The Influence of H Content on the Properties of a-C(W):H Coatings

Round 1

Reviewer 1 Report

Manuscript ID: Coatings-12-00280

The introduced manuscript under the title of “The influence of H content on the properties of a-C(W):H coatings” shows the influences of H content on the mechanical and tribological properties of a-C (W):H coatings deposited on biased Si substrates by DC sputtering.

The manuscript should be modified before publication.

1-     the model number, manufacturer name, and country of origin should be indicated for targets and the sputtering system

2-     Type and surface pretreatment of the substrate should be indicated in the first paragraph of materials and methods

3-     line no. 101, The damage effects that you would like to avoid should be determined and a supporting reference should be added

4-     The indentation rest time, and the model number of nanoindentation, adhesion, and friction testers are not mentioned. Additionally, a reference should be included to confirm why you selected 5 mN load.

5-     The applications that explain why you selected a steel counterpart should be mentioned (line no.118)

6-     Line no. 127-130, The author claims that the C content in the deposited films is increasing by increasing the CH4 flow, which contradicts with the obtained results in table 1. The effects of increasing CH4 flow on W and Ar content should be reconsidered and supported references should be added.

7-     In Fig.4, the caption should be consistent with its explanation, which means the illustrated optical images should be correlated to the H content rather than the CH4 flow. Additionally, the reason for existing adhesive failure and LC2 should be pointed out. 

8-     In Fig.5, the caption should be correlated to the H content rather than the CH4 flow.

9-     The surface roughness and sliding track should be presented by AFM image

10- The references should be updated, and the missing references should be included       

Author Response

• the model number, manufacturer name, and country of origin should be indicated for targets and the sputtering system

It was added to the text the technical data to the manuscript

• Type and surface pretreatment of the substrate should be indicated in the first paragraph of materials and methods

The description of the substrates used was added to the text.

• line no. 101, The damage effects that you would like to avoid should be determined and a supporting reference should be added.

The damage that the laser can cause is the graphitization due to the heating of the samples.

• The indentation rest time, and the model number of nanoindentation, adhesion, and friction testers are not mentioned. Additionally, a reference should be included to confirm why you selected 5 mN load.

The text was modified according with the request.

• The applications that explain why you selected a steel counterpart should be mentioned (line no.118)

The reason why a steel counterpart was selected was added to the text. It’s related with the material that this type of coatings will have as sliding partner in the type of applications in which is used.

• Line no. 127-130, The author claims that the C content in the deposited films is increasing by increasing the CH4 flow, which contradicts with the obtained results in table 1. The effects of increasing CH4 flow on W and Ar content should be reconsidered and supported references should be added.

In the text it’s not said that with the increasing of the CH4 flow there is an increase of the C content in the coatings. The increase of the CH4 flow results in an increase of the C and H atoms in the plasma, thus a reduction of C in the coatings is expected as it was observed. I did not found any relevant references that can justify the evolution of the composition with the increase of CH4 flow. In the text some of possible justification for the observed behaviour are explained. 

• In Fig.4, the caption should be consistent with its explanation, which means the illustrated optical images should be correlated to the H content rather than the CH4 flow. Additionally, the reason for existing adhesive failure and LC2 should be pointed out.

The figure caption was modified to H content.

• In Fig.5, the caption should be correlated to the H content rather than the CH4 flow.

The captions were modified to H content

• The surface roughness and sliding track should be presented by AFM image

No AFM was performed on the samples. The worn volumes were measured by a white light interferometer (Zygo NewView 7200). The roughness for all of the samples, deposited on the steel substrates are identical with a values of 0.004 ±0.001 µm. I think that adding the images will not improve significantly the manuscript.

10- The references should be updated, and the missing references should be included       

The references were checked.

Reviewer 2 Report

The authors of the research paper entitled: The influence of H content on the properties of a-C(W):H coatings is an interesting study. The idea of this work itself is quite interesting. Nevertheless, the work shows some serious issues. The following issues should be addressed:

 1.       The abbreviation a-C(W):H should be defined. I could not find in the manuscript what it means. Also new and interested readers or even undergraduate students should.

2.       The use of abbreviations is a lot and the definition of them is very poor for this manuscript. The authors should make the manuscript better readable.

3.       The English should be checked again very carefully. There are some problems with spelling, commas and some term used.   

4.       I would suggest you to avoid abbreviations in the abstract. It minors the quality of an abstract and the reading flow.

5.       Materials and Methods should state out all used devices and chemicals correctly: Devices (type, company, city, country), chemicals/gases (purity, manufacturer, city, country).

6.       How were the gas flows controlled and adjusted in this study? Please add this in chapter 2.

7.       Indicate in Figure 1 the main peak. This is missing.

8.       Line 66: -1 is not correctly indicated for the wavelength unit (uppercase it). This is happen a few time more throughout the whole manuscript.

