Tribological Properties of the 40Cr/GCr15 Tribo-Pair under Unidirectional Rotary and Reciprocating Dry Sliding

Round 1

Reviewer 1 Report

General comments

The article entitled: "Comparative Research on Tribological Properties of 40Cr / GCr15 Tribo-pair under Unidirectional Rotary and Reciprocating Dry Sliding" presents mainly tribological tests performed in two types of movement at different friction durations between 15min and 240min. In addition, the surface morphology and cross-sectional microstructures of the "40Cr" pin sample and "GCr15" disc sample for different sliding durations were examined. The microhardness was also tested.

The article was prepared carefully, and the analysis was carried out correctly. The added value of this article is the research on the microstructural evolution and microhardness gradient in tribo-layers. The paper is obviously of interest to researchers working in this field. However, some minor corrections should be made:

The article is written and arranged correctly, but please include a few minor corrections:

1. In the literature introduction, a few sentences are missing on the subject of the application of the tested tribo-pair 40Cr / GCr15.
2. On what basis were the conditions of the tribological tests selected?
3. In Figure 1, the material of the sample and counter-samples can be determined.
4. Line 93-94: There is no information on the number of microhardness measurements.
5. Figures 9 and 11 do not show the standard deviation for microhardness measurements. This information can also be placed on a table or included in descriptions of figures.

After making minor corrections, I recommend publishing the article.

Author Response

Response to Reviewer 1

Dear reviewer1:

On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript. We appreciate you very much for your positive and constructive comments and suggestions on our manuscript entitled “Comparative Research on Tribological Properties of 40Cr/ GCr15 Tribo-pair under Unidirectional Rotary and Reciprocating Dry Sliding” (coatings-1672408). Attached below are our responses to the review comments and the explanations for the revision of the paper.

Thank you and best regards.

Yours sincerely,

Jialiang Cao

Point 1: In the literature introduction, a few sentences are missing on the subject of the application of the tested tribo-pair 40Cr / GCr15.

Response 1: We accept your comment completely. It is true that we did not express them clearly in the introduction. We supplemented the application and revised the Introduction to make it more clear and distinct. (the first and last paragraph in introduction)

Point 2: On what basis were the conditions of the tribological tests selected?

Response 2: The experimental parameters used in this paper were those commonly used in the literature. We have also tried many experimental parameters. Combined with the test results, we believe that the parameters in the manuscript (150N load, 0.2m/s sliding speed) are representative and persuasive.

Point 3: In Figure 1, the material of the sample and counter-samples can be determined.

Response 3: We accept your comment completely. It will indeed give the reader a clearer picture of the friction system. We made changes in the manuscript. Besides, we added one systematic figure (Figure 1) to illustrate the workflow of experimental testing, which may make it easier for readers to understand.

Point 4: Line 93-94: There is no information on the number of microhardness measurements.

Response 4: It was an oversight on our part not to indicate the number of microhardness tests in the manuscript. Each reported hardness value was the average of at least seven effective measured results. We added experimental details in “2.2 Wear Test and Characterization” as much as possible. Thank you for reminder!

Point 5: Figures 9 and 11 do not show the standard deviation for microhardness measurements. This information can also be placed on a table or included in descriptions of figures.

Response 5: We did miss this. Each reported hardness value was the average of at least seven effective measured results. For the sake of beauty, we omitted this detail before. We added error bars in Figure 9 and Figure 11 as you suggested. Thank you!

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Title:

• The title is quite long and bulky. Please simplify.

 

Abstract:

• The abstract needs to be carefully revised by the language.
• Is a comparison of two different kinematic conditions is really fair and meaningful?
• What do you mean with the following “For the tribo-layer of pin under reciprocating sliding, the surface microstructure converges from both sides to the middle,”

 

Introduction:

• The language of the manuscript is really poor. The readability of the entire manuscript suffers from that.
• The novelty of the study should be better worked out.

