Properties of Diamond-like Tungsten-Doped Carbon Coatings Lubricated with Cutting Fluid
Round 1
Reviewer 1 Report (New Reviewer)
Comments and Suggestions for AuthorsThe article is devoted to the “fashionable” topic of studying DLC coatings doped with tungsten. The authors conducted a complex of research and testing of the coating. From the introduction and conclusion it follows that the authors recommend the use of coatings for cutting tools. In this regard, a number of questions arise from the text of the article.
1. Where in the text is table 3 and the link to tables 2 and 4?
2. In what percentage is the composition shown in Table 1?
3. In what units is the composition shown in Table 5?
4. In Fig. Figure 5 shows a discrete distribution of tungsten in the form of balls from 2 to 10 microns, therefore in Fig. 6 its content should be sawtooth from 0 to 100%. Tungsten distribution in Fig. 6 requires further explanation in connection with Figure 5.
5. It is not clear how linear wear was measured. From the text it seems that linear wear is the sum of the linear wear of the sample and the counter sample; this needs to be clarified. But then it is not clear that the wear rate of the coating is equal to zero at 50 N (fig. 13). Does the steel ball also not wear out?
6. If Figures 13 and 14 show the wear of only the sample, then it is not clear what happened to the 3 µm thick coating.
7. In Figure 13, the wear of the a-C:H:W coated sample at 50 N is 30 µm along the entire friction path. Where did the average value of 50.65 µm in Figure 14 come from? It should be explained how Figure 14 relates to Figure 13.
8. Why is there 3.7 after section 3.5, and 3.6 after 3.7?
9. It should be explained how the coating thickness of 3 microns is combined with the tungsten bead size of 2 - 10 microns
10. Figure 21b shows the friction surface of a cracked coating. All cracks pass through tungsten balls. This must be discussed from the point of view of the integrity of the coating on the cutting tool.
11. The tribological test scheme does not correspond to the operation scheme of the cutting tool, therefore the mention of the cutting tool must be removed from paragraph 7 of the conclusion.
12. There is no study of changes in the composition of the friction surface.
13. A critical attitude towards the literature review in the introduction is necessary. For example, in the description of [28], the authors write that graphitization of a diamond-like coating leads to a decrease in the friction coefficient and wear rate. If this were correct, then graphite-like rather than diamond-like coatings would be used to reduce wear. In fact, the greater the degree of graphitization, the greater the wear of the diamond-like coating.
Authors must answer the questions given and make necessary corrections to the text.
Author Response
Thank you for your all valuable comments.
1. Thank you for your valuable comment. It’s our mistake. We corrected this.
2. It is mass percentage.
3. It is mass percentage.
4. In Figure 5 we showed cross section of the sample which is gone through one of tungsten inclusions. It is reason why content of W is higher and when it goes further, through interlayer and steel, its level is low.
5. Yes, linear wear is the sum of the linear wear of the sample and counter sample. It is not equal to zero at 50 N. Steel ball and coatings at 50 N wear out as much as we can observe on Figure 21, Figure 22, Figure 24, Figure 26 and Table 7.
6. Figures 13 and 14 show the wear of sample and counter sample. Wear of coatings is showed in Figure 27 and coating did not wear because the maximum depth of the wear was 2 µm.
7. It was our mistake. We corrected it in improved version of manuscript. The wear of the a-C:H:W coated sample at 50 N is around 50 µm because there was a mistake with the description of wear for steel at 10 N and coating at 50 N (signatures changed in photo 13).
8. Thank you for your valuable comment. It’s our mistake. We corrected this.
9. After looking again at the size of the tungsten inclusions, we concluded that they were 2-5 micrometers in size. This is the average of the balls, not their volume. The measurement was made using the 2D technique.
10. After discussing this issue with people involved in machining with cutting tool, we came to the conclusion that the DLC coating, although degraded in some places, still serves its functions.
