Influences of Nitrogen Flow Rate on Microstructure, Mechanical and Tribological Properties of WCN Coatings Deposited by HiPIMS

Round 1

Reviewer 1 Report

The title is of interest and could be recommended for publication. However, the following comments must be addressed adequately by the authors:
1. In Figure 6, the different sections should be named on the micrographs.

We thank the reviewer’s comment. The different sections have been amplified in the Figure 6. The modifications in the revised manuscript are indicated in RED.
2. The results of EDS analyses from different areas of the coating, interface, and substrate should be provided.

We appreciate the reviewer’s comment. We have provided the results of EDS analyses from different areas of the coating, interface, and substrate in the Table 2. The modifications in the revised manuscript are indicated in RED.
-Higher magnification SEM images should be provided to show the structure of different parts of the coatings.
-The relationship between the coating interface and the substrate needs to be further explored.

3. Higher magnification SEM images should be provided to show the structure of different parts of the coatings.

Thanks to the reviewer’s comment, we have provided higher magnification SEM images to show the structure of different parts of the coatings embedded in Figure 6. The modifications in the revised manuscript are indicated in RED.

4. The relationship between the coating interface and the substrate needs to be further explored.

The relationship between the coating interface and the substrate have been further explored. The revisions in the manuscript are indicated in RED.

5. In the introduction, the formation of carbide coatings with other methods and their comparison should be discussed. For this purpose, review the following resources could be helpful:
--Ceramics International, 41 (2015) 9350-9360
--Neural Computing and Applications, 24 (2014) 685-694

Thanks to your kind advice and I have downloaded the essays you have mentioned. After a careful reading of the inspiring and illuminating papers I found them closely related to my research. Therefore I could refer to these two articles, which helps me a lot to improve my own paper. I’ve included the two essays you recommended into the part “References”. I really appreciate your comment and academic perspective. The modifications in the revised text and figure are indicated in RED.
6. The conclusion part needs to be abridged. The current form is lengthy and it is recommended to be written in such a quantitative manner.

We appreciate the reviewer’s valuable and careful comments. The Conclusion part has been shortened as suggested by the reviewer. The modifications in the revised manuscript are indicated in RED.
7. There are too many obsolete refs published before 2011 are cited in the ref part. Exclusion of the outdated refs is suggested.

Thanks to the reviewer’s careful comment. We have removed some of the refs before 2011. The modifications in the revised manuscript are indicated in RED.

Reviewer 2 Report

The submitted manuscript is very interesting and I find it valuable in the discussion about improving the anti-wear performance of WC coatings by incorporating the nitrogen dopant. The main advantage of the submitted paper is a complementary set of methods that will characterize the test material. The construction of the text is consistent. The experimental procedure is conducted reliably and honestly. However, in my honest opinion, the submitted paper has few weak points which require correction. Here is the list of my comments formulated after reading your paper:

1. The English is easy to read and understandable for me, nevertheless I would consider thoroughly check the wording in the text, i.e.: title has no articles, “hexagonal vacuum”, “experimental facility”, etc.)

We thank the reviewer’s comment. We have checked the manuscript thoroughly. The “hexagonal vacuum” has been change into “hexagonal vacuum chamber”, “experimental facility” has been change into “experimental setup”. The modifications in the revised manuscript are indicated in RED.

2. Line 104 “The HiPIMS power supply for WC target is operated in constant voltage mode and the peak target voltage is maintained at -820 V” Comparing this statement with the Fig. 2 is confusing. From Fig. 2 it is clear that the power supply does not constantly keep the -820 V. If I read the figure well, the discharge is triggered by the ~ -800 V voltage pulse and since the voltage is not changing during the pulse the power supply operates in a “voltage mode” keeping it steady. Another thing is that the voltage drops after the discharge to -200 V and the left side shows that just before pulse it is kept at -400 V. Can you explain the operation of the power supply in the revised text more precisely?

Thanks to the reviewer’s valuable and careful comment. I’m sorry I have made a stupid mistake. After we reviewed the original dates and found the peak target voltage is maintained at -800 V just as you have pointed out. So we have revised the values given for the peak target voltage. The voltage before pulse kept at -400 V is in order to make the target easier to glow. And the voltage drops after the discharge to -200 V and rise up to -400 V during the pulse off time. The modifications in the revised manuscript are indicated in RED.

3. The figure is put in the text far after first mentioned in the text. It should be moved. Additionally, the voltage axis description does not include its negative character.

Thanks to the reviewer’s comment, we have put the figure 2 in the appropriate position of the text. And we add the negative character in the voltage axis description.

4. Table 1 Was the temperature stabilized to 150 °C during each process of coating deposition or it was set only initially? This is crucial for the discussion of the structure of the coatings.

Thanks to the reviewer’s valuable and careful comment, the temperature is kept stable to 150 °C during each process of coating deposition. We have monitored the temperature by using a thermometer during the deposition.

