Feasibility of CaZr₄(PO₄)₆ as Radome TBC Based on Thermophysical and Thermal Cycle Performance Research

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

ABSTRACT
- Clearly state what is new.
- Reduce failure explanation to one concise sentence.

INTRODUCTION
- Define the knowledge gag including what is unknown about CaZr₄(PO₄)₆ as an EBC? and why APS processing is challenging for this composition?
- Add comparison logic between CTE, thermal conductivity, dielectric properties vs. existing EBCs.

MATERIALS AND METHODS
- Some processing choices lack justification.
- Justify why 1300 °C annealing for 20 h was chosen.
- Clarify coating thickness, roughness, and uniformity.
- Explain why no bond coat was used beyond CTE similarity.
- Explain why 34 min at temperature + 1 min quench is representative.

RESULTS AND DISCUSSION
- Discuss how phase stability compares to other NZP ceramics.
- Clarify whether 0.75% mass loss is acceptable for long-term EBC service.
- Discuss phosphate volatility (P₂O₅) risks at high temperature.
- Estimate volume or fraction of secondary phases (ZrO₂, Ca₃(PO₄)₂).
- Discuss why APS induces decomposition (particle temperature, residence time).
- Compare XRD peak broadening quantitatively.
- Estimate thermal stress using measured CTE, modulus, and ΔT.
- Reduce repetition and structure the failure mechanism into clear stages.
- Remove informal phrasing, such as in my opinion.

CONCLUSION
- Please add limitations.
- No need to include the numerical values again, given in results section.

Comments on the Quality of English Language

None

Author Response

Comments 1: Clearly state what is new.

Response 1: Thank you for pointing this out. This paper investigates the feasibility of CaZr₄(PO₄)₆ as the novel thermal barrier coating for SiO_2f/SiO₂ .

Comments 2: Reduce failure explanation

Response 2: The primary cause of coating failure and peeling is the excessive internal stress between the coating and the expansion of transverse cracks.

Comments 3: Define the knowledge gag including what is unknown about CaZr₄(PO₄)₆ as an EBC? and why APS processing is challenging for this composition?

Response 3: Due to its low expansion rate, low thermal conductivity, and high temperature resistance, CaZr₄(PO₄)₆ ceramics can be considered as a potential material for TBC. Partly decomposition of the CaZr₄(PO₄)₆ occurs during the spraying process when it is subjected to excessively high temperatures (≥10 ⁵ K) because of the plasma.

Comments 4: Add comparison logic between CTE, thermal conductivity, dielectric properties vs. existing EBCs.

Response 4: The existing TBCs such as YSZ, LMA and Yb₂SiO_5, they have significant thermal expansion differences from SiO₂f/SiO₂ and are prone to rapid failure due to thermal mismatch, making them unsuitable for coating preparation on SiO₂f/SiO₂ surfaces. meanwhile the radome coating material needs to satisfy the low dielectric loss property.

Comments 5: Some processing choices lack justification.

Response 5: Thank you for pointing this out. Modifications have been made.

Comments 6: Justify why 1300 °C annealing for 20 h was chosen.

Response 6: Thank you for pointing this out. Due to the presence of a large number of amorphous phases in the as-sprayed coating, the internal stress within these amorph.ous phases can affect the subsequent service performance of the coating. Therefore, high-temperature heat treatment is required to crystallize and release the internal stress.

Comments 7: Clarify coating thickness, roughness, and uniformity.

Response 7: The prepared CaZr₄(PO₄)₆ coating is about 200 µm, the coating structure is primarily composed of molten, semi-molten, and not melted particles.

Comments 8: Explain why no bond coat was used beyond CTE similarity.

Response 8: The role of the bonding layer is to reduce thermal mismatch and enhance the service life of the coating. Additionally, radomes have specific requirements for conductivity and dielectric properties. The commonly used Si and NiCrAlY bonding layers fail to meet the performance requirements of radomes in terms of thermal expansion properties and conductivity/dielectric properties.

