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Peer-Review Record

Effect of Bias Voltage on Structure, Mechanical Properties, and High-Temperature Water Vapor Corrosion of AlCrNbSiTi High Entropy Alloy Coatings

Coatings 2023, 13(11), 1948; https://doi.org/10.3390/coatings13111948
by Xuanzheng Wang 1, Zhong Zeng 2, Haobin Wang 2, Haiping Bai 3, Wentao Li 1, Yonghao Li 3, Ziwei Wang 1, Yanming Chen 2,* and Bing Yang 2,4,*
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Coatings 2023, 13(11), 1948; https://doi.org/10.3390/coatings13111948
Submission received: 8 September 2023 / Revised: 19 October 2023 / Accepted: 2 November 2023 / Published: 15 November 2023

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have prepared the AlCrNbSiTi high-entropy alloy coatings by magnetron sputtering.

1)      Have the authors published any other similar works?

2)      English and typographical errors should be fixed.

3)   What is the significance behind choosing the alloy and its composition? How did you confirm the multicomponent element composition?

4)      Have you measured the porosity of the coating?

5)     What about the phase formation?  Provide further details.

6)      How the coating thickness is decreased while the bias voltage is raised. The mechanism behind it should be explained in detail.

7)      How did you calculate the surface roughness from AFM images? The images appear to be in conflict.  

8)      Why is the hardness of the sample increased at 50 V and the residual stress increased at 150 V?

9)       Fig. 10 results are also not uniform.

10)  The quantum of work is not sufficient, it would be better to have the wear studies.

11)  What are the peaks of XRD at 44, 50 & 75 ?

 

Overall, the paper lacks a discussion portion. A detailed interpretation of the results must be added to the discussion section.

Comments on the Quality of English Language

English and typographical errors should be fixed.

Author Response

Comments 1: Have the authors published any other similar works?

Response 1: Our group has published one paper on mechanical properties of the same coating but deposited at different deposition parameters.

Xiangyu Zhang et al., Unravel hardening mechanism of AlCrNbSiTi high-entropy alloy coatings. Journal of Alloys and Compounds, 965, 2023, 171222.

Comments 2: English and typographical errors should be fixed.

Response 2: The English quality has been revised.

Comments 3: What is the significance behind choosing the alloy and its composition? How did you confirm the multicomponent element composition?

Response 3: High entropy alloys consist of five or more elements and form simple metal solutions rather than intermetallic compounds. In addition, the presence of Al and Si elements is conducive to improving the resistance to water vapor corrosion. We have added relevant content in the article, lines 89-102. 

The multicomponent element composition has been prooved by EDS measurement, lines 210-227.

Comments 4: Have you measured the porosity of the coating?

Response 4: In the present study we did not measure the porosity of the coating. In SEM cross-section images, Fig.2, all coatings are dense without any pores. We suppose our coatings do not have any pores, but there are pit defects.

Comments 5: What about the phase formation?  Provide further details.

Response 5: The coatings are amorphous or composed of small nano crystallites. The observed XRD peak belongs to FCC crystal lattice (200) reflection. Lines 229-233.

 Comments 6: How the coating thickness is decreased while the bias voltage is raised. The mechanism behind it should be explained in detail.

Response 6: A detailed explanation has been added in line 182-190 of the paper. As the bias increases, the coating thickness decreases for two main reasons. First, the bombardment energy is enhanced due to high bias, and the intervoid of the coating grains, if any, is filled during deposition, which promotes the densification of the coating surface, decreases the coating thickness. On the other hand, under the condition of high bias, the resputtering effect is enhanced, which leads to the decrease of deposition rate and the decrease of coating thickness.

Comments 7: How did you calculate the surface roughness from AFM images? The images appear to be in conflict.

Response 7: The root-mean-square roughness of the coatings was calculated by an AFM system software NanoScope Analysis when taking AFM pictures. We cannot find any inconsistency in the AFM images. 50 and 100 V samples appear much rougher and do have higher roughness. They are also in agreement with SEM images. 150 and 200 V samples are smoother in AFM and SEM images. In AFM images we did not use the same contrast ranges, due to too large difference in the roughness.

Comments 8: Why is the hardness of the sample increased at 50 V and the residual stress increased at 150 V?

Response 8: 

(1) There are two main reasons for the increase in coating hardness at 50 V. Compared with samples without bias, bias will promote the densification of the coating. Relevant contents have been added in lines 186-189 of the paper. In addition, there will be a small amount of hard silicide formation in the coating, which will also lead to an increase in the hardness of the coating, due to paper length reasons, this result was not added. Also, unbiased samples are misleading in the diagram and have been removed in the latest changes, Figure 7, line 263-264.

