Cyclic Oxidation Properties of the Nanocrystalline AlCrFeCoNi High-Entropy Alloy Coatings Applied by the Atmospheric Plasma Spraying Technique
Round 1
Reviewer 1 Report
The article deals with the microcrystalline and near nanocrystalline AlCrFeCoNi high-entropy alloy coatings. These coatings were applied on Inconel 718, using air plasma spraying proces. The research is focused on comparing these coatings.
In my opinion, the strong point is the use of selected methods for experimental work and thus the results achieved. These results then provide a number of interesting information about the properties of the investigated coatings, their oxidative behavior under the air atmosphere for defined cycles. There are useful information from XRD and XPS measurements.
The weaknesses are: unclear interpretation of the microhardness values of the individual coatings in Table 2. First of all, the first line does not show the units of quantities. They are listed in the table description, which is good but confusing in the table. Second, measuring the microhardness of a nanoparticle coating is misleading. The interpretation of this value is then unclear. In the case of FIG. 1, the representation of the particles is not entirely clear. Figures 1a, 1b are shown at different scales. Both figures show particles in the size of µm and hundreds of nm, respectively. There is no obvious difference in terms of morphology.
igure 3 shows a graph of atomic absorption spectroscopy. The wt% values for the individual elements should also show the standard deviation. Then the comparison of the coating elements is clear. Next, line 297 shows 11000 ° C, which is perhaps a typo that needs to be corrected. Table 3 at point B has shifted values wt% for elements Ni and O, it is also necessary to correct.
Furthermore, the text contains several formal errors. They are:
Line 91 and 167: the unit (h) for the value at the end of the line is on the next line.
Line 319: again the unit (° C) for the value at the end of the line is on the next line, the value 100 does not correspond to the previous text.
Line 371: unit (° C) is missing at value 1100.
References: for each source of literature there is a 2x number under which the literature is cited in the text. It is customary for the cited literature to begin with the author's name and not again with the citation number.
I recommend the mentioned weaknesses, especially the correction of formal errors, to supplement and clarify the manuscript.
On the other hand, I positively evaluate the last chapter, Conclusion. In the end, the results achieved are briefly described. At the end of it, the area of application use of the results is given. I think that its length and content is sufficient for the manuscript.
Author Response
Dear Reviewer,
Please see the attached file.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript coatings-1624227 Cyclic oxidation properties of the near nanocrystalline AlCrFeCoNi high-entropy alloy coatings applied by atmospheric plasma spraying technique, gives an experimental approach to evaluate high-temperature oxidation behaviour of the microcrystalline and nanocrystalline AlCrFeCoNi high entropy alloys (HEA) layers to improve the performance and stability of gas turbines and jet engine elements. The AlCrFeCoNi HEA feedstock powders were synthesized using the high-energy ball-milling method by 5 and 25 hours. The powders sizes and structural features and composition were analyzed by SEM, BF TEM and AAS techniques. The layers were deposited by air plasma spraying technique on the Inconel 718. In the article, the authors study the microstructural characteristics (FE-SEM and EDS), phase composition (XPS, XRD) of the micro- and nanocrystalline layers and oxide scale formed during cyclic oxidation (50 cycles). Both types of layers showed high homogenity and lamellar structure consisting of structural porosity and major BCC (B2) phase, and minor FCC (A1) phase. The authors confirmed that the nanocrystalline AlCrFeCoNi HEA layer had an outstanding protective coating against cyclic oxidation at 1100 ºC after 50 cycles in compare to microcrystalline HEA layer. Therefore, such nanocrystalline layer may be definitely applied against high-temperature corrosion in the hot section of gas turbine and jet engines. Generally, the article is well written; however, sometimes, it is hard to follow. The Authors should proofread the article and also pay attention on self citations (12 self citations from 30 references is too much). Moreover, before the publishing process starts, the Authors should also improve some editorial aspects to present their results more clearly. Some of the editorial comments I have highlighted below:
- Keywords should be added.
