Design, Fabrication, and Characterization of New Materials Based on Zirconia Doped with Mixed Rare Earth Oxides: Review and First Experimental Results

Round 1

Reviewer 1 Report

In the present work entitled: “Design, fabrication and characterization of new materials based on Zirconia doped with Mixed Rare Earth Oxides: review and first experimental results” the Authors combined a literature review and experimental preliminary studies on REO doped ZrO₂. I found this paper very interesting, well written and well organized. The results themselves sounds interesting and promising. However during reading a minor comments/suggestions arises as follows:

1. Line 33: There should be subscript in “ZrO2”,
2. Line 35: please use “electron beam” instead of short “e-beam”,
3. Line 37: the same comment as No.1,
4. Lines 435-437: What happened with carbon foil? Did the Authors measured carbon content in sintered material?
5. Lines 447-448: Was MCrAlY coating manufactured by EB-PVD as well? It is not clear in this place,
6. Table 6: Please use SI units over the whole manuscript (MPa or Pa instead of torr),
7. Figure 5: first of all, letter marks pasted in images are broken (cut in half). Secondly, images 5 a – e covers elemental maps. Please correct.
8. Lines 570-571: I kindly disagree with the statement that: “It can be seen that the layers did not change their morphology being similar to the sample before heat treatment.” . One can clearly observe in figure 7 b and from corresponding map, that during heat treatment in nitrogen, substrate/bondcoat became oxidized (typical oxide scale morphology between substrate and ceramic coating and enrichment of Cr – most probably Cr₂O₃ oxide scale). Why it happened under nitrogen atmosphere? Please propose explanation.
9. Figure 7: letter marks pasted in images are broken (cut in half).
10. Lines 589-590: Please explain what was the criterion used to judge whether oxidation resistance of complete TBC was satisfactory or not. Please describe this in detail.
11. Figure 11: Something is wrong with figure 11: there is no caption for mentioned figure and image itself overlap (covers) part of the text, the most probably Conclusion.
12. References: There is at least one very important article missing in the references, namely article by R. Darolia, “Thermal barrier coatings technology: Critical review, progress update, remaining challenges and prospects”, International Materials Reviews 58(6):315-348, DOI:10.1179/1743280413Y.0000000019. Please consider citation of mentioned paper.

 

Despite my comments I found present article worth publishing. Therefore, considering all above mentioned comments I recommend to publish the paper after minor revision.

 

Author Response

Thank you very much for your kind help. Your valuable observations, corrections and completions proposed are very useful for improving the proposed paper. In particular I would like to thank you for the suggestion to add in the reference list the paper suggested by you, this will help us in our future work. Please find below answers to your review report.  All changes and completions are marked in green font.

Reviewer 1.

In the present work entitled: “Design, fabrication and characterization of new materials based on Zirconia doped with Mixed Rare Earth Oxides: review and first experimental results” the Authors combined a literature review and experimental preliminary studies on REO doped ZrO2. I found this paper very interesting, well written and well organized. The results themselves sounds interesting and promising. However during reading a minor comments/suggestions arises as follows:

1. Line 33: There should be subscript in “ZrO2”. Answer: corrected.
2. Line 35: please use “electron beam” instead of short “e-beam” Answer: corrected.
3. Line 37: the same comment as No.1, Answer: corrected
4. Lines 435-437: What happened with carbon foil? Did the Authors measured carbon content in sintered material? Answer: during experiment we did not observed any damaged of the C foil and we didn’t measure the C content in the sintered material.
5. Lines 447-448: Was MCrAlY coating manufactured by EB-PVD as well? It is not clear in this place, Answer: we added the following phrase: All successive layers were deposited by EB-PVD process.
6. Table 6: Please use SI units over the whole manuscript (MPa or Pa instead of torr), Answer: we modified the units from torr to Pa and recalculated the values.
7. Figure 5: first of all, letter marks pasted in images are broken (cut in half). Secondly, images 5 a – e covers elemental maps. Please correct. Answer: We corrected the figure. Additionally we corrected also XRD spectra from fig. 6 and 8 showing clearly the existing phases.
8. Lines 570-571: I kindly disagree with the statement that: “It can be seen that the layers did not change their morphology being similar to the sample before heat treatment.” . One can clearly observe in figure 7 b and from corresponding map, that during heat treatment in nitrogen, substrate/bondcoat became oxidized (typical oxide scale morphology between substrate and ceramic coating and enrichment of Cr – most probably Cr2O3 oxide scale). Why it happened under nitrogen atmosphere? Please propose explanation. Cooling of the sample was performed in air. Most probably, during cooling, the substrate/bondcoat became oxidized.

