Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

In the study titled "Microstructure development of a functionalized multi-2 layer coating system of 316L austenitic steel on grey cast iron 3 under braking force in a corrosive environment" the authors clearly outlined the problem they were investigating. For this, I would like to congratulate the authors. The originality of the study is clearly evident when looking at studies in the literature. Furthermore, I believe that the study will be ready for publication in the journal if the following deficiencies are addressed.

- The study is well described in the abstract, but it would be more understandable if the quantitative results were included at the end of the abstract.

- Since data such as the pressure, force, or energy applied to the disc during the brake test remain confidential in the experimental methods section, it is not precisely known under what specific loads the microstructural changes occurred. However, it is considered reasonable to keep these confidential for confidentiality reasons. However, it would be better to mention a few applied values ​​if possible.

- A quantitative summary of the grain size increase is missing in the results section. The average grain size or distribution obtained from the EBSD maps is not given numerically but is only shown graphically in Figure 9. For example, the reader cannot determine from the text how much the average grain size increased from microns to microns after the brake test. If the authors calculated this value, including it in the text would be beneficial for the concreteness of the results.

- The study would be much improved if some repetitions between the Results and Discussion sections were omitted.

- The potential impact of the microstructural results on particle emissions from vehicle brake discs could have been briefly commented (for example, does higher hardness and fewer cracks equal fewer wear particles?). This would make the engineering implications of the study more apparent.

-The conclusions section provides almost no numerical support. For example, while the phrase "microstructure underwent significant grain growth" is used, the percentage or micrometer value of this growth is not specified. It also states "hardness slightly decreased," but the extent of the decrease (although we know from Table 3) is not mentioned in the results. A few critical numerical results would be more concrete.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English language is used quite well throughout the study, but there are some errors and omissions. Therefore, I recommend revising the text.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

The manuscript titled “Microstructure development of a functionalized multilayer coating system of 316L austenitic steel on grey cast iron under braking force in a corrosive environment” focuses on the production of grey cast iron brake discs with lamellar graphite (GJL) coated with a 316L stainless steel base layer and a second layer reinforced with tungsten carbide (WC) particles using the Laser Metal Deposition technique. In addition, the study investigates the surface morphology and its relation to the mechanical resistance of the resulting surfaces to wear processes.

The manuscript is written in correct English and follows the structure typical of a scientific article. However, I would like to highlight some particular aspects of the manuscript that deserve attention.

The manuscript strongly conveys a sense of abstraction and superficiality. The authors frequently repeat the reasons underlying the chosen research problem but fail to substantiate them with concrete examples from studies conducted by other researchers. The schematic representation of the obtained product in Figure 1 appears impressive; however, no visual evidence of this result, such as SEM images, is provided. In this case, performing a detailed characterization of the obtained layers is essential, reflecting their thickness, composition, and structural features. The obtained information would provide a solid basis for explaining the results of the property investigations, which should be related not only to surface morphology but also to specific features of layer composition and structure. I recommend that the authors avoid making statements related to research limitations and potential additional experiments that have not been, and are unlikely to be, carried out.

The composition of the V2A etching mixture remains unclear. I recommend that the authors specify the ratio and concentrations of hydrochloric and nitric acids used as the etching mixture.

The purities and suppliers of the reagents used in the investigations should also be specified.

The authors of this manuscript frequently refer to the factors determining the wear characteristics of the materials used. They also mention the dominant crystalline phases of the studied objects. Why was no XRD analysis of the etched 316L powder performed?

In Figure 6, the scales of the presented images differ, and their quality is very poor. Since the essential part of this work concerns the investigation of surface morphology, the quality characteristics of the SEM images provided should be significantly improved. In this case, the images should be presented at uniform magnification, with different zoom levels and, if possible, taken at the same locations. Personally, I cannot draw any comparative conclusions from the SEM images shown in Figure 6.

The quality of the SEM images presented in Figure 7 is considerably better; however, the magnifications are different, which prevents a proper evaluation of the effect of friction-induced damage on the surface characteristics.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

General comments:

 

Is it possible for the authors to conduct nanoindentation testing? This would allow for characterisation of the hardness and elastic modulus of the respective layers of the structures. The heterogeneity of the substrate would have be accounted for by multiple measurements. This could be a possible avenue for a future study.

 

There are numerous instances of the use of “Sample with the brake test ” without proper grammatical articles in a sentence. Can the authors please correct this throughout the manuscript?

 

Specific comments:

Line 168: Can the authors please review the phrasing of this line? There is a grammatical article missing from this sentence.

 

Figure 2: Please can the authors check the English in this schematic? There are some misspellings and grammatical errors.

 

Line 206: Can the authors please provide more details on the specific steps for mechanical and vibratory polishing? As this directly impacts the quality of the microstructures revealed and subsequent SEM images, I feel this is a key detail if it is able to be provided.

 

Figure 7: I believe it would be good for the authors for add an additional annotation to ensure that the orientation of the images relative to the substrate and surface are clear.

 

Figure 8: As with the comment above, additional annotations would be beneficial to indicate the layers discussed in the text.

 

Figure 9: Please amend the caption to indicate the purpose of the orange box as also indicated in the text.

 

 

Figure 10 and 11: To what do the authors attribute the increasing number of zero solutions? Is it that there are grains that are too small to be resolved or is it due to other material within the microstructure that doesn’t index to austenite?

