Review Reports - Properties of Diamond-like Coatings in Tribological Systems Lubricated with Ionic Liquid

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the submitted manuscript, the effect of using a lubricant in the form of an ionic liquid: 8 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF6) on the tribological prop-9 erties of a hydrogenated diamond-like coating (DLC) doped with tungsten a-C:H:W were investigated. In my opinion, the research content of the article is small and the experimental procedure need to be described in more detail. The review suggestions are as follows:

What the frictional experiment details, like sliding speed, duration time et al. and why was the experiment condition chosen for the present study?
Could the authors provide the SEM images of the samples (100Cr6 steel discs, DLC coating and counter-sample 100Cr6 steel ball) before and after the tribological tests for better understand the sliding process?
The discussion of the results is insufficient, like the dry friction and friction using ionic liquid, the author only provide the amplitude parameters of the 100Cr6 steel surface and with DLC coating, while there is no depth discussion for the difference between these results. Moreover, the results obtained in the paper should be compared to the data which reported previously.

Author Response

Thank you very much for your valuable comments.

What the frictional experiment details, like sliding speed, duration time et al. and why was the experiment condition chosen for the present study?

Information on the sliding velocity (0.1 m/s) was included in the original version of the article in Table 3. In accordance with the comments to the above-mentioned table, I have supplemented the information on the test duration. This particular type of test (10 N load and 100Cr6 steel countersample with a diameter of 6 mm) was selected due to its normative compliance. The 15 N load was selected by comparing the changes in friction after increasing the normative load by 50%.

Could the authors provide the SEM images of the samples (100Cr6 steel discs, DLC coating and counter-sample 100Cr6 steel ball) before and after the tribological tests for better understand the sliding process.

As suggested, SEM images before and after tribological tests were included in the article.

The discussion of the results is insufficient, like the dry friction and friction using ionic liquid, the author only provide the amplitude parameters of the 100Cr6 steel surface and with DLC coating, while there is no depth discussion for the difference between these results. Moreover, the results obtained in the paper should be compared to the data which reported previously.

Following suggestions, the discussion of the amplitude parameter results was improved to be more in-depth and valuable. These parameters after testing were also compared to those obtained after surface preparation.

Once again, thank you very much for the suggestions received. I hope I have addressed them exhaustively. However, if there are still any inaccuracies, I kindly ask for feedback, which I will gladly address.

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript No.: Coatings-3704821

Date received June 08, 2025

Title: Properties of Diamond-like Coatings in Tribological Systems 2 Lubricated with Ionic Liquid

Authors: Krystyna Radoń-Kobus and Monika Madej

Review – R1

General Comments:

The abstract presents the scope and objectives of the study clearly, focusing on the influence of the ionic liquid BMIM PF₆ on the tribological properties of tungsten-doped diamond-like carbon (DLC) coatings. The description of the experimental setup and materials is informative; however, the abstract could be improved by more clearly stating the novelty of the work and highlighting the key results quantitatively. Additionally, the significance and potential applications of the findings should be briefly outlined. At present, the abstract reads more like a condensed methods and results section, without clearly positioning the study within the broader research context or emphasizing its contribution to the field.

After carefully reviewing this paper, I recommend that it:

In Abstract – Relevant Points Identified:

On page, lines 13-15, the abstract states: “The tests were carried out for discs with a diameter of 42 mm and a height of 6 mm made of 100Cr6 steel and 100Cr6 steel discs with a DLC coating. The tests were carried out in dry friction conditions and lubricated with the ionic liquid BMIM PF6, at loads of 10 N and 15 N.”

Please revise to avoid repeating 'The tests were carried out'. For example, you could combine the ideas into one sentence for improved clarity and style."