- Have the Authors realised other different methods of the surface activation process of the substrates before the APS process. How would it affect the adhesion and interface of the layers?
- Did the authors study the adhesive properties and stresses of the layers and interface ? Did the authors try to estimate the stress state of such substrate/layer system before and after oxidation ?
- It would be better for the Readers to show the figure right after it is mentioned not after the whole text considering the figure (Figure 7,8).
- Please proofread and correct the references according to Journal’s Guide for the Authors.
- Line 16 please correct the temperature to 1100 C instead of 110 C
- Line 160 Authors realized the cyclic oxidation at 1100 C, so please correct 950 C in air
I have no more comments that I would like to point out. Generally, the article requires editorial refinement and proofreading at some points. Nevertheless, the manuscript after corrections should be of great interest to readers from the HEA alloys and layers processing field. The issues presented in the article are essential and suitable for publication in the Journal Coatings. I recommend the paper for publication after minor revision.
Author Response
Dear Reviewer,
Please see the attached file.
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper is presenting a lot of “shaky” results, which are contradictory to their own claims (particularly see my comment question 12). The measurement techniques (size by SEM and weight by scale) are so rough…DLS (for size) and TGA (for weight) are already mature techniques for delicate analysis, but the authors seem to seek a very primitive method. When it comes to phase analysis with XRD and XPS, it was a true disaster…
I should frankly say that acceptance of this paper would really lower the standard and reputation for this journal. I would recommend a rejection and prefer not to review it any more.
- Page2 line 93: What does “-75 to +15 pm” mean? A bit confusing
- Page 3 line 100, better use a table.
- Line 152, Microhardness should have a proper unit.
- Why the cyclic oxidation process is chosen? Which application scenario would require the consideration of specific “cyclic” oxidation? Why 1h oxidation in each cycle is enough? This experiment design is not clear.
- What is the original particle size for Al, Cr, Fe, Co, and Ni individually?
- Figure 1 is TOTALLY NOT scientific! Please use Dynamic Light Scanning, etc. to provide a more detailed size analysis…Otherwise, this manuscript is wrong from the data at beginning…
- For section 3.2, the coating is only several hundreds micron deep. But the hard ness is ~500HV. Then, what is the microhardness loading force and time used? You need to provide the indentation mark on the surface.
If the mark is too small, it’s not accurate for hardness; If it’s too big, it may fail the microhardness test requirements, given the coating thickness.
- Figure 3, why Fe and Ni wt.% is higher, when O is more oxidized? They will not be oxidized during milling? What is the reason?
- You said in the Experimental part that XRD is for 20-80 degree…Why your Figure 5 and Figure 6have different scales? Your data are so random…
- Figure 7: All the peak split is WRONG…For Figure 7b, C1s, clearly it should be fitted into 2 peaks… (I do not understand why it is C1s1? What does the second “1” mean?)
- Figure 9 and Table 3, instead of point scanning, please give the cross-section line scanning for element change. (See the following literature and follow what they did…)
[1] Jin, K., Pan, S., Wang, T. and Zhang, Z., 2021. Non-negligible corrosion process in a novel sulfur-based energy storage system. Journal of Power Sources, 490, p.229529.
- One major flaw in the manuscript:
Al is oxidized, and if it’s dominant (see Figure 6), it could NOT be parabolic oxidation curve (there is even no fitting to prove if it’s parabolic…)…See the following literature for the Al oxidation discussion:
[2] Pan, S., Yao, G., Guan, Z., Yu, N., Sokoluk, M. and Li, X., 2020. Kinetics and dynamics of surface thermal oxidation in Al-ZrB2 nanocomposites. Corrosion Science, 176, p.108890.
Second, based on the random XPS peak analysis, the oxidation analysis is totally unbelievable. In the XRD, we already show that Al is in Al2O3 and (Ni, Co, Cr) Al2O4. How can you only have one peak for Al2p4???
These 2 points are against the general physics and chemistry laws…
Author Response
Dear Reviewer,
Please see the attached file.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
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