9.       Line 79: CH4 is not written correctly (lowercase the 4). In addition, a few times more over the whole manuscript.

10.   Figure 3 and figure 7: The labeling in the diagrams is too small. It is difficult to read that. Please increase the font size.

11.   Figure 4: Scale bars are too small. Please make them better readable. Figure caption misspelling: micrographs – plural. There are two images shown. Moreover, which samples are used for these scratch tests shown in the images? This information is completely missing in the figure caption.

12.   How is figure 4 related with figure 6? Why there are no profilograms shown in figure 4?

13.   Figure 6 labeling: The word depth is misspelled in the x-axis labeling. Please correct this.

14.   Figure 8 is showing a calo test. However, where are the results of these tests? This test is used for determining the coating thickness. Where are the results? Why are only shown the images of two samples? What about the rest of the samples investigated in this study? Please show the other samples too and indicate the resulting coating thicknesses in a diagram.

15.   The conclusion is not suitable. It should be stated out why it was important to conduct this study and some outlook should be given at the end.

Author Response

1. The abbreviation a-C(W):H should be defined. I could not find in the manuscript what it means. Also new and interested readers or even undergraduate students should.

In the field of coatings deposited by PVD or CVD is normally accepted the use of abbreviations. The a-C(W):H was explained in the text.

2. The use of abbreviations is a lot and the definition of them is very poor for this manuscript. The authors should make the manuscript better readable.

The abbreviations and their explanation was improved in the manuscript

3. The English should be checked again very carefully. There are some problems with spelling, commas and some term used.

The English was carefully verified and improved

4. I would suggest you to avoid abbreviations in the abstract. It minors the quality of an abstract and the reading flow.

The abstract was modified according with this suggestion 

5. Materials and Methods should state out all used devices and chemicals correctly: Devices (type, company, city, country), chemicals/gases (purity, manufacturer, city, country).

The description of the equipment’s used was modified according with the request.

6. How were the gas flows controlled and adjusted in this study? Please add this in chapter 2.

The gas was introduced in the deposition chamber by mass flow controllers. This is already describe in the text.

7. Indicate in Figure 1 the main peak. This is missing.

The Raman of DLC coatings presents two main peaks D and G peaks around 1300-1400 cm-1 and 1500-1600 cm-1 respectively

8. Line 66: -1 is not correctly indicated for the wavelength unit (uppercase it). This is happen a few time more throughout the whole manuscript.

It was verified and modified in the text.

9. Line 79: CH4 is not written correctly (lowercase the 4). In addition, a few times more over the whole manuscript.

It was modified through the text.

10. Figure 3 and figure 7: The labeling in the diagrams is too small. It is difficult to read that. Please increase the font size.

The figures were modified with higher magnifications of the fonts.

11. Figure 4: Scale bars are too small. Please make them better readable. Figure caption misspelling: micrographs – plural. There are two images shown. Moreover, which samples are used for these scratch tests shown in the images? This information is completely missing in the figure caption.

The scratch tests were performed on coatings deposited on the steel substrates and this information was added to the legend.

12. How is figure 4 related with figure 6? Why there are no profilograms shown in figure 4?

The figures are not related, Fig. 4 is the optical image of a scratch test with increasing load up to 50N, Fig 6 is the indentation curve for 3 coatings with an maximum load of 5mN

13. Figure 6 labeling: The word depth is misspelled in the x-axis labeling. Please correct this.

It was corrected

14. Figure 8 is showing a calo test. However, where are the results of these tests? This test is used for determining the coating thickness. Where are the results? Why are only shown the images of two samples? What about the rest of the samples investigated in this study? Please show the other samples too and indicate the resulting coating thicknesses in a diagram.

The images of Fig. 8 are from the sliding steel balls after the tests, no calottes were performed in the samples. The images of the other samples are very similar, thus I dint think that put all images will improve significantly the manuscript.

15. The conclusion is not suitable. It should be stated out why it was important to conduct this study and some outlook should be given at the end.

The conclusions were modified.

Reviewer 3 Report

The article aims at developing an indirect method for determining the hydrogen content of physical vapour deposited a-C(W):H coatings, deposited with different flow of the reactive gas, using Raman spectroscopy and nanoindentation.  The trends observed with Raman spectroscopy and nanoindentation is correlated with the hydrogen content measured by ion beam analysis and the adhesion and tribological properties are evaluated. The article is interesting and is recommended to be published. However, some revisions are recommended to improve the quality of the manuscript.

1. Several grammatical mistakes exists in the manuscript and the authors are encouraged to carefully go through the manuscript and correct them. A few examples are provided:

·        A space is missing before “To” on line 156.

·        Start of the sentence “However, the almost reduction of Ar down to 0…” on line 134 should be changed to “However, the reduction of Ar down to almost 0…”.

