Optimization of Yb₂O₃-Gd₂O₃-Y₂O₃ Co-Doped ZrO₂ Agglomerated and Calcined Powders for Air Plasma Spraying

Round 1

Reviewer 1 Report

The authors have prepared Yb2O3-Gd2O3-Y2O3 co-doped ZrO2 powders for plasma spraying thermal barrier coatings with a spray drying process followed by calcination. They evaluated the effect of calcining temperature on the extent of chemical mixing and flowability of the powder. In particular, they investigated the powder morphology, microstructure and chemical mixing using electron microscopy techniques and x-ray diffraction. They identified that Yb2O3 required higher temperatures to fully dissolve. However, the higher temperatures lead to some necking between powder and change in powder morphology that was not desirable for flowability.

 

This work produced very nice powder feedstock that could be used for plasma spraying thermal barrier coatings. The characterization of the powder is detailed and useful for manufacturing, although the study is not fully complete without correlating with the plasma spray coatings themselves. The manuscript would be strengthened with some clarifications about their methods, and the authors should address a few minor issues before the manuscript can be published.

 

1. There are two routes proposed for preparing the powder. Has route #1 (Fig. 1) ever been attempted for this chemical system? If so, please provide a citation. Otherwise, it is not clear why it is included in this discussion as it was not used here.
2. Why is Route #2 in the figure twice? Recommend removing from part A of the figure for clarity.
3. Why was this composition used? (Citations used different amounts of dopants.)
4. Please provide information about the raw oxide powders, including size, purity, manufacturers, etc.
5. Spray drying has a lot of other parameters - were these optimized to achieve the powders here? Or were the parameters determined from another paper?
6. What equipment was used for spray drying?
7. What type of furnace was used for calcining?
8. How was the powder packed in the furnace? This seems like it would effect the extent of sintering.
9. Why was 3 hours chosen for each temperature? Why was only one temperature attempted to fully optimize the calcining process?
10. Need to introduce c-ZrO2, t-ZrO2, and m-ZrO2 earlier in the document so readers know what c, t, and m stand for.
11. Several times the authors discuss the flowability (for example, the flowability of 1500C calcined powders on page 10 line 338), but the data is not reported for all calcining temperatures. Please add a table that shows all of the flowability data for each calcining temperature to better support arguments.
12. The authors provided the apparent density and mean particle size used for plasma spray (Page 9, Line 306). Similarly, please add the fluidity values that are desirable for plasma spray with citations.
13. The calcined powder was first sieved to look at flowability. Did this result in a large loss of powder or for specific calcination temperatures? Considering that this paper is for a manufacturing audience, the efficiency of the powder production should be considered.
14. In the results section for the characterization of the agglomerated powders on page 4, there is discussion of how water affects the internal structure. The authors need a citation or provide supporting evidence to support their claim.
15. In the results section for the characterization of calcined powders at 1100°C on page 6, there is discussion about how the dark phase can still be observed due to "infiltration of liquid resin into the powders via the porous surface during sample preparation." I believe the authors are referring to the resin for mounting the specimen for polishing. The resin they are referring to should be clarified in the text.
16. The authors include the size distribution of the powder calcined at 1400°C but none of the other calcining conditions. Recommend including for all of the powder to interpret flowability data.
17. Sintering lead to a reduction in the powder size. Is it possible to just change the spray drying process to achieve larger initial particle sizes that when sintered do not affect the flowability?
18. It is unclear when the authors describe non-uniform Yb and Gd distribution in the EDS maps whether the authors are indicating second phases that haven't dissolved or whether they are dissolved but not fully mixed constituents. Please clarify in the text.
19. The first sentence in the conclusions is misleading. Only one synthesis route was "explored," as only one method was reported in the results. This should be modified.
20. The authors discuss the grain size throughout the results but it is not discussed in terms of the ability for plasma spray. Can the authors elaborate on how grain size should be optimized for plasma spray feedstock?
21. In the conclusions, 1100°C was considered the threshold temperature to dissolve, but lower temperatures were not tested. Therefore, the "threshold" could not be determined in this study.
22. The authors only look at the effect of calcining temperature but there are many other factors that could be optimized, including time at temperature, spray drying parameters, etc. What do the authors think is needed to complete this study?

