An Improved Model for Prediction of Critical Velocity of Cold-Spray by First-Principles Calculations
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
-The literature review should be expanded to explain the results obtained in similar studies and identify the researchers who conducted these studies. Consequently, the authors should detail how their work differs from these studies and the specific gap it addresses in the literature.
-Please provide the originality of the work separately as the last paragraph in the intrıduction.
-It is important to include SEM images of the initial powders. These images provide crucial insights into the morphology, particle size distribution, and surface characteristics of the powders, which can significantly influence the cold spray process. By presenting SEM images, the study can offer a more comprehensive understanding of the starting materials, thereby enhancing the validity and applicability of the predictive model.
-The study should include SEM analyses of the post-spray coating morphology and cross-sections to evaluate the quality and integrity of the coating. Analyzing particle deformation after impact would validate the model's predictions regarding critical velocity. Additionally, EDS mapping with SEM could confirm the elemental distribution and homogeneity within the coating. These analyses would provide a more comprehensive understanding of the cold spray process and further support the predictive model.
Author Response
Response to Reviewer 1
Comments 1: The literature review should be expanded to explain the results obtained in similar studies and identify the researchers who conducted these studies. Consequently, the authors should detail how their work differs from these studies and the specific gap it addresses in the literature.
Response 1: Thanks for your comments. We have revised the manuscript as your suggestion and highlighted these modifications
Comments 2: Please provide the originality of the work separately as the last paragraph in the introduction.
Response 2: Thanks for your suggestion. We have rearranged the introduction and made the originality of the work separately as the last paragraph.
Comments 3: It is important to include SEM images of the initial powders. These images provide crucial insights into the morphology, particle size distribution, and surface characteristics of the powders, which can significantly influence the cold spray process. By presenting SEM images, the study can offer a more comprehensive understanding of the starting materials, thereby enhancing the validity and applicability of the predictive model.
Response 3: Thanks for your comments. We should restate the purpose of the present work. In our present work, we predicted the critical velocity of cold spray by using first principles method which has never been used in other literatures before. And we evaluated the validity and applicability of the calculated method by comparing our results with other experimental or calculated results. Actually, it will be more visualized when the SEM images were used. But we should note that our research is a theoretical analysis in atomic scale and did not involve any experiments. The bonding mechanism of cold spray was also discussed in atomic scale. The SEM images could represent the curving shape of bonding interfaces, while in the atomic scale, the interface is flat as shown in our calculated model. The schematic diagram in Fig.1 was drawn by referring many experimental literatures and clearly show the feature of calculated models.
Comments 4: The study should include SEM analyses of the post-spray coating morphology and cross-sections to evaluate the quality and integrity of the coating. Analyzing particle deformation after impact would validate the model's predictions regarding critical velocity. Additionally, EDS mapping with SEM could confirm the elemental distribution and homogeneity within the coating. These analyses would provide a more comprehensive understanding of the cold spray process and further support the predictive model.
Response 4: Thanks for your comments. In our present work, we try to predict the critical velocity of cold spray by using first principles calculations. The critical velocity can be obtained by energy barrier calculations and the bonding mechanism can be clarified by the variation of strains during the impact process. Our purpose is to develop a simple and high-efficient theoretical method to predict the critical velocity of cold spray without experimental measurements. The validity and applicability of this method can be proved by comparing our calculated results with other experimental or simulated values. Therefore, our present work do not need experimental verifications. In fact, the SEM analyses and EDS measurements are helpful for understanding the cold spray process, but not necessary for predicting the critical velocity.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for Authors
Dear Authors,
The manuscript entitled “An Improved Model for Prediction of Critical Velocity of Cold-spray by First-principles calculations” authored by Zhang et al. aims at evaluating the reliability of predicting the critical velocity of cold spray processes for Cu/Al cases by the first-principles calculations method. They achieved the calculated critical velocity values for various combinations of Cu and Al materials in both Particle and substrate forms by comparing the reported Experimental values and predicted values and discussed them. It seems well organised computational study to understand more how to occur cold-spray coating mechanisms for the FCC structural materials like Cu and Al. However, there are some points need to be addressed to make this model clear for the readers.
1. Why did the authors decided to use Cu and Al materials in this research work?
2. What do the authors think about the twinning mechanisms of Cu materials as they are under compression forces during the cold spray coating process?
3. Have the authors consider on possible defects or oxide layers could be on a surface of Cu and Al materials? What could be the effect of these parameters on their First-principle calculation method?
4. Please check all this manuscript carefully in terms of grammar and spelling typos.
After the questions abovementioned have been answered, this paper could be a good candidate to publish in this prestigious journal.
Kind regards,
Author Response
Response to Reviewer 2
Dear Authors,
The manuscript entitled “An Improved Model for Prediction of Critical Velocity of Cold-spray by First-principles calculations” authored by Zhang et al. aims at evaluating the reliability of predicting the critical velocity of cold spray processes for Cu/Al cases by the first-principles calculations method. They achieved the calculated critical velocity values for various combinations of Cu and Al materials in both Particle and substrate forms by comparing the reported Experimental values and predicted values and discussed them. It seems well organised computational study to understand more how to occur cold-spray coating mechanisms for the FCC structural materials like Cu and Al. However, there are some points need to be addressed to make this model clear for the readers.
Comments 1: Why did the authors decided to use Cu and Al materials in this research work?
Response 1: In this work, the copper/aluminum (Cu/Al) couples were taken as an example to check the reliability of first-principles calculations on predicting the critical velocity of cold-spray due to their sufficient experimental and simulated data.
