Influence of Powder Plasticity on Bonding Strength of Cold-Sprayed Copper Coating

Round 1

Reviewer 1 Report

This is an interesting work; however, prior to proceeding to the next step, the following comments should be addressed by the authors.

1. The language of the manuscript has to be improved.

2. Provide more in-depth discussion of related previous works.

3. Authors should also provide more meaningful discussions regarding the repeatability and reproducibility of the conducted tests/analysis.

4. Expand the “Conclusion” section and present more quantitative data as the main results of the research study.

Author Response

Please refer to the track change manuscript for the wording and format modifications. Thanks for your patience.

1. The language of the manuscript has to be improved.

Re: Thanks for the reviewer’s comment. The language corrections are marked by blue color in the track change manuscript.

2. Provide more in-depth discussion of related previous works.

Re: Modifications have been made in “4.1. Powder plasticity” (page 12, 2^nd paragraph).

3. Authors should also provide more meaningful discussions regarding the repeatability and reproducibility of the conducted tests/analysis.

Re: Modification have been made in “3.1.2. Grain structure and strain state” (page 5, last sentence), “4. Discussion” (page 11, last paragraph, and adding ref. 34), “4.2. Recrystallized twin grains” (page 13, 4^th paragraph).

4. Expand the “Conclusion” section and present more quantitative data as the main results of the research study.

Re: Modifications have been made in “5. Conclusions”, page 14.

Author Response File: Author Response.docx

Reviewer 2 Report

In this manuscript the author reports ‘Influence of Powder Plasticity on Bonding Strength of Cold-Sprayed Copper Coating’. This work will be surely interesting for many researchers working in the related fields. However, there are some points that need to be addressed before possible publication.

 Recommendation: Minor as noted

1.   The novelty of the present article should be discussed in the Introduction section.

2.      The scale bars in the Fig.1 are not clearly visible to the readers.

3.   The formatting and grammatical errors in the article need to be checked carefully.

4.      The authors have cited relevant references in the Introduction section; however the manuscript needs to be highlighted with some recent reports to further broaden the impact, related literatures.

5.      The resolution of Fig.2 and Fig.3 should be increased.

6. The author should write the purpose for each test in one/two sentences (in brief) before explaining the results of the characterization techniques. Therefore, the logic and organization of this part will be enhanced.

Author Response

Please refer to the track change manuscript for the wording and format modifications. The improved figures are provided as supplemental files and not shown in the track change manuscript. Thanks for your patience.

1. The novelty of the present article should be discussed in the Introduction section.

Re: Modifications have been made in “1. Introduction” (page 2, last .paragraph).

2. The scale bars in the Fig.1 are not clearly visible to the readers.

Re: The improved Fig.1 is provided as supplemental file and not shown in the track change manuscript.

3. The formatting and grammatical errors in the article need to be checked carefully.

Re: Thanks for the reviewer’s comment. The language corrections are marked by blue color in the track change manuscript.

4. The authors have cited relevant references in the Introduction section; however the manuscript needs to be highlighted with some recent reports to further broaden the impact, related literatures.

Re: Several references, [1], [20,21], [23-25], and [29] are newly added in “1. Introduction”.

5. The resolution of Fig.2 and Fig.3 should be increased.

Re: The improved Fig.2 and Fig.3 are provided as supplemental file and not shown in the track change manuscript.

6. The author should write the purpose for each test in one/two sentences (in brief) before explaining the results of the characterization techniques. Therefore, the logic and organization of this part will be enhanced.

Re: Modification have been made in “3.1.1. Morphology” (page 4, 3^rd paragraph), “3.1.2. Grain structure and strain state” (page 5, 1^st paragraph), “3.2.1. Hardness and bonding strength” (page 8, 1^st paragraph).

Author Response File: Author Response.docx

Reviewer 3 Report

In recent years, plasma assisted sprying methods have become increasingly popular. In light of the following articles https://doi.org/10.3390/polym10050532, https://doi.org/10.3390/s18124086 and https://doi.org/10.1016/j.jwpe.2020.101150  :, Please respond to these methods and indicate the main differences with the method described in the article.  

