Erosion Performance of Atmospheric Plasma Sprayed Thermal Barrier Coatings with Diverse Porosity Levels

Round 1

Reviewer 1 Report

Dear Authors,

I have read your paper and I have some suggestions/comments, which may improve your work.

Genreal remarks:

• Please check the style of your work, e.g., references, tables. It is different than marked in template.
• You have presented 48 references, only nine have been published in ast three years. I propose to add some more. Science made big step forward in marked period of time, and there are many references showing scientific background for your work.

Introduction:

• Lines 38 and 39 - you showed inproper style of citations "[10-11]" and "[12] [13]". Should be [10,11] and [12,13]. The same in the rest of your text. "-" should be used only for citing more than two positions in one bracket.
• I propose to extend Introduction by newly reported (last three years) information.

Materials and Methods:

• Line 89 - you should be consequent in presenting abbreviations. Like earier in your work, it shoud be "homogenized oven spherodised powder (HOSP)".
• There is lack of information about used powders. Please add some information about their chemica compositions, properties, etc..
• What does it mean " carbon steel buttons" - please mark the grade of steel.
• Line 131 - change "Table I" to "Table 1" as it was numbered earier. The same in the next page.

Results and Discussion:

• I propose to move Fig. 2 after the first paragraph in this section. Figuer should be firstly mentioned in the text, than it should be presented.
• Fig. 3 - the scale bars and their white bacground look much different. Please use same size. The same comment to the rest of the text.
• In my opinion, obtained results should be more discussed with the other scientific papers. You should mark the novelty of your work with comparison with other scientists. In many places (subsections 3.1-3.3.) work is similar to the technical raport, showing results without scientific discussion.
• The subsections from 3.4. to 3.7. have been written very well, results are clearly presented and supported by scientific discussion.

Conclusions:

This part is clear. However, I propos to mark the most important concusions in points.

Best regards,
Reviewer

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Review of the article:

Erosion  performance  of atmospheric  plasma  sprayed thermal barrier coatings with diverse porosity levels

The article concerns experimental research on Erosion performance of atmospheric plasma sprayed thermal barrier coatings with various porosity levels. The authors made a number of well described samples. The parameters are given in the table in a clear way. In this work we can find a number of structures of the coatings and hardness and crack analyses.

The results show that coatings behave differently than e.g. steel during erosive wear with the jet directed at 90 and 30 degrees. In my opinion, the authors rightly conclude that the porosity of the coating has an influence on it. The conclusions are formulated in a concise and informative manner.

I think that in the article it would be worth to add a scheme or a photo of the stand to test the impact of the angle of incidence of the abrasive jet on erosive rate.

The article contains 48 literature items that have been used in the text.

I don't feel qualified to judge about the English language and style.

There are several editorial errors that should be removed:

1. Figures 2 and 12- the font used in the legend caption is too big, I would recommend to decreasing if possible.
2. The font size is different in each figure. I recommend to use the same size.
3. Figures 10, 12 and 14 - dot in the wrong place (Figure.10 instead of Figure 10.)
4. In line 98 - I recommend to use a space before the units.
5. Lines 440, 441, 449, 455 and 460 we have f.g. “Figure.13(e) instead of Figure 13(e). I recommend delete dot.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Mahade et al. investigate the influence of the deposition conditions on the microstructure and mechanical properties of the yttria stabilized zirconia thermal barrier coatings (TBC). The microstructural peculiarities (before and after the erosion impact) are investigated via scanning electron microscopy (SEM). The research work contains several important observations, particularly,

(1) the increased feed rate (for example, in the L-DH sample) results into the increased amounts of defects such as partially melted particle inclusions in the sample matrix;

(2) a standoff distance may have influence on the level of porosity;

(3) the lower density of the coatings results into the increased amount of the “vertical” (60-90 degrees) defects;

(4) the erosion rate rapidly decreases with increase of the sample density;

(5) there is a possible correlation between the low angle erosion resistance and the degree of the splat bonding.

I find this work to be written in a very detailed manner and the information to be useful for the scientific community working on TBC. I would only recommend the authors to specify in which mode the SEM images were recorded (back scattered (BSE) or the secondary (SE) electron mode).

Author Response

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Reviewer 4 Report

An interesting research idea, however, no breakthrough results have been revealed.

1. Have you been tested with thermal conductivity? If so, by what method and what were the results obtained?
2. Were the phase composition of the powders tested, please provide the results? If not, do a phase analysis and compare.
3. Very large measurement errors were shown - please comment.
4. After completion, I recommend it for publication.

