*2.2. Oxidation Test*

Isothermal oxidation tests on the MoSi2-Si3N4/SiC multi-layer coated carbon/carbon specimens were performed in a corundum tube furnace at 1773 K for 150 h. After every 10 h to a maximum time of 150 h, the samples were taken out of the furnace directly and cooled to room temperature in air. The weight loss of each specimen at room temperature was measured with an electronic balance with a sensitivity of ±0.1 mg. The percentage weight loss (wt%) was calculated using Equation (1), where *m*<sup>0</sup> and *m*<sup>1</sup> are the weights of specimens before and after the oxidation test, respectively.

$$\text{wt\%} = \frac{m\_0 - m\_1}{m\_0} \times 100\% \tag{1}$$

The thermal cyclic oxidation resistance of the MoSi2-Si3N4/SiC multi-layer coating was investigated by conducting a test between 1773 K and room temperature. In this test, the coated carbon/carbon specimens were heated at 1773 K for 10 min, removed from the furnace, and air-cooled to room temperature for each cycle. The testing was carried out in 40 cycles.

## *2.3. Characterization of the Coatings*

The morphology of the MoSi2-Si3N4/SiC multi-layer coating before and after the oxidation test was analyzed using a scanning electron microscope (SEM), equipped with an energy dispersive spectroscope (EDS). Raman spectroscopy (LabRam-1B, 633nm line of the He-Ne laser) was used to identify the chemical composition and phase transformation. An X-ray diffraction (XRD) technique was used for the crystallographic characterization of the coating before and after the oxidation test.

To analyze the elemental distribution along with the depth of the coated samples, they were sectioned using a power cutter and were then ground and polished to prepare the surface perpendicular to the top coating surface. These samples were then analyzed using the same characterization techniques stated earlier in this section.
