in the scientific literature this study addresses.]
Comments 2: [The authors mentioned the benefits of hBN and cBN but could enhance the argument by comparing these materials with other emerging alternatives for similar applications.]
Response 2: [Thank you for your valuable feedback. Your suggestion to compare hBN and cBN with other new options in similar applications is very useful. To improve the article, in the revised version, we have added explanations about some alternative materials, such as graphite, metal fluorides, and other emerging automotive materials (page 2 lines 72-77) to clarify the relative advantages and limitations of hBN and cBN compared to these alternatives. Useful references have also been added to the manuscript in this regard.]
Comments 3: [Consider adding more detail about why the specific laser cladding parameters were chosen, especially with regard to the scanning speed and pulse frequency.]
Response 3: [Thank you for your valuable comment. The selection of laser cladding parameters, particularly scanning speed and pulse frequency, was based on a combination of literature review and extensive trial-and-error experiments conducted in our laboratory (This text was added to the revised version on page 3, lines 131-136.). Our goal was to achieve an optimal balance between coating adhesion, microstructure refinement, and wear resistance.
Comments 4: [It is needed to explicitly connect the microstructural findings to the observed tribological behavior in each of the samples. How do the different morphologies of the coatings correlate with the wear behavior?]
Response 4: [Thank you for your valuable comments. The main controlling factors of the wear behavior in this study are the hBN and cBN particles, whose roles have been thoroughly discussed, particularly in Section 3.5 (Worn Surface Analysis). However, another important factor is the extension of equiaxed grains in sample MT3, which has contributed to the increased wear resistance. To address this, we have added a discussion on the role of equiaxed grains in improving wear resistance in page 7, lines 4-5 of the revised manuscript.]
Comments 5: [Some Figures could be improved by better labeling, particularly in complex diagrams like Figure 7. Consider adding more descriptive captions to ensure that each figure can be fully understood independently of the text.]
Response 5: [Thank you for your insightful suggestion. We have improved the labeling of the figures, particularly in Figure 7, to enhance clarity. Additionally, we have revised the figure captions to be more descriptive, ensuring that each figure can be understood independently of the main text.]
Comments 6: [If possible, it is suggested to compare your findings to those of similar studies in the literature. A more direct comparison would strengthen the conclusion about the unique contributions of your study.]
Response 6: [Thank you for your valuable suggestion. We have incorporated comparisons with similar studies in various sections of the results to strengthen our conclusions. Additionally, we have utilized the helpful references suggested by you to enrich the discussion.]
Comments 7: [The wear mechanism analysis can be expanded by providing further details on how these findings compare with other known tribological behaviors of coatings in similar contexts.]
Response 7: [Thank you for your valuable suggestion. We have expanded the wear mechanism analysis by providing further comparisons with known tribological behaviors of coatings in similar contexts. These additions can be found in the results section of the revised manuscript.]
Comments 8: [And referring to the following papers would be helpful to strength your analysis on wear mechanism: https://doi.org/10.1016/j.matchar.2024.114250 , https://doi.org/10.1007/s11665-023-09101-y , https://doi.org/10.1016/j.wear.2024.205609, https://doi.org/10.1016/j.triboint.2024.110189]
Response 8: [Thank you for your valuable suggestions. We have incorporated the recommended references into the revised manuscript and used them to strengthen our discussion.]
Reviewer 3 Report
Comments and Suggestions for AuthorsIn this article, the authors designed and processed a hard and self-lubricating NiCr/hBN-cBN composite coatings by laser cladding, and investigated their microstructure, hardness and wear properties. Regarding the quality and quantity presented in this manuscript, several critical and minor issues require extensive revision from the authors. As a result, I cannot recommend publishing the current version of this article.
(1) In the title of Table 2, Table 4 and Table 5, please add the unit of substance or element content in the table, such as wt. % or at. %.
(2) In Table 3, Pulse frequency (Hz) is 20 mg/s? Please check the unit and modify.
(3) In Figure 1(a), Av is redundant, please check and delete. In Figure 1(c) and Figure 1(d), please appropriately reduce the proportion of the image so that it does not obscure the chart.
(4) Line 149, please refer to the relevant references of the wear rate formula.
(5) In Figure 7, what is the SEM image of the bonding zone between the MT3 coating and the substrate? Why not enlarge the shooting and analysis? Please supplement the SEM image or explain the reason.
(6) In order to maintain consistency, please add the subscript (0.3) after the microhardness unit (HV) in the article.
(7) Please explain in this paper why the microhardness of the coating ' HAZ ' is lower than that of ' BM '?
(8) Reference [25] has not been cited.
Author Response
Dear Reviewer
Thank you for your valuable comments and guidance, which have greatly helped improve the quality of the article. We have tried to take all your comments into account as much as possible, and the desired changes have been made in the revised version. These changes are highlighted in red.
