Crack Healing in Mullite-Based EBC during Thermal Shock Cycle

Round 1

Reviewer 1 Report

Thank you very much for your interesting paper. Since crack healing phenomena of various types of EBCs during thermal shock cycle test were investigated in detail, this paper will be useful for researchers and engineers in the field of gas turbine. However, more information is required for understanding your research, I would like to ask you to revise your paper with taking account of the following comments.

Please add the schematic diagram of cross-section of the test sample. If possible, please add the layers (consist of the EBC) with components and thickness to the illustration of the sample in Figure 3 Please add the captions of “Top coat”, Bond coat” and “Substrate” to Figure 4. Cross-sectional crack growth and crack healing during the thermal shock cycle is also important. So, please add the pictures of cross-sections of the EBCs to Figures 6 and 7. (Si bond coat seems to be brittle. So, cross-sectional information of the sample after 5000 cycles should be added in order to provide us the information on the Si bond coat.)

That is all.

I would very much appreciate your contribution.

Author Response

We appreciate reviewer’s comments and suggestions.

We hereby submit a revised version. We revised our paper according to reviewer’s suggestion.

Reply to Review #1

Please add the schematic diagram of cross-section of the test sample. If
possible, please add the layers (consist of the EBC) with components and
thickness to the illustration of the sample in Figure 3 Please add the
captions of “Top coat”, Bond coat” and “Substrate” to Figure 4.

We added the notation in Fig. 3(b) as “Top coat”, Bond coat” and “Substrate” according to your helpful suggestion.  

Cross-sectional crack growth and crack healing during the thermal shock
cycle is also important. So, please add the pictures of cross-sections
of the EBCs to Figures 6 and 7. (Si bond coat seems to be brittle. So,
cross-sectional information of the sample after 5000 cycles should be
added in order to provide us the information on the Si bond coat.)

Thank you for your important suggestion and we added as Fig. 8 in the new manuscript.

Figure 8. SEM micrographs showing the microstructure on the cross-section of the EBC after thermal shock test at 1350℃:  (a) mullite, 5000cycles; (b) mullite+Yb₂SiO₅, 5000cycles ; and (c) Y₂SiO₅, 5000cycles.

Figure 8 shows the cracks on the cross-section of the mullite, mullite + Yb₂SiO₅, and Y₂SiO₅ after 5000 thermal shock cycles, as observed by SEM. Each arrow in the mullite, mullite + Yb₂SiO₅ indicate crack healed region. On the other hand, each arrow in the Y₂SiO₅ indicate cracked region. Only cracks are observed after 5000 cycles of thermal shock in the case of Y₂SiO₅ (Fig. 8(c). By contrast, after 5000 thermal shock cycles, new phases are formed to fill the gaps and cover the cracks, and this result is more pronounced in the mullite + Yb₂SiO_5.

The cracks on the cross-section as well as surface were covered with a new phase in the case of the mullite-based EBC.

We really hope our revision of paper be more appropriate for your journal.

Thank you. 

Reviewer 2 Report

Article is very interesting and describes new types of coatings. Some changes in article are unfortunetly required:

fig. 1 is not necessary - the APS process and devices are well-known more resutls from SEM should be presented not the photos from light microscope- it could increase the quality of article

In my opinion after small modification article should be published in journal "Coatings"

Author Response

We appreciate reviewer’s comments and suggestions.

We hereby submit a revised version. We revised our paper according to reviewer’s suggestion.

Reply to Review #2

Fig. 1 is not necessary - the APS process and devices are well-known more results from SEM should be presented not the photos from light microscope- it could increase the quality of article

We deleted Fig. 1 according to your suggestion. Than you.

Reviewer 3 Report

The authors have presented a study on the stability of microstructure and crack propagation within three APS deposited EBCs, deposited on SiC_f/SiC substrate, upon thermal shock loading. They concluded that among three tested EBCs, the mullite + Yb2SiO5 EBC exhibits best durability to thermal shock loading. The reviewer feels that this is an important topic and is highly relevant to environmental protection of high temperature turbine blades and compressor blades made of lightweight and high temperature resistant ceramic matrix composites. However, the reviewer feels that this manuscript should revised thoroughly for English, and the reviewers will address the following questions and suggestions before publication of the manuscript.

Comment 1:

Page 9, Lines 256-257

“We reported in a previous paper that the addition of Yb2SiO5 to the mullite results in densification of the mullite sintered body because of the formation of a eutectic molten phase [10]”.

Here Please mention the eutectic reaction along with the eutectic composition and temperature after referring to the appropriate phase diagram. How does the experimental thermal cycling temperature correspond to the eutectic temperature?

Comment 2:

Page 12, lines 299-301

“In the case of mullite, the residual displacement related to the hardness of the material was slightly increased after thermal shock, and the tangential slope of the stress–strain at unloading, which is related to the modulus of elasticity, increased after thermal shock [10,25,26,30]. Thus, the hardness and elastic modulus of the sample decreased by 5000 thermal shock cycles.”

This paragraph is confusing. In one sentence the authors claim that the modulus decreases after 5000 thermal shock cycles. On the other hand, in the previous sentence the authors mention that the tangential slope of the stress-strain at unloading increases! This contradiction must be addressed. The reduction in elastic modulus after 5000 thermal shock cycles suggest that the slope of the P-h response of the thermally cycled specimen at unloading will decrease from the P-h response at unloading of untested material.

