A Study on the Tribological Behavior of Molybdenum Disulfide Particles as Additives
Round 1
Reviewer 1 Report
This manuscript discusses tribological properties of molybdenum disulfide (MoS2) as solid lubricant. The topic is engineering useful. I have the following questions to be addressed:
1. On Page 3, why did the authors choose n-MoS2 (~ 40 nm in diameter) and c-MoS2 (~ 1.5 μm in diameter) as the analysis object? Please give reasons for your choice.
2. In Figure 1, the text is not aligned with the formula, the specific meaning is unclear, please clarify.
3. In Figure 3, the author only describes the change law of friction coefficient with time and angular velocity. Please explain the reason and content of this change.
4. Page 4, the “observed a non-linear variation of the Mf in the ratio with ω and in particular its decrease, over a critical velocity ωcr, which is due to viscosity variation of the lubricant with the temperature." Please explain clearly about this.
5. Page 5, the author mentioned that "The chemical state and composition of The elements of the rubbed and worn balls surfaces were analyzed on XSP" is incorrect. The analysis software should be X-ray photoelectron spectroscopy (XPS).
6. Page 5, the author said that "The improvements regarding the extreme load and friction reduction in the friction pairs from the four-ball tribometer could be explained by easier adsorption of the n-MoS2 on the balls’ sliding surfaces." Please clarify.
7. Page 5, "unlike using c-MoS2 particles as additive", the XPS analysis results of c-MoS2 is better presented.
8. Page 6, the authors mention that "May be due to the easier of n-MoS2 particles on the balls sliding surfaces and the formation of a MoO3-containing a protective and last of film." Please explain clearly.
Author Response
Response Letter
for Reviewer 1, Round 1
Manuscript ID coatings-1852915
entitled:
"Tribological Behavior Study of the Molybdenum Disulfide Particles, as an Additive"
by
Filip Ilie, Andreea-Catalina Cristescu
Note: The manuscript is very colorful. What was deleted is marked in red and cut, and what was entered is marked in green.
Many thanks to Reviewer 1 for the comments made and I try to answer at the questions addressed:
- Reviewer 1 asks: On Page 3, why did the authors choose n-MoS2 (~ 40 nm in diameter) and c-MoS2 (~ 1.5 μm in diameter) as the analysis object? Please give reasons for your choice.
Response 1: These dimensions were chosen as the object of analysis, because the authors had at their disposal a compact piece (lump) of MoS2, which was ground, obtaining a MoS2 powder, with particles of different sizes. The smallest MoS2 particles had the size of ~ 40 nm in diameter and we symbolized them n- MoS2, and in commerce we found MoS2 particles with the smallest size of ~ 1.5 μm in diameter and they we called MoS2 particles commercial or common (symbolized c-MoS2). As a result, in chapter 2 Materials and Methods, the first sentence was supplemented with two other sentences, namely: "It is mentioned that these dimensions were chosen for n-MoS2, as being of medium size and obtained through a simple method of grinding and selection by sieving, and the c-MoS2 ones are commercially available. By the mixing of n-MoS2 and c-MoS2 in the base oil 15W/40 Super 2 will lead to new tribological results, which is different from results obtained before from other nanoparticles." (see the manuscript)
- Reviewer 1 says: In Figure 1, the text is not aligned with the formula, the specific meaning is unclear, please clarify.
Response 2: Through the removals (marked in red) and the additions (marked in green), I hope that everything is clear now! (see the manuscript)
- Reviewer 1 says: In Figure 3, the author only describes the change law of friction coefficient with time and angular velocity. Please explain the reason and content of this change.
Response 3: By removing the sentence „The friction coefficient variation with the time, t, and angular sliding speed, ω, can be seen in Figures 3 (a) and (b).“ (marked in red and cut) and adding some paragraphs “The friction coefficient variation with the time, t, and angular sliding speed, ω can be observed in Figures 3 (a) and (b) as following of an experiment to monitor its variation (of friction coefficient). Figure 3 shows that the friction coefficients variation vs. time and angular sliding speed for all the three cases (base oil, base oil + c-MoS2, base oil + n-MoS2) are relatively small, with close shapes, which indicate that their friction process and chemical reaction is comparatively simples, no matter what is the parameter (t or ω) through which their variation is presented.