Best regards,
Authors
In this article, the authors designed and processed a hard and self-lubricating NiCr/hBN-cBN composite coatings by laser cladding, and investigated their microstructure, hardness and wear properties. Regarding the quality and quantity presented in this manuscript, several critical and minor issues require extensive revision from the authors. As a result, I cannot recommend publishing the current version of this article.
Comments 1: [In the title of Table 2, Table 4 and Table 5, please add the unit of substance or element content in the table, such as wt. % or at. %.]
Response 1: [We thank the referee for pointing this out. All analyses presented in this paper are based on weight percent (wt. %), not atomic percent (at. %). This has been reviewed and corrected throughout the manuscript.]
Comments 2: In Table 3, Pulse frequency (Hz) is 20 mg/s? Please check the unit and modify.
Response 2: [We thank the esteemed referee for his accuracy and attention to detail. The unit mentioned in Table 3 was reviewed and corrected.]
Comments 3: [In Figure 1(a), Av is redundant, please check and delete. In Figure 1(c) and Figure 1(d), please appropriately reduce the proportion of the image so that it does not obscure the chart.]
Response 3: [We thank the referee for their detailed and constructive feedback. The extra item "Av" in Figure 1(a) was removed and the aspect ratio in Figures 1(c) and 1(d) was corrected to display the graphs correctly.]
Comments 4: [Line 149, please refer to the relevant references of the wear rate formula.]
Response 4: [We thank the referee for this remark. Reference [29] has been added as a source related to the attrition rate formula on line 186.]
Comments 5: [In Figure 7, what is the SEM image of the bonding zone between the MT3 coating and the substrate? Why not enlarge the shooting and analysis? Please supplement the SEM image or explain the reason.]
Response 5: [We thank the referee for their detailed and constructive feedback. In Figure 7, image (a1) shows the general view of the coating profile, while images (a2) and (a3) ​​are enlarged images of the areas marked in (a1). This approach was adopted to more accurately represent the junction area between the MT3 coating and the substrate. The same method was applied to images (b) and (c). However, in the image of the MT3 sample (Figure 7c), as stated in the text of the manuscript, the intention is to say that the grains formed at the bottom and top of the coating are of the equiaxed type. The caption of Figure 7 has also been revised and corrected.We hope this explanation is sufficient.]
Comments 6: [In order to maintain consistency, please add the subscript (0.3) after the microhardness unit (HV) in the article.]
Response 6: [We thank the referee for this remark. The subscript (0.3) after the microhardness unit (HV) has been added and corrected throughout the article.]
Comments 7: [Please explain in this paper why the microhardness of the coating ' HAZ ' is lower than that of ' BM '?]
Response 7: [We thank the referee for their valuable feedback. The explanation regarding the reduction in hardness in the HAZ due to grain growth has been clearly added to the manuscript (page 11, lines 309-315).]
Comments 8: [Reference [25] has not been cited.]
Response 8: [We sincerely appreciate the reviewer’s attention to detail. The study by Wei et al. was mistakenly cited as reference [26]. This has now been corrected, and it is properly cited as reference [25] in the manuscript (Of course, with reference [33] in the new version of the manuscript).]
Reviewer 4 Report
Comments and Suggestions for AuthorsOverview: This manuscript investigated the simultaneous and individual effects of hBN and cBN on growing wear resistance. The article describes the material preparation, the morphology characterization of the synthesized microstructure, and the hardness measurement behavior of three types of coatings.
Reviewer comments:
- Can the author add some discussions or the control experiments on why boron nitride? Compared to other self-lubricating materials such as WS2, MoS2, and SiC, what are the additional advantages of BN?
- Have the authors characterized the subsequent durability and long-term performance of cBN/hBN coating? Is there any relevant characterization data that can be added?
- How did the author determine the hBN/cBN coating composition in MT1-3? Were the crystal structures of cBN and hBN considered? Please add more relevant discussions to help readers better understand the simultaneous effect.
- The author shows particle distributions, but will the grain size affect hardness and Wear resistance?
- Do a thorough proofreading to avoid grammatical and spelling errors and improve the quality of the manuscript. E.g. XRD wavelength is angstroms (Å), not A.
The English expression is basically good, with few confusing concepts, but some sentences are too long, and essential proofing is needed, extra attention should be paid to the details of the paper to improve its quality.
Author Response
Dear Reviewer
Thank you for your valuable comments and guidance, which have greatly helped improve the quality of the article. We have tried to take all your comments into account as much as possible, and the desired changes have been made in the revised version. These changes are highlighted in red.
Best regards,
Authors
Overview: This manuscript investigated the simultaneous and individual effects of hBN and cBN on growing wear resistance. The article describes the material preparation, the morphology characterization of the synthesized microstructure, and the hardness measurement behavior of three types of coatings.
Reviewer comments:
Comments 1: [Can the author add some discussions or the control experiments on why boron nitride? Compared to other self-lubricating materials such as WS2, MoS2, and SiC, what are the additional advantages of BN?]
