Activated Carbon Nano-Particles from Recycled Polymers Waste as a Novel Nano-Additive to Grease Lubrication

Round 1

Reviewer 1 Report

In this manuscript the preparation of activated carbon nanoparticles and their performance as an additive to grease is analyzed and presented. I suggest major revisions for this manuscript.

My comments and questions are as following.

·        Line 43 the sentence must be rewritten. As it is written it claims that one third of the universe energy originates from friction. It should be world energy consumption, not energy but energy consumption as friction dissipates energy and does not create it. And it should be on the world scale not universe scale.

·         Line 120 it should be grinder not gender. How did the authors ensure that the size after grinding is 0.5 μm?

·         There is a lack of information about the grease used to which the AC was added. The authors must provide some more information about the grease used as base for the addition of AC as well as the commercial grease they used for comparison. Like what is Shell grease, grease made out of shells?

·         The authors used acetone to remove the grease residuals. Acetone does not dissolve hydrocarbons and it can be quite inefficient, I would strongly recommend to use heptane or petroleum ether.

·         The authors wrote: “The broad peak at 2θ located between 18.04 and 20.44° corresponds to the (002) carbon plane (JCPDS card no. 87-1526) which is confirming the production of amorphous activated carbon”. How can you have diffraction of X-rays if the activated carbon is amorphous?? Additionally, you have a relatively strong (101) peak which indicates presence of highly crystalline material, which I strongly believe is graphite in this case.

·         Why did the authors perform Raman analysis on the commercial rGO and not on the newly synthesized AC?

·         The authors should provide error bars for Figure 6 and Table 2. How many repetitions were made for the tribological experiments? How many repetitions should be made based on the ASTM standards followed ?

·         The post-test SEM/EDS analysis is highly doubtful. EDS only showed C and O? There was no iron? The tests performed lead to eventual seizure of the counterparts, meaning that there was strong metal to metal contact and I would not expect a big difference between the samples. In my opinion the tests where grease with AC was used, the authors should have performed an interrupted test before seizure and analyze the interface.

·         Regarding the mechanisms, the authors claimed a change from sliding to rolling contact, there is no evidence for that in this case.

Author Response

1. Line 43 the sentence must be rewritten. As it is written it claims that one third of the universe energy originates from friction. It should be world energy consumption, not energy but energy consumption as friction dissipates energy and does not create it. And it should be on the world scale not universe scale.

Response: Thank you for your remark. We have edited line 43 to be more accurate and precise about the mentioned statistics and wording. We have also inserted reference [4] to support our claim. Modifications are highlighted in yellow.

2. Line 120 it should be grinder not gender. How did the authors ensure that the size after grinding is 0.5 μm?

Response: We are sorry for this typo, and we have fixed it. The word “gender” was corrected, and the average particle size of the ground activated carbon was written by mistake as 0.5 μm and it was corrected to its estimated value of 0.09 μm. This size is indicated in the SEM images.   

3. There is a lack of information about the grease used to which the AC was added. The authors must provide some more information about the grease used as base for the addition of AC as well as the commercial grease they used for comparison. Like what is Shell grease, grease made out of shells?

Response: Thank you for your feedback. We have modified the materials and methods section to include section 2.4 about the types of grease we used for comparison and their main specs. Modifications are highlighted in yellow.

4. The authors used acetone to remove the grease residuals. Acetone does not dissolve hydrocarbons and it can be quite inefficient, I would strongly recommend to use heptane or petroleum ether.

Response: Thank you for your helpful recommendation and we will consider applying heptane for the tribo-test research in the future work. In the current case (load carrying capacity test), it was just for rinsing the ring and roller after the test and without further investigation because we investigated only the non-seizure load values in this section. After your permission we removed this sentence to avoid unnecessary confusion for readers.

5. The authors wrote: “The broad peak at 2θ located between 18.04 and 20.44° corresponds to the (002) carbon plane (JCPDS card no. 87-1526) which is confirming the production of amorphous activated carbon”. How can you have diffraction of X-rays if the activated carbon is amorphous?? Additionally, you have a relatively strong (101) peak which indicates presence of highly crystalline material, which I strongly believe is graphite in this case.

