*3.3. Thermal Cyclic Oxidation Resistance of the Coating at 1773 K*

Figure 9a shows the SEM image of the MoSi2-Si3N4/SiC coating surface after the thermal cyclic oxidation test. It can be observed that whiskers are formed throughout the surface. The exposure of Si3N4 coatings to high temperatures can foster the formation of Si2N2O whiskers [33]. These whiskers play an essential role in increasing the oxidation resistance of the coating [34]. The cross-sectional view of the multi-layer coating in Figure 9b shows small pores, mainly caused by the rapid cooling from 1773 K to room temperature. These pores can be sealed by the formation of Si3N4 whiskers in the subsequent thermal cyclic oxidation process, which further protects the coating from high-temperature oxidation [35].

**Figure 9.** (**a**) Surface SEM image, where red arrows represent whiskers; (**b**) cross-section of multi-layer coatings after thermal cyclic oxidation test between 1773 K and room temperature 40 times.

Figure 10 shows the weight loss curve of the MoSi2-Si3N4/SiC-coated C/C specimen during its repeated thermal cycling between 1773 K and room temperature. It can be observed that the weight loss of the coated specimen was only 0.05% after 40 thermal cycles. During the thermal cycling, the coating remained intact, and no oxidation or spallation was found. This indicates that the coating exhibits excellent oxidation and thermal cyclic oxidation resistance, which can be attributed to the filling of micro cracks under oxidizing environments by abundant glassy oxides.

**Figure 10.** Thermal cycling oxidation curves of the MoSi2-Si3N4/SiC coating between 1773 K and room temperature.

#### **4. Conclusions**

The effect of Si3N4 addition in MoSi2/SiC coating on the oxidation and thermal cyclic oxidation resistance at 1773 K was investigated. The 0.5% weight loss in the MoSi2-Si3N4/SiC coating after its oxidation for 150 h in comparison to the 4.0% weight loss in the MoSi2/SiC coating after 90 h proves that the coating with Si3N4 has a better high-temperature oxidation resistance. The negligible weight loss after 40 thermal cycles between 1773 K and room temperature proves that the MoSi2-Si3N4/SiC coating has an excellent thermal cyclic oxidation resistance. The presence of Si3N4 results in a complete transformation of MoSi2 to SiO2, which results in a dense glassy SiO2 film. The SiO2 film improves the coating's resistance to high-temperature oxidation by preventing gas diffusion into the coating, thus shielding the C/C substrate from byproduct-forming gases and delaying the generation of micro cracks. The addition of Si3N4 to the multi-layer coating is found to be beneficial for both coating integrity and coating–substrate compatibility.

**Author Contributions:** Conceptualization, I.A. and Y.W.; Methodology, I.A.; Software, H.E.; Validation, M.A.S., M.U., and I.A.; Formal analysis, F.Q.; Investigation, I.A.; Resources, M.A.S. and M.U.; Data curation, I.A., F.Q; Writing—original draft preparation, I.A., F.Q., and H.E.; Writing—review and editing, all co-authors; Visualization, I.A.; Supervision, Y.W.; Project administration, Y.W.; Funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (51573087) and the Natural Science Foundation in Shandong Province (ZR2014EZ001, ZR2011EMM002).

**Acknowledgments:** The authors acknowledge the support of the National Natural Science Foundation of China and the German Academic Exchange Service (DAAD). The authors also acknowledge the support of technical staff for assisting in preparing samples and analyzing them.

**Conflicts of Interest:** The authors do not have any conflict of interest.

#### **References**


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