However, it can be seen that the base oil containing c-MoS2 and n-MoS2 particles has the friction coefficients smaller than the base oil. Although, the friction coefficient of friction pairs with base oil + n-MoS2 is the smallest, there is a discrete difference compared with that of base oil + c-MoS2. Thus, it can be said that the base oil + n-MoS2 is of more predominant anti-friction performance than base oil + c- MoS2. In addition, the base oil + n-MoS2, has the lowest coefficient of friction and due to as this one has the higher chemical activity to the base oil + c-MoS2 and the base oil.
Also, for the whole friction process is observed that the friction coefficients for all cases increased slightly after 5 - 10 min, for as after 10-15 min to fall slightly, about the same extent. The prolonged time enables heat energy to accumulate, and makes possible the further tribochemical reaction between active base oil + n-MoS2 and friction pairs materials (marked in green)” (see the manuscript), I hope that we have managed to explain the reason and content of the change in the coefficient of friction with time and angular velocity.
- Reviewer 1 says: Page 4, the “observed a non-linear variation of the Mf in the ratio with ω and in particular its decrease, over a critical velocity ωcr, which is due to viscosity variation of the lubricant with the temperature." Please explain clearly about this.
Response 4: Thank you for the suggestion! By completing the following paragraph „The non-linear variation of the Mf in the ratio with ω is explained by the fact that Mf increases with ω, until it reaches a maximum value for each of the three situations analyzed (base oil, base oil + c-MoS2 and base oil +n-MoS2) and ω corresponding to these values is considered the critical speed (ωcr), and the corresponding temperatures are critical temperatures (Tcr). This is possible because the lubricant mixture adheres to the contact surfaces, the its temperature increases very little and is sufficiently viscous, for Mf to increase to a maximum value (when ω = ωcr), after which it follows its relatively slow decrease, when the temperature, T continues to increase with the increase of ω, so and the viscosity of the lubricant decreases.”, we consider that we have explained this quite clearly. (see the manuscript)
- Reviewer 1 says: Page 5, the author mentioned that "The chemical state and composition of The elements of the rubbed and worn balls surfaces were analyzed on XSP" is incorrect. The analysis software should be X-ray photoelectron spectroscopy (XPS).
Response 5: Thanks for the observation and it was corrected with XPS. (see the manuscript)
- Reviewer 1 says: Page 5, the author said that "The improvements regarding the extreme load and friction reduction in the friction pairs from the four-ball tribometer could be explained by easier adsorption of the n-MoS2 on the balls’ sliding surfaces." Please clarify.
Response 6: Thank you for the suggestion and by removing some words and the phrase " This film is formed by the easy oxidation of the n-MoS2 in the sliding process, allowing it to be released and travel of the n-MoS2 from the valley onto the friction metallic surfaces and their isolation at the interface. ", respectively by completing the paragraph "The MoO3 oxide is obtained on this film after a rapid oxidation of nanoparticles in the sliding process, which also are maintained by the release and furnishing of the nanoparticles from the topography valleys onto the rubbing metal surfaces and their confinement at interfaces. At the same time, the analyses of surface film composition, characterized with the help of XPS and of the SEM images showed that the deposed nanoparticles form a protective film (MoO3) allowing for an increase in the load capacity of friction pairs.", I think I have clarified what you noted above! (see the manuscript)
- Reviewer 1 says: Page 5, "unlike using c-MoS2 particles as additive", the XPS analysis results of c-MoS2 is better presented.
Response 7: Thanks for the observation, but honestly I didn't quite understand what you were referring to! However, through the deletions and additions to this sentence, I hope to have cleared up the misunderstanding! (see the manuscript)
- Reviewer 1 says: Page 6, the authors mention that "May be due to the easier of n-MoS2 particles on the balls sliding surfaces and the formation of a MoO3-containing a protective and last of film." Please explain clearly.
Response 8: Thanks for the observation, but with the corrections made, I think I have clarified this misunderstanding! (see the manuscript)
Thanks for understanding!
Date: Aug. 14, 2022 Authors
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper seeks to introduce an approach ‘’ Tribological Behavior Study of the Molybdenum Disulfide Particles, as an Additive”. However, the authors should consider improving upon the quality to further highlight and emphasize.
1. Replace the word “was” with “were” in line eight (8) from the abstract.
2. Based on the understanding of what entails in an abstract, consider adding one or two lines highlighting the significance of the study.
3. Make a table of the materials used with their physical and chemical properties.
4. The introduction needs to be improved by relating to the mechanics of the studied materials and their mechanical characteristics. The references to be included are: 10.1016/j.jiec.2022.06.023, 10.1016/j.porgcoat.2022.107015.