 2. Several inconsistencies exist in the text and should be addressed, as highlighted below:

·        Currently, the sp3 hybridization is written in three different ways (i.e., “sp3” (line 43), “sp³” (line 226), and “sp³” (line 190)). Please pick one way to write it and be consistent throughout the entire manuscript. Same for ”sp2”.

·        Chemical formular are written both with and without numbers in the subscript (i.e., “CH4” (lines 79, 134, 167, etc.) and “CH₄” (lines 134, 137, 142, etc.,). Please pick one way to write it and be consistent throughout the entire manuscript. Same for “C2H2” (line 234).

·        The unit for the wave number is presented both as “cm-1” (lines 66, 67, 151, 159, etc.) and “cm^-1” (line 104, Fig. 1, etc.). Please pick one and be consistent throughout the entire manuscript.

·        References to subfigures is done both as “Figure 3a)” (lines 206, 253, etc.) and “Figure 8 (b)” (lines 250, 251, etc.). Please pick one and be consistent throughout the entire manuscript.

3. As noted by the authors, the increase of CH sp³ hybridization bonds could explain the mechanical properties (hardness, Youngs modulus, and elastic recovery) of the coatings. However, the introduction is currently lacking a comprehensive discussion on the tribological and mechanical properties of sp² and sp³ material and the scientific value of the manuscript would significantly increase if such a discussion was added. Some references might be of help, for example: Zhai, W., Carbon (2017), 119: 150–171; Babicheva, R., et al., Journal of Experimental and Theoretical Physics (2019), 129(1): 66–71.

4. The inset in Figure 6 is too small to see clearly and should be magnified. Furthermore, the symbols used for plastic deformation and elastic recovery appears to be different from those used in the main text. Please be consistent to not confuse the reader.

5. As the goal is to determine an indirect method to evaluate the hydrogen content, it would be valuable if more concrete guidelines for how the hydrogen content is estimate using the results form the Raman spectroscopy and nanoindentation could be provided. Could a mathematical formula be derived linking the hydrogen content to the intensity of the G peak or the D over G intensity ratio similar to what Casiraghi et al. (i.e., the reference mentioned on line 157) have done? Furthermore, please validate that the photoluminescence background and the hydrogen content agree with the formula mentioned on line 157.

6. Why was CH₄ chosen as the reactive gas? Would the result be different if a different reactive gas was used.

7. The discussion about the adhesion and mechanical properties is very brief and should be further extended. A schematic figure of how the increase in hydrogen content affects the elastic recovery and wear resistance should be added to help the understanding of the reader.

Author Response

1. Several grammatical mistakes exists in the manuscript and the authors are encouraged to carefully go through the manuscript and correct them. A few examples are provided:

• A space is missing before “To” on line 156.

It was corrected

• Start of the sentence “However, the almost reduction of Ar down to 0…” on line 134 should be changed to “However, the reduction of Ar down to almost 0…”.

 It was modified

2. Several inconsistencies exist in the text and should be addressed, as highlighted below:

• Currently, the sp3 hybridization is written in three different ways (i.e., “sp3” (line 43), “sp³”(line 226), and “sp³” (line 190)). Please pick one way to write it and be consistent throughout the entire manuscript. Same for ”sp2”.

It was corrected

• Chemical formular are written both with and without numbers in the subscript (i.e., “CH4” (lines 79, 134, 167, etc.) and “CH₄”(lines 134, 137, 142, etc.,). Please pick one way to write it and be consistent throughout the entire manuscript. Same for “C2H2” (line 234).

It was modified

• The unit for the wave number is presented both as “cm-1” (lines 66, 67, 151, 159, etc.) and “cm^-1” (line 104, Fig. 1, etc.). Please pick one and be consistent throughout the entire manuscript.

It was corrected

• References to subfigures is done both as “Figure 3a)” (lines 206, 253, etc.) and “Figure 8 (b)” (lines 250, 251, etc.). Please pick one and be consistent throughout the entire manuscript.

It was modified in a systematic way

1. As noted by the authors, the increase of CH sp³hybridization bonds could explain the mechanical properties (hardness, Youngs modulus, and elastic recovery) of the coatings. However, the introduction is currently lacking a comprehensive discussion on the tribological and mechanical properties of sp² and sp³ material and the scientific value of the manuscript would significantly increase if such a discussion was added. Some references might be of help, for example: Zhai, W., Carbon (2017), 119: 150–171; Babicheva, R., et al., Journal of Experimental and Theoretical Physics (2019), 129(1): 66–71.

The main objective of the study presented in this manuscript was to find a way to estimate the H content with the most used techniques for coatings characterization. However it was conclude that it was not possible to establish a direct relation of the coatings main properties and the H content. Since this was main goal of the paper, the tribological properties of DLC was not a main subject for the introduction.

2. The inset in Figure 6 is too small to see clearly and should be magnified. Furthermore, the symbols used for plastic deformation and elastic recovery appears to be different from those used in the main text. Please be consistent to not confuse the reader.

