Mechanical Properties and High-Temperature Steam Oxidation of Cr/CrN Multi-Layers Produced by High-Power Impulse Magnetron Sputtering

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The development of advanced coatings for zirconium-based alloys has got significant attention in recent years, particularly in the nuclear and aerospace industries. Zirconium alloys, such as Zr-4, are widely used due to their corrosion resistance, low thermal neutron absorption cross-section, and favorable mechanical properties. Protective coatings are being explored as a means to enhance the oxidation resistance and mechanical properties of Zr-4 alloys under such extreme conditions. HiPIMS is a promising technology for depositing advanced coatings with superior adhesion, density, and microstructural control.

In the manuscript, multilayer coatings were deposited on alloy substrates using HiPIMS. The effects of different coating structures on the microstructure, mechanical properties, and high-temperature steam oxidation resistance were systematically investigated. The authors try to optimize coating performance by leveraging the synergistic benefits of individual Cr and CrN layers.

The manuscript is quite close to the just published article of the same authors: Journal of Nuclear Materials 603 (2025) 155482, though it is further extended to the field of multilayer coatings.

Overall the paper is well written, it provides valuable insights into the design and development of advanced protective coatings for critical applications in extreme environments.

There are few language moments though. For instance, I would suggest using plural "mechanical properties" instead of property. It is more appropriate because the discussion covers multiple attributes (nanohardness, modulus of elasticity, bond strength, stress reduction, and crack propagation).

The manuscript is recommended for publication in Coatings journal.

Author Response

Comments 1:

The development of advanced coatings for zirconium-based alloys has got significant attention in recent years, particularly in the nuclear and aerospace industries. Zirconium alloys, such as Zr-4, are widely used due to their corrosion resistance, low thermal neutron absorption cross-section, and favorable mechanical properties. Protective coatings are being explored as a means to enhance the oxidation resistance and mechanical properties of Zr-4 alloys under such extreme conditions. HiPIMS is a promising technology for depositing advanced coatings with superior adhesion, density, and microstructural control.

 

In the manuscript, multilayer coatings were deposited on alloy substrates using HiPIMS. The effects of different coating structures on the microstructure, mechanical properties, and high-temperature steam oxidation resistance were systematically investigated. The authors try to optimize coating performance by leveraging the synergistic benefits of individual Cr and CrN layers.

 

The manuscript is quite close to the just published article of the same authors: Journal of Nuclear Materials 603 (2025) 155482, though it is further extended to the field of multilayer coatings.

 

Overall the paper is well written, it provides valuable insights into the design and development of advanced protective coatings for critical applications in extreme environments.

 

There are few language moments though. For instance, I would suggest using plural "mechanical properties" instead of property. It is more appropriate because the discussion covers multiple attributes (nanohardness, modulus of elasticity, bond strength, stress reduction, and crack propagation).

Comments 1: The development of advanced coatings for zirconium-based alloys has got significant attention in recent years, particularly in the nuclear and aerospace industries. Zirconium alloys, such as Zr-4, are widely used due to their corrosion resistance, low thermal neutron absorption cross-section, and favorable mechanical properties. Protective coatings are being explored as a means to enhance the oxidation resistance and mechanical properties of Zr-4 alloys under such extreme conditions. HiPIMS is a promising technology for depositing advanced coatings with superior adhesion, density, and microstructural control.   In the manuscript, multilayer coatings were deposited on alloy substrates using HiPIMS. The effects of different coating structures on the microstructure, mechanical properties, and high-temperature steam oxidation resistance were systematically investigated. The authors try to optimize coating performance by leveraging the synergistic benefits of individual Cr and CrN layers.   The manuscript is quite close to the just published article of the same authors: Journal of Nuclear Materials 603 (2025) 155482, though it is further extended to the field of multilayer coatings.   Overall the paper is well written, it provides valuable insights into the design and development of advanced protective coatings for critical applications in extreme environments.   There are few language moments though. For instance, I would suggest using plural "mechanical properties" instead of property. It is more appropriate because the discussion covers multiple attributes (nanohardness, modulus of elasticity, bond strength, stress reduction, and crack propagation).   The manuscript is recommended for publication in Coatings journal.