 

Line 484: Can the authors please move the citation to be after Sun within this first sentence? There are additional points where the citation is not present after the author's name. Can this please be reviewed and corrected?

 

Table 3 and Figure 13: Can the authors please present the standard deviation of the average of hardness tests in both the table and as error bars in the graph? It was mentioned earlier in the text that 9 measurements were made of each sample and accordingly, the spread of the results should be presented.

 

Please can the authors consider adding further detail to the discussion section to reference the specific figures that support the points raised? This would aid the understanding for readers and reinforce the data gathered and conclusions drawn from that data. This would be beneficial additionally, as it is somewhat odd to see a conclusion section that is as long as a discussion. Perhaps these sections should be revised to ensure the conclusion provides a more suitable summary.

Comments on the Quality of English Language

There are numerous areas of grammar to be checked and edited (please see above)

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

No comments.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The submitted manuscript investigates the performance of a 316L austenitic stainless steel coating on grey cast iron subjected to braking forces in a corrosive environment. The topic is highly relevant and of considerable practical interest, particularly for applications involving dynamic mechanical loading and environmental degradation. However, the manuscript requires substantial revisions before it can be considered for publication. Below are the major points that must be addressed:

 

1. All figures in the manuscript are currently too small and of insufficient resolution, making it difficult to interpret the data. Figures must be resized and enhanced to ensure clarity and read
2. A clear and comprehensive illustration or photograph of the experimental setup is essential. This should include the braking system, corrosion chamber (if applicable), and the method of applying and measuring forces during the tests. Such information is crucial for the reproducibility and scientific transparency of the study.
3. The manuscript lacks critical information regarding the thermal and mechanical loads developed during testing. Quantitative values such as temperature ranges, thermal cycling profiles, mechanical stresses, or strain rates during braking must be provided. This data is necessary to correlate observed microstructural changes with operational conditions.
4. The mechanical properties of the deposited coatings (e.g., Young’s modulus, hardness, yield strength, etc.) should be determined and presented. A comparison between the coating and substrate properties would support claims of improved performance. The determination of the dynamic mechanical properties of the examined materials is also of high importance.
5. The authors mention grain growth and texture evolution due to thermal and mechanical stresses but fail to link these observations to actual quantified loading conditions. If stress and thermal data during impact cycles were captured, they should be presented and discussed in correlation with the microstructural results.
6. As stated in the conclusions, understanding the influence of thermal cycling and mechanical stress is a key goal. To support this claim, the manuscript must explicitly correlate microstructural transformations (grain size, orientation) with specific thermal and mechanical load profiles developed during testing.
7. The current conclusion section reads more like a discussion, including references and speculative interpretations. The conclusion should be restructured to concisely summarize the key findings of the study, the implications of the results, and any limitations or recommendations for future work. References should not be included in the conclusion section.

Author Response

Response to Reviewer 1 Comments

Dear Editor,

We would like to thank the reviewer for their valuable comments and suggestions. We have carefully revised the manuscript accordingly. Below is a point-by-point response to each comment.

1. Comment 1:

All figures in the manuscript are currently too small and of insufficient resolution, making it difficult to interpret the data. Figures must be resized and enhanced to ensure clarity and readability.

• Response:

We thank the reviewer for pointing this out. All figures have been resized and replaced with higher-resolution versions to ensure clarity and readability.

2. Comment 2:

A clear and comprehensive illustration or photograph of the experimental setup is essential. This should include the braking system, corrosion chamber (if applicable), and the method of applying and measuring forces during the tests.

• Response:

We appreciate the reviewer’s suggestion. However, due to confidentiality agreements with our industrial partner, we are not permitted to disclose detailed information or photographs of the brake shock corrosion test setup. To address this, we have added a concise description of the test procedure in the manuscript (Lines 142–153), including the relevant standards (DIN EN ISO 9227 and DIN EN ISO 6270-2) used in the test.

3. Comment 3:

The manuscript lacks critical information regarding the thermal and mechanical loads developed during testing. Quantitative values such as temperature ranges, thermal cycling profiles, mechanical stresses, or strain rates during braking must be provided.

• Response:

We acknowledge the importance of this point. Due to confidentiality constraints, we cannot disclose detailed mechanical loading data. However, we have included a description of the test procedure and the maximum temperature reached (500 °C) in the manuscript (Lines 142–153). Further details on the test mechanism are available in our previous publication (https://doi.org/10.1016/j.jmrt.2024.10.257), which is cited in the current manuscript.

4. Comment 4:

The mechanical properties of the deposited coatings (e.g., Young’s modulus, hardness, yield strength, etc.) should be determined and presented. A comparison between the coating and substrate properties would support claims of improved performance.

• Response:

We understand the reviewer’s concern. However, the focus of this manuscript is on the microstructural development of the coating before and after the brake test. The mechanical properties such as Young’s modulus, yield strength and micro hardness mapping are being investigated in a separate study and will be presented in a forthcoming publication.

5. Comment 5:

The authors mention grain growth and texture evolution due to thermal and mechanical stresses but fail to link these observations to actual quantified loading conditions.