5. Rephrase the whole of paragraph 4 of section 2 because most of the statement made there does not make sense or is grammatical wrong.
6. Figure 1 is very blurred. Add the image as text but not as a snapshot. Reinsert it considering its visibility.
7. The characterization (TEM, SEM, and XPS) descriptions just under figure 1 lacks a lot of details. For instance, acceleration voltage, working range among other for SEM should be included. The author should clearly indicate all the operating conditions for which the results would be obtained.
8. Rewrite the statement “Thus, Figure 2 is can observe the evolution of extreme load depending on the percentage content and the particle size of MoS2 in the mineral oil (base oil)” to “thus, Figure 2 can be observed as an evolution of extreme load depending on the concentration and particle size of MoS2 in the base mineral oil”.
9. Poor representation of all figures. All the figures need to be reinserted as text but not snapshot or any method which has resulted in that.
10.The results of the characterization do not spell out what you actually intended it for. kindly tell us the significance of these characterization to the study.
11. Put space between any variable and its respective unit. For instance, in the conclusion, instead of 35 °C, the author represented it as 35°C. consider correcting such anomalies throughout the article.
Author Response
Response Letter
for Reviewer 2, Round 1
Manuscript ID coatings-1852915
entitled:
"Tribological Behavior Study of the Molybdenum Disulfide Particles, as an Additive"
by
Filip Ilie, Andreea-Catalina Cristescu
Note: The manuscript is very colorful. What was deleted is marked in red and cut, and what was entered is marked in green.
Many thanks to Reviewer 2 for the comments made. In addition, we made the indicated improvements, namely:
- Reviewer 2 says: 1. Replace the word “was” with “were” in line eight (8) from the abstract.
Response 1: Thanks for the directions and I changed accordingly. (see the manuscript)
- Reviewer 2 says: Based on the understanding of what entails in an abstract, consider adding one or two lines highlighting the significance of the study.
Response 2: Thank you for the suggestion, as a result I added two sentences " Therefore, the present paper reports the tribological properties of the lubricating oil with n-MoS as an additive compared with the c-MoS2 ones, and by the application the friction modeling theory using Couette flow, was possible to calculate the temperature,T, when the friction torque, Mf is maximum, on the basis of which it is obtained its value, respectively of the sliding velocity, ω, corresponding in the contact areas of the four-ball system." that highlight the significance of the study. (see the manuscript)
- Reviewer 2 says: Make a table of the materials used with their physical and chemical properties.
Response 3: At your suggestion, was introduced Table 1 with materials used and with their physical and chemical properties. (see the manuscript)
- Reviewer 2 says: The introduction needs to be improved by relating to the mechanics of the studied materials and their mechanical characteristics. The references to be included are: 10.1016/j.jiec.2022.06.023, 10.1016/j.porgcoat.2022.107015.
Response 4: Thank you for the recommendation and I inform you that I consulted the indicated references included, both in the text and in References, to improve the Introduction, completing it with the paragraphs: "Several types of MoS2 nanoparticles have been experimentally investigated as possible lubrication agents and the results suggest that this is a promising idea. Thus, MoS2 nanotubes have been experimentally evaluated in this respect and the results were compared to reference base oil, and it was found that MoS2 nanotubes significantly decreased the friction and wear compared to the base lubricant [12'].
Other research aimed the tribological behavior of bearings rolling material (alloy steel) under lubricated conditions with four various sizes of MoS2 particles as additive in a conventional lubricant [13']. It was observed that as the size of the particles de-creased, the friction coefficient and wear volume decreased which was attributed to the rolling of the small sized particles in the contact zone [13']." and "By relating to the mechanics properties of the materials studied and their mechanical characteristics is can said that the MoS2 particles mixed into a base oil it is manifest as a the electrophoretic deposition process of particles in general and in particular of MoS2 particles. This process what can be reported kinetically via the use of response surface methodology based on kinetic models, using both statistical and quantitative techniques [15', 16']. The fundamentals of electrophoretic deposition process kinetics are useful to demonstrate the influence of the process parameters on the properties of the deposited films into an interval of time. Therefore, the kinetics of the electrophoretic deposition process was initiated as a function of tension and time of deposition, to avoid significant loss of MoS2 particles, which decreases during process, followed by deposited films physicochemical characterization (XPS and SEM).
