Hardness Enhancement in CoCrFeNi_1−x(WC)_x High-Entropy Alloy Thin Films Synthesised by Magnetron Co-Sputtering

Round 1

Reviewer 1 Report

The authors investigate the Hardness Enhancement in CoCrFeNi1-x(WC)x High-Entropy Alloy Thin Films Synthesised by Magnetron co-Sputtering. However,to improve the quality of the manuscript provided suggestions made by the reviewer.

1. Authors should change the title of the manuscript and provide a more prized one.
2. What are the main contribution and novelty of the paper?
3.  List out some of the important applications of CoCrFeNi1-x(WC)x High-Entropy Alloy. 
4. Fig. 4, The Vickers hardness has been showed an monotonic 246 rise from (651 ± 20) HV to (1108 ± 34) HV following the increasing WC content, (Maximum error),  justify with specific reason.

Author Response

The authors investigate the Hardness Enhancement in CoCrFeNi1-x(WC)x High-Entropy Alloy Thin Films Synthesised by Magnetron co-Sputtering. However, to improve the quality of the manuscript provided suggestions made by the reviewer.

• Authors should change the title of the manuscript and provide a more prized one.

After discussing this point among the co-authors, we decided that the current title provides all necessary information in a sufficient manner.

• What are the main contribution and novelty of the paper?

The mixture of CoCrFeNi and WC by magnetron sputtering with widely varying range of composition is demonstrated for the first time. Also, a high homogeneity / uniformity of the element distribution is observed which can not be seen in comparable material systems in literature. It is assumed that the sputter deposition process is highly beneficial for the uniform mix of elements while the mechanical properties investigated in our work are conserved (here especially the hardness contribution of WC).

Added information in line 37 f.: “In this work we attempt to form homogeneously mixed thin films of CoCrFeNi1-x(WC)x with few or single-phase structure and variable composition by magnetron co-sputtering. Therefore, simultaneous deposition from a spark plasma sintered (SPS) CoCrFeNi target and a commercial WC target on Si(100) is carried out.”

See also line 241 f.: “The fabrication of evenly mixed CoCrFeNi1-x(WC)x thin films by simultaneous magnetron sputter deposition from two targets has been demonstrated. By variation of the deposition powers for both targets the WC content of the mixed layers has been tuned in a range of (17 ... 82) at.%.”

• List out some of the important applications of CoCrFeNi1-x(WC)x High-Entropy Alloy. 

We assume that many surface coatings with requirements of a certain hardness variability could be addressed by the alloy system we demonstrated in this work. The tuneabilty of WC content in the co-sputtering process provides the necessary tool. Nevertheless, we are aware that the sputtering process itself might not be suitable for every application and we need to point out that our work should be considered as basic research with no strong focus on certain applications.

• Fig. 4, The Vickers hardness has been showed an monotonic 246 rise from (651 ± 20) HV to (1108 ± 34) HV following the increasing WC content, (Maximum error),  justify with specific reason.

WC has a rather high intrinsic hardness ranging from 1300 – 2200 HV. The HEA+WC mix is assumed to have a hardness value averaged from the content weighted individual hardness of CoCrFeNi and WC and so it seems reasonable to see a monotonic rise of the alloy hardness with increasing WC content.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments to the author

In this manuscript, the authors report a new fabrication method of the CoCrFeNi high-entropy alloy films with addition of WC to improve their hardness. A serial of CoCrFeNi_1-x(WC)_x thin films with high homogeneity and tunable stoichiometry were synthesized by magnetron co-sputtering, through which the WC content of the mixed films was tuned by adjusting the deposition powers for both targets. The influence of the WC content on the structural features and mechanical properties were systematically investigated. The manuscript was well presented in the experimental part with an impressively detailed description of fabrication and characterization processes, and it contains sufficient amount of work. Taken overall, I would recommend the publication of this paper in the “coatings” after several concerns are properly addressed.

1. The authors need to give a more comprehensive discussion about the motivation and novelty of this paper in the introduction section.
2. It appears that there are grain size differences between films prepared under various deposition power ratios. In Fig. 5, their hardness values were analyzed as a function of the WC content, without mentioning grain size dependence. Please take into account the grain size effect on harness.
3. It has been reported that the substrate temperature also had a notable influence on the structure and mechanical property of high-entropy alloy thin films. What’s the substrate temperature in the present deposition processes?
4. What does “normal deposition geometry” (line 65) refer to when preparing the unmixed constituents? Is it different from the geometry for the HEA-WC films (a schematic figure may be helpful for the readers)? If it is, does this geometry difference affect the the corresponding structure and properties between different films?
5. The latest papers regarding HEA films, PVD technics and nanoindentation are suggested for discussion: ACS Appl. Mater. Interfaces 2021, 13, 55712−55725; Int. J. Plast. 142 (2021) 102997.
6. The XPS data is suggested to be given, and thus the determination of WC content based on XPS data needs to be addressed.
7. Line 112-116, if Cr has the highest sputter yield, then more Cr atoms will be ejected from CoCrFeNi target upon Ar bombardment, resulting a higher Cr content in the films; while, this seems to be contradictory with the authors statements.
8. Line 172, what is the “parallel lamellar structure” in the corresponding figure? It’s better to be labelled for clarification as the reviewer can not see anything. Similarly, in Fig. 3, the grain size can not be clearly observed and compared.