Author Response

Please see the attachment. Thank you very much for reviewing my manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

Authors have attempted to demonstrate an optimized synthesis route for Yb2O3-Gd2O3-Y2O3 co-doped ZrO2 (YGYZ) feedstock by investigating feedstock microstructure, flowability, apparent density, chemical homogeneity and composition.

My major objection to this work is that the manuscript merely presents the powder study and there are no coatings deposited and investigated. Therefore, I think the manuscript does not meet the standards and scope of Coatings journal. However, if authors can show the coating preparation from at least the optimized feedstock and demonstrate that these are promising feedstocks for plasma spraying then it may be considered for publication.  Apart from this I have some more suggestions which authors should work on before sending it forward for publication.

1. Indeed, extensive researches have been conducted on YGYZ. However, most of these 61 studies are limited to its properties investigations (phase stability, thermos-physical properties, corrosion resistance) and there is no open report focusing on the preparation and 63 optimization of YGYZ powders for thermal spraying, especially for air plasma spraying 64 (APS) which is the most widely used technique for coating preparation

Authors should provide about 8-10 references and clearly summarize them in terms of what has been done when it comes to depositing TBCs using YGYZ. Now (as you see from the above paragraph) authors are only saying extensive studies have been conducted without any support from literature.

2. Figure 1. Two routes for the synthesis of 10YGYZ feedstock (a) and experimental procedure 79 of this work (b)

It seems that authors are trying to explain the procedures that are typically employed in literature for producing these types of feedstock (Route #1 and Route #2). If this is the case, then please clarify this in the text as well as in the figure and in the figure caption. Also, please provide the appropriate references.  

3. The presentation of results from section 3.1 onwards are chaotic and disorganized. Authors should compare all the results (e.g., shape, size, phases, density, flowability etc.) of 10YGYZ agglomerated powders as well as 10YGYZ calcined powders at all temperatures in one or two tables and then discuss those results around those tables.

Author Response

Please see the attachment. Thank you very much for reviewing my manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

The article “Optimization of Yb₂O₃-Gd₂O₃-Y₂O₃ co-doped ZrO₂ agglomerated and calcined powders for plasma sprayable feedstock” shows the results of research in very relevant field of thermal barrier coatings development. Detailed studies of the possibility of obtaining powders for plasma sprayable feedstock in the system Yb₂O₃–Gd₂O₃–Y₂O₃–ZrO₂ have been performed. However some changes are needed to improve the manuscript.

• The main scientific significance of the work is to identify the possibility of incorporating Gd and Yb ions together with Y ions into the structure of ZrO₂ leading to the stabilization of the cubic ZrO₂ and the formation of a single-phase material. The manuscript provides a comparative analysis of ionic radii and atomic masses of Gd and Yb with Y but not with Zr. It may be necessary to provide a table with data for Gd, Yb, Y and Zr to present a more detailed analysis in this field.
• The Figure 4 does not contain EDX results as well as mapping results of Zr, Y, Gd and Yb elements distribution.
• A large elongated aggregate is observed in the Figure 5d. There is no explanation with what this is connected.
• The Authors only indirectly mentions the relationship between the firing temperature of powders and changes in their properties such as fluidity, apparent density, etc. But these properties are not indicated for all temperatures. It is necessary to present these values in a table or on a graph so that there is a clear dependence on temperature.
• The possibility of obtaining of powder with optimal properties at a temperature of 1300 ° C, when increasing the holding time does not mention in the “Conclusion” section.

Author Response

Please see the attachment. Thank you very much for reviewing my manuscript.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have addressed the concerns of the reviewers. The addition of the plasma spray coatings strengthened the paper.