In Situ Heating TEM Study of the Interaction Between Diamond and Cu-Rich CoCrCuFeNi High-Entropy Alloy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present an interesting study that contributes to the research community's understanding of structural and phase transformations at the interface between a high-entropy alloy (HEA) with the composition CoCrCu2FeNi and single-crystal diamond upon heating.

In my opinion addressing the following recommendations would further enhance the scientific quality of the work:

1) Objectives and Applications:  Clarify the justification for the chosen temperature of 900°C in the Materials and Methods section and formulate a clear statement of the study's specific objectives and their connection to potential applications.

2) Sentence Revision: The sentence on page 9 that says "Continuity loss at the HEA–diamond interface is most  likely to be associated with reduced specific surface area of the coating because of surface tension." needs justification. Rephrase it to clearly explain the finding.

3) The text lacks a description of the method for estimating the size of crystallites.

4) Misprints: 

- Line 116 ".....residual pressure in the microscope column was 10-6 Pa."

- Abbreviations: SEM  for Linear dichroism.

Comments on the Quality of English Language

I think my English is of the same quality, so there is always room for improvement.

Author Response

We are greatly thankful to the reviewer for the constructive comments that helped us to improve our research paper. In the revised manuscript, we have made the desirable changes to amend our manuscript and to conform it to the reviewer's recommendations. Our humble responses to the reviewer's comments are given below.

All principal corrections in the text are highlighted in yellow. All English improvements are highlighted in green.

Reviewer 1

The authors present an interesting study that contributes to the research community's understanding of structural and phase transformations at the interface between a high-entropy alloy (HEA) with the composition CoCrCu₂FeNi and single-crystal diamond upon heating.

In my opinion addressing the following recommendations would further enhance the scientific quality of the work:

Comment 1

Objectives and Applications:  Clarify the justification for the chosen temperature of 900°C in the Materials and Methods section and formulate a clear statement of the study's specific objectives and their connection to potential applications.

Response

Thank you for your comment. The maximum test temperature of 900 °C is the typical temperature for hot pressing or pressureless sintering of metal-diamond composite materials (in particular, segments for diamond tools) containing copper in large quantities. Moreover, reaching higher temperatures could lead to partial melting of the coating material or evaporation of copper in the vacuum environment of the microscope. We have added the motivation for choosing the testing temperature in the Materials and Methods section and added references to relevant publications.

The maximum test temperature of 900 °C was selected as it is the typical temperature for hot pressing or pressureless sintering of metal-diamond composite materials, specifically segments for diamond tools, which contain copper in substantial quantities [7, 26–28].

To clarify the connection between the purpose of the study and the potential application, we have added a brief comment at the end of the Introduction section.

Understanding the processes taking place at the HEA-diamond interface allows us to optimize the manufacturing regimes of the metal-diamond composites. It will be possible to improve the performance of HEA-based diamond cutting tools, ensure high bonding strength of diamond to binder, and protect diamond from excessive degradation.

Comment 2

Sentence Revision: The sentence on page 9 that says "Continuity loss at the HEA–diamond interface is most  likely to be associated with reduced specific surface area of the coating because of surface tension." needs justification. Rephrase it to clearly explain the finding.

Response

Thank you for your comment. The sentence was poorly connected with the whole text. We think that the removal of this sentence from the article would not compromise the significance of this subsection.

Comment 3

The text lacks a description of the method for estimating the size of crystallites.

Response

Thank you for your comment. For this purpose we used the linear intercept method. The corresponding changes have been made to the text of the article, in the Materials and Methods section.

The TEM microstructure images were used to estimate the crystallite size using the linear intercept method.

Comment 4

Misprints:

- Line 116 ".....residual pressure in the microscope column was 10-6 Pa."

- Abbreviations: SEM  for Linear dichroism.

Response

Thank you for careful study of our paper. We apologize for the mentioned typos. Line 116 and the Abbreviations section have been corrected accordingly.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This submission deals with the in-situ TEM characterization of diamond and CoCrCuFeNi high entropy alloy (HEA) composites upon heating. The authors present the structural transformations occurring at various temperatures and investigate diffusion phenomena and interaction of HEA with diamond.

