Enhancing the Microhardness of Coatings Produced by Cold Gas Dynamic Spraying through Multi-Reinforcement with Aluminum Powders Containing Fullerenes and Aluminum Nitride

Round 1

Reviewer 1 Report

This work reports on the preparation of Al-C60-AlN coatings produced by high-energy ball milling and cold gas-dynamic spraying. The microstructure of the milled powders and coatings were characterized. The hardness of the prepared coatings was measured. This work is original and provides some interesting results, it can be accepted for publication after some revisions.

1) The detailed information of cold spraying, such as gas pressure and spraying velocity, should be provided.

2) In the inset of Fig. 2b, some small particles were denoted as C60 fullerence particles. The authors should provide evidence to confirm these nanoparticles being C60.

3) Page 6, How to confirm the content of AlN in the prepared coatings. The detailed calculation should be provided in Materials and Methods.

4) In the present study, C60 fullerenes are used to prepare coatings. The influence of C60 fullerenes on the quantity and hardness of such coatings is required to discuss.

Minor editing of English language required

Author Response

We would like to take this opportunity to express our sincere thanks to the Reviewer who identified areas of our manuscript that needed corrections and modification. We found Reviewer's feedback very constructive, and we agreed with comments and corrections. Below are detailed responses to the Reviewer's notes and indications of following it. Attached manuscript marked with all the changes made during the revision process. We hope that Reviewer will find these revisions relevant to expectations. Many comments and recommendations will also be taken into account in future studies.

Reviewer #1

1) The detailed information of cold spraying, such as gas pressure and spraying velocity, should be provided.

We agree with this comment. Detailed information on the CGDS modes has been added to the "Materials and Methods" section.

2) In the inset of Fig. 2b, some small particles were denoted as C60 fullerence particles. The authors should provide evidence to confirm these nanoparticles being C60.

Additional Raman spectroscopy studies were performed to confirm the presence of C60 particles (see Fig. 2c). The manuscript now includes the results of combination light scattering and their discussion.

3) Page 6, How to confirm the content of AlN in the prepared coatings. The detailed calculation should be provided in Materials and Methods.

Quantitative analysis of AlN content in the coating was performed by processing X-ray diffraction data using the DIFFRAC.EVA v2.0 program with ICDD PDF2. This was accomplished using the reference intensity ratio (RIR) method implemented in the software. Information about this was added to the "Materials and Methods" section.

4) In the present study, C60 fullerenes are used to prepare coatings. The influence of C60 fullerenes on the quantity and hardness of such coatings is required to discuss.

A discussion on the influence of C60 fullerenes on the quantity and hardness of coatings has been added to the manuscript.

Reviewer 2 Report

The topic is interesting and would appeal to the readership of the Journal. However, the paper in the present form is not ready to be published within the scientific community. Some comments are required in various sections of the manuscript. Detailed explanations are also required when the authors describe and discuss the results. The discussion of the results is poor and needs to be extended.
The following, some points should be revised before I can recommend the paper for publication.

1.    The abstract is dispersive and not focused on the topic. A concise and factual abstract is required. The abstract should state briefly the purpose of the research, the principal results and major conclusions. I suggest modifying the abstract, accordingly.
2.    The authors say that: “The advantage of employing microparticles for reinforcing coatings lies in the high deposition efficiency of such mixtures, with a high degree of ceramic particle transfer into the coating.” Some of the references used for the discussion are not recent and additional studies were carried out in literature in the last years. I can suggest, for example, adding and discussing the following:
-    Experimental Study of Functionalized Polymer Matrix Composite with Multi-Material Metal Coatings Produced by Means of Cold Spray Technology, Key Eng. Mater. 813 (2019) 267–272. https://doi.org/10.4028/www.scientific.net/KEM.813.267.

The authors also say: “Another issue associated with cold gas-dynamic spraying of polydisperse powder mixtures, including those containing nanoscale reinforcement, is the high instability of particles of this size in the gas stream, leading to significant dispersion during spraying [17, 18].” On this topic I can suggest to improve your discussion by adding new references that discussed the mechanisms ruling adhesion in cold spraying.