Response: Thank you for your comment. As stated in the results and discussion section, the shape of peaks from X-ray test is broad and the description of the broad peak in the XRD spectrum specifies the amorphous carbon material, while the sharpness of the peak is a characteristic for crystalline carbon materials. Accordingly, our prepared matrix has mainly amorphous structure as its two characteristics peaks are broad peaks. Regarding the relatively strong peak (101), it is still identified as broad peak that confirms the amorphous structure of prepared carbon material.

6. Why did the authors perform Raman analysis on the commercial rGO and not on the newly synthesized AC?

Response: We are sorry that we did not add the Raman analysis of AC. We have included the Raman test results for the synthesized AC samples and the results were discussed at the modified manuscript.

7. The authors should provide error bars for Figure 6 and Table 2. How many repetitions were made for the tribological experiments? How many repetitions should be made based on the ASTM standards followed ?

Response: We appreciate your important remark. We have inserted Figure 7a (beside table 2) to demonstrate average WSD values with standard deviation (error bars). We have followed ASTM D2266 standard for wear prevention characteristics and it instructs to take two measurements at 90° to each other for wear scar: one measurement with the striations and the other across the striations. The arithmetic average of the six measurements (two on each test ball) as the scar diameter in millimeters is recorded. We have repeated each test three times and calculated the average WSD. Based on your remark, we have inserted figure 7a to include the standard deviation for each sample.

On the other hand, We have followed similar steps to the ASTM D2509 for figure 6 which did not state any certain number of repetitions for a test, however, we have conducted the same tests two times to ensure that the non-seizure load step for each for same sample is the same in each trial. Therefore, we did not add error bars in figure 6.

8. The post-test SEM/EDS analysis is highly doubtful. EDS only showed C and O? There was no iron?

Response: We appreciate your remark. Considering the scope of our paper, we have focused on comparing the adsorption of both C and O elements to the wear scar surface for the samples lubricated with carbonaceous nano-additives to explain SEM images and justify the effectiveness of AC in the wear results. Therefore, we were not concerned with adding the results of other elements in the analysis. We calculated percentages of C and O with respect to each other.

The tests performed lead to eventual seizure of the counterparts, meaning that there was strong metal to metal contact and I would not expect a big difference between the samples. In my opinion the tests where grease with AC was used, the authors should have performed an interrupted test before seizure and analyze the interface.

We would like to mention that we worked according to ASTM D2266 which instructs to run an interrupted test at a constant speed of 1200 ± 60 rpm for 60 min ± 1 min. Then the balls are removed, rinsed, and examined for WSD measurement. The test was not meant to reach seizure as per standard procedure. It was as you have recommended an interrupted test.  

9. Regarding the mechanisms, the authors claimed a change from sliding to rolling contact, there is no evidence for that in this case.

Response: We appreciate this valuable comment. We have based our claim on two observations: the obtained SEM results for the wear surface and the findings of other researchers in the review of literature. In figure 9a, sliding contact effect is being observed in case of base grease. It is seen that the surfaces of wear scar in case of nano additives are much smoother than the case of base grease which can’t be effectively reached in case of sliding contact. It was mentioned in previous research works that mechanism of antifriction and anti-wear in case of AC nano additives for cutting fluids is rolling friction rather than sliding friction. We have cited the relevant works who supports our claim “Reference 24 https://doi.org/10.5109/4480728”. we are not sure in our work 100% as it needs ultra-thin interferometry to observe such a mechanism, so we have modified the text in the revised manuscript accordingly.

Please see the attachment. Thank you very much.

Author Response File: Author Response.pdf

Reviewer 2 Report

Strong suggestion to simplify your english. The sentences are sometimes very long and complex, they don't have to be complicated for the reader. Complicated english words don't make the research better, but make it harder for a reader to discover the merit of your work.  Keep it simple.

There are some suggestions in the text. Try to reduce long paragraphs full of wt%, percentages of change, numbers... in long sentences.  Instead, put those in easy to read graphs.

Some images of the production method (grinder ?) and what the particles look like optically, without magnification, can make it more real for the reader.

Also images of the triboequipment used could be helpful.

Comments for author File: Comments.pdf

Author Response

Dear Respectable Reviewer,

On behalf of the authors, I would like to express my deepest gratitude for your valuable comments and the careful checking of the paper. We believe that the modifications that we have made based on your comments have improved the quality of the article.

Kindly find below the detailed responses to your comments and an information on the modifications made.