11. Thank you for your valuable comment. We removed it from paragraph 7.
12. After tribological tests, the presence of phosphorus, nitrogen, nickel and iodine was observed in the friction tracks. This is due to the presence of these elements in the cutting fluid used during tribological tests.
13. Thank you for you valuable comment. We focused more at the introduction. We added more example of using DLC and corrected description about graphitization of a diamond-like carbon coating.
Reviewer 2 Report (Previous Reviewer 1)
Comments and Suggestions for AuthorsThe manuscript describes studies of the properties of the 100Cr6 steel and DLC coatings alloyed with tungsten (a-C:H:W). There are the following remarks:
1. How was the roughness of the steel substrates controlled?
2. Chemical composition of the coating and substrates in weight or atomic percentage?
3. What are the quantitative parameters (voltage, etc.) of deposition of coatings and Cr sublayer?
4. The authors must be present the results of Raman spectroscopy of coatings and analyze them.
5. How were cross sections obtained to determine thickness?
6. There is little analysis or description throughout all Section 3. There are no comparisons with other literary sources. More discussion is needed. Alternatively, authors should add a Discussion section after Section 3 and compare their results with other literature. What is better and what is worse compared to other studies in the world.
7. Section 2 should describe how the plastic work and elastic work were defined.
8. What is the chemical composition of the friction tracks after tribological tests?
9. What is the specific volumetric wear after all tribological tests?
10. DLC coatings are characterized by the formation of tribofilms. Very often they are several nanometers thick. Have the authors observed this in their samples?
Author Response
Thank you for your all valuable comments.
1. All samples were ground and polished in the same way. Then, using confocal microscopy, the roughness parameters of these surfaces were checked and compared, which were similar for all samples. The average values are given in the text in section 2.1.
2. It is weight percentage.
3. We did not make these coatings in our laboratory. The coatings were made at Oerlikon Balzers. It is reason why we cannot answer more about quantitative parameters because it is company secret.
4. EDS analysis confirmed the detection of tungsten. Raman spectroscopy confirmed the presence of bonds characteristic of DLC. We were unable to determine tungsten using the spectroscope that we have at our university.
5. The sample was cut using a saw. Then, a metallographic specimen was made from the cut piece. After grinding and polishing the specimen, it was placed in a scanning microscope and the thickness of the coating and interlayer was determined.
6. There is currently no work reported on the use of Swisscool 3000 in the study of DLC coatings. It is in the end of Introduction.
7. We described in section 2 plastic and elastic work.
8. After tribological tests, the presence of phosphorus, nitrogen, nickel and iodine was observed in the friction tracks. This is due to the presence of these elements in the cutting fluid used during tribological tests.
9. The volumetric wear after tribological tests was described using Leica software.
10. No, we did not observe this in our samples.
Reviewer 3 Report (New Reviewer)
Comments and Suggestions for AuthorsThe manuscript is generally well written, however, the following comments need to be addressed before the final acceptance:
1. The novelty of the manuscript is not clear. How the manuscript differs from the existing literature should be written in objectives.
2. It is written on Page 2: The cutting tool industry has become an essential market for protective coatings. Sufficient references should be provided. The use of DLC coatings in lightweight automotive materials machining is very popular, therefore, a few references should be added to strengthen the application.
3. The authors discussed the tribological properties of (a-C:H:W). How about the W-DLC without Hydrogen? Numerous papers are available on the tribological behavior of W doped in a non-hydrogenated DLC matrix. A comparison between hydrogenated and non-hydrogenated W-DLC will be required to strengthen the introduction.
4. The literature on the W-DLC in lubricated sliding should be summarised in the introduction.
5. The authors used cutting fluids in the tribological experiments. However, a high temperature is always observed during the machining of titanium and Inconel with W-DLC. Therefore, a glimpse of the high-temperature tribological behavior of W-DLC should also be discussed in the introduction.