The choice of TEM ,especially in-situ studies is adequate to fully characterize the structural changes occurring during heating. The results are presented in a good, comprehensive way and the discussion and conclusions are readily drawn. I recommend the manuscript's publication in Metals, provided the authors address several points, as of below:

Lines 139-140: How did authors check the non-existence of carbide phases at the interfacial region between HEA/diamond? -please provide evidence 

Figure 4: authors should explain what the nanoparticles existing inside diamond comprise of, in terms of structure and elemental composition

TEM figures should be labelled as (a), (b), ... for clarity reasons

Figure 7: the last SAD pattern from Cr7C3 is confusing. Authors should labelled it more clearly and use a small font size for reflections/planes

Author Response

Comment 1

Lines 139-140: How did authors check the non-existence of carbide phases at the interfacial region between HEA/diamond? -please provide evidence

Response

The hypothesis about the absence of carbides at the interface was made on the basis of three considerations. First, the magnetron coating was applied under fairly mild conditions. The surface temperature of the diamond target was approximately 100-150 °C, which is insufficient for the formation of carbides. Second, selected area electron diffraction patterns taken in areas adjacent to the interface did not demonstrate the presence of theoretically expected carbides (Cr₂₃C₆, Cr₇C₃, Cr₃C₂, Fe₃C). An example of such diffraction is shown in the article. Third, we have studied high-resolution TEM images directly at the interface. Interplanar distances from FFT images also indicate that the FCC solid solution phase is directly adjacent to the interface with diamond. We would like to add the high-resolution TEM images of HEA-diamond interface in Figure A. “FFT2” area is given in Figure A-1 as an example. Actually, we studied a fairly large area of interface and did not find any spots that could be attributed to carbides.

Figure A – High resolution TEM image of diamond-HEA interface (1) with FFT images from diamond (2) and HEA coating (3)

Comment 2

Figure 4: authors should explain what the nanoparticles existing inside diamond comprise of, in terms of structure and elemental composition.

Response

Thank you for your fair comment. In fact, at low annealing temperatures (approximately from 100 to 700 °C), some nanoparticles of dark contrast can be seen on the diamond surface. We believe, that these nanoparticles are gallium-based. Gallium slightly contaminated the sample during ion etching. It is probable that the nanoparticle formation is due to the evaporation and crystallization of gallium itself or one of its compounds. Unfortunately, contamination of this kind is inevitable when preparing a sample using this method. However, we can state that the gallium content in the original lamella was very low and it did not have a significant effect on the processes occurring at the HEA-diamond interface.

Unfortunately, Gatan TEM heating holder can not be used for EDX study. For this reason, we were unable to accurately identify the composition and structure of these nanoparticles. However, we have recorded a video demonstrating the processes during heating from 20 to 200 °C. It shows that these nanoparticles appear almost immediately at the start of the heating, that is, at very low temperatures. We believe that under these conditions, the occurrence of any processes in the HEA-diamond system is very unlikely.

Below we provide an image of the lamella at a temperature of 200 °C (Figure B). We will also try to add a video of nanoparticles precipitation in the temperature range of 20-200 °C to our response.

However, we fully agree that the fact of nanoparticles precipitation on the diamond part of the lamella is important to mention in the text of the article. Therefore, we will add a corresponding comment in the text.

Page 6. “Furthermore, it is important to note that dark contrast nanoparticles appeared on the diamond part of the lamella during the heating process. We believe their appearance at the very beginning of the heating experiment is due to the presence of small amounts of gallium on the surface of the lamella, which remained after ion etching”.

Comment 3

TEM figures should be labelled as (a), (b), ... for clarity reasons

Response

Thank you for your comment. We agree with it.

We have replaced most of the pictures in the article. And we have added letter labelling to make them easier to perceive.