(2) The final residual stress is determined by the competition of these two above factors, lines 254-263. The high energy ions produce ion peening effect on the substrate, producing defects and also causing the increase of residual pressure stress. However, at the same time, the mobility of the surface adsorbed atoms rises since the collision of high-energy ions to the surface of the substrate, which also promotes the fusion of defects and reduces the residual stress. The residual stress increased at 150 V, the reason is that the high energy ion shot peening effect is stronger than the defect fusion effect, and the residual stress increases.

Comments 9: Fig. 10 results are also not uniform.

Response 9: We agree. In fact, the adhesion force of metal coatings is affected by many factors. The measurement of adhesion force is a very probabilistic process. We suppose to get more uniform data it is necessary to make at least a few tens measurements. So, we think, these data should be consedered as more qualitative.

Comments 10: The quantum of work is not sufficient, it would be better to have the wear studies.

Response 10: At present, the work mainly focuses on the water vapor corrosion resistance and other mechanical properties of the coating. However, your suggestion has given us a great inspiration, and we will do more in-depth research in this aspect in the following work.

Comments 11: What are the peaks of XRD at 44, 50 & 75 ?

Response 11: The peaks at 44, 50 , 75 belong to 304 stainless steel substrate.

Comments 12: Overall, the paper lacks a discussion portion. A detailed interpretation of the results must be added to the discussion section.

Response 12: We agree. More discussion of the results has been added to the paper to explain the results, including but not limited to lines 175-182, 185-191, 194-206.

Thank you for valuable comments.

4. Response to Comments on the Quality of English Language

Point 1: English and typographical errors should be fixed.

Response 1: We agree. The English quality of the whole article has been checked and the relevant errors corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, AlCrNbSiTi high entropy alloy (HEA) coatings are prepared by magnetron sputering at different bias voltages; and the influence of bias on coating morphologies; structure and performance are investigated. The paper is well organized and the results are well presented. The quality of the images is also suitable. It is suggested to consider the following before accepting.

The novelty and purpose of the research should be clearly stated in the abstract and introduction.

Some parts of the article, such as the introduction, should be revised for consistency. So, the introduction should be written better and needs minor revisions. Also, the first paragraphs presented are primarily general and general information. At the end of the introduction, a suitable summary of the importance of the present issue should be provided.

Use the following resources to deepen the introduction and discussion. Hardening-softening of Al0.3CoCrFeNi high-entropy alloy under nanoindentation. Air plasma-sprayed high-entropy (Y0.2Yb0.2Lu0.2Eu0.2Er0.2) 3Al5O12 coating with high thermal protection performance. Outstanding wear resistance of plasma sprayed high-entropy monoboride composite coating by inducing phase structural cooperative mechanism.

The research method section seems long and boring due to the lack of division into several sub-sections. It is suggested to use several subsections in addition to summarizing this section.

On what basis are the coating parameters selected? Also, how is the quality of the coating checked?

What is the purpose of presenting figures 4 and 5 (comparison of fracture cross-section and surface EDS results)?

In the graphs of Figure 8, there is a sample of 0 V. Is this correct?

Why is the change process of elastic modulus and hardness, especially in the first sample, completely different?

How is microhardness and elastic modulus obtained? How has the reproducibility of these results been checked?

How is the residual stress (Figure 9) calculated? These items should be presented in the research method section.

There are many pictures in Figure 12 and some of them are not clear. Better-quality images should be used.

The conclusion is very brief and superficial and needs to be rewritten.

Comments on the Quality of English Language

***

Author Response

Comments 1: The novelty and purpose of the research should be clearly stated in the abstract and introduction.

Response 1: Relevant content has been added in article. Lines 12-17, 89-96.

Fuel cladding tubes are devices used in reactors to encapsulate fuel clots and transmit heat to coolants. However, zirconium alloy materials which are widely used in the fuel cladding pipe of pressurized water reactor have noticeable safety risks in resisting design basis accidents. Therefore, it is very important to improve the corrosion resistance of fuel envelope tubes to high temperature water vapor oxidation. High entropy alloy is considered to be a potential protective coating material for cladding tubes.

At present, there are some defects in the protective coating technology of cladding material, which limits its large-scale use. In order to meet the requirements of accident resistant cladding materials, high entropy alloy coating is a feasible choice. High entropy alloy has four major effects different from traditional alloys, namely, high entropy effect in thermodynamics, lattice distortion effect in crystallography, hysteresis diffusion effect in dynamics and cocktail effect in performance, which makes it have good mechanical properties and corrosion resistance, and has great application potential in cladding tube protection.