6. What is the hydrogen content in the coatings? The H contents should be measured and mentioned in the manuscript because the tribological property of DLC coatings largely depends on the hydrogen contents.
7. The loads were used 10 and 50 N. The reason for choosing the loads should be mentioned.
8. What is the ASTM standard of the methodology?
9. Figures 23 and 24 are not clear. A clearer picture is required.
10. The mechanisms of the W with cutting fluids are not clear. The discussion should be written in the shed of chemical reactions between W, H, and cutting fluids (additives in cutting fluids?). The mechanisms pertaining to abrasive and adhesive wear those are mentioned in the manuscript are already well known in the literature.
Comments on the Quality of English LanguageGood
Author Response
Dziękuję za wszystkie cenne uwagi.
1. Na końcu Wstępu napisaliśmy: Obecnie nie ma żadnych doniesień o zastosowaniu Swisscool 3000 w badaniach powłok DLC. Jest to więc nowość rękopisu.
2. Dziękuję. Dodaliśmy kilka referencji na temat powłok DLC w motoryzacji.
3. Dziękuję bardzo za cenne uwagi. Przedmiotem badań były właściwości tribologiczne powłok DLC domieszkowanych wolframem. W artykule nie analizowaliśmy wpływu zawartości wodoru w badanej powłoce ze względu na braki sprzętowe. Z tego powodu nie możemy porównywać właściwości testowanej przez nas powłoki z właściwościami powłok bez wodoru. W najbliższej przyszłości (po zakupie odpowiedniego sprzętu) planowane są publikacje zawierające oznaczanie zawartości wodoru.
4. Dziękuję. Dodaliśmy więcej referencji na temat W-DLC w smarowanych prowadnicach.
5. Dziękuję bardzo za cenne uwagi. Przedmiotem badań były właściwości tribologiczne powłok w temperaturze otoczenia. W najbliższej przyszłości planowana jest publikacja kolejnego artykułu dotyczącego oceny właściwości powłok W-DLC smarowanych cieczami obróbczymi w podwyższonych temperaturach.
6. Dziękuję za cenny komentarz, jednak zawartość wodoru nie była tematem tego artykułu. Nie posiadamy odpowiedniego sprzętu do oznaczania zawartości wodoru w badanej powłoce.
7. Zastosowano obciążenie 10 N zgodnie z normą. Obciążenie 50 N to największe obciążenie, jakie można zastosować w naszych urządzeniach, co symuluje wyższe obciążenia i również zostało wybrane z tego powodu.
8. Standardy metodyki to:
PN-EN ISO 21920-2:2022-06 – struktura geometryczna powierzchni
ISO 19403 – kąt zwilżania
ASTM E2546 – twardość
ASTM G99 – badania tribologiczne
9. Zmieniliśmy zdjęcia. Mamy nadzieję, że teraz są już jasne.
10. Ze względu na fakt, że artykuł został napisany w Inżynierii Mechanicznej, skupiliśmy się na właściwościach mechanicznych i tribologicznych badanych materiałów. Nie dysponujemy wystarczającym sprzętem, aby określić specyficzny mechanizm pomiędzy W a zastosowanym chłodziwem (w śladach ścierania osadzały się dodatki z chłodziw). Jak wspomnieliśmy we wcześniejszej odpowiedzi, nie dysponujemy sprzętem, który pozwoliłby nam określić ilość wodoru w danej powłoce.
Round 2
Reviewer 1 Report (New Reviewer)
Comments and Suggestions for AuthorsNext time, describe the testing and measurement methodology in more detail.