It was modified in the figure according to the text and magnified

3. As the goal is to determine an indirect method to evaluate the hydrogen content, it would be valuable if more concrete guidelines for how the hydrogen content is estimate using the results form the Raman spectroscopy and nanoindentation could be provided. Could a mathematical formula be derived linking the hydrogen content to the intensity of the G peak or the D over G intensity ratio similar to what Casiraghi et al.(e., the reference mentioned on line 157) have done? Furthermore, please validate that the photoluminescence background and the hydrogen content agree with the formula mentioned on line 157.

The development of a formula that can be used for estimate the H content in metal doped DLC coatings would require more depositions. It’s necessary to known how the metal content influences the Raman photoluminescence and the hardness. In the coatings deposited for this work the variation of the metal content was very low from 2 up 5 at. %. I think to try to develop a formula that can be used to estimate the H content will be very difficult or even impossible, since the influence of the metal selected will vary for the element selected.

4. Why was CH₄chosen as the reactive gas? Would the result be different if a different reactive gas was used.

The CH4 gas was chosen, since it was used before in the development several type of coating systems, therefore it’s from our interest to know how the gas we use will influence the H content of the coatings.

7. The discussion about the adhesion and mechanical properties is very brief and should be further extended. A schematic figure of how the increase in hydrogen content affects the elastic recovery and wear resistance should be added to help the understanding of the reader.

The main objective with this work was to study the influence of the H content in the coatings and to if was possible to estimate indirectly the H content. The evaluation of the adhesion by scratch test is mainly to see if there will be any delamination problems during the tribological characterization. Therefore, no great discussing is made for the scratch test characterization. A small discussion how the H content influences the wear performance was added to the conclusions.

Reviewer 4 Report

The article is of interest to the DLC research area. Detailed studies have been carried out and useful results have been obtained. However, an explanation of the results obtained is necessary. The questions are below.

1. Hydrogen content in at. % in table 1 looks quite impressive. However, if we convert at. % to wt. %, it turns out that the hydrogen content from the first to the last line varies from 1.1% wt. up to 3% wt. The content of tungsten varies from 49 to 42.7% wt. The carbon content varies from 45.5 to 55.5 % wt. Thus, in weight. %, the carbon content increases with an increase in the hydrogen content by about 20%, and the tungsten content decreases by about 13%. In this regard, it remains unclear why the density decreases by almost 1.9 times, given that hydrogen is contained in DLC not in the form of gas pores, but in chemical compounds.

2. It is incomprehensible that the DLC wear rate decreases approximately by a factor of 3 with an increase in the hydrogen content. Its maximum content is only 3% wt. The basis of DLC remains carbon and tungsten. Only the state of carbon can change, which can change from the sp3 to the sp2 state during friction, which leads to an increase in the wear rate. Maybe the authors will be useful article: : Iosif Gershman 1, Alexander Mironov, Alexei Mezrin, Elena Torskaya, Tatyana Kuznetsova, Vasilina Lapitskaya and Alexander Rogachev. Effect of sp3–sp2 Transformation on the Wear Rate of the DLC Coating. Lubricants 2022, 10, 85.  The mechanism of the influence of hydrogen on the wear rate should be described in more detail and given in the Conclusion section.

Author Response

1. Hydrogen content in at. % in table 1 looks quite impressive. However, if we convert at. % to wt. %, it turns out that the hydrogen content from the first to the last line varies from 1.1% wt. up to 3% wt. The content of tungsten varies from 49 to 42.7% wt. The carbon content varies from 45.5 to 55.5 % wt. Thus, in weight. %, the carbon content increases with an increase in the hydrogen content by about 20%, and the tungsten content decreases by about 13%. In this regard, it remains unclear why the density decreases by almost 1.9 times, given that hydrogen is contained in DLC not in the form of gas pores, but in chemical compounds.

The density is also related with the bond structure of the material, and with the increase of the H content in the coatings substantial changes in the bonding structure occur. This can be corroborated by the variation of the hardness from 16 to 6 GPa, that is substantial. The Raman also confirms that that there are significant changes in the bond arrangement and the SP3 SP2 content. From a small research the Density of carbon can vary from amorphous: 1.8–2.1 g/cm³ graphite: 2.267 and diamond: 3.515 g/cm³. Therefore just by changing the bonding arrangement, the SP2 SP3 content and the presence of other elements, the changes in the density are not so impressive. I would say that the changes in the density of the coatings are in the range that we can expect 3.9 to 2.1 g/cm³.

