Vacuum Tribological Properties of W-S-N Coatings Synthesized by Direct Current Magnetron Sputtering
, Khuram Yaqoob 3, Amir Mukhtar 3, Filipe Fernandes 1,2,4,*, Andrey Bondarev 5, Fabio Ferreira 1, Abbas Al-Rjoub 1 and Albano Cavaleiro 1,2
Round 1
Reviewer 1 Report
The present manuscript shows the second stage of another publication, with interesting results. Maybe only one paper with both results could be increse the impact of their research. However, the analysis of the N in the WSN system obtained by MS; additional to tribological properties of WSN is suficient for this publication. Congratulations for the excelent work
Author Response
Dear reviewer, please see the replies in attached file.
Response to the comments of the reviewers
Title: Effect of annealing heat treatment on the composition, morphology, structure and mechanical properties of the W-S-N coatings
coatings-1947083
First of all, we would like to highly appreciate and thank the Reviewers for managing the time for their critical review of the manuscript and all the suggestions for improvements. All the comments were carefully considered, and corrections and improvements were made accordingly. We are providing point by point response for the comments received from the Reviewer.
The changes made in the manuscript are highlighted in the respective file.
Reviewer #1:
The present manuscript shows the second stage of another publication, with interesting results. Maybe only one paper with both results could be increased the impact of their research. However, the analysis of the N in the WSN system obtained by MS; additional to tribological properties of WSN is sufficient for this publication. Congratulations for the excellent work.
Thank you for the encouraging remarks. Regarding one single article, the authors agree with this comment but the idea of 2 separate articles was:
- The previous article was getting too lengthy
- Vacuum tribological properties of WSN coating have never been reported before so, the idea was to present these novel results in form of short communication
Reviewer 2 Report
The authors investigated the tribological behaviors of TMN coatings under vacuum environments. The results are valuable, but there are some issues need to be modified.
1. In the abstract, the authors claimed that the tribological behaviors are usually ignored. However, Martin's research group conducted a series of studies on the tribological behavior of MoS2 coating since 1990s, including under vacuum condition. "Surface and Coatings Technology 62(1–3) ,1993, 406-411".
2. The repeatability of the friction tests should be provided.
3. The functional mechanisms should be also further discussed.
Author Response
Dear reviewer, please see the replies in attached file.
Response to the comments of the reviewers
Title: Effect of annealing heat treatment on the composition, morphology, structure and mechanical properties of the W-S-N coatings
coatings-1947083
First of all, we would like to highly appreciate and thank the Reviewers for managing the time for their critical review of the manuscript and all the suggestions for improvements. All the comments were carefully considered, and corrections and improvements were made accordingly. We are providing point by point response for the comments received from the Reviewer.
The changes made in the manuscript are highlighted in the respective file.
Reviewer #2:
The authors investigated the tribological behaviors of TMN coatings under vacuum environments. The results are valuable, but there are some issues need to be modified.
- In the abstract, the authors claimed that the tribological behaviors are usually ignored. However, Martin's research group conducted a series of studies on the tribological behavior of MoS2 coating since 1990s, including under vacuum condition. "Surface and Coatings Technology 62(1–3) ,1993, 406-411".
The reviewer is right about the reports of MoS2 in vacuum environments. There are also reports of WS2, W-S-C, Mo-S-C, Mo-S-N and Mo-Se-C coatings tribological properties in vacuum environments. But these coating do not behave similar and the properties vary from system to system.
The present work is focusing on the tribological performance of “W-S-N” coatings in vacuum environment which has never been explored before. Thus, this was required to be explored.
- The repeatability of the friction tests should be provided.
All the tests were repeated twice. The authors find it more adequate to show friction curves from one of the tests rather than showing mean/average values. For reference, the friction curves obtained from both tests of WSN12.5 coating are shown in the figure R.1.
Figure R.1: Repeatability test results of WSN12.5 coating
- The functional mechanisms should be also further discussed.
The idea of this manuscript is to present results in the form of short communication. Thus, detailed discussion has not been reported. Also, for a detailed discussion of the mechanisms behind the obtained results, further analysis like Raman spectroscopy, scanning electron microscopy micrographs and transmission electron microscope analysis of the wear tracks must be done. This can be a part of future studies but for this article, the authors wanted to disseminate novel results in form of communication for clear elucidate the readers about what we want to communicate.