Comments 9: Explain why 34 min at temperature + 1 min quench is representative.

Response 9: According to the GBT 42259-2022, it is the universal experimental system.

Comments 10: Discuss how phase stability compares to other NZP ceramics.

Response 10: The property of maintaining phase stability at 1200 °C is consistent with the characteristics of the NZP family.

Comments 11: Clarify whether 0.75% mass loss is acceptable for long-term EBC service.

Response 11: According to the GB/T 42259-2022, The TBCs can be considered in a state where the experiment can continue until it loses 30% of its mass.

Comments 12: Discuss phosphate volatility (P₂O₅) risks at high temperature.

Response 12: Thank you for pointing this out. due to the trace presence of P₂O₅, a small amount of harmful gas may be released at high temperatures, and it is necessary to stay away from people.

Comments 13: Estimate volume or fraction of secondary phases (ZrO₂, Ca₃(PO₄)₂).

Response 13: After refining the peak area, in Fig 5 (a, 2), the coating was annealed at 1300 °C for 20 h, the proportion of secondary phase is about  ZrO₂ (7 mol %), Ca₃(PO₄)₂ (1 mol%), CaZrO₃ (1 mol%); in Fig 5 (a, 3), the coating was thermal shocked, the proportion of secondary phase is about  ZrO₂ (7 mol %), Ca₃(PO₄)₂ (1 mol%), CaZrO₃ (1 mol%).

Comments 14: Discuss why APS induces decomposition (particle temperature, residence time).

Response 14: Because the core temperature of the plasma is extremely high (≥10 ⁵ K), the partial decomposition of powder could happen during the powder goes through the plasma flame.

Comments 15: Compare XRD peak broadening quantitatively.

Response 15: As shown in Fig 5, Select peak (202) to compare FWHM values, FWHM of as-sprayed coating is 0.63,  FWHM of the annealed coating is 0.14, FWHM of thermal shocked coating is 0.3. By comparison, it can be concluded that amorphous phase in as-sprayed coating recrystallize after the anneal process,and then the gain size grow during the thermal-shock process. 

Comments 16: Estimate thermal stress using measured CTE, modulus, and ΔT.

Response 16: According to σ = E * α * ΔT,  the value of E is 36.8 GPa, the value of α uses the highest value for numerical values 5.55×10^-6 K^-1, the value of ΔT is 860 °C, finally the value of σ is 175.6 MPa, The maximum thermal stress inside the coating is approximately 175.6 MPa.

Comments 17: Reduce repetition and structure the failure mechanism into clear stages.

Response 17: Thank you for pointing this out. Clear stage division has been made in the paper.

Comments 18: Remove informal phrasing, such as in my opinion.

Response 18: Thank you for pointing this out. Have been deleted.

Comments 19:  Please add limitations.

Response 19: Limit the research scope to CaZr₄(PO₄)₆  bulks and coatings.

Comments 20: No need to include the numerical values again, given in results section.

Response 20: Thank you for pointing this out. I mainly want to summarize the data at the conclusion to make the writing smoother.

 