Response: Dear Professor, Thank you for your careful review and valuable comments on our manuscript. We are pleased to learn that you found this work to provide valuable insights into the design and development of advanced protective coatings for critical applications in extreme environments.   Regarding the language issue you raise, it is true that, as you say, the term "mechanical properties" should be used in the plural to cover several properties (e.g. nanohardness, elastic modulus, bond strength, stress reduction and crack propagation). We have revised the relevant part of the text to reflect this more accurately. We also thank you for pointing out the similarities between our study and the recently published article Journal of Nuclear Materials 603 (2025) 155482. We have added a citation to this article in the manuscript and further clarified the extensions and innovations of this study in the field of multilayer coatings to differentiate between the two papers and to highlight the unique contribution of our study. We appreciate your suggested revisions and have adjusted the manuscript accordingly.   Thank you again for taking the time out of your busy schedule to carefully review our paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The results of the work show once again the positive aspect of the manufacture of multilayers. High-quality test results were obtained. This allowed an in-depth discussion supported by a theoretical scheme of oxidation processes. This indicates the high scientific level of this study.

The comments on the work are rather technical in nature.

1. The title of the work should be corrected by adding, for example, the word ‘produced’:

Mechanical properties and high temperature steam oxidation of

Cr-based coatings and their multilayers produced by high power pulse

magnetron sputtering’.

2. Combining the results line in Figure 3b does not make logical sense. A presentation as in Figure 3a should be used.

3. The conclusion regarding a smaller number of pores after oxidation with an increase in the number of layers is not well clocked in Figure 7. 

Author Response

Comments 1: The title of the work should be corrected by adding, for example, the word ‘produced’: Mechanical properties and high temperature steam oxidation of Cr-based coatings and their multilayers produced by high power pulse magnetron sputtering’.

Response: Dear Professor, Thank you for your valuable comments on our manuscript. We understand your suggestion and agree that the phrase "produced by high power pulse magnetron sputtering" should be added to the title to better reflect the content and methodology of the study. Therefore, we have revised the title accordingly and the new title is: "Mechanical properties and high temperature steam oxidation of Cr-based coatings and their multilayers produced by high power pulse magnetron sputtering".   Thank you again for your careful review and support.

Comments 2: Combining the results line in Figure 3b does not make logical sense. A presentation as in Figure 3a should be used.

Response: Dear Professor, Thank you for your careful review and valuable suggestions on our manuscript. Regarding the combining of results in Fig. 3b, we understand the reviewer's concern about the logic of the graphical presentation. Fig. 3a shows the hardness and elastic modulus of different coating structures, while Fig. 3b shows the ratio of hardness to elastic modulus derived from nanoindentation testing.   In particular, the H/E ratio is commonly used to assess the plastic deformation and abrasion resistance of a coating, while the H3/E2 ratio is proportional to the yield stress of the material and can be used as an indicator of the toughness of the coating. In this study, Figure 3b aims to show that CrN coatings are less ductile and may be more susceptible to crack sprouting during high temperature steam oxidation, a phenomenon that supports the subsequent high temperature steam oxidation results. We have chosen to present these ratios in Fig. 3b to highlight the potential for cracking of coatings during scratch testing and high temperature steam oxidation, and to provide a theoretical basis for subsequent analysis.   Once again, we thank the reviewers for their valuable comments, and we have re-explained Fig. 3b in the text to ensure that the logic of this section is clearer.

 

Comments 3: The conclusion regarding a smaller number of pores after oxidation with an increase in the number of layers is not well clocked in Figure 7.

Response: Dear Professor, thank you for your careful review and valuable comments on our manuscript. Regarding the conclusion that the number of pores decreases with increasing number of layers after oxidation, we have re-examined the data presentation in Fig. 7 and found that the conclusion is not well presented in Fig. 7. We have therefore decided to remove this conclusion and rephrase Figure 7 to ensure consistency between the data and the conclusion.   Thank you again for your careful review and support.

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

 

Dear Authors,

I find your article very interesting and well-prepared.

But there can be no methodological errors in it, so I propose the following necessary changes and additions:

1.      Removal of Tab.1 from the work.

The X-ray microanalysis method of determining the chemical composition in microareas using an energy-dispersive X-ray spectrometer (EDS) does not allow for the quantitative assessment of the so-called light elements (in this case N and O) in a standard-free analysis - and such was used in this work. This is a purely methodological limitation! A different approach or method must be used here. I therefore propose to absolutely remove Tab. 1 and all references to these results from the work. After all, in this work, the analysis of qualitative results of these elements (e.g. changes in concentration along the line in combination with mapping) is enough to understand the physicochemical phenomena and the discussion.

2.       Supplementation of the phase analysis results Figure 6.

This supplementation is to concern a greater number of details concerning the phases specified in Figure 6. It is necessary because these details are the subject of analysis in the further part of the work (also in the discussion of results), e.g. it concerns the α-Zr(O) phase and it is not in Figure 6, etc.

3.       Please specify the measurement errors - Figure 5 and their discussion.

Good luck!