2. It is incomprehensible that the DLC wear rate decreases approximately by a factor of 3 with an increase in the hydrogen content. Its maximum content is only 3% wt. The basis of DLC remains carbon and tungsten. Only the state of carbon can change, which can change from the sp3 to the sp2 state during friction, which leads to an increase in the wear rate. Maybe the authors will be useful article: : Iosif Gershman 1, Alexander Mironov, Alexei Mezrin, Elena Torskaya, Tatyana Kuznetsova, Vasilina Lapitskaya and Alexander Rogachev. Effect of sp3–sp2 Transformation on the Wear Rate of the DLC Coating. Lubricants 2022, 10, 85. The mechanism of the influence of hydrogen on the wear rate should be described in more detail and given in the Conclusion section.

One important point that we must have in consideration when we are analysing tribological results of DLC coatings by pin on disk is that there are several factors that might influence the results. The humidity, temperature, the sliding conditions and the sliding partner material have significant influence on the wear and friction performance. Also in the pin on disk testing the formation of a transfer layer on the sliding ball can have major role on the tribological performance. In the manuscript the intention was not to have a detailed study of the tribological performance but just an approach to see if the addition of H to the coatings will influence the coatings tribological performance. Thus with the addition of H to the coatings it was found that it influences the friction and wear performance of the coatings. One of the possible factors that influences the behaviour observed is the formation of and well adherent transfer layer in the steel ball as the H content increases. More detailed about the

Round 2

Reviewer 2 Report

The authors of the research paper entitled: The influence of H content on the properties of a-C(W):H coatings is performed the first revision round. The revision was done not very good and I am not satisfied with some answers. For this moment, I cannot give a suggestion to accept this paper. Let me answer in details to the problematic points.

1. The abbreviation a-C(W):H should be defined. I could not find in the manuscript what it means. Also new and interested readers or even undergraduate students should.

In the field of coatings deposited by PVD or CVD is normally accepted the use of abbreviations. The a-C(W):H was explained in the text.

Answer to this comment: Of course, this term is accepted. But, it is not about the term itself. If a reader is interested in your work, but not familiar with the terms of the research, authors should write a paper in the way to attract new interested readers. In addition, it was not explained before it was just added during the revision.   

5. Materials and Methods should state out all used devices and chemicals correctly: Devices (type, company, city, country), chemicals/gases (purity, manufacturer, city, country).

The description of the equipment’s used was modified according with the request.

Answer to this comment: Sorry, this is not done in all cases! For example: The tribological behavior was evaluated using a pin-on-disc tribometer (CSM Instruments) in room conditions. – Correct would be: The tribological behavior was evaluated using a pin-on-disc tribometer (type - this company is producing a few types of tribometers, CSM Instruments, Needham Heights, MA, USA) in room conditions. Or this one: A Raman micro-spectrometer (HORIBA Jobin-Yvon, Xplora) with a laser wavelength of 532 nm was used to characterise the structure of the coatings. Correct: A Raman micro-spectrometer (Xplora – state out the correct type, because Xplora is a Series of AFMs from this company, HORIBA Jobin-Yvon GmBH, Bensheim, Germany) with a laser wavelength of 532 nm was used to characterise the structure of the coatings. There are more problems with that still open.

6. How were the gas flows controlled and adjusted in this study? Please add this in chapter 2.

The gas was introduced in the deposition chamber by mass flow controllers. This is already describe in the text.

Answer to this comment: Usually, mass flow controller are used for it. I cannot find one mentioned in the text of chapter 2. In chapter 2 is even no single word related with gas! Some gas related points are mentioned in the abstract, introduction and chapter 3. It makes a big bow around chapter 2!

7. Indicate in Figure 1 the main peak. This is missing.

The Raman of DLC coatings presents two main peaks D and G peaks around 1300-1400 cm-1 and 1500-1600 cm-1 respectively

Answer to this comment: This is still not indicated in the spectra of figure 1.

12. How is figure 4 related with figure 6? Why there are no profilograms shown in figure 4?

The figures are not related, Fig. 4 is the optical image of a scratch test with increasing load up to 50N, Fig 6 is the indentation curve for 3 coatings with an maximum load of 5mN

Answer to this comment: What is Figure 4 needed for? Where is the profilogram for the stratch tests in figure 4? Why figure 6 is not showing scratch test images? I really have problems how this paper is structured. There seems to be no real common thread in this manuscript, which can be seen by this arrangement of figures and tables.

14. Figure 8 is showing a calo test. However, where are the results of these tests? This test is used for determining the coating thickness. Where are the results? Why are only shown the images of two samples? What about the rest of the samples investigated in this study? Please show the other samples too and indicate the resulting coating thicknesses in a diagram.

The images of Fig. 8 are from the sliding steel balls after the tests, no calottes were performed in the samples. The images of the other samples are very similar, thus I dint think that put all images will improve significantly the manuscript.

Answer to this comment: There is no trend to seen in the images for the other samples? According to figure 7 there should be. Otherwise, I also need to ask here, for what figure 8 is useful to have?

Author Response

The authors of the research paper entitled: The influence of H content on the properties of a-C(W):H coatings is performed the first revision round. The revision was done not very good and I am not satisfied with some answers. For this moment, I cannot give a suggestion to accept this paper. Let me answer in details to the problematic points.