Answer: We fully agree with your observation and did the corresponding modifications, thank you. Here is the modified text: One can observe in figure 7 b and from the corresponding map (figure 7 d), that during heat treatment, most probably during cooling in air, the substrate/bondcoat became oxidized, showing typical oxide scale morphology between substrate and ceramic coating and enrichment of Cr, most probably with the formation of Cr₂O₃ oxide.

1. Figure 7: letter marks pasted in images are broken (cut in half). We corrected the figure.
2. Lines 589-590: Please explain what was the criterion used to judge whether oxidation resistance of complete TBC was satisfactory or not. Please describe this in detail. Answer: we introduce the following text: This test shows the number of heating and cooling cycles that a coated material may support without delamination of the coating layers. Comparison between thermal shock values of new coatings architecture with existing ones may be a method to assess their potential application in aeronautics.
3. Figure 11: Something is wrong with figure 11: there is no caption for mentioned figure and image itself overlap (covers) part of the text, the most probably Conclusion. We corrected the figure.
4. References: There is at least one very important article missing in the references, namely article by R. Darolia, “Thermal barrier coatings technology: Critical review, progress update, remaining challenges and prospects”, International Materials Reviews 58(6):315-348, DOI:10.1179/1743280413Y.0000000019. Please consider citation of mentioned paper. Answer: we introduce this as reference 1.

Reviewer 2 Report

This paper provides a very interesting summary of oxide materials for thermal barrier applications and puts forward an interesting approach of using monazite source materials rather than producing the insulating coating materials from the individual lanthanides. The paper is very well structured and provides adequate references although it lacks of some fundamental works to complete their summary and justify their approach. The approach of investigation is well designed and the experimental methods very adequate to allow a sufficient while not very deep interpretation of the results. I therefore recommend major revision. I am quite confident that addressing the following comments will substantially improve the overall quality of the paper and increase further the interest to the readers.

General comments: Please, replace everywhere “perowskite” by “perovskite”. Also check for some English language issues (like decimal points instead of decimal commas, legends in the figures, etc.)

Specific comments

Page 3, end of paragraph 1. The authors shall recall that in addition to the loss of phase stability of YSZ above 1200°C, the main critical issue is CMAS attack that lowers the thermal insulating power [e.g. Boissonnet et al. J. Europ. Ceram. Soc. 40 (2020) 2042-2049]. This is in fact one of the real justifications to develop new thermal barrier coating materials (and derived processes). The other justification is to obtain materials that with even lower thermal conductivities given the increased operating temperatures of the turbines.

Page 8, between “Sc2O3-20%mol CeO2 and the perovskites ad pyrochlores” the authors shall introduce the use Erbia-doped YSZ by EB-PVD that lower the thermal conductivity and stabilize the t’-phase (link with the above comment). In addition, these erbia-doped YSZ EB-PVD coatings develop thicker oxide layers upon oxidation that lower even further their thermal diffusivity (conductivity) [e.g. Boissonnet et al. Surf. Coating Technol. 389 (2020) 25566 and references therein].

Page 10, lune 334: remove square bracket ]

Page 16, lines 524-525. The authors provide extremely accurate values without any standard deviation, e.g. NiCrAl with 448,9 nm thickness. Also, is this nm or µm?

Figure 5: Cannot see scale bars adequately + remove spectra and provide semiquantitative element concentrations. Also, the labels and the X-ray maps are not properly shown. Please amend.

Figure 6 and Figure 8: It does not seem that the Kalpha2 was removed from the patterns. This may lead to potential misinterpretation. Can the authors please check again their results?