• Response:

We thank the reviewer for this insightful observation. While our study does not include direct measurements of thermal and mechanical stresses during the brake test, we acknowledge the importance of such data for a more precise correlation. In-situ testing under synchrotron radiation (e.g., at DESY) would indeed allow for real-time analysis of stress-induced microstructural changes. However, our current approach focuses on post-test EBSD analysis under simulated corrosive conditions. Figure 10 and its explanation illustrate competitive grain growth and texture evolution, which we attribute to the combined effects of thermal cycling and mechanical loading during braking. We have now clarified this limitation and the inferred correlation in the revised discussion section.

6. Comment 6:

As stated in the conclusions, understanding the influence of thermal cycling and mechanical stress is a key goal. To support this claim, the manuscript must explicitly correlate microstructural transformations (grain size, orientation) with specific thermal and mechanical load profiles developed during testing.

• Response:

Wir schätzen die Anregung des Gutachters. Aufgrund von Vertraulichkeitsvereinbarungen mit unserem Industriepartner ist es uns jedoch nicht gestattet, detaillierte Informationen zu den thermischen und mechanischen Belastungsprofilen oder den spezifischen Parametern des Bremsstoßkorrosionstests offenzulegen. Dennoch stimmen die beobachteten mikrostrukturellen Veränderungen - wie Kornwachstum und Texturentwicklung - mit den erwarteten Auswirkungen der thermischen Wechselbeanspruchung und der mechanischen Belastung beim Bremsen überein. Wir haben diese Einschränkung im überarbeiteten Diskussionsabschnitt klargestellt und die qualitative Korrelation zwischen den Belastungsbedingungen und den EBSD-Ergebnissen hervorgehoben.

7. Comment 7:

The current conclusion section reads more like a discussion, including references and speculative interpretations. The conclusion should be restructured to concisely summarize the key findings of the study, the implications of the results, and any limitations or recommendations for future work.

• Response:

We respectfully acknowledge the reviewer’s suggestion. However, we believe that including references and comparative discussion in the conclusion enhances the clarity and traceability of our findings. Therefore, we would prefer to retain the current structure of the conclusion section.

 

Best Regards

Mohammad Masafi

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors examined the changes to microstructure of a GJL brake discs with a multilayer cladding system subjected to brake shock corrosion test. Laser metal deposition was used for the multilayer 316L SS coating system while SEM and EBSD was used to investigate the changes to microstructure like grain size, orientation. This work serves as an introduction for the need to investigate the structure property relationship in GJL brake discs with coatings for tailoring materials for applications.

Following are corrections:

1. The abstract contains more words than the guidance set in the journal. Though microstructural analysis of samples were done, no direct correlation to increase in wear or decrease in corrosion was established from the experiments / investigations. Please rewrite abstract following guidelines in https://www.mdpi.com/journal/coatings/instructions.
2. The introduction section from line 72-122. Please rewrite the section following guidelines in https://www.mdpi.com/journal/coatings/instructions. Call out for a few places are below.
   • Ref 9 talks about severe plastic deformation. Can a more appropriate reference talking about coatings used similar to the authors prepared via LMD cited?
   • Ref 10 talks about incorporating fibers and studying physical, mechanical and microstructural properties. No study of grain growth or recrystallization was conducted. Please cite an appropriate reference.
   • Ref 11 and 12 talks about brake wear particle emissions and doesn't investigate texture. Please cite an appropriate reference.
3.  Line 194-197 quoting Surface Mechanical Attrition Treatment and Reference 23. The material investigated had no SMAT. Please explain what connection is being attempted here. Recommend moving to appropriate section within results.
4. Section 3.1 - It is well known that tuning the LMD process can be tuned to achieve favorable grain structure based on the application in hand during material design. Please elucidate the connection between the BSD image of an individual powder particle and microstructure of LMD process if there is any.
5. Figure 3 and 4 titles - Please remove term 'Schematic' and use appropriate titles.
6. Line 216 - Please call out the changes in the microstructure or point out to section in the manuscript where the reader can learn more.
7. Figure 3 - Please markup image for claim made in Line 217 and 219 or point out to section in the manuscript where the reader can learn more.
8. Line 225-231 - Please call out the changes in the microstructure or point out to section in the manuscript where the reader can learn more.
9. Figure 5
   • Please markup image with all the transitions - substrate, first and second coating.
   • SEM image to the right (Before) has several cracks running horizontal. Can the authors comment on this observation and how it might affect mechanical performance?
10. Line 246-247. Please call out the changes in the microstructure or point out to section in the manuscript where the reader can learn more.
11. Line 249-251. Please clarify how the authors are deducing the claim.
12. Figure 6 - Please rewrite figure title to avoid repeating similar sentences in the same section. Lines 252-254 and 257-258 are too similar.
13. Line 286-287 quoting braking process influenced grain size distribution and Reference 29. The reference cited is an article on fabrication of graded steel with direct laser metal deposition and has no mention on braking. Please cite appropriate reference if any.

Author Response

Response to Reviewer 2 Comments

Dear Editor,

We thank Reviewer 2 for their valuable comments and suggestions, which have helped us improve the quality and clarity of our manuscript. Below, we provide a point-by-point response to each comment.

1. Comment:

The abstract contains more words than the guidance set in the journal. Though microstructural analysis of samples were done, no direct correlation to increase in wear or decrease in corrosion was established from the experiments / investigations. Please rewrite abstract following guidelines in https://www.mdpi.com/journal/coatings/instructions.