1. The abbreviation a-C(W):H should be defined. I could not find in the manuscript what it means. Also new and interested readers or even undergraduate students should.

In the field of coatings deposited by PVD or CVD is normally accepted the use of abbreviations. The a-C(W):H was explained in the text.

Answer to this comment: Of course, this term is accepted. But, it is not about the term itself. If a reader is interested in your work, but not familiar with the terms of the research, authors should write a paper in the way to attract new interested readers. In addition, it was not explained before it was just added during the revision.   

The text was improved and the abbreviations were explained in the abstract and also the introduction.

1. Materials and Methods should state out all used devices and chemicals correctly: Devices (type, company, city, country), chemicals/gases (purity, manufacturer, city, country).

The description of the equipment’s used was modified according with the request.

Answer to this comment: Sorry, this is not done in all cases! For example: The tribological behavior was evaluated using a pin-on-disc tribometer (CSM Instruments) in room conditions. – Correct would be: The tribological behavior was evaluated using a pin-on-disc tribometer (type - this company is producing a few types of tribometers, CSM Instruments, Needham Heights, MA, USA) in room conditions. Or this one: A Raman micro-spectrometer (HORIBA Jobin-Yvon, Xplora) with a laser wavelength of 532 nm was used to characterise the structure of the coatings. Correct: A Raman micro-spectrometer (Xplora – state out the correct type, because Xplora is a Series of AFMs from this company, HORIBA Jobin-Yvon GmBH, Bensheim, Germany) with a laser wavelength of 532 nm was used to characterise the structure of the coatings. There are more problems with that still open.

The description of the equipment’s was modified. Some of the information is difficult to obtain, since some of the equipment’s used have more than 10 years and in this way the information asked is difficult to obtain.

1. How were the gas flows controlled and adjusted in this study? Please add this in chapter 2.

The gas was introduced in the deposition chamber by mass flow controllers. This is already describe in the text.

Answer to this comment: Usually, mass flow controller are used for it. I cannot find one mentioned in the text of chapter 2. In chapter 2 is even no single word related with gas! Some gas related points are mentioned in the abstract, introduction and chapter 3. It makes a big bow around chapter 2!

It was added the model of the mass flow controllers used. This is the way that normally is described the process of add the gas to the deposition chamber in many journals.

1. Indicate in Figure 1 the main peak. This is missing.

The Raman of DLC coatings presents two main peaks D and G peaks around 1300-1400 cm-1 and 1500-1600 cm-1 respectively

Answer to this comment: This is still not indicated in the spectra of figure 1.

It was added a line for the typical positions for each peak D and G.

1. How is figure 4 related with figure 6? Why there are no profilograms shown in figure 4?

The figures are not related, Fig. 4 is the optical image of a scratch test with increasing load up to 50N, Fig 6 is the indentation curve for 3 coatings with an maximum load of 5mN

Answer to this comment: What is Figure 4 needed for? Where is the profilogram for the stratch tests in figure 4? Why figure 6 is not showing scratch test images? I really have problems how this paper is structured. There seems to be no real common thread in this manuscript, which can be seen by this arrangement of figures and tables.

The images of Figure 4 are optical micrograph of the scratch test from 0 to 50N with an increasing load of 100 N/cm. This test is normally used to assess quality of the adhesion. Normally if extensive delamination start at low values the coatings probably will have delamination problems when tested by pin on disk with loads of 5 or 10N.

Figure 6 is not related with Figure 4, Figure 6 is related with the hardness measurement Figure 5.

1. Figure 8 is showing a calo test. However, where are the results of these tests? This test is used for determining the coating thickness. Where are the results? Why are only shown the images of two samples? What about the rest of the samples investigated in this study? Please show the other samples too and indicate the resulting coating thicknesses in a diagram.

The images of Fig. 8 are from the sliding steel balls after the tests, no calottes were performed in the samples. The images of the other samples are very similar, thus I dint think that put all images will improve significantly the manuscript.

Answer to this comment: There is no trend to seen in the images for the other samples? According to figure 7 there should be. Otherwise, I also need to ask here, for what figure 8 is useful to have?

Figure 8 shows something that could be quite important to the tribological performance of the coatings that is the adherent tribolayer formed in the sliding ball. In the case of the ball wear rate it will avoid the wear of the ball, if this transfer layer is well adherent to the ball. Depending on the characteristics of this transfer layer it can influence significantly the coatings wear performance.