I would very much appreciate it if you could add a diagram of the apparatus used to obtain copper coatings. This will greatly simplify the understanding of the whole process of applying these layers

Do the authors know the dimensions of the grains used as precursors in obtaining coatings. If so, please very much include these values, if not, such research should be conducted. Investigating the grain size would give a different insight into the results obtained and perhaps enable the study of another aspect, which is the study of the dependence of the properties of the tested coatings on the size of the precursor grains.  In my opinion, this would have raised the scientific quality of the reviewed article. 

The authors used purified and unpurified precursors for the preparation of coatings. Did the authors perform the purification themselves if so, how? Such information should be from the Materials and Methods section. If not, the purification procedure should be provided by the manufacturer, unless it is a trade secret. 

Author Response

Please refer to the track change manuscript for the wording and format modifications. Thanks for your patience.

1. In recent years, plasma assisted sprying methods have become increasingly popular. In light of the following articles https://doi.org/10.3390/polym10050532, https://doi.org/10.3390/s18124086 and https://doi.org/10.1016/j.jwpe.2020.101150:, Please respond to these methods and indicate the main differences with the method described in the article.

Re: Cold spraying is a rapid kinetic deposition process in which feedstock powders are accelerated to high velocities ranging from 300 to 1200 m/s in a supersonic jet of compressed inert gas, which is preheated to a temperature exceeding 300°C [Ref. 12,23]. Upon impacting a substrate, sprayed powders deform in a solid state at temperatures below their melting point. The bonding of cold-spray powder with a substrate is highly influenced by powder’s plastic deformation (plasticity). Hence, in the present study, cold spray deposition is a dry and physical process. There is no wet and chemical reaction involved in deposition as the above references mentioned by the reviewer.

2. I would very much appreciate it if you could add a diagram of the apparatus used to obtain copper coatings. This will greatly simplify the understanding of the whole process of applying these layers

Re:

The above picture shows an illustration of cold spray system for reference. (Please see the attached word file)

3. Do the authors know the dimensions of the grains used as precursors in obtaining coatings. If so, please very much include these values, if not, such research should be conducted. Investigating the grain size would give a different insight into the results obtained and perhaps enable the study of another aspect, which is the study of the dependence of the properties of the tested coatings on the size of the precursor grains. In my opinion, this would have raised the scientific quality of the reviewed article.

Re: The grain boundaries of Cu powder feedstocks before and after annealing treatment are shown in Fig. 2 and Fig. 3, respectively. The grain growth is shown with the formation of low-angle grain boundary (page 7, 1^st paragraph) and the corresponding influence on the coating properties is discussed in page 12, 2^nd paragraph of the revised manuscript.

4. The authors used purified and unpurified precursors for the preparation of coatings. Did the authors perform the purification themselves if so, how? Such information should be from the Materials and Methods section. If not, the purification procedure should be provided by the manufacturer, unless it is a trade secret.

Re: The purity of the three Cu powder feedstocks used in the present study is ≥99.9% which satisfies the requirements of commercial pure Cu powder and is set as the same condition for the discussion of powder plasticity. Commercial Cu feedstocks are commonly fabricated through electrolysis (EP) and atomization and yes, the process parameters are generally trade secret.

Author Response File: Author Response.docx

Reviewer 4 Report

Cold spraying is a process where the particle kinetic energy has been exploited to make a dense coating. The bonding of particles to the substrate occurs only when the velocity of sprayed particles exceeds a critical value. This critical velocity is highly related to the material combination selected for the coating. The critical velocity of some of the metal-to-metal combinations has been widely reported also many metallic materials have been successfully reported/coated by cold spraying. Having said that, the authors (Wang et al) in the submitted paper to Coatings emphasized the various Cu feedstocks and the influences of reactions on the bonding strength. The coating microstructure and mechanical conductivity properties were evaluated.

The work is well presented, and the manuscript is well written with reasonable copper coatings insights drawn from the morphological images of original and annealed electrolyzed powder and other precursors. However, some revision is required before rendering a final decision.