Comments 2: Some parts of the article, such as the introduction, should be revised for consistency. So, the introduction should be written better and needs minor revisions. Also, the first paragraphs presented are primarily general and general information. At the end of the introduction, a suitable summary of the importance of the present issue should be provided.

Response 2: The introduction part of the article has been carefully revised, and some contents have been added/deleted to appropriately summarize the introduction, including but not limited to lines 49-50, 54-59, 89-102.

Comments 3: Use the following resources to deepen the introduction and discussion. Hardening-softening of Al0.3CoCrFeNi high-entropy alloy under nanoindentation. Air plasma-sprayed high-entropy (Y0.2Yb0.2Lu0.2Eu0.2Er0.2) 3Al5O12 coating with high thermal protection performance. Outstanding wear resistance of plasma sprayed high-entropy monoboride composite coating by inducing phase structural cooperative mechanism.

Response 3: Thank you for your references, which have been used to refine the introduction and discussion sections, including but not limited to lines 175-182, 186-191, 194-206.

Comments 4: The research method section seems long and boring due to the lack of division into several sub-sections. It is suggested to use several subsections in addition to summarizing this section.

Response 4: We have rewritten the Experimental details. The item has been divided into several parts and more details have been added. Including calculation methods, testing techniques, repeatability and other related content. Lines 123-150.

Comments 5: On what basis are the coating parameters selected? Also, how is the quality of the coating checked?

Response 5:

(1) High entropy alloys consist of five or more elements and form simple metal solutions rather than intermetallic compounds. This parameters was chosen for this reason. In addition, the presence of Al and Si elements is conducive to improving the resistance to water vapor corrosion. We added relevant content in the article, lines 96-102. As for the preparation parameters of the coating, such as pressure, power, sputtering time, etc., the selection is based on our previous studies.

(2) We observe the surface and cross section of the coating using an electron microscope. We observe whether the coating is dense, uniform, whether there are cracks, inclusions, and so on to make a preliminary judgment of the quality of the coating. After that mechanical properties were tested to further characterize the quality of the coating.

 Comments 6: What is the purpose of presenting figures 4 and 5 (comparison of fracture cross-section and surface EDS results)?

Response 6: EDS results for both surface and cross section are provided to support the "uniform distribution of elements in the coating". As shown in Figure 4 and 5, a uniform distribution of elements can be observed on both the cross section and the surface. This proves that the elements are evenly distributed not only in the horizontal direction, but also in the thickness direction, and it is inferred that the elements are evenly distributed throughout the coating. We also show the EDS maps in these figures to compare with Al distribution after water vapor corrosion test, Fig.14, where change in distribution is observed.

Comments 7: In the graphs of Figure 8, there is a sample of 0 V. Is this correct?

Response 7: The sample of 0 V has been removed considering that its presence would cause confusion, Figure 7, lines 263-264 In this study all samples were deposited under the bias.

Comments 8: Why is the change process of elastic modulus and hardness, especially in the first sample, completely different?

Response 8: The original figure included elastic modulus and hardness data for a 0V sample and substrate. The substrate is the first sample, which is why the change in the data of the first sample is completely different from that of subsequent samples. They were all removed in the new figure, Figure 7, lines 263-264.

Comments 9: How is microhardness and elastic modulus obtained? How has the reproducibility of these results been checked?

Response 9: Hardness and elastic modulus were measured using a nanoindentation instrument (NANO G200). The test uses the continuous stiffness method, and the penetration depth does not exceed 10% of the coating thickness, in order to reduce the influence of substrate effects. Each sample was measured at 6 points and averaged. In the latest version of this article, lines 142-145, the relevant details have been added.

Comments 10: How is the residual stress (Figure 9) calculated? These items should be presented in the research method section.

Response 10: The residual stress is tested by the curvature method, using the stress tester (Supu FST2000) to obtain the curvature change of the specimen before and after the coating deposition. Each sample was measured 3 times and the average value was taken. The residual stress of the coating is calculated according to Stoney equation. In the latest version of this article, lines 133-139, the relevant details have been added.

Comments 11: There are many pictures in Figure 12 and some of them are not clear. Better-quality images should be used.

Response 11: Agree. Clearer images have been added.

Comments 12: The conclusion is very brief and superficial and needs to be rewritten.

Response 12: We agree. According to your suggestion, the conclusion has been rewritten. Lines 355-385.

Thank you for you valuable comments.