Please highlight the novelty of the study more explicitly in the abstract. It would be helpful to briefly state what distinguishes this work from previous research—such as the use of BMIM PF₆ with tungsten-doped DLC coatings, or any unique experimental approach or findings
In Introduction:
On page 1-2, lines 40-47 describe the general properties of DLC coatings, please consider including representative quantitative values from the literature (e.g., hardness in GPa, coefficient of friction ranges, wear resistance data, etc.). Providing such data would give the reader a clearer understanding of the tribomechanical performance of DLC coatings and strengthen the scientific grounding of the discussion
In the section discussing the effects of doping elements (lines 49–55), it would significantly improve the scientific value of the introduction to include specific quantitative effects from the literature—such as how much tungsten reduces residual stresses or how nitrogen affects thermal stability. Similarly, when mentioning the benefits of ionic liquids (lines 56–71), it would be helpful to cite studies that quantify improvements in wear resistance or friction reduction, to better illustrate their potential in tribological applications.
Please explicitly state in the conclusions what is novel about this study, and how it advances current research in tribological systems using DLC coatings and ionic liquids.
Materials and Method.
On page 3 lines 86-88 seems to be an inconsistency in the description of the surface preparation process. You mention that abrasive papers were used in the order up to 1200 μm, but later state that the final abrasive paper used was of 2500 grit. Please clarify the full range of grit sizes applied and ensure that the description is consistent."
Then, please indicate the method and equipment used to measure the surface roughness parameters (Sa, Sq, Sz) after the surface preparation. This information is important for the reproducibility and validation of the results.
According to MDPI guidelines, please provide the full identification of the equipment used, including the manufacturer, city, and country, for example, for the Leica DCM 8 confocal microscope and the Phenom XL SEM, TRB3 tribometer etc. This improves the transparency and reproducibility of the experimental methods.
Results
On page 6, please improve the description of Figure 3 by clearly explaining what is shown in the SEM image and the linear chemical composition profile. In addition, consider providing the quantitative elemental composition (in atomic or weight percent) for the identified elements (C, W, Fe, Cr), as obtained from the EDS analysis. This would offer a clearer picture of the coating’s structure and confirm the effectiveness of the doping process.
Pages 8-9, please specify the total duration or time of the tribological tests, in addition to the sliding distance. Also, it would be valuable to clarify whether the coefficient of friction (COF) was monitored throughout the test and if distinct run-in and steady-state phases were observed and analyzed. This information is crucial for interpreting frictional behavior over time.
In Section 3.5, please clarify how the wear area values (in µm²) were calculated. Was the area determined based on a 2D profile cross-section, or was a 3D surface analysis performed using the confocal microscope software? Also, specify the criteria or method used to define the boundary of the wear track. This information is necessary to properly interpret and reproduce the wear measurements.
Conclusions
Please consider strengthening the conclusions by explicitly stating the main scientific contribution of this study and how it differs from existing work. Also, it would be beneficial to include a brief comment on the potential applications of the findings, as well as the relevance of the observed synergy between the DLC coating and BMIM PF₆ in real-world tribological systems.

General Remark: The manuscript addresses a relevant and timely topic in the field of tribology, focusing on the performance of tungsten-doped DLC coatings lubricated with the ionic liquid BMIM PF₆. The experimental approach is methodically structured, and the study presents promising results regarding the reduction of friction and wear. However, the manuscript would benefit from several improvements to enhance its scientific rigor and clarity. Specifically, the novelty of the work should be more clearly articulated, both in the abstract and conclusions. Some methodological details are missing or inconsistent (e.g., surface preparation sequence, wear area calculation, and equipment specifications), which affects the reproducibility and transparency of the results. Quantitative references from the literature are also needed to support general claims made about the properties of DLC coatings and ionic liquids.

Overall, the study has potential, but revisions are necessary to improve its quality and impact.

If the authors address the above points with sufficient detail and clarity, the manuscript could be suitable for publication.

Comments on the Quality of English Language

I am not in a position to assess the quality of the English language used, as I do not have the necessary competence in this regard.

Author Response

Thank You very much for Your valuable comments!

On page, lines 13-15, the abstract states: “The tests were carried out for discs with a diameter of 42 mm and a height of 6 mm made of 100Cr6 steel and 100Cr6 steel discs with a DLC coating. The tests were carried out in dry friction conditions and lubricated with the ionic liquid BMIM PF6, at loads of 10 N and 15 N.”

I changed these sentences, I hope that now it is more clear.

Please highlight the novelty of the study more explicitly in the abstract. It would be helpful to briefly state what distinguishes this work from previous research—such as the use of BMIM PF₆ with tungsten-doped DLC coatings, or any unique experimental approach or findings

In accordance with the comments, I tried to emphasize the novelty of the presented article - the lack of tribological tests of tungsten-doped DLC coatings with this ionic liquid.

On page 1-2, lines 40-47 describe the general properties of DLC coatings, please consider including representative quantitative values from the literature (e.g., hardness in GPa, coefficient of friction ranges, wear resistance data, etc.). Providing such data would give the reader a clearer understanding of the tribomechanical performance of DLC coatings and strengthen the scientific grounding of the discussion

As suggested, more detailed data on the mechanical properties of the tested coatings were included.