Author Response

In this manuscript, the authors report a new fabrication method of the CoCrFeNi high-entropy alloy films with addition of WC to improve their hardness. A serial of CoCrFeNi_1-x(WC)_x thin films with high homogeneity and tunable stoichiometry were synthesized by magnetron co-sputtering, through which the WC content of the mixed films was tuned by adjusting the deposition powers for both targets. The influence of the WC content on the structural features and mechanical properties were systematically investigated. The manuscript was well presented in the experimental part with an impressively detailed description of fabrication and characterization processes, and it contains sufficient amount of work. Taken overall, I would recommend the publication of this paper in the “coatings” after several concerns are properly addressed.

• The authors need to give a more comprehensive discussion about the motivation and novelty of this paper in the introduction section.

Added information in line 29 f.: “WC as a material system with a high intrinsic hardness appears to be a promising candidate for increasing the hardness of CoCrFeNi in a co-sputtering process. WC-CoCrFeNi mixtures fabricated by vacuum hot pressing sintering (VHPS) have already been reported. While a hardness increase of more than 200% is observed an elemental separation has been shown by EDX measurements and multiple phase formation was detected via XRD. To fabricate uniformly mixed thin films with few or single-phase structure and variable composition a co-sputtering approach is demonstrated in this work.”

• It appears that there are grain size differences between films prepared under various deposition power ratios. In Fig. 5, their hardness values were analyzed as a function of the WC content, without mentioning grain size dependence. Please take into account the grain size effect on harness.

Quote from line 177 f.: “On samples PR0.5 - PR4 no prominent features can be resolved, even with higher resolution. For PR8 a fine-grained structure is observed with an averaged grain size of () nm.”

With this we want to point out that samples PR0.5 to PR4 do not show morphological features suitable to be investigated in a sense of “grains” even for higher magnification in the SEM. Therefore, it is omitted to take a grain size effect into account since it seems not reasonable when 4 out of 6 samples compared do not show the necessary features.

• It has been reported that the substrate temperature also had a notable influence on the structure and mechanical property of high-entropy alloy thin films. What’s the substrate temperature in the present deposition processes?

Added information in Line 64: “… with respect to the surface normal and kept at room temperature during the deposition.”

• What does “normal deposition geometry” (line 65) refer to when preparing the unmixed constituents? Is it different from the geometry for the HEA-WC films (a schematic figure may be helpful for the readers)? If it is, does this geometry difference affect the the corresponding structure and properties between different films?

Added information in Line 72: “… respective target facing the substrate surface parallel so that a deposition with normal incidence is conducted.”

• The latest papers regarding HEA films, PVD technics and nanoindentation are suggested for discussion: ACS Appl. Mater. Interfaces 2021, 13, 55712−55725; Int. J. Plast. 142 (2021) 102997.

Thank you for giving hint to these recent publications. We read them with interest and decided to cite, see Line 35 f. “Recently, magnetron sputtering has been demonstrated to be a promising technique to form HEA thin films of homogeneous thickness and element distribution [16,17].”

• The XPS data is suggested to be given, and thus the determination of WC content based on XPS data needs to be addressed.

Showing the XPS data in the first submission has been omitted due to the amount of data. For full investigation of all samples, it would be necessary to provide the core levels of Co, Cr, Fe, Ni, W and C for all six mixed thin films (36 spectra) + 6 spectra for the pure HEA and WC references. To keep the manuscript in a short and well-arranged state we decided to include the availability of these data within the “Data Availability Statement” (Line 267). For the calculation of the elemental content from XPS measurements we refer to [16] (Line 105 f.).

• Line 112-116, if Cr has the highest sputter yield, then more Cr atoms will be ejected from CoCrFeNi target upon Ar bombardment, resulting a higher Cr content in the films; while this seems to be contradictory with the authors statements.

The statement in Line 119-125 (now) refers to the Ar+ bombardment step which was conducted prior to the XPS analysis in UHV. Judging from the sputter yield calculator we have cited, Cr has the highest value among the HEA elements (Co, Cr, Fe, Ni). While this could lead to an increased deposition of Cr during the fabrication of the thin films, this will also reduce the Cr content to the highest amount during the cleaning step. Therefore, we assume, that the reduced Cr content detected in XPS is related to the sputter yield. Using the less surface sensitive method EDX, we see no relevant difference in the Co, Cr, Fe and Ni content, which is evidence, that the Cr depletion is only present in the surface region to which the XPS method is limited.

