Thermal Qualification of the UHTCMCs Produced Using RF-CVI Technique with VMK Facility at DLR

Round 1

Reviewer 1 Report

The paper reports a decent work on the effect of the simulated supersonic airflow on the ZrB2-impregnated UHTCMC materials. There are several minor issues that need attention before publication:

• It is not clear what is the main aim of the work. The authors performed the simulated supersonic airflow test at different conditions on the same material. They did not compare the results to reference ZrB2-free materials, or any other materials. In addition, the ablation mechanism was described with the help of findings published by numerous earlier works. The authors should make it clear what is the aim of the present paper at the end of introduction.
• What was the final composition of the UHTCMC materials? How much of ZrB2 was in the material in relation to Cf and PyC? This should be clearly stated in the paper.
• The authors should explain all the symbols given in Table 1.
• It is not clear what “sample #1 and sample #2” mean in Table 2. What is the difference between the samples?
• The authors should clearly explain what it the purpose of measuring/reporting back temperatures measured by thermocouple? How this affects the present work/results?
• The authors formulated many conclusions/statements by presenting a single SEM image in Fig. 13b, such as presence of Fe and Si, presence of liquid boria on some of remaining carbon fibres, presence of porous ZrO2 layer after volatilisation of B2O3, etc. The figure provided does not confirm either of this, and this should be better presented or evidenced.
• What was the possible effect of Fe and Si contaminants on the ablation behaviour of the samples? Although their presence was reported, but their potential effect was not considered.
• The authors claimed in the conclusion that “all the samples survived such severe testing conditions with a reasonable ablation rate, which....”. Well, based on what it was concluded that “reasonable ablation rate” was achieved? What materials/conditions the authors compare the present results to? As mentioned earlier, they did not investigate the ablation of ZrB2-free samples, so perhaps they should at least compare the results with some earlier investigations to make such a conclusion.
• It is not clear to me what is the point of reporting Fig. 14. This should be better explained in the paper.
• There is some wrong numbering of the figures given in the text, at the beginning of page 9....

Author Response

Reviewer 1:

Point 1: It is not clear what is the main aim of the work. The authors performed the simulated supersonic airflow test at different conditions on the same material. They did not compare the results to reference ZrB2-free materials or any other materials. In addition, the ablation mechanism was described with the help of findings published by numerous earlier works. The authors should make it clear what is the aim of the present paper at the end of introduction.

Response 1: The main aim of the work is to screen the UHTCMC produced by the RF-CVI method for rocket nozzle and TPS applications. The present test has been carried out using the propellant mixtures using test conditions matching the real-time rockets. These have been incorporated in the abstract and the end of the introduction. Please see the attachment for further details.

 

Point 2: What was the final composition of the UHTCMC materials? How much of ZrB2 was in the material in relation to Cf and PyC? This should be clearly stated in the paper.

Response 2: The detailed sample composition is provided at the beginning of section 3.1, again marked with the comment and highlighted. Please see the attachment for further details.

 

Point 3: The authors should explain all the symbols given in Table 1.

Response 3: The details of the symbols have been incorporated in the table itself. Please see the attachment for further information.

 

Point 4: It is not clear what “sample #1 and sample #2” mean in Table 2. What is the difference between the samples?

Response 4: Sample #1 and #2 are the two samples of similar composition, size, and shape but test using different propellant concentrations. The information is added in Table 2 for clarity of the readers. Please see the attachment for further details.

 

Point 5: The authors should clearly explain what it the purpose of measuring/reporting back temperatures measured by thermocouple? How this affects the present work/results?

Response 5: The significance of the back face temperature has been included at the end of page 6, again marked with the comment and highlighted. Please see the attachment for further details.

 

Point 6: The authors formulated many conclusions/statements by presenting a single SEM image in Fig. 13b, such as the presence of Fe and Si, presence of liquid boria on some of remaining carbon fibres, presence of porous ZrO2 layer after volatilisation of B2O3, etc. The figure provided does not confirm either of this, and this should be better presented or evidenced.