Reviewer 3 Report

Comments and Suggestions for Authors

Review on coatings-2617483-peer-review-v2-1

The reviewed paper presents preliminary results on the high-entropy coating deposited by magnetron sputtering as the corrosion protection. However, all the paper is more report of preliminary tests than a scientific paper. I have many remarks, but I would like to first emphasise the 3 most important:

1.       After the introduction, you should add the aim of work. In the introduction you should explain what the motivation of coating composition was. What results were expected.

2.       It was very annoying during reding that the composition of the substrate was the secret information. Even the XRD reflects where described as the substrate. The substrate also influences the obtained results. In one place I have found that it was 304 steel, but the EDS results in Fig. 5 indicate a high silicon content (silicon wafer?). In another place, the EDS result revealed high content of Co and W (Table 1) but after the corrosion test the EDS in Table 2 indicates the Fe and Mn elements. It was very confusing.

3.       Resolution of measurements, number of repetitions, and error bars. This is the basic knowledge. Is it justified to write that the coating thickness was e.g. 1.21 micrometers. What was the method of thickness determination? How many repletion you did? Maybe it was just 1.2 +/- something? I have the same questions for other data. For example, the elastic modulus were: 18.19 GPa and 232.4 GPa. Why you have different precision of measurement (0.01 and 0.1 GPa)? In some figures (3,8) you added error bars in some figures not (6,9,10). Why? How many repetitions of measurements you did?

EDS in Table 1 and 2 the precision 0.01 % (what % at. or wt.?) but in Fig. 6 precision 1%....

Additional remarks:

Introduction

Explain abbreviations: TRISO, CANDU

Experimental part

Please add the composition of the substrate and details of substrate preparation (griding, polishing, etc.). Also add the details of scratch test (indenter type, load range etc.)

EDS is not a power spectrometer; XRD can give information about phase composition. Please explain precisely all the abbreviations.

Change 'In this figure' to 'in Figure 1'

Results and Discussion

What was the origin of the porosity of the coating visible in Fig.2?

Figure 3 does not add any new information. The thickness of coating is shown in Fig. 2, while the deposition rate is only thickness divided by 60 min (1 hour deposition).

Figure 8 – y axe description is 'samples' Volts are not units of 'samples'. Other figure captions should also be improved. For example, Fig. 2 it should be more precisely described where is SEM of surface, cross section, where AFM, the abbreviation Rq should be explain; Figure 5 - What does it mean cross sectional EDS results? In Fig. 5 the SEM image is also visible. In Fig. 13 it is not clear after what time the SEM pictures were taken…

Lines 182-184, about Fig. 6, proportions of element (Al, Cr) or content of element. The proportion generally is between something.

'Corrosion pits are believed to be due to corrosion of coatings.' This is a fact. Pits mean corrosion. Pits are formed when some regions become more prone to oxidation (anodic) in comparison to surrounding (cathodic), which means that the composition of the substrate has the crucial influence on pit formation.

Why cross-section in Fig. 14 is after 100 hours of corrosion, whereas the surface observations were done after 10, 50, 140 and 200 hours of corrosion. Table 2 should be connected to Fig. 13.

You do not have any evidence of Al2O3 formation. Only Al surface enrichment. On the other hand, why the EDS analysis showing Al enrichment has been chosen in the case of coating obtained with 200 v bias voltage, whereas from Fig. 6 it is known that the sample (200V) has the lowest Al content.

The discussion part should be improved.

Author Response

Comments 1: After the introduction, you should add the aim of work. In the introduction you should explain what the motivation of coating composition was. What results were expected.

Response 1: According to your suggestion, the introduction part has been modified, introducing the purpose, motivation and expected results of this work. Details are added in lines 89-102 of this article.

Comments 2: It was very annoying during reading that the composition of the substrate was the secret information. Even the XRD reflects where described as the substrate. The substrate also influences the obtained results. In one place I have found that it was 304 steel, but the EDS results in Fig. 5 indicate a high silicon content (silicon wafer?) . In another place, the EDS result revealed high content of Co and W (Table 1) but after the corrosion test the EDS in Table 2 indicates the Fe and Mn elements. It was very confusing.

Response 2: We agree. This is not a secret at all. We have added necessary information. We used single crystal Si (100), WC carbide and 304 stainless steel substrates. The coatings deposited on the single crystal Si (100) were used to observe the surface and cross section morphology of the coatings. The coating deposited on 304 stainless steel substrate is used for XRD testing, mechanical properties testing, residual stress testing and water vapor corrosion testing of the coating. The coatings deposited on the WC carbide is used for scratch testing. Details are added in lines 105-112 of this article.