Reply to Reviewer# 2 comments:

1. Strong suggestion to simplify your english. The sentences are sometimes very long and complex, they don't have to be complicated for the reader. Complicated english words don't make the research better but make it harder for a reader to discover the merit of your work. Keep it simple.

Response: We are sorry for leaving such an impression. It was not in our intention to make the research look better by using long sentences with complicated English words. We have revised the manuscript to check for long and complicated sentences and simplified what we believe is unnecessarily complex. Thank you for your helpful remark.

2. There are some suggestions in the text. Try to reduce long paragraphs full of wt%, percentages of change, numbers... in long sentences. Instead, put those in easy to read graphs.

Response: We have considered your valuable suggestions in the text and all corresponding modifications are highlighted in yellow.

3. Some images of the production method (grinder ?) and what the particles look like optically, without magnification, can make it more real for the reader. Also images of the triboequipment used could be helpful.

Response: Thank you for your comment. We have included here in the report and in the graphical abstract images of the grinder and AC particles beside rGO particles. We also attached in the revised manuscript the customized test rig (tribotester) we developed for load carrying capacity calculation. It is inspired by Brugger apparatus. We have followed similar steps to ASTM D 2509 to find the load carrying capacity.

4. 33% of fuel energy is dissipated in friction, not 'nearly half'. Where is the reference for 80% of mechanical failure results from wear ?

Response: Thank you for your note. We have revised the references and corrected the percentages accordingly. We have cited the reference we used for the later percentage (reference4)

5. what is 6006zz ?

Response: 6006zz rolling bearing is a single row deep groove ball bearing type with double shielding. We have added this information to the revised manuscript. Thank you for your caring and time to revise the introduction section thoroughly.

6. do you mean 'grinder' ?

Response: Yes, and so sorry for this typo. We have corrected it in the revised manuscript.

7. be more precise. 60 minutes +/- xxx seconds ?

Response: Thank you for helping increase the quality of text. We have corrected it in the revised manuscript.

8. This is NOT a Timken test, but what is sometimes called a 'timpken' test but more correctly named a Lubricity test. The real Timken test uses blocks on ring, but you describe a roller-on-ring, which is a very different method. Also, the Timken test uses increments of 5 lbs, not 500 gramms.  I think you are confusing two test methods. Please verify this. Also, in a standard Timken test, you do not apply increasing load on the same samples, you have to run each load level on fresh rings and blocks. It seems you are mixing two test methods here, clarify this to the reader and do not call it a 'TIMKEN' test.  Add in an image of the setup, if it is non-standard.  (refer to ASTM D2509 for the complete standard)
9. you may not call these results 'TIMKEN OK LOADS' as they are not performed per the ASTM D2509 geometry and load increments. Please rename or redescribe this test, not confusing it with the ASTM method.

Response: Again, we appreciate your helpful remarks to enhance the quality of text. We have in our labs a customized tribotester similar in its structure and principle of work to Brugger apparatus (DIN 51347-1). We have compared between Timken tester and Brugger tester and chose to conduct similar approach to ASTM D2509 in executing the test except for the values of test loads. We have clarified all steps in the revised manuscript, and we also added an image of the tribotester setup.

10. more correct is to say an increase of 25% (referred to the 20 N OK load of the untreated grease). Adding 5N to 20 is in fact 25%, not 20%. Same for the next sentence : improvement of 50% (from 20 to 30 N) and of 75% (20 to 35).

Response: Thank you for your valuable correction. We have modified the paragraph accordingly.

11. It would be easier to have an overview, if you put an extra column in table 2, with the relative wear scar values. Assume the 1. Lithium grease to be 100, and calculate the relative wear (WSD and Wear area) compared to that, then add those to the table, so it already has the percentages of improvement in a more easy to read overview.

Same suggestion as for the WSD : add the COF values as a reference to one grease, the Lithium grease COF = 100.

Response: We have updated the table of WSD to include the percentages of reduction for each sample result according to your helpful note. We have also created another table for COF results with similar information.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The comments and questions I had very addressed in an acceptable manner. In my opinion the manuscript can be published. One final thought. I would suggest to the authors to perform Raman analysis on the worn surfaces instead of EDS, EDS can be quite inaccurate with low atomic mass elements (e.g. carbon and oxygen). The most probable friction (and wear) reduction mechanism is the presence of graphitic layers on the sliding interface. Graphite has a layered crystal structure that is anisotropic with low shear strength between its layers, i.e. it is lubricious.