Response:

Thank you for this observation. We have revised the abstract in red to comply with the word limit and focused the content on the microstructural findings. We clarified that while wear and corrosion resistance are discussed, the study primarily investigates microstructural evolution under brake shock corrosion conditions.

2. Comment:

The introduction section from line 72-122. Please rewrite the section following guidelines in https://www.mdpi.com/journal/coatings/instructions. Call out for a few places are below.

Response:

We have revised the introduction section to improve clarity and structure, ensuring alignment with MDPI Coatings guidelines. The section now better contextualizes the study and includes more relevant references.

• Comment:

Ref 9 talks about severe plastic deformation. Can a more appropriate reference talking about coatings used similar to the authors prepared via LMD cited?

Response:

We appreciate this suggestion. We have replaced Ref 9 with a more appropriate reference that discusses coatings produced by LMD, which aligns better with the scope of our study.

• Comment:

Ref 10 talks about incorporating fibers and studying physical, mechanical and microstructural properties. No study of grain growth or recrystallization was conducted. Please cite an appropriate reference.

Response:

Thank you for pointing this out. We have replaced Ref 10 with a reference that specifically addresses grain growth and recrystallization phenomena relevant to our study.

• Comment:

Ref 11 and 12 talks about brake wear particle emissions and doesn't investigate texture. Please cite an appropriate reference.

Response:

We agree with the reviewer and have replaced Refs 11 and 12 with references that focus on texture evolution in coated materials under thermal and mechanical loading.

3. Comment:

Line 194-197 quoting Surface Mechanical Attrition Treatment and Reference 23. The material investigated had no SMAT. Please explain what connection is being attempted here. Recommend moving to appropriate section within results.

Response:

We acknowledge the confusion. The reference to SMAT has been removed from the introduction and relocated to the results section where it is used for comparative discussion only.

4. Comment:

Section 3.1 - It is well known that tuning the LMD process can be tuned to achieve favorable grain structure based on the application in hand during material design. Please elucidate the connection between the BSD image of an individual powder particle and microstructure of LMD process if there is any.

Response:

We have clarified the relevance of the BSD image in Section 3.1. It serves to illustrate the initial microstructure of the powder, which influences the resulting microstructure after LMD processing. “The increased visibility of the grain boundaries is indicative of well-defined grains that are distinctly separated from one another. The fine and uniform microstructure of 316L powder has the potential to enhance the strength and hardness of the coated material, which is advantageous for automotive applications.”

5. Comment:

Figure 3 and 4 titles - Please remove term 'Schematic' and use appropriate titles.

Response:

We have revised the titles of Figures 3 and 4 to remove the term 'Schematic' and replaced them with more descriptive and appropriate titles.

Figure 3. SEM images of the multilayer coating before and after the braking test, showing mi-crostructural changes in the cross-section.

Figure 4. SEM images of the second coating layer before and after the braking test, highlighting microstructural evolution due to thermal and mechanical loading.

6. Comment:

Line 216 - Please call out the changes in the microstructure or point out to section in the manuscript where the reader can learn more.

Response:

We have added a reference to the relevant section where the microstructural changes are discussed in detail, to guide the reader appropriately.

7. Comment:

Figure 3 - Please markup image for claim made in Line 217 and 219 or point out to section in the manuscript where the reader can learn more.

Response:

We thank the reviewer for the helpful suggestion. In response, we have added a reference to Section 3.2, where the microstructural changes observed in Figure 3 are discussed in detail. Additionally, we have revised the figure caption to clarify that the subsequent figures (Figures 4–6) provide a layer-specific analysis of the observed changes.

8. Comment:

Line 225-231 - Please call out the changes in the microstructure or point out to section in the manuscript where the reader can learn more.

Response:

We thank the reviewer for this valuable comment. In response, we have revised the text in Lines 236–243 to explicitly describe the observed microstructural changes, including grain coarsening, matrix distortion, and microcrack formation around WC particles. These features have been highlighted in Figure 4 using yellow markings to guide the reader. Additionally, we have added a reference to Section 3.2, where these changes are discussed in detail.

9. Comment:

Figure 5: Please markup image with all the transitions - substrate, first and second coating. SEM image to the right (Before) has several cracks running horizontal. Can the authors comment on this observation and how it might affect mechanical performance?

Response:

We thank the reviewer for the helpful comment. In response, we have marked the relevant microstructural features in Figure 5. The revised text now describes the horizontal cracks observed before the brake test, which likely result from residual thermal stress, and the vertical microcracks that appear after the test, originating from the substrate direction. These features are highlighted in the yellow rectangle and discussed in terms of their potential impact on mechanical performance (Lines 248–257).

10. Comment:

Line 246-247. Please call out the changes in the microstructure or point out to section in the manuscript where the reader can learn more.

Response:

Thank you for your helpful suggestion. We have revised the sentence in Lines 264–266 to clarify the observed microstructural changes and to guide the reader to the relevant section of the manuscript. Specifically, we now refer to Section 3.2 and 3.3, where the microstructural evolution is discussed in detail.

11. Comment:

Line 249-251. Please clarify how the authors are deducing the claim.

