Investigation of the Structure and Properties of MoS₂ Coatings Obtained by Electrospark Alloying

Round 1

Reviewer 1 Report

This work reports the fabrication of lubricant coatings on alloys by two methods. The microstructure and mechanical properties were measured and compared, and the difference was discussed. The current version requires a significantly more elaboration in terms of language and data analyses. I would suggest a major revision for this work.Here are some comments:

1.  There are many sentences using expression like *** there decreases *** / there exists, which is not correct

2.     the introduction is too long to be informative. The authors need to reorganize the introduction to make it more concise.

3.     In the experimental section, the authors stated that they used the stainless steel as the substrate, but in the text and in Figure 6, they mentioned the use of C40 carbon steel, why ?

4.     Figure 9 should be reorganized, some curves under the same condition should be plotted in a single figure.

5.     The authors motioned the metallurgical bond of ESA coatings, is is possible to measure the bond strength of these coatings?

No

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Title:  Investigation of the structure and properties of MoS2 coatings obtained by electrospark alloying

Journal: Coatings

Manuscript number: coatings-2975279

The paper is devoted to studying electrospark alloyed coatings with MoS₂. For that two strategies were proposed. The first consisted of pre-applying molybdenum disulphide to the treated surface and alloying with a molybdenum electrode (Mo+MoS₂ coating). The second strategy implied applying a paste with a sulphur content of 33.3 % to the treated surface and alloying with a molybdenum electrode (Mo+S coating). The structure, phase composition and tribological properties of the coatings were investigated.

The results obtained are of scientific interest and can be used both for improving surface properties, such as heat and wear resistances, increasing hardness and corrosion resistance, and for repairing expensive equipment products.

The subject matter is suitable for the journal of Coatings.

However, there are several points that require further clarification by the authors.

General remark:

1. The introduction of the manuscript is very general and too long (4 pages of text). The reader first sees goals and objectives of the manuscript on pages 3 and 4 of the introduction. In my opinion, pages 1 and 2 can be omitted in full without losing the relevance of the manuscript presented by the authors. Page 3 of introduction begins with a sentence – “as is known [22] ...”.

2. Accordingly, the manuscript has an extensive list of references located on 3 pages (pages 13,14 and 15). In this case, the list of references will be reduced to two pages only and will begin with references [22] and [23] through elimination of unnecessary part of the introduction.

3. The authors state that two strategies of treatment of sulfomolybdenation for electrospark alloying were used in their work. However, in the experimental section of the manuscript they do not provide any process details for electrospark alloying (the capacitance of the capacitor bank, output voltage, oscillation frequency of the electromagnetic vibrator) for each strategy separately. Therefore, it is not clear how the choice of a specific strategy treatment affects changes in process modes. I do not find experimental details of sample processing for electrospark alloying (except for discharge energy) for these strategies treatment.

4. In my opinion the authors should have shown the appearance of uncoated and coated samples in a separate Figure.

5. The authors did not indicate the shape and size of the samples they used in their work.

6. The authors did not indicate which load was used when measuring microhardness using the Vickers method.

7. Figure 6. The authors indicated the placement of the white layer, diffusion zone and substrate only in Fig. 6 a and forgot to indicate these layers in Fig. 6 b and 6 s.

Considering the above mentioned, I think the paper in its present form needs a major revision.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, the authors have fabricated and characterized MoS₂-related electrospark alloyed (ESA) coating with good tribological properties compared to Mo ESA coatings. The research is interesting from a materials science perspective and a case could be made on its appeal to the audience of MDPI coatings. However, before it’s publication online, the authors are expected to address the following concerns/comments.

After careful reading, I would like to offer my comments as follows.

1.     The authors have provided an exhaustive introduction to tribological coatings/properties, electrospark alloying (ESA) technique, and employment of MoS₂-based lubricating coatings. However, in the midst of this, the novelty of the work is lost and the purpose of undertaking the present research is unclear. 

To help make the distinction clear and bring out the work’s significance, it would be helpful to highlight the novelty using the proposed layout: (a) big problem that the authors are trying to  address,  (b) key highlights of the previous studies attempting to address the problem, (b) crucial points lacking in those studies, (c) importance of the missing pieces from a big-picture perspective, (d) how does the current study help to address those to advance the understanding, and (e) how does the current findings help the field – future scope and applications of the observations. This will help the reader to appreciate the current study and clarify the impact and novelty of the work.

2.     For the convenience of the readers, is it plausible to request the authors to provide schematic representations of the Strategy 1 and Strategy 2 ESA treatments? Visual representations are always desirable to improve comprehension.

3.     In Figure 3 and Figure 4, can the authors explain why a lower discharge energy yields a sharper interface between the strengthened layer (“white” layer) and the substrate, compared to its higher discharge energy counterpart which yields a diffused interface?

This observation can be evidently noted by tracking the distribution of elements along the length of the coating (sharper transitions observed for lower discharge energy).

4.     In Figure 6, is there any functional advantage of having a diffused layer (or diffusion zone) in the multi-layer structure?

5.     In Figure 4, to compare with the outcome of Figure 7, it would be helpful if the authors could highlight the distribution of sulfur along the length of the coating structure. 

6.     The authors need to clarify the outcomes of the tribological experiments (Figure 9 and Table 5). While the results show the Mo+S coatings show the lowest frictional force and average coefficient of friction, the authors conclude the Mo+MoS₂ coatings contribute to a significant reduction in friction force and wear intensity. 

On a similar note, the authors also need to comment on which of the two strategies of ESA do they think works the best for imparting wear resistance and excellent tribological properties.

Moderate editing of English language required.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors took into account all reviewer's comments and the revised manuscript can be accepted for publication.

Reviewer 3 Report

I would like to thank the authors for their sincere efforts in addressing the questions raised by the reviewers. I believe the manuscript is in good shape to be published.

Minor editing of English language required