*3.1. Microstructure of Coating before the Oxidation Test*

The XRD pattern of the as-coated inner SiC layer is shown in Figure 1a. Three significant peaks at 2θ ≈ 35.6◦, 60.0◦, and 71.7◦correspond to the (111), (220), and (311) crystalline planes of β-SiC, indicating the formation of the β-SiC crystalline plane during the pack cementation process. The low-intensity peak at 2θ ≈ 33.6◦ indicates the existence of stacking faults (SF) in the β-SiC coating. The stacking faults might be induced due to the thermal stresses produced in the coating process [24].

**Figure 1.** Structure of the inner SiC coating: (**a**) XRD pattern; (**b**) surface SEM image.

Figure 1b shows the surface microstructure of the as-coated inner SiC layer. Figure 2 shows the XRD analysis of the as-coated specimen, i.e., the MoSi2-Si3N4/SiC-coated carbon/carbon specimen before the oxidation test. MoSi2, Si3N4, and SiC phases are apparent. The more vigorous peak intensity of the MoSi2 phase in Figure 2 indicates that it is relatively more abundant in the slurry.

**Figure 2.** XRD pattern of MoSi2-Si3N4/SiC coated carbon/carbon specimen before the oxidation test.

Figure 3a shows the surface microstructure of the as-coated MoSi2-Si3N4 outer layer before the oxidation test. The surface of the coating has no micro-cracks, indicating that the as-prepared coating is dense and homogeneous. In Figure 3b, the EDS pattern of the area marked with the red square in Figure 3a clearly shows that the smooth area of the as-coated MoSi2-Si3N4 is composed of the elements Si, N, and Mo.

**Figure 3.** Surface of the as-coated MoSi2-Si3N4 outer layer: (**a**) SEM image; (**b**) EDS pattern corresponding to the marked red square in Figure 3a.

Figure 4 shows an overlay of element line scanning over the SEM image of the cross-section of MoSi2-Si3N4/SiC coated C/C composite before the oxidation test. The multi-layer coating has an average thickness of 195 ± 3 μm. It can be observed from the line scan that the as-prepared coating is composed of a MoSi2-Si3N4outer layer, SiC inner layer, and SiC transition layer, of thicknesses 40 ± 5, 61 ± 2, and 93 ± 2 μm, respectively. The SiC transition layer, formed possibly due to the infiltration of Si into the carbon/carbon substrate at high temperature, increases the oxidation and thermal cyclic oxidation resistance of the coating. The lack of voids between the outer and inner coating layers in Figure 4 indicate two things:


**Figure 4.** Line scans of the cross-section of C/C composite coated with MoSi2-Si3N4/SiC.
