Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript investigates the influence of Zr addition on the microstructure, mechanical behavior, and thermal stability of Cu-Zr nanocrystalline films prepared by magnetron co-sputtering. The topic is relevant and the data are potentially useful for understanding alloying effects in Cu-based thin films.

• The authors should better highlight what is new in their work,e.g., differences in sputtering conditions, quantitative strengthening analysis, or annealing behavior.
• Although the Methods mention EDS, the results do not include any EDS maps or spectra to confirm the Zr content .
•     The figures show trends in grain size but no uncertainty is indicated. The authors should report error bars based on multiple measurements, especially for nanoindentation and Scherrer calculations.

• The annealing was conducted only up to 300 °C. The authors should justify why such a low temperature range was selected and whether oxidation was controlled.
• The conclusions are currently formatted as bullet points that largely repeat the results. They should be rewritten as concise paragraphs emphasizing the key insights and broader implications for future studie..
• The authors should better justify the choice of the magnetron co-sputtering technique. A short discussion on the advantages of magnetron sputtering (e.g., uniformity, control of composition, low substrate temperature, suitability for alloys) compared to other deposition methods would improve the clarity and scientific motivation of the study.

Author Response

Comments 1: The manuscript investigates the influence of Zr addition on the microstructure, mechanical behavior, and thermal stability of Cu-Zr nanocrystalline films prepared by magnetron co-sputtering. The topic is relevant and the data are potentially useful for understanding alloying effects in Cu-based thin films.

Response 1:

Thank you for your positive evaluation of the manuscript.

 

Comments 2: The authors should better highlight what is new in their work, e.g., differences in sputtering conditions, quantitative strengthening analysis, or annealing behavior.

Response 2:

Thanks for your important and considerate comments. According to your suggestions, we have rewritten the conclusions of the revised manuscript and added descriptions related to the highlight work in the article. The revisions in the manuscript have been highlighted in red.

 

Comments 3: Although the Methods mention EDS, the results do not include any EDS maps or spectra to confirm the Zr content .

Response 3:

Thank you for the reviewer's comments. We have added a figure showing the variation of Zr content with Zr target power in the revised manuscript (Fig. 1). As can be seen from the graph, with the increase of Zr target power, the Zr content in the alloy thin film gradually increases from 0 at.% to 4.1 at.%.

 Figure 1. Relationship between global composition and Zr target Power in Cu-Zr alloy films.

Please see the attachment. Figure 1 can be found in the attachment.

 

Comments 4: The figures show trends in grain size but no uncertainty is indicated. The authors should report error bars based on multiple measurements, especially for nanoindentation and Scherrer calculations.

Response 4:

Thank you for the reviewer's suggestion. The error bars have been added to all experimentally measured results in the revised manuscript, such as hardness and elastic modulus. In contrast, no error bars were added to data obtained through formula calculations, including grain size and intragranular solute content. On the one hand, based on the authors' literature survey [1-3], error bars are rarely attached to such calculated data. On the other hand, these formula-derived data possess certainty, making the addition of error bars probably inappropriate.

References:

1. Souli, G.C. Gruber, V.L. Terziyska, J. Zechner, C. Mitterer. Thermal stability of immiscible sputter-deposited Cu-Mo thin films. J. Alloys Compd. 2019, 783: 208-218.
2. S.C. Pun,W.B. Wang, A. Khalajhedayati, J.D. Schuler, J.R. Trelewicz, T.J. Rupert. Nanocrystalline Al-Mg with extreme strength due to grain boundary doping. Mater. Sci. Eng. A 2017, 696: 400-406.
3. V.N. Vamsi, N.P. Wasekar, G. Sundararajan. Influence of heat treatment on microstructure and mechanical propertiesof pulse electrodeposited Ni-W alloy coatings. Surf. Coat. Technol. 2017, 319: 403-414.

 

Comments 5: The annealing was conducted only up to 300 °C. The authors should justify why such a low temperature range was selected and whether oxidation was controlled.

Response 5:

Thank you for the reviewer's suggestion.Due to the relatively low stability of nanocrystalline Cu, structural instability of nanocrystalline pure Cu can occur even at room temperature. Therefore, the annealing temperature selected in this work is much lower than that for traditional coarse-grained Cu alloys. In addition, the purpose of annealing in this study is to investigate the stability of Cu-Zr alloy thin films. According to the experimental results in the manuscript, the grain size of all Cu-Zr alloy thin films grew to varying degrees at 300°C, indicating that this temperature is sufficient for the research presented herein. Regarding the issue of oxidation control, prior to the annealing experiments, the Cu-Zr alloy thin film samples were first placed in quartz tubes for vacuum sealing (as shown in Fig. 2), and then transferred to a vacuum annealing furnace for the annealing process. Relevant descriptions have been added to the experimental section of the revised manuscript.