Reviewer 4 Report

The authors obtained interesting results on the effect of hydrogen on the properties of DLC. An interesting and even unexpected result should be noted - a decrease in the wear rate, despite a decrease in the hardness and density of the coating. Transfer layers on friction surfaces are always formed, this is an attribute of friction. The effect of transfer layers on friction and wear has been written about for the past 130 years. It is not enough to explain the decrease in the wear rate by the formation of transfer layers on rubbing surfaces, since they are always formed, including at a low hydrogen content. These layers are always different. Tribological properties are influenced not by “several” factors, as the authors write, but it is difficult to find such a factor that would not affect tribological properties. The processes occurring during friction are too complex and ambiguous. Therefore, the authors can be forgiven for the lack of an adequate explanation of the reason for the decrease in the wear rate. But it is necessary to give an adequate explanation for the decrease in density by 1.9 times. The authors explain this by an increase in the content of hydrogen, as a light element, and a decrease in the content of tungsten, as a heavy element. Such an explanation cannot be correct, since the maximum hydrogen content was only 3 wt. %, and the tungsten content decreased slightly from 49 to 42.7 wt. %. This could not lead to a decrease in density by 1.9 times. In response to the reviewer, the authors write that the density of carbon can vary from 3.515 g/cm3 (diamond) to 1.8 g/cm3 (amorphous). This may correspond to the fact that as the hydrogen content increases, the diamond-like phase disappears. In this case, the question arises, why are DLCs needed if they have such good tribological properties without a diamond-like phase?.

Without an adequate explanation of at least some of the results obtained, the results themselves may be questionable. Therefore, the authors need to explain at least one result obtained, probably the easiest way is to reduce the density. Only after that the article can be published.

Author Response

The authors obtained interesting results on the effect of hydrogen on the properties of DLC. An interesting and even unexpected result should be noted - a decrease in the wear rate, despite a decrease in the hardness and density of the coating. Transfer layers on friction surfaces are always formed, this is an attribute of friction. The effect of transfer layers on friction and wear has been written about for the past 130 years. It is not enough to explain the decrease in the wear rate by the formation of transfer layers on rubbing surfaces, since they are always formed, including at a low hydrogen content. These layers are always different. Tribological properties are influenced not by “several” factors, as the authors write, but it is difficult to find such a factor that would not affect tribological properties. The processes occurring during friction are too complex and ambiguous. Therefore, the authors can be forgiven for the lack of an adequate explanation of the reason for the decrease in the wear rate. But it is necessary to give an adequate explanation for the decrease in density by 1.9 times. The authors explain this by an increase in the content of hydrogen, as a light element, and a decrease in the content of tungsten, as a heavy element. Such an explanation cannot be correct, since the maximum hydrogen content was only 3 wt. %, and the tungsten content decreased slightly from 49 to 42.7 wt. %. This could not lead to a decrease in density by 1.9 times. In response to the reviewer, the authors write that the density of carbon can vary from 3.515 g/cm3 (diamond) to 1.8 g/cm3 (amorphous). This may correspond to the fact that as the hydrogen content increases, the diamond-like phase disappears. In this case, the question arises, why are DLCs needed if they have such good tribological properties without a diamond-like phase?.

Without an adequate explanation of at least some of the results obtained, the results themselves may be questionable. Therefore, the authors need to explain at least one result obtained, probably the easiest way is to reduce the density. Only after that the article can be published.

Besides the variation of the coatings composition with the increase of the CH4 flow. There are several factor that may influence the coatings density. Thus it’s difficult to have and clear answer, since the density will depend on the coatings composition, bonding and morphology. The Raman analysis of the coatings shows that significant changes in the coatings structure occurs. The coatings can change from diamond like type to more graphitic or even some polymeric type of bonding that can occurs simultaneously. The following paper shows that significant variations of density for DLC type coatings can occur.

Naoto Ohtake, Masanori Hiratsuka, Kazuhiro Kanda, Hiroki Akasaka, Masanori Tsujioka, Kenji Hirakuri, Atsushi Hirata, Tsuguyori Ohana, Hiroshi Inaba, Makoto Kano, Hidetoshi Saitoh,” Properties and Classification of Diamond-Like Carbon Films” Materials 2021, 14, 315. https://doi.org/10.3390/ma14020315

Round 3

Reviewer 4 Report

The authors did not explain their results.

I declare for the third time that the authors obtained a very interesting but contradictory result, for example, a decrease in wear with a decrease in hardness. Therefore, I would not like to give a negative review. To do this, the authors need to answer one question. A simple calculation shows that:

- with a coating density of 3.9 g/cm3 and a tungsten content of 49 wt. %, the density of a part of the coating excluding tungsten (mainly carbon) is about 2.1 g/cm3; - with a coating density of 2.1 g/cm3 and a tungsten content of 42.7 wt. %, the density of a part of the coating excluding tungsten (mainly carbon) is about 1.26 g/cm3.

The authors report that “With the increasing H content, the mechanical properties of the coatings tend to be determined by the polymer-like structure due to more C-H sp³ hybridized bonds”.