 

 

Section 2:

• Please present all experimental data with mean values and error bars.
• Please add Fe as balance in table 1.
• What was the acting contact pressure?
• Figure 1 can be deleted.
• Please provide more experimental details related to the used characterization techniques.

 

Section 3:

• How did you calculate the COF for the stroke in linear reciprocating experiments? Did you consider the 0-velocity regions at the end of the reversal points? “(the CoF of reciprocating sliding includes static friction and dynamic friction caused by the change of motion direction, resulting in a large fluctuation of CoF)”
• The following statement is questionable or at least misleading. Please check and revise “During reciprocating sliding, the possibility of the wear debris remaining in the contact area increased due to the bidirectional motion and the related alternating stress”
• The shown error bars in Figure 2 are huge? Why? Comparing with the frictional evolution shown, this is somehow surprising.
• The following statement misses some connection to the overall text “we show that the wear problem has to take into account the particle detachment process, and the flow of particles inside the contact until their ejection,”
• The following analysis is possible “it is difficult for laboratory devices to collect wear debris at present”. Please check the following article “Oil film lifetime and wear particle analysis of laser-patterned stainless steel surfaces”
• The entire paragraph starting at line 134 is rather speculative.
• Please present all SEM micrographs with scale bars only.
• Related to the microstructural evolution of steel substrates, the following study should be taken into consideration “Superior Wear-Resistance of Ti3C2Tx Multilayer Coatings”
• For plastic deformation, the cross-sectional analysis is more meaningful.
• The cross-sectional analysis should be performed with higher magnification.
• In addition, a TEM analysis of the tribo-layer would be desirable.
• How meaningful are the Vickers hardness maps considering the rather small layer thickness? Nanoidentaiton could shed more light on it

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Cao et al.,

 

The manuscript “Comparative Research on Tribological Properties of 40Cr/GCr15 Tribo-pair under Unidirectional Rotary and Reciprocating Dry Sliding” (coatings-1672408) by Cao et al. were carried out unidirectional rotary and reciprocating sliding experiments of 40Cr pin/ GCr15 disc tribo-pair on MFT-5000 Rtec friction and wear tester under same test condition of 0.2m/s sliding speed and 150N load. The topic is interesting, but I think this article should reconsider after proper changes in major revision for publication in Coatings. Some of my specific comments are:

 

1. In the keywords section (line 20), why the authors only include 40Cr steel, but not include GCr15 steel too? It seems to also include GCr15 steel since it is also evaluated in the present study.
2. In the first paragraph of introduction section (line 22-46), the authors are recommended to split it into two paragraphs and elaborate it with a different main topic to discuss. With the first paragraph after the split become a general overview and the second paragraph after the split become previous research found in the literature.
3. Same as previous comments, in the second paragraph of introduction section (line 47-63), the authors are recommended to split it into two paragraph and elaborate it with different main topics to discuss. With the first paragraph after the split discuss more in-depth about friction and wear behaviors of structural steel research and the second paragraph after the split become the objective of the current study.
4. The state of the art and the significance of the present study are not clearly present, the authors should highlight it more advanced in the introduction section (line 22-63).
5. Based on the observations I reviewed from the introduction section (line 22-63), I do not see such a strong novelty deliver in the present study. Can the authors convey the important points of the novelty? Also, I suggest the authors highlight it more advance in the introduction section.
6. Since this present study concern regarding metal tribo-pair, I would encourage and advise the authors to adopt some of the additional references related to tribological study of metal-on-meta bearing in the introduction section published by MDPI as follow:

• • The Effect of Bottom Profile Dimples on the Femoral Head on Wear in Metal-on-Metal Total Hip Arthroplasty. https://doi.org/10.3390/jfb12020038
  • Tresca Stress Simulation of Metal-on-Metal Total Hip Arthroplasty during Normal Walking Activity. Materials (Basel). 2021, 14, 7554. https://doi.org/10.3390/ma14247554