Page 17, lines 567/568: why was the heat treatment performed in nitrogen? Why not in air or in Ar or in vacuum? Therefore, Figure 7 shown internal nitridation? Oxidation? In the substrate. Why is this happening? How much can this affect the thermal conductivity of the system? Also, there is problem with the legends (please amend).

Page 18, roughness. I’d recommend that the authors indicated that the values 0.448 and 0.521 are VERY SIMILAR, thus proving that…Indeed, I do not think that there is a marked difference of values when considering the measurement technique employed.

Figure 10: can the authors please replace “pirometru” by “pyrometer”? I guess this is the meaning.

Page 19: what is the figure in the middle of the page? Please amend.

Author Response

Thank you very much for your kind help. Your valuable observations, corrections and completions proposed are very useful for improving the proposed paper. Thank you for the suggestions to complete the reference list with the 2 papers mentioned by you, this will help us in our future work. Please find below answers to your review report.  All changes and completions are marked in red font.

 

Comments and Suggestions for Authors

This paper provides a very interesting summary of oxide materials for thermal barrier applications and puts forward an interesting approach of using monazite source materials rather than producing the insulating coating materials from the individual lanthanides. The paper is very well structured and provides adequate references although it lacks of some fundamental works to complete their summary and justify their approach. The approach of investigation is well designed and the experimental methods very adequate to allow a sufficient while not very deep interpretation of the results. I therefore recommend major revision. I am quite confident that addressing the following comments will substantially improve the overall quality of the paper and increase further the interest to the readers.

General comments: Please, replace everywhere “perowskite” by “perovskite”. Also check for some English language issues (like decimal points instead of decimal commas, legends in the figures, etc.)

Answer: all these issues were corrected.

Specific comments

Page 3, end of paragraph 1. The authors shall recall that in addition to the loss of phase stability of YSZ above 1200°C, the main critical issue is CMAS attack that lowers the thermal insulating power [e.g. Boissonnet et al. J. Europ. Ceram. Soc. 40 (2020) 2042-2049]. This is in fact one of the real justifications to develop new thermal barrier coating materials (and derived processes). The other justification is to obtain materials that with even lower thermal conductivities given the increased operating temperatures of the turbines. Answer: we introduced the proposed reference and text.

Page 8, between “Sc2O3-20%mol CeO2 and the perovskites ad pyrochlores” the authors shall introduce the use Erbia-doped YSZ by EB-PVD that lower the thermal conductivity and stabilize the t’-phase (link with the above comment). In addition, these erbia-doped YSZ EB-PVD coatings develop thicker oxide layers upon oxidation that lower even further their thermal diffusivity (conductivity) [e.g. Boissonnet et al. Surf. Coating Technol. 389 (2020) 25566 and references therein]. Answer: we introduced the proposed reference

Page 10, line 334: remove square bracket ] Answer: corrected

Page 16, lines 524-525. The authors provide extremely accurate values without any standard deviation, e.g. NiCrAl with 448,9 nm thickness. Also, is this nm or µm?

Answer: We remade the measurements by checking 100 areas for each sample, averaging and computing standard deviation and we changed the text as follow: NiCrAlY bonding layer with 0.556 ± 0.072 µm thickness, YSZ with 1.984 ± 0.151 µm layer thickness, LZO layer thickness 4.245 ± 0.119 µm and GZO with a layer thickness of 4.618 ± 0.191 µm.

 

Figure 5: Cannot see scale bars adequately + remove spectra and provide semiquantitative element concentrations. Also, the labels and the X-ray maps are not properly shown. Please amend. Answer: Fig 5 was corrected

 

Figure 6 and Figure 8: It does not seem that the Kalpha2 was removed from the patterns. This may lead to potential misinterpretation. Can the authors please check again their results?

 

We checked the results and remade the graphics to be more clear. Kalpha2 was stripped before search and match in HighScore Plus 3.0.e software by using Rachinger method (W.A. RACHINGER, A correction for the α1 : α2 doublet in the measurement of widths of X-ray diffraction lines, J. Sci. Instrum., 25, (1948), 254 - 259.)

Answer: we corrected figures 6 and 8

 

Page 17, lines 567/568: why was the heat treatment performed in nitrogen? Why not in air or in Ar or in vacuum? Therefore, Figure 7 shown internal nitridation? Oxidation? In the substrate. Why is this happening? How much can this affect the thermal conductivity of the system? Also, there is problem with the legends (please amend).

