Ablation-resistant carbide Zr0.8Ti0.2C0.74B0.26 for oxidizing environments up to 3,000 °C

Ultra-high temperature ceramics are desirable for applications in the hypersonic vehicle, rockets, re-entry spacecraft and defence sectors, but few materials can currently satisfy the associated high temperature ablation requirements. Here we design and fabricate a carbide (Zr0.8Ti0.2C0.74B0.26) coating by reactive melt infiltration and pack cementation onto a C/C composite. It displays superior ablation resistance at temperatures from 2,000–3,000 °C, compared to existing ultra-high temperature ceramics (for example, a rate of material loss over 12 times better than conventional zirconium carbide at 2,500 °C). The carbide is a substitutional solid solution of Zr–Ti containing carbon vacancies that are randomly occupied by boron atoms. The sealing ability of the ceramic’s oxides, slow oxygen diffusion and a dense and gradient distribution of ceramic result in much slower loss of protective oxide layers formed during ablation than other ceramic systems, leading to the superior ablation resistance.

The work is convincing in terms of the data presented. The discussion of the relevance of the work, as well as the interpretation, is overall solid. I believe this paper will spark interest in investigating other quarternary compounds for hypersonic applications as it represents a "new" promising material class.
I believe the work of the authors is well described in this overall excellent paper. Not merely the presentation of some interesting results, the authors perform a relatively in depth analysis of the new compound. For example, see Figure 6c and d, where atomic placement is shown. Some comments: Page 3, line 75 -there is no matching parenthesis.
Page 4 -what is the heat flux of the ablation flame? This is a standard way to compare data. Also, what opening was used on the gas tip? 3 mm dia? 5 mm dia? Why did the tests last "approximately" 60 secs and 120 secs as opposed to exactly 60 secs or 120 secs? What was the error in time measurement?

Reviewers' comments:
Reviewer #1 (Remarks to the Author): This manuscript titled "Highly ablation resistant Zr0.8Ti0.2C0.74B0.26 carbide for extremely oxidizing environments up to 3000°C" by Dr. Xiong and co-authors report a new carbide (Zr0.8Ti0.2C0.74B0.26) coating by reactive melt infiltration and pack cementation onto a C/C composite. The results showed that the coating after oxidation has good sealing ability due to its low oxygen diffusion and a dense and gradient distribution of ceramic. Although the materials design and results are interesting, the inherent shortcomings should be overcame before its publication.
(1) During the RMI process, the solution of molten Zr-Ti was infiltrated into the porous C/C composite. However, the author should supply the information of melt-viscosity of Zr-Ti, because the infiltration height is associated to the melt viscosity. Also how the pore size and porosity for the NIP?
We are happy to clarify in the text. Actually, the viscosities of pure Zr and Ti at (3) In the section of ablation mechanism, the authors had better supply cross-section morphology after ablation. It can clearly see the change and variation of oxide layer after ablation.
Following the referees comments, the cross-section morphology after ablation have been investigated by SEM and EPMA. Some interesting observations under the surface have been made and are illustrated in Fig. 6f and g in the revised manuscript. Consequently the Results section is modified as indicated in red text. (4) With the increase in ablation temperature, more amorphous phases occurred When the test finished and the ablation temperature decreased to the room temperature shortly, little crystallization of oxides on the surface of layer occurred, indicating a quenching process happened which caused the formation of amorphous phases. Consequently, we have added the following contents in "protective mechanisms' of the revised manuscript.
"Additionally, the XRD results suggest the presence of more amorphous phases, with the ablation temperature increasing from 2,000 to 3,000°C, due to the quenching of more liquid-solid phases of oxide layer at the end of the ablation test (some oxides under the surface may remain solid at 3,000 °C within the limited ablation time, due to the thermal barrier provided by the Zr-O-Ti ceramic system 37) ." Reviewer #2 (Remarks to the Author): I believe this paper will spark interest in investigating other quarternary compounds for hypersonic applications as it represents a "new" promising material class.
I believe the work of the authors is well described in this overall excellent paper. Not merely the presentation of some interesting results, the authors perform a relatively in depth analysis of the new compound. For example, see Figure 6c and d, where atomic placement is shown.
Many thanks for your supporting comments, which encourages us greatly.