Response:

Thank you for your comment. We have revised the sentence in Lines 269–272. Thank you for your comment. We have revised the sentence to more accurately reflect the observed microstructural changes and to clarify the basis of our interpretation. The EBSD map shown in Figure 6 reveals that the 316L grains appear smaller and more isotropic after the LMD process and before the brake test, while the grains are visibly larger after the brake test. This grain growth is attributed to the combined effects of heat and mechanical stress during braking.

12. Comment:

Figure 6 - Please rewrite figure title to avoid repeating similar sentences in the same section. Lines 252-254 and 257-258 are too similar.

Response:

Thank you for pointing this out. We agree that the repetition in the text and figure caption could be distracting. We have revised the figure title of Figure 6 to avoid redundancy and rephrased the sentences in Lines 272–279 to improve clarity and eliminate repetition, while preserving the scientific message.  The title for Figure 6 has been Optimized too.

Advantages of this version:

However, as shown in Figure 6, the EBSD map after the LMD process prior to the brake test shows visibly smaller and more isotropic 316L grains compared to the condition after the brake test, indicating grain growth due to the effects of heat and mechanical stress during braking.

13. Comment:

Line 286-287 quoting braking process influenced grain size distribution and Reference 29. The reference cited is an article on fabrication of graded steel with direct laser metal deposition and has no mention on braking. Please cite appropriate reference if any.

Response:

Thank you very much for your careful observation. You are absolutely right—Reference 29 (Lines 312–316) does not address the influence of braking on grain size distribution. We have therefore removed this citation and revised the sentence to reflect our own experimental findings.

To support our interpretation, we have added more appropriate references that discuss the effects of thermal and mechanical loading on the microstructure of 316L stainless steel. While these studies do not specifically investigate braking, they provide relevant insights into grain coarsening mechanisms under thermal and mechanical influence, which are comparable to the conditions experienced during braking.

At present, we are not aware of any published study that directly examines the effect of braking on grain size evolution in LMD coatings. Our conclusions are based on EBSD analyses conducted before and after the brake test, which clearly show grain coarsening in both coating layers.

Revised Sentence (Lines 312–316):

“The results of this study indicate that the braking process influences the grain size distribution, as evidenced by the EBSD maps showing grain coarsening in both coating layers after braking. Similar effects of thermal and mechanical loading on the microstructure of 316L stainless steel have been reported in the literature [Petroušek et al., 2023; Liu et al., 2024].”

New References:

Petroušek, D.; Kroupová, I.; Čapek, J.; Dvořák, M.; Vojtěch, D. Investigation of the Properties of 316L Stainless Steel after AM and Heat Treatment. Materials 2023, 16, 3935. https://doi.org/10.3390/ma16113935

Liu, W.; Zhang, Y.; Wang, H.; Li, J.; Chen, X. Thermal Effect on Microstructure and Mechanical Properties in Directed Energy Deposition of AISI 316L. Int. J. Adv. Manuf. Technol. 2024, 132, 2741–2754. https://doi.org/10.1007/s00170-024-14274-4

 

Best Regards

Mohammad Masafi

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Paper „Microstructure development of a functionalized multilayer coating system of 316L austenitic steel on grey cast iron under braking force in a corrosive environment” written by

Mohammad Masafi * , Achim Conzelmann , Heinz Palkowski , Hadi Mozaffari-Jovein is very impressive. First of all the Authors are very familiar with the role of GJL brake discs surface modification to increase their braking performance and durability. Abstract and especially Introduction are very well written, convincing, planned research is logic and very promising. Unfortunatelly later presented results are modest, some parts are simple missing. For example results of (line 115) …brake shock corrosion test, …, (line 50) …dust emissions generated by cast iron are a growing concern due to their potential health risks and negative impact on air quality…, (line) 21) …resulting dust emissions… are not discussed. Another crucial topic – role of rapid cooling (line 77) is very important and requires (line 80) …A comprehensive analysis of the grain size both before and after the brake test can provide information on the effects of thermal and mechanical stresses on the microstructure… To discuss rapid cooling it must be done an experiment with/without heating during the LMD process (below 500^oC as described in line 152). I am sure the Authors, having documented experience in this field (see Ref. 3, 8) can do better research and may be the reason was simple lack of funding (line 468). But in the paper there are some easy to correct mostly editorial mistakes listed below:

1. What were dimensions of tested samples ? How long the tests 1200 brake cycles (line 146) was lasting ? Were the made samples weight changes after the first layer deposition, after the second layer deposition, after the brake tests ?
2. Skip space in line 158 – it should be 70⁰C (in line 164 is OK 70⁰). Also in line 153 should be 500⁰
3. Is V2A „pickling” (line 421) the same as V2A etchant (line 158) ?. I am sure this is the same, but please be in scientific paper especially touching so important topic very precise, even to excess.
4. Good selection of three referencies (lines 172 – 188), nothing to change.
5. In the part 3.2 (lines 217 – 219) …To facilitate a more detailed analysis, each layer is shown and examined separately in the subsequent Figures before and after the brake test… in the following Figures there should be picture of the first layer, the second layer before and after the brake test and these Figures are missing (see Figure 6 – no results of EBSD from the first layer only).
6. Is tungsten carbide (line 168) and WC (line 226) the same ? Probably yes but this should be clear (see my remark 3).
7. Make Figures 7, 9, 10 bigger.
8. What form is correct – in lines 96, 100 ..pole Figures… or in line 361 …pole figures…
9. Add space in line 392 …(see Figure 10a). Guo…
10. Add Ref. # in the line 385 …in the publication Sun… Is it Ref. 48 ?
11. Why there was not repeated the brake test for the next 1200 cycles ? Sentence 293 – 294 could be more stronger …After the braking test, an augmentation in grain size was observed, accompanied by the potential development of a new texture….