Reviewer 2 Report

Comments and Suggestions for Authors

1.    In the introduction, please emphasize the research gap to be addressed. The objectives and scope should be clarified in a way that the completion/success of this work can be evaluated after considering the presented results and conclusions. The scope should state the independent variables and the studied ranges, specifically.
2.    In the conclusion section, please also include the reason for the observed results. 
3.    The following points should be elaborated on:
3.1.    On page 1, “radome” in the first paragraph of the introduction.
3.2.    On page 2, lines 7-8: “However, SiO2f/SiO2 cannot endure high temperatures above 1100 °C under high load conditions.” Why? Please also provide credible references.
3.3.    On page 2, lines 14-15: “…it needs to meet the following conditions:” Please provide credible references.
3.4.    In Table 1, please clarify the definition of the “Feeding rate (%).” The kind of percentage is unclear (e.g., mol%, wt%, vol%, etc.).
3.5.    In Table 2, the meaning of composition in % (mol%, wt%, vol%, etc.) is unclear.
4.    The last three paragraphs of page 7, “Fig. 6 illustrates … to the denser” discussing the results in Fig. 6, in particular, the influence of the density or porosity on the thermal conductivity, thermal diffusivity, and constant-pressure specific heat capacity are unclear. This requires clear data on the density or porosity of the ceramic at various temperatures, e.g., by including the data in Figure 6.
5.    Provide support information and references for the following:
5.1.    On page 7, “Possible explanations for this change in trend include enhanced heat radiation at higher test temperatures, leading to higher measurement values, or the chemical reaction involving the graphite coating adhered to the sample surface. The presence of the graphite coating serves to ensure complete and uniform laser pulse absorption during thermal diffusivity measurements. As a consequence, the decomposition of the graphite coating at elevated temperatures may introduce errors into the measurement results. Careful consideration of these factors is crucial for accurately interpreting the thermal diffusivity data obtained in the study.”
5.2.    On page 7, “In comparison to the bulk material, the coating structure prepared using the APS method exhibits higher porosity. The increase in porosity results in a reduced phonon mean free path during the heat conduction process, thereby leading to lower thermal conductivity of the coating compared to the denser bulk.”
5.3.    On parge 8, “…, a non-linear curve change is observed, as depicted in the variation of the thermal expansion coefficient shown in Fig.7 (b). This behavior might be associated with stress relaxation accumulated within the sprayed coating over this temperature range; Within the temperature range of 800-900 °C, a sudden increase in the slope of the expansion rate curve is observed.”
5.4.    On page 8, “Notably, the thermal expansion coefficient of the m-ZrO2 phase in this temperature range is approximately 5×10-6 K-1, leading to an abnormal expansion rate increase in the coating; conversely, in the range of 1000-1100 °C, the expansion rate curve declines. This decrease can be attributed to the crystallization of the amorphous phase in the coating, resulting in volume shrinkage; after reaching 1100 °C, the m-ZrO2 undergoes an endothermic transformation into t-ZrO2. This transformation is accompanied by a further shrinkage of the crystal phase volume, resulting in a subsequent decrease of the thermal expansion rate, as shown in Fig.7 (a).”
5.5.    On page 9, the first paragraph, the criteria for preferable mechanical properties are unclear.
6.    Provide references for equations (2), (5), and (6). Also, please define σ in equation (5).
7.    Avoid bulk citation, e.g., [1-7], [13-22], and [23-30] (page 2). Instead of grouping too many references together, these references should be disaggregated by attributing different concepts, methods, and findings, specifically.
8.    Other comments:
8.1.    The font size of the content in section 2 is noticeably inconsistent.
8.2.    Ensure the consistency of the quantity format. For example, 5×10^-6 K^-1 (on page 8) and 2.07∙10^-6 K^-1 to 5.55∙10^-6 K^-1 (on page 12) are inconsistent.
8.3.    Ensure that the unit is expressed correctly, for example, Wm^-1∙K^-1 (on page 12) should be corrected.
8.4.    I suggest providing a scale bar for Figure 8.

Author Response

Comments 1: In the introduction, please emphasize the research gap to be addressed. The objectives and scope should be clarified in a way that the completion/success of this work can be evaluated after considering the presented results and conclusions. The scope should state the independent variables and the studied ranges, specifically.

Response 1: Expected to increase the operating temperature of the radome (SiO_2f/SiO₂) to above 1100 °C, the research defined within the scope of thermal barrier coatings.

Comments 2: In the conclusion section, please also include the reason for the observed results.

 Response 2: The reason for the observed results is to prove CaZr₄(PO₄)₆ coating with low thermal expansion coefficient and thermal conductivity is the radome TBCs potential candidate for SiO_2f/SiO₂ application.

Comments 3.1:  On page 1, “radome” in the first paragraph of the introduction.