Reviewer 3 Report
The article needs significant corrections. Introduction is too general. Details are required. A modest number of cited works by other scientists. The poorly and inaccurately described methodology needs to be significantly improved. Discussion of the results of the work insufficient, no explanation of the obtained results. The summary should be significantly enriched, it is insufficient for the requirements of the journal. Overall, the work is scientifically poor.
Author Response
Dear reviewer, please see the replies in attached file.
Response to the comments of the reviewers
Title: Effect of annealing heat treatment on the composition, morphology, structure and mechanical properties of the W-S-N coatings
coatings-1947083
First of all, we would like to highly appreciate and thank the Reviewers for managing the time for their critical review of the manuscript and all the suggestions for improvements. All the comments were carefully considered, and corrections and improvements were made accordingly. We are providing point by point response for the comments received from the Reviewer.
The changes made in the manuscript are highlighted in the respective file.
Reviewer #3:
The article needs significant corrections. Introduction is too general. Details are required. A modest number of cited works by other scientists. The poorly and inaccurately described methodology needs to be significantly improved. Discussion of the results of the work insufficient, no explanation of the obtained results. The summary should be significantly enriched, it is insufficient for the requirements of the journal. Overall, the work is scientifically poor
With all due respect, the authors want to bring it to the notice of the reviewer that this manuscript has been submitted as a communication and not a full-length article. Therefore, this manuscript does not contain all the details and just focuses on one domain of results i.e., Vacuum tribological properties. The reason for this focus is that literature lacks reports on these investigations and authors should take in to account that a) the properties of TMD based solid lubricants can be different in dry environments (here, dry N2) and vacuum. Thus, analysis in all environments is important for a systematic study and selection for specific applications. The message is that dry N tests cannot be taken as granted estimate the behaviour of the films in vacuum conditions. This is the procedure often used in the literature and communication alerts for it.
- b) the coating that displayed superior properties in other environments is not the one showing superior properties in vacuum. It should be mentioned that this comparison of performance is strictly made for WSN coatings in diverse environments.
- c) overall, WSN coatings show good frictional properties in vacuum as well.
Reviewer 4 Report
The manuscript explores the tribological properties of magnetron sputtered W-S-N coatings in the ideal vacuum conditions.
The paper is well-organized, thorough descriptions of the experimental are provided, as well as discussion on the results. Figures are clear to understand, as well as tables.
The English form is overall good, with just some minor spell checks required.
To aid the readability of the manuscript, the reviewer suggests dividing it into sections (Introduction, Material and Methods, etc.), as clearly indicated in the “Instructions for Authors” section of the journal.
After performing such minor revisions, the reviewer thinks the paper can be accepted for publication in Coatings.
Author Response
Dear reviewer, please see the replies in attached file.
Response to the comments of the reviewers
Title: Effect of annealing heat treatment on the composition, morphology, structure and mechanical properties of the W-S-N coatings
coatings-1947083
First of all, we would like to highly appreciate and thank the Reviewers for managing the time for their critical review of the manuscript and all the suggestions for improvements. All the comments were carefully considered, and corrections and improvements were made accordingly. We are providing point by point response for the comments received from the Reviewer.
The changes made in the manuscript are highlighted in the respective file.
Reviewer #4:
The manuscript explores the tribological properties of magnetron sputtered W-S-N coatings in the ideal vacuum conditions. The paper is well-organized, thorough descriptions of the experimental are provided, as well as discussion on the results. Figures are clear to understand, as well as tables.
The English form is overall good, with just some minor spell checks required.
English revision has been done in the manuscript.
To aid the readability of the manuscript, the reviewer suggests dividing it into sections (Introduction, Material and Methods, etc.), as clearly indicated in the “Instructions for Authors” section of the journal. After performing such minor revisions, the reviewer thinks the paper can be accepted for publication in Coatings.
The suggested changes have been made in the manuscript.
Round 2
Reviewer 2 Report
I suggest this manuscript to be accepted in present form.
Author Response
Thank you for your evaluation.
Reviewer 3 Report
Dear authors, the article needs a few significant corrections:
1) ''The coating with the lowest N content (14.6 at. %) displayed the lowest friction, specific wear rate and wear scar depth under vacuum conditions'', why???.
2) "Despite superior sliding performance at room temperature (35 % humidity), 200 áµ’C and dry nitrogen conditions, the performance of WSN12.5 coating deteriorated vacuum environment." why???
3) "The coating with the lowest nitrogen concentration i.e., WSN5 coating, displayed the lowest friction coefficient." why is this happening?