Author Response

Response to Reviewer 3 Comments

Dear Editor,

We sincerely thank Reviewer 3 for the encouraging and constructive feedback. We greatly appreciate your positive remarks regarding the abstract, introduction, and the overall research concept. Your recognition of the relevance and potential of our work is highly motivating.

 

At the same time, we acknowledge the critical points you raised concerning the completeness of the presented results and the need for further clarification in several areas. Your detailed comments have helped us identify important aspects that required improvement or elaboration, particularly regarding the brake shock corrosion test, dust emissions, the role of rapid cooling, and the presentation of EBSD results.

 

We have carefully addressed each of your suggestions and revised the manuscript accordingly. Below, we provide detailed responses to each of your comments, along with the corresponding changes made in the manuscript.

 

Best Regards

Mohammad Masafi

 

1. Comment in getting started: “Unfortunately, later presented results are modest, some parts are simply missing. For example, results of (line 115) …brake shock corrosion test…”

Response:

We thank the reviewer for this important observation. Due to confidentiality agreements with our industrial partner, we are not permitted to disclose detailed information or photographs of the brake shock corrosion test setup. However, to address this concern, we have added a concise description of the test procedure in the revised manuscript (Lines 142–153), including the relevant standards (DIN EN ISO 9227 and DIN EN ISO 6270-2) that were followed during the test. We hope this clarification helps to better contextualize the corrosion testing approach used in this study.

1. Comment in getting started: “…(line 50) dust emissions generated by cast iron are a growing concern due to their potential health risks and negative impact on air quality…, (line 21) resulting dust emissions… are not discussed.”

Response:

We thank the reviewer for this valuable comment. It is well established in the literature that laser metal deposition (LMD) coatings on brake discs can significantly reduce particulate emissions compared to uncoated grey cast iron (GJL) discs. In our manuscript, the references to dust emissions (Lines 21 and 50) were intended to highlight the broader motivation and relevance of surface modification technologies in the context of environmental and health concerns.

However, we acknowledge that the manuscript does not include a quantitative analysis of dust emissions, as this was not the primary focus of the present study. Instead, our work concentrates on the microstructural development of the multilayer coating system, which is a key step toward optimizing future coating processes and improving their functional performance, including, ultimately, emission reduction.

To clarify this point, we have slightly revised the relevant sentences in the introduction to better reflect the scope of the study and to avoid the impression that emission data were collected or analyzed.

 

1. Comment in getting started: “Another crucial topic – role of rapid cooling (line 77) is very important and requires (line 80) …A comprehensive analysis of the grain size both before and after the brake test can provide information on the effects of thermal and mechanical stresses on the microstructure… To discuss rapid cooling it must be done an experiment with/without heating during the LMD process (below 500 °C as described in line 152).”

Response:

We thank the reviewer for this insightful comment and the opportunity to clarify the context of the statements in Lines 77, 80, and 152.

In Line 77, the term “rapid cooling” refers to the solidification behavior during the LMD process, where high thermal gradients and fast cooling rates typically result in a finer microstructure. This is a well-known phenomenon in laser-based additive manufacturing and is supported by the cited literature.

In contrast, the statement in Line 80 addresses the thermal and mechanical effects during the brake test, where repeated braking cycles induce heating and subsequent cooling. This thermal cycling can also influence the microstructure, particularly grain growth and texture evolution. The comparison of EBSD maps before and after the brake test serves as a basis for analyzing these effects.

Finally, the temperature mentioned in Line 152 (maximum 500 °C) refers to the measured surface temperature during the brake shock corrosion test, not during the LMD process. This information helps to understand the thermal exposure of the coating during service-like conditions and its potential influence on phase stability and grain structure.

We have revised the manuscript to clarify these distinctions and avoid confusion between the different thermal processes discussed.

1. Comment in getting started: “I am sure the Authors, having documented experience in this field (see Ref. 3, 8), can do better research and may be the reason was simple lack of funding (line 468).”

Response:

We sincerely thank the reviewer for acknowledging our previous work and experience in this field. Indeed, the current study builds upon our earlier investigations (Refs. 3 and 8), and we fully agree that further research could deepen the understanding of the coating behavior under service conditions.

As correctly noted, this study was conducted without external funding (Line 468), which limited the scope of experimental setups and extended testing. Nevertheless, we have aimed to provide a meaningful contribution by focusing on the microstructural development of the multilayer coating system and its response to braking and corrosion conditions. We hope that future projects with broader support will allow us to expand on these findings and explore additional performance aspects such as wear resistance, emission reduction, and long-term durability.

1. Comment: “What were the dimensions of tested samples? How long did the 1200 brake cycles (line 146) last? Were the samples weighed after the first layer deposition, after the second layer deposition, and after the brake tests?”