5. Coatings preparation Was the planetary table rotating during the deposition or kept steady?

Thanks to the reviewer’s comment, the planetary table was kept steady during the all the coatings preparation.

6. OES measurements How were the spectra collected? It should be indicated in the experimental procedure what was the acquisition time, how many plasma pulses were summed to obtained presented spectra, if they were averaged, etc?

Thanks to the reviewer’s comment, the OES spectra were collected by the probe of the OES spectrometer located in front of the inspection windows. The acquisition time and the statistical mode have been added in the manuscript and the modifications are indicated in RED.

7. How high was the collimator placed? Was it at the height where substrates were mounted?

Thanks to the reviewer’s valuable and careful comment, the mounting height of the collimator relative to the substrates is very important. In our experiments, the mounting height of the collimator is the same as the substrates.

8. Line 170 “The total working pressure increases with increasing inflow nitrogen, which leads to more collision and produces more ions [15].” As I briefly checked the 15th references nothing is dealing with your statement. Please provide proper literature. This is true that in relatively higher pressure the total number of inter-molecular collision events increases, but it does not mean that number of ions also increases. It depends on which side of the Paschen curve we are on with the pressure. In my honest opinion, this explanation is very speculative. In the condition of higher pressure, we can obtain a larger population of excited species but the excitation of individual species can be relatively low because of a high tendency to losing the kinetic energy by a high number of non-elastic collisions. I agree the discussion about the free path is necessary here, but the authors focused on it so deep that forgot about other factors which could increase the number of electrons like the mechanism of secondary electron creation of nitrogen or the thermo-electrons emission from the poisoned target material.

Thanks to the reviewer’s valuable and careful comment, your incisive advice concerning the relationship between deposition pressure and the number of ions. I deeply agree with you and realize the inadequacy of this part. Definitely I have made revisions as you advised. And about the 15^th reference, I am so sorry I have made a stupid mistake. I have replaced this reference by a correct one. The revisions in the manuscript are indicated in RED.

9. what kind of current is presented in the figure? Either ion or electron? The authors in the many fragments of the text discuss their results basing on ion population increase/decrease. Are the electrons irrelevant in the discussion?

Thanks to the reviewer’s valuable and careful comment, the current presented in the figure is ion current. In this paper, we did not take into account the electronic current. In our future studies, we will systematically discuss the states and roles of electrons in plasmas.

10. This figure is not easy to read especially when comparing the intensity of indicated lines. I have to trust by heart what the authors have written in the text while the figure itself should speak. I strongly recommend creating inserts or additional graph panels presenting the evolution of the intensity of chosen lines in a function of nitrogen flow rate. Moreover, the exact position of indicated lines is impossible to read from the figure. I would consider creating an additional table with spectral data of indicated lines: position, intensity, levels.

Thanks to the reviewer’s valuable and careful comment, we have add an additional table-Table 3 to present the spectral data of indicated lines: position, intensity, species. The revisions in the manuscript are indicated in RED.

11. XPS analysis The authors provided many XPS spectra, what about the chemical composition of coatings? Was it calculated?

Thanks to the reviewer’s valuable and careful comment, the chemical composition of coatings has been presented in the new table- Table 2. The revisions in the manuscript are indicated in RED.

12. I would strongly recommend the authors create an additional table provided summarized results of material characterization: deposition rate, chemical composition, grain size, roughness, etc. It is helpful for the readers to find the most important information of the studied material when quickly reading the manuscript. I believe it raises the further citation index of the paper.

Thanks to the reviewer’s valuable and careful comment, we have added an additional table which provided summarized results of material characterization: deposition rate, chemical composition, grain size, roughness, etc. The revisions in the manuscript are indicated in RED.

13. Line 296 I agree with the explanation given by the authors that more excited plasma activates the lateral growth of crystallites and adatoms mobility, nevertheless, I would extend their discussion of the mechanism of defecting the growing films under the bombardment of more excited plasma species. In my opinion, it corresponds well with the thread and obtained results.

We thank the reviewer’s advice. Your incisive advice concerning the mechanism of defecting the growing films under the bombardment of more excited plasma species indeed makes sense. I deeply appreciate the incisive perspective of the reviewer, which will profoundly contribute to my future research. But in the research of this article, we did not do experiments or analyses related to this topic, Could we research and discuss in-depth in our future experiment?

14. The Conclusion section I do not like the construction of this section. I agree that this section should also refer to the research in a summarized manner but the authors made an abstract of the Results section. In my honest opinion, this section should provide a direct reference to the research goal, present the study in an overall context of the taken topic, describe the studied mechanisms in general meaning, show the significance of their finding and possible future actions.

We appreciate the reviewer’s valuable and careful comments. The Conclusion part has been shortened as suggested by the reviewer. The modifications in the revised manuscript are indicated in RED.

Round 2

Reviewer 1 Report

The authors addressed all my comments and therefore I can recommend publication of their revised manuscript in Coatings.