Author Response

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Author Response File: Author Response.pdf

Reviewer 5 Report

The paper “Erosion performance of atmospheric plasma sprayed thermal barrier coatings with diverse porosity levels”, by Satyapal Mahade et al., present an interesting discussion related to coatings with various porosities obtained via atmospheric plasma sprayed method. The paper is well written, and the information is clearly presented.

The paper is recommended for publication with minor revisions:

1. In section 2 is inserted a remark for particles velocity and particles temperature. Which particles? You refer to the plasma species or the resulted material? If is the resulted material, in which nucleation point you consider it “particle-like shape”?
2. Considering an input power of 100kW, the inserted powder will be melted inside the plasma discharge, and it will start to nucleate further in new morphologies, depending of course of the input parameters. What is the plasma temperature? How is influenced the new coating from the plasma heat, because the substrate is placed at 150 mm distance?
3. What gas type was used for the plasma?
4. In Figure 3, authors should insert in the image description, which is top view and which is cross-section view.
5. In Figure 4, authors should insert in the image description, which is top view and which is cross-section view.
6. In Figure 5, authors should insert in the image description, which is top view and which is cross-section view.
7. In Figure 6, authors should insert in the image description, which is top view and which is cross-section view.
8. Authors are asked to maintain the same size of the writing on the images (scale, etc), for all images in the paper.
9. In Figure 11, the error bars are high. Can we consider the result, even if the errors are high?
10. Line 421- authors are asked to re-formulate the title. In the present form it looks like a code, not a title.
11. Line 459- authors are asked to re-formulate the title. The same like above, in the present form it looks like a code, not a title.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors,

Thank you for detailed response and relevant improvements in manuscript.

I proposed to present conclusions in points. However, ofcourse, I give the liberty to authors to present their conclusions in this manner, as it is in the paper.

All my comments were ansered, and in my opinion the paper could be published in this state.

Congratulations for good paper.

Best regards,
Reviewer

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

If we do not know the phase composition of the powders used, we are not sure what influenced the formation of the coating thermal barriers. This is the fundamental knowledge one should have to interpret the obtained results. Commercial powders also need to be tested before being used for research.
It is obvious that: "There was an overall observed decrease in erosion resistance with increasing porosity content of the TBC coatings. This becomes particularly acute once the porosity exceeds 20%." - did the obtained results allow for a wider assessment of the observed phenomenon? Please comment on the summary.
When examining the microstructure, we must know what exactly it is composed of to draw appropriate conclusions about the processes taking place during the experiment.
What I miss in my work is to prove the relationship between the materials used in the experiment to the obtained results.

Author Response

Comments brought by the reviewer are fair. 

Extensive phase changes are known to occur during plasma spraying of YSZ powder particles that lead to reduction or elimination of monoclinic phase in the final deposited coatings. The co-author has reported this in an earlier publication:- 

[1] Curry et al - https://doi.org/10.1007/s11666-013-0014-9 

where the monoclinic content in the as sprayed coatings was between 3-5 wt% regardless of the starting powder phase content. Furthermore a recent study by [2] Sharma et al:- https://doi.org/10.1016/j.jeurceramsoc.2020.10.062indicates a similar result after plasma spraying. Sharma et al further concludes that it is the microstructure and porosity of the coatings that dominates their properties and that phase content has minimal influence. 

It is true that monoclinic phase may increase with isothermal or cyclic exposure at high temperatures and that the volumetric change associated with the change in phase may influence the durability of the coating. This however requires significant time at high temperature [1,2].

In an earlier work, the co-author has evaluated the phases present in the same high porosity (>20%) coatings that were studied here after furnace cyclic testing. See Curry et al - https://doi.org/10.3390/coatings10100957

In that case, the amount of monoclinic phase in the thermally cycled coatings was at or below the detection limit of the XRD equipment after 437-630 hours exposure. It was considered that as these coatings display marginal monoclinic content after thermal cycling and them being produced with colder coating conditions more likely to maintain monoclinic phase from the powder feedstock; a phase analysis for the as sprayed coatings may not yield further information

In this current work, the focus is on the as-sprayed coatings and their resistance to erosion damage due to varying porosity levels and microstructural features. As such an extensive phase analysis was not deemed essential at this stage, before any significant thermal exposure. It is something that would be considered for an erosion study after heat treatment or thermal cycling of coating samples. 

Moreover we must admit that we now have neither the samples or resources from this already completed study to enable such an analysis.

Regarding the second comment, previously published work that has looked at the influence of varying porosity content on erosion resistance in YSZ coatings is now several decades old and contains less process and coating information as presented here. The authors felt that this subject should be revisited in order to have baseline evaluation to which future coating types could be more easily compared.

Author Response File: Author Response.pdf