7. Can the authors explain the basis of conducting Unidirectional Rotary and Reciprocating sliding study? I see in line 55-57 the authors have been describing it, but it is not clearly understood. It is important to explain it more in-depth for the reason of performing Unidirectional Rotary and Reciprocating sliding on the present study.
8. Why does the current study only study dry sliding? various recent literature explain the importance to deliver dry and lubricated condition even carry out tribological evaluations experimentally in the reason of lubrication is very crucial and significantly affect the results. Can the authors explain the reason for only conducting dry sliding in the present study? If not, I recommended the authors to give additional experimental data from the lubricated condition.
9. In the materials and experimental methods section (line 64-94), the authors should add one systematic figure to illustrate the workflow of experimental testing in the present study to make the reader more interested and easier to understand rather than only using dominant text and specific figures to explain.
10. The author must provide a detailed specification and use condition more detail regarding all tools used in the research carried out so that the reader can estimate the accuracy and differences in the results that the authors describe due to the use of different tools in future studies.
11. In the results section, authors are advised to compare the results they obtain with previous similar/identical experimental studies if it is possible.
12. Results explanation from authors present manuscript would be advance if the authors provide explanation in the perspective of analytical and/or computational study. I suggest the authors can present this explanation to make a more comprehensive discussion on the current manuscript.
13. In the last paragraph of the discussion section (line 95-360), the authors should add of one paragraph about the limitations of the research conducted after line 360.
14. The conclusion of the present manuscript is not solid, further elaborate is needed. Also, arrange it into paragraph(s), not point-by-point as present in current form of the manuscript.
15. Further research needs to be explained in the conclusion section.
16. To improve the quality of English used in this manuscript and make sure English language, grammar, punctuation, spelling, and overall style are correct, further proofreading is needed. As an alternative, the authors can use the MDPI English proofreading service for this issue.
17. Please make sure the authors have used the Coatings, MDPI format correctly. The authors can download published manuscripts by Coating, MDPI, and compare them with the present author's manuscript to ensure typesetting is appropriate.

 

I am pleased to have been able to review the author's present manuscript. Hopefully, the author can revise the current manuscript as well as possible so that it becomes even better. Good luck for the author's work and effort.

 

Best regards,

The Reviewer

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you very much for your revision. 

 

The quality of the manuscript has certainly improved. However, the following aspects should be considered: 

 

• Please shorten the title to "Tribological Properties of 40Cr/GCr15 Tribo-pair under Unidirectional Rotary and Reciprocating Dry Sliding"
• There is an important paper by Grützmacher et al. shedding more light on experimental research conducted under rotational sliding mode. Please refer to: "The influence of centrifugal forces on friction and wear in rotational sliding"
• Figure 1 should be still improved. 
• Please calculate the acting contact pressures. Just the normal loads are not very meaningful. 
• What is the explanation for the observed trend in the hardness results? The decreasing tendency can be correlated with the observed oxide scale. In this regard, you may refer to: "Oxide formation, morphology, and nanohardness of laser‐patterned steel surfaces"

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Cao et al.,

 

After carefully reading the author's revised manuscript entitled "Comparative Research on Tribological Properties of 40Cr/GCr15 Tribo-pair under Unidirectional Rotary and Reciprocating Dry Sliding" (coatings-1672408) by Cao et al., The authors have been made significant improvements in the revised manuscript. Also, all of the issue in my review report have been addressed precisely.

 

With my pleasure, I recommend the manuscript should be accepted for publication on Coatings.

 

Best regards,

The Reviewer

Author Response

Dear reviewer3:

       We thank you very much for the effort in reviewing the previous version of the manuscript. Your suggestions have enabled us to further improve our work.

       Thank you for the kind advice again！

Best regards.

Yours sincerely,

Jialiang Cao

Round 3

Reviewer 2 Report

Thank you very much for revising the manuscript. 

 

It can be recommended for acceptance now.