In the section discussing the effects of doping elements (lines 49–55), it would significantly improve the scientific value of the introduction to include specific quantitative effects from the literature—such as how much tungsten reduces residual stresses or how nitrogen affects thermal stability. Similarly, when mentioning the benefits of ionic liquids (lines 56–71), it would be helpful to cite studies that quantify improvements in wear resistance or friction reduction, to better illustrate their potential in tribological applications.

As suggested, more detailed data on the influence of tungsten doping on DLC coatings and the positive aspect of using ionic liquids in tribological systems were included.

Please explicitly state in the conclusions what is novel about this study, and how it advances current research in tribological systems using DLC coatings and ionic liquids.

In relation to this question and question 2, information was added regarding the novelty of the presented research. The conclusions also added the possibilities of applications of combining DLC coatings with ionic liquids.

On page 3 lines 86-88 seems to be an inconsistency in the description of the surface preparation process. You mention that abrasive papers were used in the order up to 1200 μm, but later state that the final abrasive paper used was of 2500 grit. Please clarify the full range of grit sizes applied and ensure that the description is consistent."

Yes, there was a mistake in the description of the surface preparation. The necessary corrections have been made.

Then, please indicate the method and equipment used to measure the surface roughness parameters (Sa, Sq, Sz) after the surface preparation. This information is important for the reproducibility and validation of the results.

As described in the materials and methods section, the amplitude parameters were determined using Leica software.

According to MDPI guidelines, please provide the full identification of the equipment used, including the manufacturer, city, and country, for example, for the Leica DCM 8 confocal microscope and the Phenom XL SEM, TRB3 tribometer etc. This improves the transparency and reproducibility of the experimental methods.

Thank you for your feedback, any gaps have been corrected in accordance with MDPI rules.

On page 6, please improve the description of Figure 3 by clearly explaining what is shown in the SEM image and the linear chemical composition profile. In addition, consider providing the quantitative elemental composition (in atomic or weight percent) for the identified elements (C, W, Fe, Cr), as obtained from the EDS analysis. This would offer a clearer picture of the coating’s structure and confirm the effectiveness of the doping process.

The description of Figure 3 has been corrected. More detailed information has been added regarding the course of the linear analysis of the chemical composition of the tested coating. Unfortunately, due to certain imperfections related to the operated equipment, presenting a precise quantitative analysis of the elements would not be justified for the research value of the article.

Pages 8-9, please specify the total duration or time of the tribological tests, in addition to the sliding distance. Also, it would be valuable to clarify whether the coefficient of friction (COF) was monitored throughout the test and if distinct run-in and steady-state phases were observed and analyzed. This information is crucial for interpreting frictional behavior over time.

Information on the test duration and continuous friction coefficient measurement is included in the revised version of the article.

In Section 3.5, please clarify how the wear area values (in µm²) were calculated. Was the area determined based on a 2D profile cross-section, or was a 3D surface analysis performed using the confocal microscope software? Also, specify the criteria or method used to define the boundary of the wear track. This information is necessary to properly interpret and reproduce the wear measurements.

In section 3.5, a method for calculating the wear surface value was added. It was determined using a 2D cross-section. Unfortunately, I did not fully understand the question: "Also, specify the criteria or method used to define the boundary of the wear track.". I would very much appreciate if you could explain what criteria you mean?

Please consider strengthening the conclusions by explicitly stating the main scientific contribution of this study and how it differs from existing work. Also, it would be beneficial to include a brief comment on the potential applications of the findings, as well as the relevance of the observed synergy between the DLC coating and BMIM PF₆ in real-world tribological systems.

In accordance with the comments presented, the conclusions were supplemented with the necessary information regarding the differences between previous studies and the possibilities of applying this connection in the real tribological world.

Once again, thank you very much for all your valuable comments. I agree with most of them and have taken them into account. However, if there are still inconsistencies, I kindly ask you to clarify the occurring inconsistencies. I will gladly address them.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

This article is acceptable in its current form.

Reviewer 2 Report

Comments and Suggestions for Authors

I have carefully reviewed the authors’ responses to the comments and the corresponding revisions made in the manuscript. I am satisfied that the authors have adequately addressed all the suggestions and concerns raised in the previous review round. The revised version significantly improves the clarity, completeness, and scientific quality of the manuscript.

I therefore recommend acceptance of the manuscript in its current form.