3.    Which is the aim of your work? Can you define the scope of the work within the introduction section? It was not clearly described.
4.    As for the cold spray facility, is it a low or a high-pressure instrument?
5.    How did you choose the cold spray process parameters? Can you report all the cold spray process parameters used, in a Table, please?
6.    Did you observe the powder agglomeration or the powder cluster formation of the composite powder AlMg6/C60?
7.    Can you show the macrograph images of the sprayed coatings, please?
8.    How did you measure the thickness of the coating?
9.    How did you measure the coating porosity?
10.    Careful reading of the text should be done in order to suppress typo errors; can you check them, please?

Careful reading of the text should be done in order to suppress typo errors; can you check them, please?

Author Response

We would like to take this opportunity to express our sincere thanks to the Reviewer who identified areas of our manuscript that needed corrections and modification. We found Reviewer's feedback very constructive, and we agreed with comments and corrections. Below are detailed responses to the Reviewer's notes and indications of following it. Attached manuscript marked with all the changes made during the revision process. We hope that Reviewer will find these revisions relevant to expectations. Many comments and recommendations will also be taken into account in future studies.

Reviewer #2

1. The abstract is dispersive and not focused on the topic. A concise and factual abstract is required. The abstract should state briefly the purpose of the research, the principal results and major conclusions. I suggest modifying the abstract, accordingly.

Changes reflecting the reviewer's comments have been made to the abstract.

2. The authors say that: “The advantage of employing microparticles for reinforcing coatings lies in the high deposition efficiency of such mixtures, with a high degree of ceramic particle transfer into the coating.” Some of the references used for the discussion are not recent and additional studies were carried out in literature in the last years. I can suggest, for example, adding and discussing the following:

- Experimental Study of Functionalized Polymer Matrix Composite with Multi-Material Metal Coatings Produced by Means of Cold Spray Technology, Key Eng. Mater. 813 (2019) 267–272. https://doi.org/10.4028/www.scientific.net/KEM.813.267.

The authors also say: “Another issue associated with cold gas-dynamic spraying of polydisperse powder mixtures, including those containing nanoscale reinforcement, is the high instability of particles of this size in the gas stream, leading to significant dispersion during spraying [17, 18].” On this topic I can suggest to improve your discussion by adding new references that discussed the mechanisms ruling adhesion in cold spraying.

We agree with this comment. The Introduction section has been reworked, incorporating relevant references provided by the reviewer.

3. Which is the aim of your work? Can you define the scope of the work within the introduction section? It was not clearly described.

The aim and scope of the work are defined in the Introduction section. This information has been added to the manuscript.

4. As for the cold spray facility, is it a low or a high-pressure instrument?

Low-pressure spraying equipment was utilized. The working gas (air) pressure was 0.8 MPa.

5. How did you choose the cold spray process parameters? Can you report all the cold spray process parameters used, in a Table, please?

Process parameters for CGDS were selected based on the experience of coating deposition as provided in [https://doi.org/10.1134/S102833581802009X, https://doi.org/10.1007/s11041-019-00430-1]. A more detailed description of spraying parameters has been added to the manuscript in the "Materials and Methods" section. Further research on the impact of spraying process parameters on the structure and properties of coatings will be performed in the future.

6. Did you observe the powder agglomeration or the powder cluster formation of the composite powder AlMg6/C60?

 AlMg6/C60 composite powder particles essentially consist of agglomerates comprising a matrix alloy and fullerenes. This information has been added to the manuscript.

7. Can you show the macrograph images of the sprayed coatings, please?

Macroscopic images of sprayed coatings shaped like cones are presented in the inset of Figure 4.

8.How did you measure the thickness of the coating?

Coating thickness (cone height) was measured using a micrometer. This information has been added to the "Materials and Methods" section.

9.How did you measure the coating porosity?

Quantitative assessment of porosity was not conducted. However, SEM analysis of microstructure images allowed for a qualitative assessment indicating low porosity in the coatings.

10. Careful reading of the text should be done in order to suppress typo errors; can you check them, please?

The manuscript text has been reviewed and revised.

Round 2

Reviewer 1 Report

All questions have been answered. I recommend the revised manuscript for publication.

Minor editing of English language required

Reviewer 2 Report

The comments of the reviewer were satisfied and the publication of the manuscript in the present form is suggested.