Response 3.1: radome is the antenna housing.

Comments 3.2:  On page 2, lines 7-8: “However, SiO2f/SiO2 cannot endure high temperatures above 1100 °C under high load conditions.” Why? Please also provide credible references.

Response 3.2: [5] Jiliang Cia, Fengdan Cuib, Lili Zhangc, Jian Zhangd, Yi Lve, Yingmin Zhaof*,Hao Zhangg , Study on the Highe-Temperature Mechanical of Al₂O_3f/SiO₂ and SiO_2f/SiO₂ Ceramic Matrix Composites, Solid State Phenomena, 330 (2022) 91-98.

Comments 3.3: On page 2, lines 14-15: “…it needs to meet the following conditions:” Please provide credible references.

Response 3.3: [1]Yiming Wu; Du Hong; Xin Zhong; Yaran Niu; Xuebin Zheng, Research progress on hafnium-based thermal barrier coatings materials, Ceramics International. 49 (2023) 21133-21141.

[9] Wanye X U，Peng L I，Qiu Y，et al．Electrical performance analysis of metal space frame radome with structural deformation [J]．Journal of Mechanical Engineering，2016，52（1）：57．

Comments 3.4:  In Table 1, please clarify the definition of the “Feeding rate (%).” The kind of percentage is unclear (e.g., mol%, wt%, vol%, etc.).

Response 3.4: It has been changed from “%” to “g/min”.

Comments 3.5: In Table 2, the meaning of composition in % (mol%, wt%, vol%, etc.) is unclear.

Response 3.5: It has been changed from “a %” to “mol %”.

Comments 4: The last three paragraphs of page 7, “Fig. 6 illustrates … to the denser” discussing the results in Fig. 6, in particular, the influence of the density or porosity on the thermal conductivity, thermal diffusivity, and constant-pressure specific heat capacity are unclear. This requires clear data on the density or porosity of the ceramic at various temperatures, e.g., by including the data in Figure 6.

Response 4: Thank you for pointing this out. This explanation mainly refers to 

[25] J. Yuan, J. Sun, J. Wang, Z. Hao, X.J.J.o.A. Cao, Compounds, SrCeO₃ as a novel thermal barrier coating candidate for high–temperature applications, 740 (2018).

Comments 5.1: On page 7, “Possible explanations for this change in trend include enhanced heat radiation at higher test temperatures, leading to higher measurement values, or the chemical reaction involving the graphite coating adhered to the sample surface. The presence of the graphite coating serves to ensure complete and uniform laser pulse absorption during thermal diffusivity measurements. As a consequence, the decomposition of the graphite coating at elevated temperatures may introduce errors into the measurement results. Careful consideration of these factors is crucial for accurately interpreting the thermal diffusivity data obtained in the study.”

Response 5.2: Thank you for pointing this out. Modifications have been made,

 [25] J. Yuan, J. Sun, J. Wang, Z. Hao, X.J.J.o.A. Cao, Compounds, SrCeO₃ as a novel thermal barrier coating candidate for high–temperature applications, 740 (2018).

Comments 5.2: On page 7, “In comparison to the bulk material, the coating structure prepared using the APS method exhibits higher porosity. The increase in porosity results in a reduced phonon mean free path during the heat conduction process, thereby leading to lower thermal conductivity of the coating compared to the denser bulk.”

Response 5.2: [25] J. Yuan, J. Sun, J. Wang, Z. Hao, X.J.J.o.A. Cao, Compounds, SrCeO₃ as a novel thermal barrier coating candidate for high–temperature applications, 740 (2018).

Comments 5.3: On parge 8, “…, a non-linear curve change is observed, as depicted in the variation of the thermal expansion coefficient shown in Fig.7 (b). This behavior might be associated with stress relaxation accumulated within the sprayed coating over this temperature range; Within the temperature range of 800-900 °C, a sudden increase in the slope of the expansion rate curve is observed.”