Response 1: [Thank you for your comment. The reasons for choosing hBN over other lubricants have been added to the manuscript (page 2, lines 72-86). Some of these reasons are also given below: h-BN was chosen for its unique properties, including:
(1) Unlike MoS₂ and WS₂, which oxidize at high temperatures and lose their lubricity, hBN remains stable up to much higher temperatures.(2) In oxidizing environments and high temperatures, MoS₂ and WS₂ oxidize to MoO₃ and WO₃, which do not have lubricity. In contrast, h-BN is more chemically stable and undergoes less degradation at high temperatures.(3) hBN has a layered structure similar to MoS₂ and WS₂, which reduces the coefficient of friction.(4) Unlike some materials such as MoS₂, which may react with the NiCr matrix and form unwanted compounds, h-BN does not react significantly with NiCr, contributing to the stability of the coating.(5) Although SiC is known as a wear-resistant material, it lacks lubricity and is more of a phase It acts as a hardener, not a solid lubricant.]
Comments 2: [Have the authors characterized the subsequent durability and long-term performance of cBN/hBN coating? Is there any relevant characterization data that can be added?]
Response 2: [Thank you for your valuable feedback. In this study, our primary focus was on investigating the initial wear behavior and the effect of hBN and cBN on the hardness and wear resistance of the coatings. However, we fully acknowledge the importance of long-term durability and sustained performance of the coatings. The wear tests conducted in this study were performed under specific load and sliding speed conditions, and the results demonstrated a significant improvement in wear resistance for the hBN-cBN coating. However, for a more comprehensive evaluation of long-term durability, additional tests such as extended wear experiments, high-temperature performance assessments, or fatigue wear testing could provide further insights. Our research on this coating is ongoing, and the good suggestion of the esteemed referee will certainly be helpful in our future studies.]
Comments 3: How did the author determine the hBN/cBN coating composition in MT1-3? Were the crystal structures of cBN and hBN considered? Please add more relevant discussions to help readers better understand the simultaneous effect.
Response 3: [Thank you for your comment, esteemed referee. We have examined different compositions in the initial tests. The combination of the three samples occurred based on various reasons, which are mentioned below:
(1) The amount of 12% by volume of cBN was chosen because: cBN has a very high hardness, but in large amounts it may increase the brittleness of the coating. In the initial tests, a value higher than 12% led to an increase in residual stress cracks, which was due to the difference in the coefficient of thermal expansion between cBN and the NiCr matrix. Values lower than 12% reduced the positive effect of cBN on hardness and wear resistance.
(2) The amount of 2% by volume of hBN was chosen for the following reasons: hBN has lubricating properties due to its layered structure and facilitates friction reduction. In amounts higher than 2%, hBN caused an excessive decrease in the hardness of the coating, which was determined in the microhardness tests. At levels below 2%, the lubricating effect of hBN on the friction coefficient decreased and its effect on wear reduction became less noticeable.] [Also, a summary of the mentioned items was added to the experiments section (page 5 lines 160-164). In addition, the most important factor that distinguishes hBN and cBN is the difference in their crystal structure. We have investigated this issue especially in the analysis of the wear results. So that we attribute the weak bonding of hBN to its hexagonal, layered and van der Waals structure, which causes self-lubrication against the grinding ball friction. On the other hand, we attribute the strong bonding of cBN to the hard, cubic and diamond-like structure of cBN, which causes resistance to the grinding ball friction.]
Comments 4: The author shows particle distributions, but will the grain size affect hardness and Wear resistance?
Response 4: [Thank you for your insightful comment. We acknowledge that grain size plays a crucial role in determining both hardness and wear resistance of the coatings. In our study, we observed that the presence of hBN and cBN influences the microstructural evolution, particularly in terms of grain refinement and phase distribution. In the hBN-cBN coating, the increased driving force for inhomogeneous nucleation led to the formation of a larger area of fine equiaxed grains, which significantly contributed to the observed improvement in hardness and wear resistance. The finer grains enhance hardness due to the Hall-Petch effect, where smaller grain sizes act as barriers to dislocation motion, leading to increased strength. Additionally, grain refinement contributes to wear resistance by enhancing load distribution and reducing localized stress concentrations, which minimizes material removal during wear testing. In summary, the focus of the increase in hardness and wear resistance is based on two conclusions in this paper: first, the distribution of cBN and hBN particles. and second, the expansion of the concentric grains. The role of both in the results is given in detail. Other items, such as page 11, lines 30-307, and page 15, lines 418-431, were also added to the manuscript. We appreciate your valuable suggestion and hope that these clarifications meet your expectations.]
Comments 5: [Do a thorough proofreading to avoid grammatical and spelling errors and improve the quality of the manuscript. E.g. XRD wavelength is angstroms (Å), not A.]
Response 5: Thank you for your valuable comment. We have not only corrected the mentioned issue regarding the XRD wavelength unit (Å instead of A) but also thoroughly proofread and revised the entire manuscript to eliminate grammatical and spelling errors, thereby improving its overall quality. We appreciate your careful review and constructive feedback.
Round 2
Reviewer 1 Report
Comments and Suggestions for Authors The manuscript has been improved.