Comparative Study of Tribomechanical Properties of HiPIMS with Positive Pulses DLC Coatings on Different Tools Steels

Round 1

Reviewer 1 Report

The authors have compared hardness and tribological properties of ta-C and WC:C coatings on 3 different tool steels. The work has similarity to other published results, so need to consider following points before publication:

1. Mention the novelty of this work. Previously sufficient work has been done on deposition of DLC and WC:C coatings using both UMBS and HIPIMS. It is of common knowledge that DLC coating has higher sp3 content therefore higher hardness, lower friction, better wear resistance than graphitic WC:C. This paper concludes the same.
2. line 147: Is Cr interlayer applied for both ta-C and WC:C coatings? Is it applied simultaneously/before/after WC interlayer?
3. line 155: Not clear if during ta-C deposition, both C targets are used at the same time or not. Does this coating have Cr interlayer? or a Cr:C interlayer? or a WC:C interlayer?
4. Please provide the cross-section of both the ta-C and WC:C coatings schematically indicating base interlayer, graded interlayer/s (if any) and top layer. SEM images are the standard requirement for the same.
5. What is the reason of using both Cr and WC interlayer for both the coatings? It is known that individual Cr and W/WC graded interlayers are good enough for such purposes. 
6. line 206: Not possible to understand coating thickness from calotest measurement as shown in Figure 3. Please provide the SEM images of the cross-sections for both coatings applied on all three substrates. 
7. Figure 4: Mention X and Y axis of these graphs. Why WC interlayer was applied on top of Cr interlayer? is it for increasing adhesion?
8. Table 4: ta-C coatings is harder and more wear resistant than WC:C due to its graphitic nature. They both have same interlayers. Then why ta-C shows almost half Lc2 and Lc3 values as compared to WC:C?
9. Fig 8 and 9 do not add any value as the results are given in Table 4 already.
10. Figure 10: Why the WC:C coatings have higher friction in run-in period than ta-C coating?
11. Figure 10: Why both coatings show different friction coefficient during steady state period particularly for Vancron? This is not observed for other two substrates.
12. Line 456: The higher hardness of the coating is not related to the process metallurgy of the steels used, rather it is related to the coating microstructure, particularly sp3 content and the interlayers. The same is applied for wear coefficient, better for ta-C than WC:C. So, authors need to mention in the discussion/conclusion section that what is the advantage of using such high quality tool steels rather than ordinary 304 or 316L SS as substrate. 
13. There are some minor grammatical errors, such as gran size instead of grain size, de instead of the etc. This should be taken care of.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript presents a good investigation of the tribological properties of HIPIMS with positive pulses DLC coatings. The work is comprehensive and very interesting to read. Given below are my comments to improvise upon the quality of the manuscript.

1. I suggest adding a few statements on the tribology of DLC coating in the introduction. You can cite the following works in the introduction.

https://iopscience.iop.org/article/10.1088/1757-899X/149/1/012124/meta

2. How did you measure the Ra value discussed in line 104? What was the exact value observed?
3. Please explain the reason for the failure modes discussed in lines 185 – 188.
4. Can you please add the results/pictures of the wear tracks observed from the confocal microscopy (line 196)?
5. What are calotest measurements, please add a few statements in line 205.
6. What do you mean by the anchoring layer (line 218)?
7. Please add axis labels in fig 4.
8. Line 242: What are the mentioned breathing modes?
9. Please check your manuscript for typos. I am able to spot some errors such as ‘Theorical analyses’ (line 278), ‘gran size’ (line 282), ‘it also be noted’ (line 412), etc.
10. How did you perform the scratch test? (line 293)
11. Please increase the resolution of fig 10.
12. How did you measure the width of the wear track? (line357)

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

In this paper, DLC coatings on different tool steels were systematically investigated using multiple characterisation methods, which exhibited good results. The discussion, however, was not sufficient enough to support the results obtained. This paper may require major revision before it can be considered for publication.

1. Actually, this paper showed more results than those regarding tribological properties. The article title can be reconsidered.
2. In Section 2.3, the materials that both pin and disk represent should be stated. The hardness and surface roughness of the friction pair should be also stated.
3. What is the reason to choose the parameters (40 N, 200 rpm and 20000 cycles) in the tribological tests? Please explain it.
4. In Fig. 8, it is better to magnify the damage spots in the micrographs corresponding to the critical loads.
5. In Fig. 9, what is the main factor that causes the fluctuation in COF curves when using coated samples? Please explain it.
6. In Fig. 10, the fonts and scale bars in the optical micrographs are not clear.
7. Lines 393-394, the confocal microscope is in fact able to measure the wear area of cross section along the wear track. The wear volume can be therefore calculated through the wear area multiplying the wear track length. Eq. 1 may need to be updated in this case.
8. The discussion part is too short to support the results obtained.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have addressed all the comments and the quality of the manuscript has been improved.

Please note that some amswers to the comments and their references included in the response to comments section (point 8 and point 10), are not included/missed in the main manuscript version 2. 

Author Response

Response to Reviewer 1 Comments – Round 2

Point 1: Please note that some answers to the comments and their references included in the response to comments section (point 8 and point 10), are not included/missed in the main manuscript version 2.

Response 1: The comments and the references of the points 8 and 10 have been included in the new version of the paper.

Reviewer 2 Report

Thank you for taking into account my comments and revising the manuscript accordingly.

Author Response

There were no more points to review in this second round.

Thank you for your comments and suggestions.