According to ref. [15', 16'] the kinetic models can reproduce experimental data well alongside some other semi-empirical equations of kinetics, which shall help in comprehending the process of thermal degradation in-depth, respectively it will be a useful tool in anticipation of thermal stability, avoiding thermal degradation of the MoS2 products, for the industrial applications.", and the two indicated references were added to References. (see the manuscript)
- Reviewer 2 says: Rephrase the whole of paragraph 4 of section 2 because most of the statement made there does not make sense or is grammatical wrong.
Response 5: Thank you for your observation and I inform you that it has been corrected and I consider it to be correct now. (see the manuscript)
- Reviewer 2 says: Figure 1 is very blurred. Add the image as text but not as a snapshot. Reinsert it considering its visibility.
Response 6: Thank you, I corrected it and I hope that it is now much more visible. (see the manuscript)
- Reviewer 2 says: The characterization (TEM, SEM, and XPS) descriptions just under figure 1 lacks a lot of details. For instance, acceleration voltage, working range among other for SEM should be included. The author should clearly indicate all the operating conditions for which the results would be obtained.
Response 7: Thank you for your observation and several useful functional characteristics have been added (in parenthese) at obtain the results. (see the manuscript)
- Reviewer 2 says: Rewrite the statement “Thus, Figure 2 is can observe the evolution of extreme load depending on the percentage content and the particle size of MoS2 in the mineral oil (base oil)” to “thus, Figure 2 can be observed as an evolution of extreme load depending on the concentration and particle size of MoS2 in the base mineral oil”.
Response 8: Thank you for the help and I rewrote the statement "Thus, Figure 2 is can observe the evolution of extreme load depending on the percentage content and the particle size of MoS2 in the mineral oil (base oil)", as you indicated, i.e. "Thus, Figure 2 can be observed as an evolution of extreme load depending on the concentration and particle size of MoS2 in the base mineral oil". (see the manuscript)
- Reviewer 2 says: Poor representation of all figures. All the figures need to be reinserted as text but not snapshot or any method which has resulted in that.
Response 9 Thank you, I corrected all the figures and I hope that now are much more visible. (see the manuscript)
- Reviewer 2 says: The results of the characterization do not spell out what you actually intended it for. kindly tell us the significance of these characterization to the study.
Response 10: Thank you for the comment, but through the corrections made and the additions made, we consider that the significance of these characterizations for the current work has been explained! (see the manuscript)
- Reviewer 2 says: Put space between any variable and its respective unit. For instance, in the conclusion, instead of 35 °C, the author represented it as 35°C. consider correcting such anomalies throughout the article.
Response 11: Thank you and sorry for this mistake and it has been corrected! (see the manuscript)
Thanks for understanding!
Date: Aug. 14, 2022 Authors
Author Response File: Author Response.pdf
Reviewer 3 Report
The work presents an investigation on tribological characteristics of MoS2 lubricating particles using four-ball tribometer. The paper is well-written and can be accepted in its current form.
Author Response
Response Letter
for Reviewer 3, Round 1
Manuscript ID coatings-1852915
entitled:
"Tribological Behavior Study of the Molybdenum Disulfide Particles, as an Additive"
by
Filip Ilie, Andreea-Catalina Cristescu
Many thanks to Reviewer 3 for accepting the work in its initial form.
Note: You will notice that the manuscript is very colorful, because the other reviewers asked for more explanations and clarifications, and I had to bring a lot of additions. So, what was deleted is marked in red and cut, and what was entered is marked in green!
Thanks for understanding!
Date: Aug. 14, 2022 Authors
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Acceptable
Author Response
Response Letter
for Reviewer 2, Round 1
Manuscript ID coatings-1852915
entitled:
"Tribological Behavior Study of the Molybdenum Disulfide Particles, as an Additive"
by
Filip Ilie, Andreea-Catalina Cristescu
Thank you for suggested for authors, the manuscript as 'Acceptable' after the first review, but analyzing and checked what 'Can be improved', I inform you that I have added paragraphs or sentences to each section to improve and more the paper and is marked with blue color (see the manuscript).
Also, three more references were introduced, which led to their renumbering both in the text (with the necessary comments to improve the Introduction section) and in References (see the manuscript).
I hope that now you will be much more satisfied with the additions made to the manuscript!
Thanks for understanding!
Date: Aug. 23, 2022 Authors
Author Response File: Author Response.pdf