Author Response

We appreciate reviewer’s comments and suggestions.

We hereby submit a revised version. We revised our paper according to reviewer’s suggestion.

Page 9, Lines 256-257

“We reported in a previous paper that the addition of Yb2SiO5 to the mullite results in densification of the mullite sintered body because of the formation of a eutectic molten phase [10]”.

Please mention the eutectic reaction along with the eutectic composition and temperature after referring to the appropriate phase diagram. How does the experimental thermal cycling temperature correspond to the eutectic temperature?

Thank you for your important indication. We revised it in the new manuscript as follows;

The coexistence of three phase L(liquid phase) + D(Yb₂Si₂O₇) + M(Al₆Si₂O₁₃) is expected at the temperature range of 1500~1505^oC in the phase diagram of Yb₂Si₂O₇-Al₆Si₂O₁₃ system. Although the test temperature in this study was lower than 1500^oC, repeated thermal cycling testing at 1350^oC is believed to result in increasing temperature.

Page 12, lines 299-301

“In the case of mullite, the residual displacement related to the hardness of the material was slightly increased after thermal shock, and the tangential slope of the stress–strain at unloading, which is related to the modulus of elasticity, increased after thermal shock [10,25,26,30]. Thus, the hardness and elastic modulus of the sample decreased by 5000 thermal shock cycles.”

 This paragraph is confusing. In one sentence the authors claim that the modulus decreases after 5000 thermal shock cycles. On the other hand, in the previous sentence the authors mention that the tangential slope of the stress-strain at unloading increases! This contradiction must be addressed. The reduction in elastic modulus after 5000 thermal shock cycles suggest that the slope of the P-h response of the thermally cycled specimen at unloading will decrease from the P-h response at unloading of untested material.

Thank you for your indication. We made a mistake describing results of Fig. 10(a). We revised it in the new manuscript as follows;

In the case of mullite, ….which is related to the modulus of elasticity, decreased after thermal shock [10,25,26,30].

Reviewer 4 Report

This paper has a highly scientific value to invest EBC coatings on SiC composite materials.

Figure 9 (c) is non-existent.

The cracks were discussed by using only surface figures of the coatings. For example, author insisted that the mechanical indentation load–displacement was changed by the crack healing effects. However, the conditions of cracks inside of the coatings were un-known. The surface cracks were covered by the healing effects, but the inside cracks have a possibility to be existed.

Therefore, it is recommended that cross - section figures of the coatings are investigated. If there are the cross-section figures of the coatings before and after the thermal shock tests, the scientific value of this paper will be enhanced considerably.

Author Response

We appreciate reviewer’s comments and suggestions.

We hereby submit a revised version. We revised our paper according to reviewer’s suggestion.

Figure 9 (c) is non-existent.

Even though we insert figure 9(c) during submission, unfortunately, I am sorry you couldn’t find it. Let me submit again on Figure 9(c) in the new manuscript.

The cracks were discussed by using only surface figures of the coatings. For example, author insisted that the mechanical indentation load–displacement was changed by the crack healing effects. However, the conditions of cracks inside of the coatings were un-known. The surface cracks were covered by the healing effects, but the inside cracks have a possibility to be existed.

Therefore, it is recommended that cross - section figures of the coatings are investigated. If there are the cross-section figures of the coatings before and after the thermal shock tests, the scientific value of this paper will be enhanced considerably.

Thank you for your important suggestion and we added it as Fig. 8 in the new manuscript.

Figure 8. SEM micrographs showing the microstructure on the cross-section of the EBC after thermal shock test at 1350℃:  (a) mullite, 5000cycles; (b) mullite+Yb₂SiO₅, 5000cycles ; and (c) Y₂SiO₅, 5000cycles.

Figure 8 shows the cracks on the cross-section of the mullite, mullite + Yb₂SiO₅, and Y₂SiO₅ after 5000 thermal shock cycles, as observed by SEM. Each arrow in the mullite, mullite + Yb₂SiO₅ indicate crack healed region. On the other hand, each arrow in the Y₂SiO₅ indicate cracked region. Only cracks are observed after 5000 cycles of thermal shock in the case of Y₂SiO₅ (Fig. 8(c). By contrast, after 5000 thermal shock cycles, new phases are formed to fill the gaps and cover the cracks, and this result is more pronounced in the mullite + Yb₂SiO_5.

The cracks on the cross-section as well as surface were covered with a new phase in the case of the mullite-based EBC.

Round 2

Reviewer 3 Report

The assumption of formation of molten eutectic phase at a temperature (1350 deg C) lower than the eutectic transformation temperature (1500 deg C) seems speculation to me. Everything else is fine.

Author Response

I appreciate reviewer’s comments and suggestions.

We revised according to reviewer’s suggestion as follows;

1.We deleted the paragraphs mentioning eutectic phase according to reviewer’s comment because we don’t have either simulation result or experimental measurement data on the surface temperature at this stage. The discussion will be covered only by XRD result.

2. We added references that reviewer mentioned. Thank you. 

We really hope our revision of paper be more appropriate for your journal.

Thank you.