Comments 2: What do the authors think about the twinning mechanisms of Cu materials as they are under compression forces during the cold spray coating process?
Response 2: It is a normal phenomenon for copper to have the twinning deformation under compression forces during cold spray processes because of its low stacking fault energy. The TEM observation has proved the twinning deformation in copper particles (see Ref. 31 in the revised manuscript). The twinning deformation will provide shear displacement that drives the interfaces turn to the bonding configuration. Therefore, the twinning deformation of copper materials is beneficial for decreasing the energy barrier of cold-spray bonding.
Comments 3: Have the authors consider on possible defects or oxide layers could be on a surface of Cu and Al materials? What could be the effect of these parameters on their First-principle calculation method?
Response 3: This is a good question. We have investigated effects of the vacancy and the interfacial oxygen atom on energy barrier for cold-spray bonding and some valuable results are obtained. The calculated result shows that the vacancy can decrease the energy barrier but the interfacial oxygen atom will significantly increase the energy barrier. This result is not included in present work and we will prepare another manuscript to discuss the effect of defects (the vacancy and the interfacial oxygen atom) on the bonding mechanism of cold-spray.
Comments 4: Please check all this manuscript carefully in terms of grammar and spelling typos.
Response 4: Thanks for your reminding. We have checked the manuscript again and corrected the grammar errors and spelling typos.
After the questions abovementioned have been answered, this paper could be a good candidate to publish in this prestigious journal.
Kind regards
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for Authors
Although this publication focuses on the prediction of critical velocity for cold-spray bonding using first-principles calculations, the absence of Scanning Electron Microscopy (SEM) characterization is a significant limitation. SEM analysis could provide valuable insights into the microstructural changes and bonding quality at the atomic level following the cold-spray impact, which would complement the theoretical findings. Specifically, SEM could reveal details about surface morphology, particle deformation, and bonding interfaces, which are crucial for validating the bonding mechanisms proposed in this study. Considering that "Coatings" is a journal emphasizing practical applications and material performance, I believe that the lack of SEM characterization makes this submission less suitable for publication in this context. Including SEM analysis would strengthen the work by offering empirical evidence to support the predictions and enhance its relevance to the target audience. Additionally, the introduction section remains inadequate, as it does not provide sufficient information even about the material itself. Why are these materials important, and in which field are they used? What tests were conducted on these materials, and for what purpose? What were the results? What is the problem encountered, and what is your proposal and contribution in this study? These aspects are missing. I recommend that the authors review similar publications to address these shortcomings.
Author Response
Response to Reviewer 1
Comments 1: Although this publication focuses on the prediction of critical velocity for cold-spray bonding using first-principles calculations, the absence of Scanning Electron Microscopy (SEM) characterization is a significant limitation. SEM analysis could provide valuable insights into the microstructural changes and bonding quality at the atomic level following the cold-spray impact, which would complement the theoretical findings. Specifically, SEM could reveal details about surface morphology, particle deformation, and bonding interfaces, which are crucial for validating the bonding mechanisms proposed in this study. Considering that "Coatings" is a journal emphasizing practical applications and material performance, I believe that the lack of SEM characterization makes this submission less suitable for publication in this context. Including SEM analysis would strengthen the work by offering empirical evidence to support the predictions and enhance its relevance to the target audience. Additionally, the introduction section remains inadequate, as it does not provide sufficient information even about the material itself. Why are these materials important, and in which field are they used? What tests were conducted on these materials, and for what purpose? What were the results? What is the problem encountered, and what is your proposal and contribution in this study? These aspects are missing. I recommend that the authors review similar publications to address these shortcomings.
Response 1: We understand your comments and agree with your view. SEM observation can provide useful information on bonding behavior of cold-spray materials. For experimental verification, we should measure the impact velocity of spray particles and observe the bonding interface by SEM to determine the critical velocity. But we don’t need to repeat these studies because many researches have done this work. Many literatures have reported the measured or calculated critical velocity of Cu/Al cold-spray systems, and they conducted SEM observations to prove that the spray particle can bond with substrates successfully under critical velocity (see references in Table 1 in the revised manuscript). Therefore, whether the spray particles can bond with the substrates or not and how their shape change is not the focus of our research. Actually, we did nothing experiments in present work. We just proposed a simple and high-efficient calculated method to predict the critical velocity of cold-spray and compared with the existed data. If our method is proved to be reliable, we can predict the critical velocity of any cold-spray systems without using any empirical parameters, which makes the prediction more scientific.
In the introduction section, we did not describe the background of cold-spray materials, since we reported a calculated method to predict the critical velocity for cold-spray rather than for a specific cold-spray process. We chose the Cu/Al materials as an example just because their abundant experimental data for comparison. Our present work focuses on the calculated method rather than the cold-spray process for a specific material. For the predicted method of critical velocity, the commonly used method is finite element analysis (FEA). This method greatly depends on the mesh size and numerical algorithm, which makes significant deviation in the calculated values. Additionally, FEA requires massive materials parameters. Therefore, the accuracy of calculation is a problem. Recently, the molecular dynamics (MD) method has been applied to predict the critical velocity. But the calculated value has a great gap with experimental values. The shortcomings of each method have been pointed out directly in the introduction section (see the description in introduction section marked by red color). Then we propose predicting the critical velocity by using first-principles method which requires nothing parameter input. Thanks for your comments.
Author Response File: Author Response.pdf