Comments 3: Resolution of measurements, number of repetitions, and error bars. This is the basic knowledge. Is it justified to write that the coating thickness was e.g. 1.21 micrometers. What was the method of thickness determination? How many repletion you did? Maybe it was just 1.2 +/- something? I have the same questions for other data. For example, the elastic modulus were: 18.19 GPa and 232.4 GPa. Why you have different precision of measurement (0.01 and 0.1 GPa)? In some figures (3,8) you added error bars in some figures not (6,9,10). Why? How many repetitions of measurements you did?

EDS in Table 1 and 2 the precision 0.01 % (what % at. or wt.?) but in Fig. 6 precision 1%....

Response 3: We have corrected all data, added experimental uncertainties in the text or figures (Figs. 6, 9, 10) and removed unnecessary digits. We did 3-6 measurements per sample for all methods. The thickness was measured by SEM technique.

Comments 4: Explain abbreviations: TRISO, CANDU

Response 4: Corrected. Lines 39, 77.

CANDU is Canadian heavy water uranium reactor, TRISO is Tristructural isotropic (fuel particles).

Comments 5: Please add the composition of the substrate and details of substrate preparation (griding, polishing, etc.). Also add the details of scratch test (indenter type, load range etc.)

Response 5: The coatings were deposited on the polished single crystal Si (100), WC carbide and 304 stainless steel substrates. The surface of the substrates was polished by a manufacturer and was not additionally mechanically treated, lines 106-112.

The multifunctional material surface performance test instrument (Huahui MFT-4000) was used to obtain the scratch toughness of the coating and the adhesion force. During the scratch test, the indenter scratches the coating at a constant speed, and the load increases uniformly from 0 N to the 150 N. The loading speed is 100 N/min, and the scratch distance is 5 mm, lines 145-150.

 Comments 6: EDS is not a power spectrometer; XRD can give information about phase composition. Please explain precisely all the abbreviations.

Response 6: Corrected.

Comments 7: Change 'In this figure' to 'in Figure 1'

Response 7: Corrected.

Comments 8: What was the origin of the porosity of the coating visible in Fig.2?

Response 8: We suppose the pits on the surface of the coating are caused by the loss of large particles under the bombardment by high-energy ions of the sample surface under the bias voltage during the sputtering. Please refer to lines 177-182 for details.

Comments 9: Figure 3 does not add any new information. The thickness of coating is shown in Fig. 2, while the deposition rate is only thickness divided by 60 min (1 hour deposition).

Response 9: We agree. Deleted.

Comments 10: Figure 8 – y axe description is 'samples' Volts are not units of 'samples'. Other figure captions should also be improved. For example, Fig. 2 it should be more precisely described where is SEM of surface, cross section, where AFM, the abbreviation Rq should be explain; Figure 5 - What does it mean cross sectional EDS results? In Fig. 5 the SEM image is also visible. In Fig. 13 it is not clear after what time the SEM pictures were taken…

Response 10: All figures and their captions have been corrected.

Comments 11: Lines 182-184, about Fig. 6, proportions of element (Al, Cr) or content of element. The proportion generally is between something.

Response 11: Corrected.

Comments 12: 'Corrosion pits are believed to be due to corrosion of coatings.' This is a fact. Pits mean corrosion. Pits are formed when some regions become more prone to oxidation (anodic) in comparison to surrounding (cathodic), which means that the composition of the substrate has the crucial influence on pit formation.

Response 12: We agree. Deleted.

Comments 13: Why cross-section in Fig. 14 is after 100 hours of corrosion, whereas the surface observations were done after 10, 50, 140 and 200 hours of corrosion. Table 2 should be connected to Fig. 13.

Response 13: We made a mistake. EDS was taken after 200 h, that is at the highest corrosion. Corrected.

Comments 14: You do not have any evidence of Al2O3 formation. Only Al surface enrichment. On the other hand, why the EDS analysis showing Al enrichment has been chosen in the case of coating obtained with 200 v bias voltage, whereas from Fig. 6 it is known that the sample (200V) has the lowest Al content.

Response 14: We agree. Regarding the formation of Al2O3, this study failed to provide sufficient support, and the relevant content has been deleted. In fact, all samples showed diffusion and enrichment of Al elements to the coating surface after 200 hours of corrosion, not only 200 V samples.

Comments 15: The discussion part should be improved.

Response 15: We agree. The discussion section of the full paper has been carefully revised and add more detailed explanations of the results, including but not limited to lines 175-182, 186-191, 194-206.

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