My specific points are below:

·         What is the potential application for this technique? Usually, it is believed that cold spray is an attractive way of enhancing the conductivity and erosive resistance properties. Does this hold good here, if so, please justify?

·         In Table, 1 it may be a good idea to also include the shape of the powders obtained from different precursors.

·         Can the irregular morphology experience higher in-flight velocity than those with an average particle size of spherical ones?

·         Do the materials have good adhesion strength for all the precursors?

·   The mechanism involved in deposition on the substrate and its various parameters involved in coatings are well reviewed in the literature (i.e. doi.org/10.1016/j.jmrt.2021.07.130; doi.org/10.1016/j.ceramint.2022.03.266; doi.org/10.1016/j.jmrt.2022.01.039). Please include these reported citations and discuss them in the appropriate section.

·         The abbreviations used in Table 2 need to be explained.

·         What is the effect of sold spray gas intel temperature on Cu particle impact with N2?

·         How are the reported feedstock in this work classified as (hard, soft feedstock versus hard, soft substrate)?

·         Make sure the discussions made in Section 4 are not like the previous section stated elsewhere.

Author Response

Please refer to the track change manuscript for the wording and format modifications. Thanks for your patience.

1. What is the potential application for this technique? Usually, it is believed that cold spray is an attractive way of enhancing the conductivity and erosive resistance properties. Does this hold good here, if so, please justify?

Re: Yes, cold spray is an attractive way of enhancing the conductivity and erosive resistance by coated with ductile pure metal or alloy. Some references regarding its potential application are added as [20,21].

2. In Table, 1 it may be a good idea to also include the shape of the powders obtained from different precursors.

Re: The descriptions of the shape of the powders are added in Table 1.

3. Can the irregular morphology experience higher in-flight velocity than those with an average particle size of spherical ones?

Re: Under similar powder size distribution, the powders with irregular morphology would experience higher velocity than those with spherical one because of higher drag force for irregular powder in gas stream (ref. [27-29]). However, the powder velocity should be mostly affected by powder size. Some discussions and references are added in “4.1. Powder plasticity” (page 12, 2^nd paragraph).

4. Do the materials have good adhesion strength for all the precursors?

Re: The employment of brittle or non-ductile materials in cold-spray process is detrimental to adhesion strength, e.g., ceramic material. Hence, the adhesion strength of cold-sprayed Cu coating on alumina or zirconia substrate is not good and usually a prior metallization treatment on the surface of ceramic substrate is necessary for the adhesion improvement.

5. The mechanism involved in deposition on the substrate and its various parameters involved in coatings are well reviewed in the literature (i.e. doi.org/10.1016/j.jmrt.2021.07.130; doi.org/10.1016/j.ceramint.2022.03.266; doi.org/10.1016/j.jmrt.2022.01.039). Please include these reported citations and discuss them in the appropriate section.

Re: The above reports have been appropriately added in “1. Introduction”.

6. The abbreviations used in Table 2 need to be explained.

Re: The abbreviations used in Table 2 have been explained in Table caption.

7. What is the effect of sold spray gas intel temperature on Cu particle impact with N2?

Re: The increase of gas temperature could increase both the velocity and temperature of Cu powder and also increase the plastic deformation of Cu powder upon impact by decrease of critical velocity.

8. How are the reported feedstock in this work classified as (hard, soft feedstock versus hard, soft substrate)?

Re: Statements such as (Hard, soft feedstock versus hard, soft substrate) are used to describe the dissimilar deformability between powder and substrate. Materials with higher hardness usually reveal less deformability than lower ones. The hardness of Cu (approximately 350 HV) is typically higher than that of Al6061 (approximately 100 HV). Hence, cold-sprayed Cu coating on Al6061 substrate is classified as hard feedstock/soft substrate. Modifications have been made in “1. Introduction” (page 2, 2^nd paragraph).

9. Make sure the discussions made in Section 4 are not like the previous section stated elsewhere.

Re: Thanks for the suggestion. Some sentences are deleted as seen in page 12, 13 and 14.

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

In this reviewer’s opinion, the revised version is OK. In which, the authors have taken my queries into a consideration and revised the manuscript.