Response 7: All the features are presented using different pictures.

Figure 13 b&e -shows the carbon fibre remaining embedded in the porous ZrO2 layer. Figure d & g provide higher magnification of the condensed oxide embedded with B2O3

Figure 13 c & f shows the liquid Boria spread on the oxide layer. The presence of Fe and Si was only a minor contaminant

These details have been explained using various SEM images and explanations in the previous paragraph on page 9.

Again marked with the comment and highlighted. Please see the attachment for further details.

Point 7: What was the possible effect of Fe and Si contaminants on the ablation behaviour of the samples? Although their presence was reported, but their potential effect was not considered.

Response 7: Only traces of Fe and Si are present on the surface of the ablated surface and no significant interaction of these elements observed on the through thickness of the oxide layer along with phase composition. Again there is no or less residence time for these elements for any reaction.

Again rephrased to benefit the reader, again marked with the comment and highlighted in the last paragraph of page 6. Please see the attachment for further details.

 

Point 8: The authors claimed in the conclusion that “all the samples survived such severe testing conditions with a reasonable ablation rate, which....”. Well, based on what it was concluded that “reasonable ablation rate” was achieved? What materials/conditions the authors compare the present results to? As mentioned earlier, they did not investigate the ablation of ZrB2-free samples, so perhaps they should at least compare the results with some earlier investigations to make such a conclusion.

Response 8: There are two types of ablation rate generally estimated in the literature, either by mass or linear surface roughness. In the present testing, the authors have reported the second type based on surface roughness. Due to the accumulation of the residual fuels and ablation reaction product on the test material surface, estimating mass change is not appropriate.

There are no published results on these materials with similar test conditions. Comparing with other test conditions could be misleading and not correct. This is the first such report. The test reference and conditioning will be published in the following publication by DLR.

Please see the attachment for further details.

 

Point 9: It is not clear to me what is the point of reporting Fig. 14. This should be better explained in the paper.

Response 9: The explanation has been provided on page 9, end of the first paragraph (line 271-276), again marked with the comment and highlighted. Please see the attachment for further details.

 

Point 10: There is some wrong numbering of the figures given in the text at the beginning of page 9...

Response 10: Corrected the figure numbers, again highlighted. Please see the attachment for further details.

Author Response File: Author Response.docx

Reviewer 2 Report

The subject of the manuscript is very interesting. The authors prepared a manuscript about the thermal qualification of the UHTCMCs produced using the RF-CVI technique with the VMK facility at DLR. ​In my opinion, this is an essential topic.

In my opinion, this manuscript does provide sufficient, interesting, and necessary in the area to be considered for publication.

Personally, I think that the title of the manuscript is acceptable. The title fully describes the subject matter of the article. In my judgment, the presented information in the manuscript is useful and reliable. From my point of view, the authors proposed very interesting information about ultra-high-temperature ceramic matrix composites (UHTCMCs) based on carbon fibre (Cf) have been shown to offer excellent temperature stability exceeding 2000oC in highly corrosive environments, which are prime requirements for various aerospace applications.

There is not a shadow of a doubt that the scientific quality of the manuscript is good. Subjectively, I assume that the figures and tables included in the manuscript are of good quality and require no corrections.

As far as I am concerned, the authors of the manuscript should necessarily refer to the other publications in the "Introduction" section. It seems to me that there are many interesting works about the situation and perspectives of thermal qualification of the UHTCMCs produced using RF-CVI technique that has been published in recent years, which could be added to as the "Introduction" part in the manuscript. The authors can use more papers to support the work and demonstrate the knowledge behind it. Especially, that in their work, they have cited only 31 literature items.

English language and style are fine/minor spell check required.

I am convinced that the paper can be published.

Author Response

Thanks for the reviewer 2's positive comments. There is no major corrections mentioned. The spell checks were done.