 Figure 2. Image of the Cu-Zr alloy film sample sealed in a quartz tube.

Please see the attachment. Figure 2 can be found in the attachment.

 

Comments 6: The conclusions are currently formatted as bullet points that largely repeat the results. They should be rewritten as concise paragraphs emphasizing the key insights and broader implications for future study.

Response 6:

Thank you for the reviewer's suggestions. We made modifications in the revised manuscript. The revisions in the manuscript have been highlighted in red.

 

Comments 7: The authors should better justify the choice of the magnetron co-sputtering technique. A short discussion on the advantages of magnetron sputtering (e.g., uniformity, control of composition, low substrate temperature, suitability for alloys) compared to other deposition methods would improve the clarity and scientific motivation of the study.

Response 7:

Thank you for the reviewer's suggestion. We have added relevant descriptions in the introduction section of the revised manuscript. The revisions in the manuscript have been highlighted in red.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a valuable and systematic study on the effect of Zr alloying on Cu nanocrystalline films. The work follows a logical structure, employing appropriate characterization methods (XRD, TEM, nanoindentation) to investigate the microstructure, mechanical properties, and thermal stability. The subsequent analysis, which successfully deconvolutes the strengthening mechanisms and provides a model for the enhanced thermal stability based on grain boundary (GB) segregation, is commendable. However, the manuscript is currently unsuitable for publication due to several critical errors and inconsistencies in the Methods and Results sections. These errors, detailed below, undermine the reproducibility and credibility of the work. Fortunately, most appear to be significant typos rather than flaws in the scientific method. If the authors can meticulously correct these issues and address the minor points of clarification, the paper will be a strong candidate for publication.

• The introduction provides good context for alloying Cu with elements like Ti , Ta, and Cr. However, it never explicitly states why Zr was chosen for this study. Is it known to have low miscibility, a strong thermodynamic driver for GB segregation, or a large atomic mismatch? A brief sentence justifying the choice of Zr would strengthen the introduction.
• The authors state a 10 mN force was used to ensure the depth "did not exceed the total film thickness" (2 µm). This is a very weak justification. To avoid substrate effects, indentation depth should ideally be <10-15% of the film thickness (i.e., <200-300 nm). Please state the actual indentation depth (or range of depths) achieved with the 10 mN load and confirm that this depth is sufficiently shallow to preclude substrate effects.
• The calculated total strengthening (solid line in Fig 9) visibly underestimates the experimental data points (∆ HTotal, black squares), especially at higher Zr concentrations. This discrepancy is not mentioned. The authors should add a brief comment on this. Does it suggest an underestimation by one of the models, or the presence of another minor, unquantified strengthening mechanism?
• This (Ref [22]) is an empirical theory from 1978 and may not be familiar to all readers. Please add a brief descriptor, such as "Using Yu's theorem [22], an empirical model relating lattice parameters to covalent electron properties..."
• The discussion provides an excellent model for stability, showing Zr segregation increases with annealing temperature (Fig 10). The results (Fig 8) show hardness still decreases slightly upon annealing, which is attributed to residual stress release. These two points should be explicitly linked. The authors could strengthen the discussion by stating that the observed softening (due to stress release) occurs despite the opposing effect of increased (and strengthening) grain boundary segregation, highlighting that stress release is the dominant mechanism for the alloyed films during annealing.

Author Response

Comments 1: The manuscript presents a valuable and systematic study on the effect of Zr alloying on Cu nanocrystalline films. The work follows a logical structure, employing appropriate characterization methods (XRD, TEM, nanoindentation) to investigate the microstructure, mechanical properties, and thermal stability. The subsequent analysis, which successfully deconvolutes the strengthening mechanisms and provides a model for the enhanced thermal stability based on grain boundary (GB) segregation, is commendable. However, the manuscript is currently unsuitable for publication due to several critical errors and inconsistencies in the Methods and Results sections. These errors, detailed below, undermine the reproducibility and credibility of the work. Fortunately, most appear to be significant typos rather than flaws in the scientific method. If the authors can meticulously correct these issues and address the minor points of clarification, the paper will be a strong candidate for publication.