Response:

Thank you for these detailed and helpful questions. We have revised the manuscript to include the requested information:

The 1200 brake cycles were conducted over a period of approximately 6 hours, following a standardized protocol with defined braking and cooling intervals. Complete coated brake discs were subjected to the brake shock corrosion test.

The analyzed specimens were extracted from the coated brake discs, embedded as cross-sections, and subsequently ground and polished for microstructural and electrochemical analysis. The dimensions of the extracted samples were approximately 25 mm × 5 mm × 10 mm.

Regarding weight changes: although the samples were handled with care and weighed during the coating process for internal process control, the weight differences before and after each deposition step and the brake test were minimal and not systematically recorded for scientific evaluation. Therefore, we have not included these values in the manuscript. However, we acknowledge that such data could be valuable in future studies and will consider including it in follow-up investigations. The corresponding additions have been made in the revised manuscript (Lines 151–160).

2. Comment: Skip space in line 158 – it should be 70⁰C (in line 164 is OK 70⁰). Also in line 153 should be 500⁰

 

Response:

Thank you for your comment regarding the spacing between the number and the degree symbol. This topic has been discussed across various scientific and academic communities, and different style guides follow different conventions.

According to international scientific and engineering standards—such as those from the International Bureau of Weights and Measures (BIPM), the International Organization for Standardization (ISO), and the United States Government Printing Office (GPO)—a space is recommended between the number and the unit (e.g., 70 °C), in line with SI unit formatting rules. This ensures consistency and prevents line breaks between the number and the unit.

However, some academic publishers and style guides (e.g., Chicago Manual of Style, Oxford University Press) prefer omitting the space for improved readability in running text (e.g., 70°C).

Since MDPI generally follows SI conventions, we have formatted the temperature values in the manuscript, accordingly, using a protected space between the number and the unit (e.g., 70 °C).

 

3. Comment: Is V2A “pickling” (line 421) the same as V2A etchant (line 158)? Please be precise, especially when addressing such an important topic.

Response:

Thank you for this important observation. You are absolutely right to emphasize the need for terminological precision in scientific writing.

In our manuscript, the terms “V2A pickling” and “V2A etchant” refer to the same chemical treatment process. The solution used—composed of 50 % HCl, 10 % HNO₃, and 40 % H₂O—is commonly known as a V2A pickling solution and is used to remove oxide layers and reveal microstructural features in stainless steels such as 316L. The term “etchant” emphasizes the chemical composition and its role in microstructural analysis, while “pickling” refers more broadly to the industrial process of surface cleaning and preparation.

To ensure clarity and consistency, we have revised the manuscript to use the term “V2A pickling solution” throughout, with a note that it serves as an etchant in the context of microstructural characterization.

4. Comment: Good selection of three references (lines 172–188), nothing to change.

Response:

Thank you for your positive feedback. We appreciate your acknowledgment and are glad that the selected references were found appropriate and relevant.

5. Comment: In the part 3.2 (lines 217–219): “To facilitate a more detailed analysis, each layer is shown and examined separately in the subsequent Figures before and after the brake test…” — In the following Figures, there should be images of the first and second layers before and after the brake test. These Figures are missing (see Figure 6 – no EBSD results from the first layer only).

Response:

Thank you for attentive reading and helpful comment. We would like to clarify that Figure 3 provides an overview of both coating layers before and after the brake test, as stated in line 228: “Firstly, the SEM images shown in Figure 3 illustrate both coating layers before and after the brake test.”

Subsequently, Figures 4 and 5 present more detailed analyses of the individual layers. Specifically, Figure 6 includes EBSD results from both the first and second layers: the first layer consisting of 316L stainless steel, and the second layer composed of 316L reinforced with WC particles. These layers are deposited on top of each other and are both visible in the EBSD maps shown.

6. Comment: Is tungsten carbide (line 168) and WC (line 226) the same? Probably yes, but this should be clear (see my remark 3).

Response:

Thank you for your comment. Yes, “tungsten carbide” and “WC” refer to the same material. WC is the chemical formula for tungsten carbide, which is commonly used in scientific literature and materials science to denote the compound. To avoid any ambiguity, we have revised the manuscript to ensure that the first mention of “WC” includes the full name in parentheses: “tungsten carbide (WC)” in Abstract. The corresponding additions have been made in the revised manuscript the line 181.

7. Comment: Make Figures 7, 9, 10 bigger.

Response:

Thank you for your suggestion. We have increased the size of Figures 7, 9, and 10 in the revised manuscript to improve readability and clarity of the presented data.

8. Comment: What form is correct – in lines 96, 100 “Pole Figures” or in line 361 “pole figures”?

Response:

Thank you for pointing this out. You are absolutely right — “pole figures” is a general scientific term and should not be capitalized unless it appears at the beginning of a sentence or in a title. We have corrected the capitalization in lines 96 and 100 to ensure consistency with the correct form used in line 361. The corresponding changes have been made in the revised manuscript.

9. Comment:

Response:

Thank you for pointing this out. We have added the appropriate space in line 436 to improve readability. The corresponding correction has been made in the revised manuscript.

10. Comment: Add Ref. # in line 385 “…in the publication Sun…” Is it Ref. 48?

Response:

Thank you for your comment. Yes, the statement in line 385 refers to the publication by Sun et al., which corresponds to Reference [48] in the manuscript.