     

Author Response

Comments 1: Removal of Tab.1 from the work.The X-ray microanalysis method of determining the chemical composition in microareas using an energy-dispersive X-ray spectrometer (EDS) does not allow for the quantitative assessment of the so-called light elements (in this case N and O) in a standard-free analysis - and such was used in this work. This is a purely methodological limitation! A different approach or method must be used here. I therefore propose to absolutely remove Tab. 1 and all references to these results from the work. After all, in this work, the analysis of qualitative results of these elements (e.g. changes in concentration along the line in combination with mapping) is enough to understand the physicochemical phenomena and the discussion.

Response: Dear Professor, Thank you for your careful review and valuable comments on our study. Regarding the deletion of Table 1 and related content, we understand the reviewers' concern about the potential for error in the quantitative analysis of light elements (e.g., N and O) by energy dispersive X-ray spectrometry (EDS). Indeed, in the absence of standard analyses, quantitative results from EDS may not be entirely accurate, especially for the quantitative assessment of these light elements. While EDS presents challenges for accurate quantitative analysis of light elements in the absence of standard conditions, the method can still be used for semi-quantitative analysis. We note that many relevant studies have analysed elements such as nitrogen and oxygen using EDS and have obtained reliable semi-quantitative data [1-4]. Semi-quantitative analyses can show trends in the relative concentrations of elements, which are still valuable for understanding physicochemical phenomena and for discussion.   In our study, although the accuracy of the quantitative results may have been affected, the semi-quantitative data still supported our analysis of concentration variations along the line. Considering that the data in Table 1 are part of our study, they reflect the actual results obtained during our experiments. Although these data may not be absolutely accurate quantitative values, they are still important for understanding the experimental process, data trends and comparisons with other results. Therefore, we prefer to keep Table 1, but will explicitly state the semi-quantitative nature of these data in the text.   Thank you again for your careful review and support.   References：

[1]. Li, Z.; Liu, C.; Chen, Q.; Yang, J.; Liu, J.; Yang, H.; Zhang, W.; Zhang, R.; He, L.; Long, J.; Chang, H. Microstructure, high-temperature corrosion and steam oxidation properties of Cr/CrN multilayer coatings prepared by magnetron sputtering. Corrosion Science 2021, 191, 109755.

[2]. Deng, J.; Geng, D.; Sun, Q.; Song, Z.; Sun, J. Steam oxidation of Cr-coated zirconium alloy claddings at 1200 °C: Kinetics transition and failure mechanism of Cr coatings. Journal of Nuclear Materials 2023, 586, 154684.

[3]. Yang, J.; Wang, B.; Hu, S.; Zhao, F.; Han, B.; Shang, L.; Cui, Y.; Wang, S.; Yun, D.; Xu, D. Microstructural understanding of CrN layer as a barrier against Cr-Zr interdiffusion in Cr-coated zirconium alloy for accident tolerant fuel claddings. Materials Characterization 2023, 205, 113242.

[4]. Han, X.; Xue, J.; Peng, S.; Zhang, H. An interesting oxidation phenomenon of Cr coatings on Zry-4 substrates in high temperature steam environment. Corrosion Science 2019, 156, 117-124.

 

Comments 2: Supplementation of the phase analysis results Figure 6. This supplementation is to concern a greater number of details concerning the phases specified in Figure 6. It is necessary because these details are the subject of analysis in the further part of the work (also in the discussion of results), e.g. it concerns the α-Zr(O) phase and it is not in Figure 6, etc.

Comments 3: Please specify the measurement errors - Figure 5 and their discussion.

Response: Dear Professor, thank you for your careful review and valuable comments on our manuscript. Regarding the measurement error of the weight gain curve in Fig. 5, during the high temperature steam oxidation test, the sample was heated from 40 °C to 1200 °C at a heating rate of 40 K/min and the weight data were recorded every 6 seconds. In this test, errors in the weight gain curve can arise from several sources. Firstly, there may have been small variations in temperature control, and although a high heating rate (40 K/min) was used in this experiment, this factor was taken into account in the design of the experiment and in the analysis of the data. In addition, changes in the atmosphere during the experiment (e.g. water vapour flow rate, etc.) can affect the rate and extent of the oxidation reaction and thus the shape and value of the weight gain curve. We therefore tightly controlled the experimental atmosphere to ensure that it remained stable throughout the test.   Despite these potential sources of error, we have taken rigorous experimental control measures, used high-precision equipment to ensure the reliability and accuracy of measurements, ensured the accuracy and completeness of data recording, and employed appropriate data processing methods to minimise random errors.   Thank you again for your careful review and support.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The authors of this work have obtained the Cr, CrN, and (Cr/CrN)n multilayer coatings on the surface of Zr-4 alloy using the high power impulse magnetron sputtering. It was shown that the (Cr/CrN)40 multilayer coating has the highest resistance to high temperature steam oxidation, higher toughness and nanohardness. Based on microstructural studies of coating samples after high temperature steam oxidation tests, the authors explained the behaviour of each coating during vapor oxidation at 1200 °C. Nevertheless, I suggest the author should address the following issues before publication:

1. Despite the fact that the title of the paper reflects its content, there is a mistake in it. Authors should pay more attention to the title of the paper.