Taking into account the low density of the carbon part of the coating, a 2.5-fold decrease in hardness, especially with a high hydrogen content, taking into account the formation of graphite during friction, taking into account the above quote from the authors, can the resulting coating be considered diamond-like? If it is possible or not, then why?

Author Response

 Dear Reviewer

We thank again the reviewer for the useful comments. We tried in previous revisions to clarify some of the doubts and comments raised by the reviewers. We think that, perhaps, we did not succeed, perhaps because we did not understand very well the comments. We will try to comment on the text the reviewer sent to us, as follows:

I declare for the third time that the authors obtained a very interesting but contradictory result, for example, a decrease in wear with a decrease in hardness. Therefore, I would not like to give a negative review. To do this, the authors need to answer one question. A simple calculation shows that:

- with a coating density of 3.9 g/cm3 and a tungsten content of 49 wt. %, the density of a part of the coating excluding tungsten (mainly carbon) is about 2.1 g/cm3; - with a coating density of 2.1 g/cm3 and a tungsten content of 42.7 wt. %, the density of a part of the coating excluding tungsten (mainly carbon) is about 1.26 g/cm3.

The authors report that “With the increasing H content, the mechanical properties of the coatings tend to be determined by the polymer-like structure due to more C-H sp³ hybridized bonds”.

The calculation of the distribution of the different “phases” in the coatings is complicated since it should depend on the way how the different elements are distributed (mainly C, W and H). W can be bonded with C, forming nanoparticles (with very small sizes, in some cases less than 1.5 nm), and this ability can depend of the presence of H. Therefore, W can be atomically integrated in the C-based structure or as very tiny nanoparticles. However, we agree with the reviewer about the approximative analysis of the possible density of the C-rich zones in the coating. What we do not understand is why this contradicts what we discussed in the paper. We even think that this confirms our interpretation, i.e. we got a value of 2.1 g/cm³ which is a value compatible with DLCs with hardness of the same level as the ones we measured (~16 GPa). For the coatings with the highest H content, more importantly deposited with very high CH₄ flow, there is a significant decrease of the density of the C-rich part (down to 1.26 g/cm³) which is in agreement with what we reported in the paper as having more and more a polymer-like structure, justifying the extreme reduction of the hardness (and Young´s modulus)

Taking into account the low density of the carbon part of the coating, a 2.5-fold decrease in hardness, especially with a high hydrogen content, taking into account the formation of graphite during friction, taking into account the above quote from the authors, can the resulting coating be considered diamond-like? If it is possible or not, then why?

Again, we agree with the reviewer that it is questionable that the coatings with the highest H contents should be considered as DLCs due to their low H values. However, this is a long debate among people depositing C-based coatings. When the aim of the study is to study a variation in a parameter that can lead to significant variations in the mechanical properties (as was the case in our work) of the C-based coatings, it is usual in the literature do not distinguish between the coatings in terms of nomenclature, i.e. keep the DLC denomination for all of them (see for example, “The role of substrate bias and nitrogen doping on the structural evolution and local elastic modulus of diamond-like carbon films, S R Polaki, K Ganesan, S K Srivastava, M Kamruddin and A K Tyagi, 2017, J. Phys. D: Appl. Phys. 50 175601, DOI 10.1088/1361-6463/aa6492” - relation of density and mechanical properties can also be checked in this paper). This denomination is widely accepted to describe coatings deposited by different processes, pure or alloyed with different elements. This results that DLC coatings can present a wide range of mechanical properties such as Hardness and Young modulus, with different tribological performance showing different levels of friction and different wear mechanism.

Finally, if we well understood, the main question of the reviewer is the fact that the coatings with the lower hardness values showed higher wear resistance. This is something that we often observed particularly with low friction self-lubricating coatings when tested by pin-on-disk. This is a repetitive tribological test with multi-passages in the same surface. The issue of the transfer layers have a predominant role on the wear behaviour. In fact, for the coatings showing lower friction and wear coefficients the contact is mainly between the modified surface of the coating and the transfer layer in the ball, which is also based on carbon. Therefore, the abrasive/adhesive effects of the ball is much lower, giving rise to a lower wear rate of both elements of the tribological pair.

Dear Editor

The reviewers of this manuscript were very helpful, contributing to improve significantly the quality of the manuscript. The subject of the research presented in this manuscript has results that can be different others that are published. The coatings that are study in this manuscript, DLC type, present a variety of properties and performances that are dependent on the deposition process used and in the conditions that were selected to characterize the tribological behaviour of the coatings. Also this type of coatings can present very different densities. In this paper we determine the density of the coatings combining the results obtained by IBA and the measurement of the thickness by profilometer.

One reviewer that is the only one that continues from the start with the same questions. We are answering to the doubts that he has the best way we can. I think than he didn’t demonstrate properly the doubts about the coatings density. Concerning the tribological performance it’s possible to have different results, since the test conditions also influences the performance and in many others cases also there is no relation of the hardness with the tribological performance.