4) "The WSN12.5 coating that displayed the best sliding properties in roomtemperature humid air, 200 áµ’C and dry nitrogen sliding displayed the worst frictional properties in vacuum." why is this happening?
5) "After N introduction, the specific wear rate suddenly drops one order of the magnitude, why? showing the beneficial influence of N-alloying.
6) "From the lowest to the highest N content, the specific wear rate progressively increased being the highest value of 2.4 x 10-7 mm3/Nm achieved by WSN20." why has increased? "The lowest value of 3.7 x 10-8 mm3/Nm was observed for the WSN5 coating." why has it lowered?
A scientific article requires a few words of explanation of obtained research results.
Author Response
Dear reviewer please see in attach the reply to your questions.
Response to the comments of the reviewers
Title: Effect of annealing heat treatment on the composition, morphology, structure and mechanical properties of the W-S-N coatings
coatings-1947083
First of all, we would like to highly appreciate and thank the Reviewers for managing the time for their critical review of the manuscript and all the suggestions for improvements. All the comments were carefully considered, and corrections and improvements were made accordingly. We are providing point by point response for the comments received from the Reviewer.
Reviewer #3:
Dear authors, the article needs a few significant corrections:
Questions 1 and 3:
1: ''The coating with the lowest N content (14.6 at. %) displayed the lowest friction, specific wear rate and wear scar depth under vacuum conditions'', why???.
3: "The coating with the lowest nitrogen concentration i.e., WSN5 coating, displayed the lowest friction coefficient." why is this happening?
It is a well-known fact about TMD based coatings that higher the TMD content or chalcogen atom/metal atom ratio, lower will be the friction and wear in vacuum atmospheres. This has been observed in almost all the literature on different TMD based coatings. This happens because with the removal of moisture, the easy shear tribolayer formation becomes more convenient. As the sliding of TMDs is due to these easy shear tribolayers, higher the possibility of their formation, higher will be friction and wear resistance.
Questions 2 and 4:
2: "Despite superior sliding performance at room temperature (35 % humidity), 200 áµ’C and dry nitrogen conditions, the performance of WSN12.5 coating deteriorated vacuum environment." why???
4: "The WSN12.5 coating that displayed the best sliding properties in room temperature humid air, 200 áµ’C and dry nitrogen sliding displayed the worst frictional properties in vacuum." why is this happening?
As all the testing reported in this work is related to the vacuum testing, so when performance of WSN12.5 is compared with WSN5 coating, the factors of higher S/W ratio or WS2 content comes in to play. Higher ratio makes the sliding and tribolayer formation easy, leading to low COF and wear. Whereas, for the comparison with WSN20 coating, the authors based on their experience with TMD coatings think that the higher hardness of WSN12.5 might have resulted in formation of some hard particles, leading to higher friction. When hard worn particles are accumulated in the wear track as 3rd bodies, efficient tribolayer formation is hindered. This hindrance in the tribolayer formation leads to higher friction. Despite this explanation, authors strongly think that this should be explored in detail via Raman spectroscopy and HRTEM studies of the wear tracks. This suggestion can be part of some future work but as this work is a short communication, this cannot be included here.
Questions 5:
5) "After N introduction, the specific wear rate suddenly drops one order of the magnitude, why? showing the beneficial influence of N-alloying.
Wear rate is strongly dependent on the load bearing capabilities of the coatings. The pure sputtered TMD coatings (WS2 here) are highly porous. The high porosity makes these coatings very soft with inferior load bearing capabilities. Thus, during sliding, pure coatings wear off fast. This is actually one of the main reasons that doping of TMDs is done / was experimented by Voevodin et. al in 1999.
Questions 5:
6) "From the lowest to the highest N content, the specific wear rate progressively increased being the highest value of 2.4 x 10-7 mm3/Nm achieved by WSN20." why has increased? "The lowest value of 3.7 x 10-8 mm3/Nm was observed for the WSN5 coating." why has it lowered?
The increments in the wear rate are again related to the decrements in the efficiency of the tribolayer formation and S/W ratios. It should be noted that the wear rates of WSN12.5 and WSN20 are not so different and so was the S/W ratios.
A scientific article requires a few words of explanation of obtained research results.
The authors tried to address the questions of the respected reviewer. The answers of similar questions have been combined together. The authors would like to mention that this is a communication and not a full-length article. A communication should just disseminate the initial results and not address the in-depth mechanisms or discussions behind the reported results/phenomenon. The questions raised by the reviewer are very legitimate but, being a communication, the detailed discussion have been avoided.