 

 

Answer: Cooling of the sample was performed in air. Most probably, during cooling, the substrate/bondcoat became oxidized.. Here is the text proposed. One can observe in figure 7 b and from corresponding map, that during heat treatment, most probably during cooling in air, the substrate/bondcoat became oxidized, showing typical oxide scale morphology between substrate and ceramic coating and enrichment of Cr, most probably with the formation of Cr₂O₃ oxide.

 

Page 18, roughness. I’d recommend that the authors indicated that the values 0.448 and 0.521 are VERY SIMILAR, thus proving that…Indeed, I do not think that there is a marked difference of values when considering the measurement technique employed. Answer: we corrected the text as suggested:

From the roughness investigations results presented in fig. 9, a slight change in the mean surface roughness from 0.448 μm of the Nimonic 80A substrate to 0.521 μm for the coated Nimonic substrate may be observed. These values are very similar, thus proving that the coatings grow uniformly following the substrate morphology

 

Figure 10: can the authors please replace “pirometru” by “pyrometer”? I guess this is the meaning.

Answer: the word pyrometer was corrected

 

Page 19: what is the figure in the middle of the page? Please amend. Answer: we corrected it.

Round 2

Reviewer 2 Report

The authors have provided an adequate rebuttal to the queries and comments I raised. Yet, there are few issues that need to be amended as follows.

Page 8, line 264: write correctly “evaporation”.

Page 15, Table 8: PDF file instead of PDFL file

Page 16, line 527: The morphology of these powders are presented in fig. 4. Replace are by is.

 

FOR THE SECOND TIME !!!!! Figure 5: Cannot see scale bars adequately + remove spectra. Remove also the yellow frames at the back of the images

 

References

There is a problem with the double reference number and the references into brackets. This is indicative of a very poor detailed rebuttal. Such poor detail is also observed in the incorrect referencing of references 1, 19 and 42 that MUST BE AMENDED as follows:

1): it is Darolia R. instead of [R. Darolia

19) Boissonnet et al. J. Eur. Ceram. Soc. 40, 2020, 2042-2049. The authors are requested to write the names of all the authors and the style changes with respect all the other references.

42) Boissonnet et al. Surf. Coating Technol, 2020, 389 25566. The authors are requested to write the names of all the authors.

Author Response

Dear Reviewer,

Thank you very much for your kind help, observations, corrections and completions proposed Please find below answers to your review report.  All changes and completions are marked in blue font.

Page 8, line 264: write correctly “evaporation”. Answer: we corrected.

Page 15, Table 8: PDF file instead of PDFL file. Answer: we corrected.

Page 16, line 527: The morphology of these powders are presented in fig. 4. Replace are by is.

Answer: we replaced.

 FOR THE SECOND TIME !!!!! Figure 5: Cannot see scale bars adequately + remove spectra. Remove also the yellow frames at the back of the images

Answer: We modify fig. 5 and the scale bars in white colour may be seen for all SEM pictures.

Also, in fig. 4 we removed the EDS spectra from the background and modify the figure capture accordingly. As a consequence, in the text line 528-529 we changed as follow: EDS analysis (not shown) confirms the presence of REOs doping elements uniformly distributed in the aggregates.

 References

There is a problem with the double reference number and the references into brackets. This is indicative of a very poor detailed rebuttal. Such poor detail is also observed in the incorrect referencing of references 1, 19 and 42 that MUST BE AMENDED as follows:

1): it is Darolia R. instead of [R. Darolia. Answer: we corrected.

19) Boissonnet et al. J. Eur. Ceram. Soc. 40, 2020, 2042-2049. The authors are requested to write the names of all the authors and the style changes with respect all the other references. Answer: we corrected.

42) Boissonnet et al. Surf. Coating Technol, 2020, 389 25566. The authors are requested to write the names of all the authors. Answer: we corrected.

 

Round 3

Reviewer 2 Report

Thank you for the rebuttal. Everything is now fine. Just delete "Elsevier" in Ref. 19. Thank you.