Reviewer 2 Report (Previous Reviewer 1)
Comments and Suggestions for Authorsno comments
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript describes studies of the properties of the 100Cr6 steel and DLC coatings alloyed with tungsten (a-C:H:W). There are the following remarks:
1. In the caption to table 2, not all words are in English
2. How was the roughness of the steel substrates controlled?
3. Chemical composition of the coating in weight or atomic percentage?
4. The authors must be present the results of Raman spectroscopy of coatings and analyze them.
5. Why was this tungsten content chosen? Authors must explain their choices.
6. The same terms must be used throughout the manuscript—either coating or film.
7. How were cross sections obtained to determine thickness?
8. Have tribological tests been carried out without Swisscool 3000 lubrication?
9. In section 3.1 there is practically no description of the structure.
10. The authors write “Microstructure studies indicated the presence of tungsten inclusions on the surface of the a-C:H:W coating, in the form of beads ranging from 2 µm to 10 µm in diameter.” - show these inclusions in Figure 5. And also show the spectra of just such tungsten inclusions
11. Why didn’t the authors in section 2 write anything about the Cr sublayer applied before applying a-C:H:W coating?
12. In Fig. 6 and 7 it is necessary to add dimensional segments.
13. There is little analysis or description throughout all Section 3. There are no comparisons with other literary sources. The manuscript is more like a scientific report than an article. More discussion is needed. Alternatively, authors should add a Discussion section after Section 3 and compare their results with other literature.
14. Section 2 should describe how the plastic work and elastic work were defined.
15. What is the chemical composition of the friction tracks after tribological tests?
16. What is the specific volumetric wear after all tribological tests?
17. DLC coatings are characterized by the formation of tribofilms. Very often they are several nanometers thick. Have the authors observed this in their samples?
18. In Figures 15 - 22 the dimensional segment is poorly visible.
19. In their conclusions, the authors cite the following “…… was characterized by a heterogeneous structure…..”. However, the manuscript (in section 3) says nothing about this. The conclusions should not contain new results not previously presented.
20. It is not clear which results relate to tribological tests with cutting fluid lubricant.
21. The title of the article does not fully disclose the essence of the described results. The title of the article needs to be changed.
22. Authors should highlight the novelty and relevance of their results.
Author Response
Good evening. I would like to thank you very much for your valuable comments. Below I present the answers to the questions and add the corrected article.
- Thank you for your valuable comment, we corrected words to English.
- All samples were ground and polished in the same way. Then, using optical microscopy, the roughness parameters of these surfaces were checked and compared, which were similar for all samples. The average values are given in the text in section 2.1.
- It is weight percentage.
- EDS analysis confirmed the detection of tungsten. Raman spectroscopy confirmed the presence of bonds characteristic of DLC. We were unable to determine tungsten using the spectroscope that we have at our university.
- The coatings were applied by an external company (section 2.1) that incorporated this tungsten content in the coating application process.
- We choose coating.
- The sample was cut using a saw. Then, a metallographic specimen was made from the cut piece. After grinding the specimen, it was placed in a scanning microscope and the thickness of the coating and interlayer was determined.
- The device is equipped with a friction force sensor that turns off when the friction force exceeds 25 N. Three attempts (without lubrication with 50 N load for steel) were made during which it turned off at the same moment.
- We added something about this structure.
- We marked these inclusions in the Figure 5 and showed the spectra of this tungsten inclusions.
- We added: Before applying the DLC coating, a chrome interlayer was applied. Chromium was used to obtain better adhesion of the coating to the substrate.
- We added dimensional segments in figures 6 and 7.
- There is currently no work reported on the use of Swisscool 3000 in the study of DLC coatings. It is in the end of Introduction.
- We described in section 2 plastic and elastic work.
- After tribological tests, the presence of phosphorus, nitrogen, nickel and iodine was observed in the friction tracks. This is due to the presence of these elements in the cutting fluid used during tribological tests.
- The volumetric wear after tribological tests was described using Leica software.
- No, we did not observe this tribofilms in our samples.
- We have changed it to more clear.
- We added it to section 3.
- All of results/tests were with cutting fluid lubricant.
- We changed it to: Properties of tungsten-doped diamond-like carbon coatings lubricated with cutting fluid.
- Thank you for your valuable comment.
Author Response File: 
Author Response.pdf