Comment 4

Figure 7: the last SAD pattern from Cr₇C₃ is confusing. Authors should labelled it more clearly and use a small font size for reflections/planes

Response

We completely agree with your comment. We have corrected the labelling on the Cr₇C₃ SAD image. A reasonable font size was chosen, and the image scale was slightly changed to improve readability.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The present manuscript deals with in-situ TEM study of interaction between diamond and HEA with increased Cu content. It contains valuable results predominantly obtained by TEM. Although the results are clear and detailed, a major revision is mandatory before the manuscript can be recommended for publication. The comments are given below:

1. The diamond structure is homogeneous at room temperature, as Fig. 2 shows. After annealing at 500 °C (and temperatures above), dark grey globular formations are visible in the diamond structure. They resemble (Fe,Co,Ni,Cu) clusters described in the interface zone. What is the authors’ opinion of such formations? It should be described and discussed in the manuscript. The video shows that after certain time of annealing, those formations dissolve. The mechanism should also be incorporated into the scheme in Fig. 11.
2. Compared to the detailed TEM results, the XRD pattern in Fig. 9 seems to be incomplete. Please, assign the Miller indices to the individual peaks.
3. Some formal editing is also required:

- line 116: 10-6 Pa ---> 10^-6 Pa

- please highlight the colored frames around the electron diffraction images more clearly. Some are very thin loosing the connection to the original image.

Author Response

We are greatly thankful to the reviewer for the constructive comments that helped us to improve our research paper. In the revised manuscript, we have made the desirable changes to amend our manuscript and to conform it to the reviewer’s recommendations. Our humble responses to the reviewer’s comments are given below.

All principal corrections in the text are highlighted in yellow. All English improvements are highlighted in green.

Reviewer 3

The present manuscript deals with in-situ TEM study of interaction between diamond and HEA with increased Cu content. It contains valuable results predominantly obtained by TEM. Although the results are clear and detailed, a major revision is mandatory before the manuscript can be recommended for publication. The comments are given below:

Comment 1

The diamond structure is homogeneous at room temperature, as Fig. 2 shows. After annealing at 500 °C (and temperatures above), dark grey globular formations are visible in the diamond structure. They resemble (Fe,Co,Ni,Cu) clusters described in the interface zone. What is the authors’ opinion of such formations? It should be described and discussed in the manuscript. The video shows that after certain time of annealing, those formations dissolve. The mechanism should also be incorporated into the scheme in Fig. 11.

Response

Thank you for your valuable comment. This is, in fact, a phenomenon that we did not focus on when describing the processes, but it is certainly worth mentioning. We have observed the nanoparticles precipitation at very low temperatures – right after the start of heating and below 200 °C. We believe, that these nanoparticles are gallium-based. Gallium slightly contaminated the sample during ion etching. Nanoparticles could form through the evaporation and crystallization of gallium itself or one of its compounds. We suppose that the gallium content in the lamella was very low and it did not have a significant effect on the processes occurring at the HEA-diamond interface.

Below we provide an image of the lamella at a temperature of 200 °C (Figure B). We will also try to add a video of nanoparticles precipitation in the temperature range of 20-200 °C to our response.

However, we fully agree that the fact of nanoparticles precipitation on the diamond part of the lamella is important to mention in the text of the article. Therefore, we will add a corresponding comment in the text.

Page 6. “Furthermore, it is important to note that dark contrast nanoparticles appeared on the diamond part of the lamella during the heating process. We believe their appearance at the very beginning of the heating experiment is due to the presence of small amounts of gallium on the surface of the lamella, which remained after ion etching”.

Figure B – HEA-diamond lamella at 200 °С with nanoparticles precipitated at the diamond side.

Comment 2

Compared to the detailed TEM results, the XRD pattern in Fig. 9 seems to be incomplete. Please, assign the Miller indices to the individual peaks.

Response

Thank you for your comment. Figure 9 was changed according to your recommendation.

Comment 3

Some formal editing is also required:

- line 116: 10-6 Pa ---> 10-6 Pa

Response

Thank you for noticing the typo. This one and some others were corrected in the revised version of the manuscript.

Comment 4

- please highlight the colored frames around the electron diffraction images more clearly. Some are very thin loosing the connection to the original image.

Response

Thank you for your comment. We have changed Figure 7. We have rearranged the SAD images, added the letter labeling and highlighted the colored frames with thicker lines.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have reflected all the comments and questions in the revised manuscript, therefore I can recommend it for publication.