Investigation on Sliding Tribological Properties of Laser Cladding Alloy Coating for Subway Wheels
Round 1
Reviewer 1 Report
This manuscript presents an interesting application of laser-surface interaction for developing high mechanical resistance materials. The topic is fascinating and within the journal's scope.
Besides that, the topic is relevant, but the presentation needs significant improvement in English and narrative. The authors should consider getting an English professional editor to revise the manuscript.
It was hard to understand what the authors wanted to convey in several sections.
The above is a significant weakness of this manuscript, which, in the view of this review, is not acceptable for publication.
Please, have a look at the attachment and read the below comments
Highlights in red are examples of English grammar improvements
Highlights in yellow are examples of a better message delivery
Line16: what do the authors mean?
Line 28-34: it is hard to read, and it can be written, concise to the point
Lines 54-57; it is hard to read. Should be improved
Line 57: microscopy analysis rather than microscopic analysis
Line 57-58: surface characterization techniques such as ….
Line 59. Full stop before starting Vickers microhardness
Lines 73-82: hard to read
Lines 83-88 should be improved
Figure 5: quality should be improved and a legend describing each picture should be added. That would help the discussion.
Figure 6: quality should be improved and a legend describing each picture should be added. That would help the discussion.
Figure 7: Description: Fe-Based elemental mapping by EDS. A legend describing each picture should be added. That would help the discussion.
Figure 8: Description: Fe-Based elemental mapping by EDS. A legend describing each picture should be added. That would help the discussion.
Lines 151-157. It is tough to read and understand what the authors want to communicate.
There are missing details in Figures 11 and 12
Lines 239 and 241: Ga is a new element introduced here; what was not detected on the EDS? And reflected in Tables 6, 7, 8 & 9.
Line 298: the presence of carbide in mention. Why there were they not identified by XRD?
Line 344: which is 50 %, what? More? Less? explain
Comments for author File: 
Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
coatings-1948316
Investigation on Sliding Tribological Properties of Laser Cladding Alloy Coating for Subway Wheel
Good meticulous work for present day research. This work is a good study on investigation on sliding tribological properties of laser cladding alloy coating for subway wheel. As a whole, the manuscript adds significant contribution to the existing literature in terms of coating system i.e., Fe-based and Ni-based, coating treatment and its applications on subway wheel. The work is following the standard procedure and technique to produce the desired results. This work also analyzed the microstructure and morphology, phase types, interfacial element distribution and segregation, and hardness of the Ni-based and Fe-based alloy coatings. English expression should be improved and carefully organized. The rest of the results and discussion are acceptable and well-presented. Include the below references to add more value for the publication.
https://www.degruyter.com/document/doi/10.3139/120.111272/html
https://link.springer.com/article/10.1007/s40864-017-0072-2
https://doi.org/10.1016/j.wear.2005.01.018
How could the authors have missed out these important contributions in this field? The technical depth is very much appropriate for the general knowledgeable individuals working in the same field.
Comments for author File: 
Comments.docx
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The article highlights peculiarities of the laser cladding technology to repair the wheel. The authors investigated Ni-based and Fe-based alloy coatings. They studied microstructure and mechanical properties of the coating, as well as its wear resistance. However, insufficient attention has been paid to the damage of the rolling surface due to the formation of shelling (spalling) on the surface of restored wheels.
The article is interesting, but a number of shortcomings should be corrected:
1. Table 3 should be checked, as the Ni-based powder should have the highest Ni content, while the Table gives Nb (Bal.).
2. What material was used as a counterbody, as well as its mechanical properties, should be indicated.
3. Contact stresses that best characterize the wear process should be added to Table 5.
4. The text in Figures 2, 13, 14, 15, 16, 17, 18, and 19 cannot be recognized.
5. Figures 4, 6, 7, 8 and 10 are given before their mention in the text, and this should be corrected.
6. It should be explained on the basis of Table 6 how the Cr content in the Ni-based coating reached 8.41% (the Cr content in the original powder is 0.26%, and the Cr content in ER9 steel is also 0.26%).
7. There are no references to Figures 11, 12, and 14 in the text of the manuscript.
8. It should be explained where the Ga-containing phases came from, if this element was not present either in ER9 steel or in Ni-based and Fe-based powders.
9. It should be explained why the cladding with a hardness 2.8 times higher than the hardness of an ER9 steel wheel should be applied. At such a high hardness, intensive wear of the rail will occur. Replacing or cladding worn rails is more problematic than wheels. Therefore, the ratio between the hardness of the wheel and the rail should be 0.9-1.1. This ratio ensures optimal wear of the wheel-rail pair.