• Line 172, what is the “parallel lamellar structure” in the corresponding figure? It’s better to be labelled for clarification as the reviewer can not see anything. Similarly, in Fig. 3, the grain size can not be clearly observed and compared.

Added additional image of PR8 with higher magnification to supplementary material. The lamellar structure should be clearly visible there.

As discussed before, for Pr0.5 to PR4 it is difficult to talk about a grain structure at all since the morphology of those samples is extremely flat and featureless. Even in a very small field of view (high magnification) discrete grains cannot be identified. Therefore, we find a scientific discussion based on grain size and related effects not applicable.

Author Response File: Author Response.pdf

Reviewer 3 Report

Hardness Enhancement in CoCrFeNi1-x(WC)x High-Entropy Alloy Thin Films Synthesised by Magnetron co-Sputtering

 

Authors: Holger Schwarz, Thomas Uhlig, Thomas Lindner, Thomas Lampke, Guntram Wagner and Thomas Seyller

 

The paper contains results of research on synthesis of CoCrFeNi layers with addition of WC using magnetron co-sputtering technique. The aim of the work was investigation on the influence of addition of WC on hardness of deposited layers of CoCrFeNi.

The paper is interesting. However, there is no information (examples) on their potential use. I also have the following questions:

1. Have you tested the adhesion of deposited layers to the substrate? What can you say about it?
2. What about the porosity of the layers?
3. Have you observed atom segregation at the grain boundaries?
4. What was the grain size of the metallic matrix? What was the grain size of tungsten carbide?
5. There seems to be a slight difference between the composition of the layer and the target. Is it possible to obtain the same composition of the layer and the target (when, probably, different components have different evaporation temperatures)? How this can be achieved?

Author Response

The paper contains results of research on synthesis of CoCrFeNi layers with addition of WC using magnetron co-sputtering technique. The aim of the work was investigation on the influence of addition of WC on hardness of deposited layers of CoCrFeNi. The paper is interesting. However, there is no information (examples) on their potential use. I also have the following questions:

Regarding the potential use, we assume that many surface coatings with requirements of a certain hardness variability could be addressed by the alloy system we demonstrated in this work. The tuneabilty of WC content in the co-sputtering process provides the necessary tool. Nevertheless, we are aware that the sputtering process itself might not be suitable for every application and we need to point out that our work should be considered as basic research with no strong focus on certain applications.

• Have you tested the adhesion of deposited layers to the substrate? What can you say about it?

Adhesion behaviour has not been investigated. We can only comment that none of the discussed samples shows indication for a dissolution of the sputtered thin film and the substrate (in the form of cracks, wrinkles, local stripping etc.).

• What about the porosity of the layers?

As described in the SEM section we are not able to see discrete features like grains, pores etc. for samples PR0.5 to PR 4. For PR8 and PR20 we see increasing grain size with reduced WC content / increased HEA percentage. However, no statement about the trend of porosity can be made from the investigations we made.

• Have you observed atom segregation at the grain boundaries?

Judging from the EDX analysis we concluded that the atoms of each element are homogeneously distributed in the layers and therein in the grains. No trace of segregation can be noted. As mentioned before and in the manuscript (line 177 f.) we do not see distinguishable grains in samples PR0.5 to PR04 even for higher magnifications. For the other samples increasing grain size is denoted but no change in the elemental distribution was observed.

• What was the grain size of the metallic matrix? What was the grain size of tungsten carbide?

For the mixed thin films, no distinction between CoCrFeNi and WC can be made within the grains observed for PR8 and PR20 and we find that all six elements are evenly distributed in all samples. Therefore, it is not possible to comment on the individual grain size of the separated materials. For the CoCrFeNi target used an analysis of the grain size in the target and of sputtered thin films is given in [16]. An investigation of the WC target regarding initial grain size has not been conducted.

• There seems to be a slight difference between the composition of the layer and the target. Is it possible to obtain the same composition of the layer and the target (when, probably, different components have different evaporation temperatures)? How this can be achieved?

The composition of the targets has not been measured in this work. For the WC target we must stick to the manufacturers statement of a 1:1 stoichiometry of tungsten and carbon. The composition of the sintered CoCrFeNi target has been investigated in [16] (therein called ‘Alloytarget’). There we found that XPS also shows a Cr depletion for sputtered thin films to ca. 19% after an Ar+ bombardment step in UHV while Ni, Fe and Co values are between 23 – 30 %. The bulk stoichiometry has been measured using X-ray fluorescence to be approx. 25% for each element. Concluding from this we think that the HEA content of the mixed films shown in our manuscript resembles that of the sputtering target according to XPS (surface) and EDX (bulk) measurements.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I didn't see any supplementary document showing the image of PR8

Author Response

We included the supplementary data in the .RaR-file uploaded after the first revision. Please find it in the attachements of this reply as well.

Author Response File: Author Response.pdf

Reviewer 3 Report

I think that the paper can be accepted now.

Author Response

Thank you very much.