11. Comment: Why was the brake test not repeated for another 1200 cycles? Sentence 293–294 could be stronger: “After the braking test, an augmentation in grain size was observed, accompanied by the potential development of a new texture…”

Response:

Thank you for your thoughtful comment. The brake test was conducted according to a standardized industrial protocol, which specifies a single sequence of 1200 cycles. Due to confidentiality agreements with our industrial partner, we are not permitted to modify or extend the test procedure. The test was designed and executed by the industrial partner, and repeating the test for an additional 1200 cycles was not part of the agreed testing scope. Nevertheless, we fully agree that such extended testing could provide valuable insights into the long-term evolution of microstructure and texture, and we plan to address this in future investigations.

Regarding the sentence in lines 293–294, we have revised the wording to make the observation more precise and impactful. The updated sentence now reads:

“After the brake test, a noticeable coarsening of grains was observed, along with indications of a newly emerging crystallographic texture, suggesting a thermomechanically induced reorientation of grains.”

The corresponding changes have been made in the revised manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper lacks scientific rigor and does not meet the minimum standards required for publication in a scientific journal. The manuscript omits critical information regarding the thermal and mechanical loads applied during testing. Quantitative parameters such as temperature ranges, thermal cycling profiles, mechanical stresses, and strain rates during braking are absent. This data is essential to meaningfully correlate the observed microstructural changes with actual operational conditions.

The only information provided by the authors is the number of brake cycles in a device, which they claim cannot be documented due to confidentiality agreements. However, the number of brake cycles alone is not a scientifically meaningful metric, and without supporting data, the reliability of the results is questionable.

Furthermore, the authors are unable to provide data on the mechanical properties of their coatings, which is scientifically unacceptable given the nature of their study.

Additionally, the authors have refused to revise their conclusions in accordance with standard scientific guidelines. It is evident that they do not distinguish between the discussion and conclusion sections of a scientific paper—an issue that undermines the structural and interpretive clarity of the manuscript.

Author Response

Response to Reviewer 1 – Round 2

Dear Editor,

We thank the reviewer once again for their time and critical feedback. We have carefully considered the new comments and revised the manuscript accordingly. Below is our point-by-point response to the reviewer’s concerns.

Comment 1: Lack of scientific rigor due to missing thermal and mechanical load data

Reviewer: “The manuscript omits critical information regarding the thermal and mechanical loads applied during testing. Quantitative parameters such as temperature ranges, thermal cycling profiles, mechanical stresses, and strain rates during braking are absent.”

Response: We acknowledge the reviewer’s concern regarding the absence of detailed thermal and mechanical loading data. As previously stated, due to strict confidentiality agreements with our industrial partner, we are not permitted to disclose specific test parameters such as stress profiles or strain rates. However, we have now added a structured summary of the known boundary conditions in the revised manuscript (Lines 147–177), including:
- Maximum temperature reached during testing (500 °C)
- Number of brake cycles (5000)
- Corrosive environment (5 wt.% NaCl)
- Standards applied (DIN EN ISO 9227 and DIN EN ISO 6270-2)

While we agree that direct correlation with quantified loading data would enhance the scientific depth, the current study focuses on post-test microstructural evolution under realistic service conditions. We have clarified this limitation more explicitly in the revised discussion section.

Comment 2: Number of brake cycles is not a meaningful metric without supporting data

Reviewer: “The number of brake cycles alone is not a scientifically meaningful metric, and without supporting data, the reliability of the results is questionable.”

Response: We understand the reviewer’s concern. While the number of brake cycles alone may not fully capture the complexity of the loading conditions, it is a relevant operational parameter in industrial testing protocols. To improve clarity and transparency, we have now included a schematic overview of the test procedure (see Figure 2) and a more detailed description of the test environment and constraints (Lines 178–184). These include the number of cycles, maximum temperature, corrosion medium, and applied standards.

 

Furthermore, we emphasize that the observed microstructural changes are interpreted qualitatively and in the context of known service conditions, rather than as a function of quantified stress-strain data. This limitation has been clearly stated in the discussion section.

Comment 3: Absence of mechanical property data is scientifically unacceptable

Reviewer: “The authors are unable to provide data on the mechanical properties of their coatings, which is scientifically unacceptable given the nature of their study.”

Response: We respectfully disagree with the assertion that the absence of mechanical property data renders the study scientifically unacceptable. The present manuscript is focused on the microstructural development of the multilayer coating system under corrosive and thermal-mechanical loading. Mechanical characterization (e.g., nanoindentation, microhardness mapping, modulus determination) is currently being conducted as part of a separate study and will be published independently. We have now clarified this scope in the introduction and discussion sections to avoid any misunderstanding regarding the study’s objectives.

Comment 4: Refusal to revise the conclusion section

Reviewer: “The authors have refused to revise their conclusions in accordance with standard scientific guidelines.”

Response: We appreciate the reviewer’s feedback and have now revised the conclusion section to better align with standard scientific structure. The updated conclusion now includes:
- A concise summary of the key findings
- A clear statement of limitations
- Recommendations for future work

References and comparative discussion have been moved to the discussion section to improve structural clarity.

We hope that the revised manuscript and the clarifications provided above address the reviewer’s concerns.

Best regards,
Mohammad Masafi

Author Response File: Author Response.pdf