10. By increasing the hardness of the rolling surface, wear resistance can be increased, but an important factor that also affects the lifetime of the wheel is the damage of the rolling surface due to the formation of shellings (spalling) and sliders. The thinning of the rim thickness due to wear is three times less than the thinning of the rim due to reprofiling to eliminate damage to the rolling surface. The damage of the rolling surface depends on the crack growth resistance characteristics of the material. Authors should note this in the text of the manuscript. They can use literature data, for example https://doi.org/10.1007/s11003-013-9557-7, https://doi.org/10.5604/01.3001.0012.0662
11. The titles of sections 3.4 and 3.5 are identical, and this should be corrected.
12. In the introduction, the authors should pay more attention to the issue of the workability of the wheel-rail pair.
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
The authors presented interesting investigations concerning tribological properties of laser cladding alloy coating for subway wheel. Due to the practicality of the presented research – in my opinion, their results are worth considering publishing in "Coatings”. Unfortunately, due to the fact that I found many inaccuracies in the introduction, methodology, results analysis and conclusions I suggest VERY major review of this work.
My detailed remarks:
1. p.1, l. 28: Scuffing is a very extreme situation for wheels - what about other forms of wear when the wheel "slides" over the rail? Maybe it is worth adding a few words on this topic?
2. p.2, l. 64-65: Why were Fe-based and Ni-based powders used? In the introduction, you did not write about their applicability in railway practice. Please add a justification.
3. Fig.1: unfortunately, a photo of this size is completely unreadable.
4. p. 2, l. 73-74: Authors wrote that “the quality of the coating of the melted samples (Figure 3) is free of excessive cracks, pores and other defects” - how can you prove it? Only on the basis of macroscopic study (Fig. 3)? It is not enough to be sure that there are no structural defects (in my opinion).
5. p. 3, l. 87: “at least three experiments were carried out” - do you mean that three repeats of the tribological test were done for each sample?
6. p. 3: The friction test conditions are imprecise. What was the counter-sample for the analyzed materials? Please show the diagram of the friction pair in the experiment. What were the reference samples? The same steel but uncoated?
7. Tab. 5: please justify why the load of 10N was applied.
8. Fig. 4: I am afraid that the drawing in this form adds nothing to the analysis. The difference between them is really subtle. Without microscopic pictures of the cross-sections of the tested surfaces, the interpretations of wear mechanisms is like “reading tea leaves” here.
9. p. 4, l. 119: how did you recognize spalling? The condition for its appearance is the presence of martensite in the microstructure. In your opinion, is it possible here?
10. Fig. 5 –all first of all, there is no spalling here – post-spalling traces look different and post-spalling material losses are macroscopic visible. Here we have pictures with a very high magnification. What you marked as spalling are micro-grooves resulting from plastic deformation during abrasion. Above the picture you wrote that there is no spalling here - and then you mark it in the pictures (?)
11. Figs. 5 and 6: these pictures are so small and of such poor quality that it is difficult to say whether there are traces of adhesive wear (or not) there.
12. Fig. 6: Same as above, in my opinion there is no spalling here.
13. Fig. 7: What follows from this that the oxygen atoms are more concentrated in the dark gray area? Please compare the concentration of oxygen and iron. Is this location of iron oxides a consequence of specific wear mechanisms ?
14. p. 6, l. 158-160: clear signs of wear indicate more intensive wear and therefore no surface strengthening.
15. par. 3.4: what are the statistics for your averages - what values are standard deviations or confidence intervals? It may be that there are no statistically proven differences between the COF for the two types of samples. This is crucial information - please add it to the Tab. 10.
16. What do you mean using the term of “grinding stage”? I suppose you think about “running-in” (?)
17. On what basis are the conclusions regarding the metallurgical phases formulated? There is no analysis on this issue in this article!
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Thanks, to the authors to work extensively on the manuscript to improve its quality
Good luck
Author Response
Thanks for your careful checks. We are sorry for our carelessness. Based on your comments, we have made the corrections to make the unit harmonized within the whole manuscript. We appreciate for Reviewers’ warm work earnestly.
Reviewer 3 Report
The authors took into account almost all comments of the reviewer and made appropriate corrections to the manuscript.
However, a number of shortcomings need to be corrected:
1. The text in Figure 18 cannot be recognized.
2. The authors should explain why they chose the ceramic counterbody during wear tests, since the hardness of the counterbody is significantly higher than the hardness of the rail.
3. The authors report that “In this paper, the sliding friction wear performance of the coating itself is examined, and the optimal wear matching of the wheel track is not considered for the time being.” The obtained results lose their importance for the cladding of wheels with Fe-based coating, since the wheel cannot be considered separately from the rail in terms of wear resistance. Therefore, the authors should note that Fe-based coating is not suitable the cladding of wheels.
Author Response
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Reviewer 4 Report
I still think part of the surface wear analysis could be better presented. For example, signs of wear clearly indicate abrasion - which could be seen in the figures (e.g. 6 or 10). However, these problems do not disqualify this paper. In my opinion, the authors' corrections are sufficient to publish the results of this research in "Coatings".
Author Response
Thanks for your careful checks. We are sorry for our carelessness. Based on your comments, we have made the corrections to make the unit harmonized within the whole manuscript. We appreciate for Reviewers’ warm work earnestly.