Response 1:

Thanks for your important and considerate comments. According to your suggestions, we had carefully revised the manuscript in order to avoid errors and misunderstandings, and added descriptions related to comments 2-6 in the article. The revisions in the manuscript have been highlighted in red. Thank you again for your professional evaluation.

 

Comments 2: The introduction provides good context for alloying Cu with elements like Ti , Ta, and Cr. However, it never explicitly states why Zr was chosen for this study. Is it known to have low miscibility, a strong thermodynamic driver for GB segregation, or a large atomic mismatch? A brief sentence justifying the choice of Zr would strengthen the introduction.

Response 2:

Thank you for the reviewer's professional suggestion. We have added relevant descriptions in the introduction section of the revised manuscript.The revisions in the manuscript have been highlighted in red.

 

Comments 3: The authors state a 10 mN force was used to ensure the depth "did not exceed the total film thickness" (2 µm). This is a very weak justification. To avoid substrate effects, indentation depth should ideally be <10-15% of the film thickness (i.e., <200-300 nm). Please state the actual indentation depth (or range of depths) achieved with the 10 mN load and confirm that this depth is sufficiently shallow to preclude substrate effects.

Response 3:

Thank you for the reviewer's comments. According to your suggestions, we have added the displacement-load curves of the nanoindentation tests for the alloy films, as shown in Fig. 1. In the figure, except that the maximum indentation depth of the pure Cu film slightly exceeds 300 nm, the indentation depths of the other alloy films are all within 300 nm. This indicates that the measurement of the hardness of the Cu-Zr alloy films in this paper is not affected by the substrate.

The revisions in the manuscript have been highlighted in red.

Figure 1. The displacement-load curves of the Cu-Zr alloy films.

Please see the attachment. Figure 1 can be found in the attachment.

 

Comments 4: The calculated total strengthening (solid line in Fig 11) visibly underestimates the experimental data points (∆HTotal, black squares), especially at higher Zr concentrations. This discrepancy is not mentioned. The authors should add a brief comment on this. Does it suggest an underestimation by one of the models, or the presence of another minor, unquantified strengthening mechanism?

Response 4:

Thank you for the reviewer's suggestion. In the original manuscript, both the black squares and black solid line in Figure 9 represent the measured hardness increment (∆H_Total). The black solid line is a fitting curve derived from the black squares, and this processing was done to more clearly show the trend of change. It may be that the description in the text was not clear enough, which caused misunderstanding. We have added supplementary explanations in the revised manuscript. The revisions in the manuscript have been highlighted in red.

 

Comments 5: This (Ref [22]) is an empirical theory from 1978 and may not be familiar to all readers. Please add a brief descriptor, such as "Using Yu's theorem [22], an empirical model relating lattice parameters to covalent electron properties..."

Response 5:

Thank you for the reviewer's suggestion. According to your suggestions, we have added relevant introductions to Yu's theorem in the revised manuscript. Proposed by Chinese scientist Yu Ruihuang in 1978, this theory is part of the Empirical Electron Theory of Solids and Molecules (EET) and is currently widely used in the prediction of material structures and properties[1, 2].

For crystal structures with known lattice parameters, EET can provide the distribution of electrons on the bonding network and the state of atoms in the crystal, which can be used to calculate the binding energy, melting point, alloy phase diagram, etc. of the crystal. This formula can also be used to calculate the relationship between the lattice constant of a solid solution and the solute content. The calculation results have better applicability than Vegard's law.

References

1. J.B.Guo, Z.Z. Guo, W. Zhang, H. Han, C.Y.  The application of EET in high-entropy alloy. J. Phys.: Conf. Ser. 2023, 2478: 122081.
2. B.Q.Fu,  Liu, Z.L. Li. Calculation of the surface energy of bcc-metals with the empirical electron theory. Appl. Surf. Sci. 2009, 255 (20): 8511-8519.

 

Comments 6: The discussion provides an excellent model for stability, showing Zr segregation increases with annealing temperature (Fig 12). The results (Fig 10) show hardness still decreases slightly upon annealing, which is attributed to residual stress release. These two points should be explicitly linked. The authors could strengthen the discussion by stating that the observed softening (due to stress release) occurs despite the opposing effect of increased (and strengthening) grain boundary segregation, highlighting that stress release is the dominant mechanism for the alloyed films during annealing.

Response 6:

Thank you for the reviewer's suggestions. In accordance with your suggestions, we have added the relevant descriptions in the discussion section of the article. The revisions in the manuscript have been highlighted in red.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I recommend the publication of this article. 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors answered all comments, and the revised version of the manuscript can be accepted in the present form.