2. Figures should be presented after they have been mentioned in the text.

3. The cross-sectional images of the coatings show high roughness of the coatings (Figure 1). What is the reason for this, if they were ground and polished during the preparation process?

4. What is indicated in Figure 1 by the yellow dotted line? It is worth mentioning this in the text or its caption.

5. It is worth identifying all phase peaks on the XRD patterns (Figure 2).

6. “… the high hardness and fracture toughness of multilayer coatings are mainly due to the growth of two materials with different moduli on the surfaces of each other …” What are “moduli”?

Author Response

Comments 1: Despite the fact that the title of the paper reflects its content, there is a mistake in it. Authors should pay more attention to the title of the paper.

Comments 2: Figures should be presented after they have been mentioned in the text.

Comments 3: The cross-sectional images of the coatings show high roughness of the coatings (Figure 1). What is the reason for this, if they were ground and polished during the preparation process?

Comments 4: What is indicated in Figure 1 by the yellow dotted line? It is worth mentioning this in the text or its caption.

Comments 5: It is worth identifying all phase peaks on the XRD patterns (Figure 2).

Comments 6:  “… the high hardness and fracture toughness of multilayer coatings are mainly due to the growth of two materials with different moduli on the surfaces of each other …” What are “moduli”?

Response: Dear Professor, Thank you for your careful review and valuable comments on our manuscript. The term 'modulus' refers to the elastic modulus, which describes the stiffness or ability of a material to resist deformation when stressed. Specifically, the higher the elastic modulus, the less the material will deform under external forces. Therefore, in multi-layer coatings, materials with different elastic modulus are able to create synergistic effects between the different layers of the coating by interacting with each other, resulting in increased stiffness and fracture toughness.   Thank you again for your careful review and support.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

I accept your explanations with understanding, I believe that thanks to the changes the article has gained a lot in precision and analysis of the research results obtained. 

Unfortunately, the explanations regarding the analysis of light elements, O and N, are not fully correct. The fact that in other publications there is such a presentation of results does not mean that these are properly methodically carried out studies using an energy dispersive spectrometer (EDS).

 If quantitative results must be presented in Table 1, it is not enough to add a note that they are semi-quantitative. I propose a more extensive explanation of the problems with the analysis of light elements in the measurement methodology using EDS (2-3 sentences) and in each case determining the errors of these measurements and presenting them in Table 1.

 Good luck!  

Author Response

Comments 1:

I accept your explanations with understanding, I believe that thanks to the changes the article has gained a lot in precision and analysis of the research results obtained.

 

Unfortunately, the explanations regarding the analysis of light elements, O and N, are not fully correct. The fact that in other publications there is such a presentation of results does not mean that these are properly methodically carried out studies using an energy dispersive spectrometer (EDS).

 

 If quantitative results must be presented in Table 1, it is not enough to add a note that they are semi-quantitative. I propose a more extensive explanation of the problems with the analysis of light elements in the measurement methodology using EDS (2-3 sentences) and in each case determining the errors of these measurements and presenting them in Table 1.

 

Response:

Dear Professor, Thank you very much for your careful review and valuable comments on our paper. We fully understand your concerns about the accuracy of light element O and N analyses and the problems that may be encountered with Energy Dispersive Spectrometry (EDS) when measuring light elements. Although EDS is a widely used analytical tool, in specific cases, such as light element analyses, it may be necessary to combine it with other more accurate analytical methods to obtain more reliable results.

 

After careful consideration of your comments, we have decided to follow your previous suggestion to remove Table 1 from the manuscript. We recognise that although we have attempted to clarify the limitations of the EDS measurements of light elements by adding semi-quantitative annotations, this is not sufficient to adequately reflect the uncertainties and potential errors in the measurements. Furthermore, as you say, concentration changes along the line combined with mapping are sufficient to understand the physicochemical phenomena and discussion. Therefore, in order to avoid misleading the reader or triggering misinterpretation of the data, we believe that deleting Table 1 is a more prudent approach.

 

 

Thank you again for your careful review and support.

Author Response File: Author Response.pdf