Response 5.3: Thank you for pointing this out. Modifications have been made. In the range of 400 to 800 °C, a non-linear curve change is observed, as depicted in the variation of the thermal expansion coefficient shown in Fig.7 (b). This is attributed to the manifestation of the negative thermal expansion (NTE) behavior of CaZr₄(PO₄)₆ ceramics within the temperature range. The combined effect of CaZr₄(PO₄)₆ and ZrO₂ results in a reduction of the macroscopic expansion rate of the coating.

[13] N. Chakraborty, D. Basu, W.J.J.o.t.E.C.S. Fischer, Thermal expansion of Ca_(1-x)Sr_xZr₄(PO₄)₆ ceramics, 25 (2005) 1885-1893.

Comments 5.4:  On page 8, “Notably, the thermal expansion coefficient of the m-ZrO2 phase in this temperature range is approximately 5×10-6 K-1, leading to an abnormal expansion rate increase in the coating; conversely, in the range of 1000-1100 °C, the expansion rate curve declines. This decrease can be attributed to the crystallization of the amorphous phase in the coating, resulting in volume shrinkage; after reaching 1100 °C, the m-ZrO2 undergoes an endothermic transformation into t-ZrO2. This transformation is accompanied by a further shrinkage of the crystal phase volume, resulting in a subsequent decrease of the thermal expansion rate, as shown in Fig.7 (a).”

Response 5.4: 

[27] Z.G. Xue, S. Wang, J.M.J.F.C. Pang, Effect of Heat Treatment on Thermal Expansion of Zirconia Mateerials, (2018).

[28] V.P.J.P.o.t.S.S. Gorelov, High-Temperature Phase Transitions in ZrO₂, 61 (2019) 1288-1293.

Comments 5.5: On page 9, the first paragraph, the criteria for preferable mechanical properties are unclear. 

Response 5.5: Thank you for pointing this out,the content of the relevant paper has been revised. What I want to express here is that CaZr₄(PO₄)₆ coating has similar mechanical properties to cordierite coating which has already been put into practical using, so CaZr₄(PO₄)₆ holds potential for utilization in similar environments where cordierite has been successfully employed(used as high temperature wave material ).

Comments 6: Provide references for equations (2), (5), and (6). Also, please define σ in equation (5).

Response 6: 

equations (2)--[25] J. Yuan, J. Sun, J. Wang, Z. Hao, X.J.J.o.A. Cao, Compounds, SrCeO₃ as a novel thermal barrier coating candidate for high–temperature applications, 740 (2018).

equations (5)--[24] G.H.J.J.o.t.E.C.S. Xu, Lanthanum–titanium–aluminum oxide: A novel thermal barrier coating material for applications at 1300 °C, (2011).

equations (6)--[29] L. Guo, Z. Yan, Z. Li, J. Yu, F.J.S. Ye, C. Technology, GdPO₄ as a novel candidate for thermal barrier coating applications at elevated temperatures, 349 (2018) 400-406.

Comments 7: Avoid bulk citation, e.g., [1-7], [13-22], and [23-30] (page 2). Instead of grouping too many references together, these references should be disaggregated by attributing different concepts, methods, and findings, specifically.

Response 7: Thank you for pointing this out. The content of the relevant paper has been revised.

Comments 8: 

Other comments: 8.1. The font size of the content in section 2 is noticeably inconsistent. 8.2. Ensure the consistency of the quantity format. For example, 5×10-6 K-1 (on page 8) and 2.07∙10-6 K-1 to 5.55∙10-6 K-1 (on page 12) are inconsistent. 8.3. Ensure that the unit is expressed correctly, for example, Wm-1∙K-1 (on page 12) should be corrected. 8.4. I suggest providing a scale bar for Figure 8.  

Response 8: Thank you for pointing this out. The content of the relevant paper has been revised.

Reviewer 3 Report

Comments and Suggestions for Authors

Please revise the manuscript as suggested by the reviewer in the annotated pdf file attached along with.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf