*3.2. Microstructure of Coating Post-Oxidation Test: Oxidation Resistance Evaluation*

Figure 5 shows the XRD spectrum of the MoSi2-Si3N4/SiC coating after the oxidation test at 1773 K. It is clear that the oxidation test resulted in the formation of several new phases. The MoSi2 phase of

the MoSi2-Si3N4/SiC coating has undergone complete transformation, producing some new phases, SiO2, Mo5Si3, and MoO3. The appearance of large fractions of Si-based new phases indicates that MoSi2 is completely oxidized. The partial oxidation of Si3N4 at oxidation temperature has resulted in the formation of the Si2N2O phase that is in accordance with the XRD pattern [26]. The possible reactions are as follows.

$$\text{SiC (s)} + 2\text{O}\_2\text{(g)} \rightarrow \text{SiO}\_2 + \text{CO}\_2 \text{ (g)}\tag{2}$$

$$\text{\textbullet\text{\textbulletMoSi}\_2\text{ (s)} + \text{7O}\_2\text{ (g)} \rightarrow \text{Mo}\_5\text{Si}\_3\text{ (s)} + \text{7SiO}\_2\text{ (s)}\tag{3}$$

$$2\text{Mo}\_5\text{Si}\_3\text{ (s)} + 2\text{IO}\_2\text{ (g)} \rightarrow 10\text{MoO}\_3\text{ (g)} + 6\text{SiO}\_2\text{ (s)}\tag{4}$$

$$2\text{MoSi}\_2\text{ (s)} + 7\text{O}\_2\text{ (g)} \rightarrow 4\text{SiO}\_2\text{ (s)} + 2\text{MoO}\_3\text{ (g)}\tag{5}$$

$$\rm Si\_3N\_4 \text{ (s)} + \rm 3O\_2 \text{ (g)} \rightarrow \rm 3SiO\_2 \text{ (s)} + 2N\_2 \text{ (g)}\tag{6}$$

$$\text{Si}\_3\text{N}\_4\text{ (s)} + \text{SiO}\_2\text{ (s)} \rightarrow 2\text{Si}\_2\text{N}\_2\text{O (s)}\tag{7}$$

**Figure 5.** XRD pattern of MoSi2-Si3N4/SiC-coated C/C composites after oxidation at 1773 K.

Mo5Si3 and SiO2 play an important role in increasing the oxidation resistance of the coating. The Mo5Si3 phase increases the coating flexural strength and the coating-substrate compatibility and enhances the creep strength [27]. The SiO2 phase, owing to its inherent low viscosity, good fluidity and low oxygen permeability, serves as a barrier against oxygen attack. In Figure 6, the Raman spectroscopy result also confirms the formation of SiO2 in the MoSi2-Si3N4 coating after oxidation at 1773 K. It can be found that the peak with wave number of 228 and 414 cm−<sup>1</sup> is the crystallite while the wave number of 1034 cm−<sup>1</sup> is the amorphous silica, which is accordance with the XRD result.

**Figure 6.** Raman spectrum of the MoSi2-Si3N4/SiC coating after oxidation at 1773 K for 150 h [26].

The effect of Si3N4 on the oxidation resistance of the multi-layer coating was investigated from the microstructure analysis and weight loss of the coatings constituting the MoSi2 and MoSi2-Si3N4 outer layers. Figure 7a shows the microstructure of the MoSi2 surface of the MoSi2/SiC multi-layer coating after the oxidation test. The incomplete formation of silica (SiO2) was due to the non-uniform agglomeration of MoSi2 particles in the brushing process [28]. These particles degraded the surface of the coating and gradually reduced its oxidation resistance. As a result, some deep cracks occurred and propagated through the randomly deposited MoSi2.

Figure 7b shows the SEM micrograph of the cross-section of MoSi2/SiC multi-layer-coated carbon/carbon composites. The cross-section is rough, porous, and oxidized. This is because of the large CTE difference between SiC and MoSi2, leading to the incomplete formation of SiO2 and pore/cavity formation [25,29]. These pores provide the diffusion channel for oxygen that penetrates the substrate and causes the failure of the coating. It is also noteworthy that the infiltration of oxygen via the cavities would reduce the thickness of the coating. Therefore, the porous morphology of the MoSi2/SiC coating is detrimental for its oxidation resistance.

Figure 7c shows the microstructure of the outer surface of the MoSi2-Si3N4/SiC coating after the oxidation test at 1773 K. A dense glassy SiO2 layer on the surface is evident, which prevents the carbon/carbon matrix from oxygen penetration and protects the coating from oxidation. This makes the MoSi2-Si3N4/SiC coating oxidation-resistant and an ideal choice for high-temperature applications.

In Figure 7d, the cross-sectional morphology of the post-oxidation tested MoSi2-Si3N4/SiC coating exhibits excellent compatibility between the coating and the substrate, which is responsible for the high bonding and flexural strengths of the coating in high-temperature environments [30,31]. Furthermore, the absence of voids/holes at the coating–substrate interface indicates that the coating is resistant to high-temperature rupture. In addition, Si3N4 forms a suitable combination with MoSi2 in the slurry, which helps to minimize the CTE difference between the outer MoSi2-Si3N4 layer and the inner SiC-coated carbon/carbon substrate. Therefore, Si3N4 plays a vital role in increasing the oxidation protective ability of outer coatings at high temperatures [32]. The formation of microdefects (i.e., microcracks and micropores) is due to the volatilization of MoO3. From experimental observations, these pores are far from detrimental to the coating and can be cured by the glassy SiO2 layer at high temperatures.

**Figure 7.** SEM image of MoSi2/SiC coating after oxidation at 1773 K: (**a**) surface, (**b**) cross-section; SEM image of MoSi2-Si3N4/SiC coating after oxidation at 1773 K: (**c**) surface, (**d**) cross-section.

Figure 8 shows a comparison of weight losses for the two coatings, with and without Si3N4, after the oxidation test at 1773 K. The MoSi2-Si3N4/SiC coating experienced a weight loss of 0.9% after 150 h of oxidation treatment while the MoSi2/SiC coating suffered a relatively high weight loss of 4.0% even after 90 h of oxidation.

**Figure 8.** Isothermal oxidation curves of MoSi2-Si3N4/SiC-coated C/C composites in the air at 1773 K.

The absence of Si3N4 in the MoSi2/SiC coatings causes the pest disintegration of MoSi2 which results in the coating's degradation at high temperatures. The increasing weight loss with oxidation time is due to the insufficient amount of SiO2 that results in the formation of large cracks on the surface of the coating. This severely affects the coating surface and allows oxygen access to the C/C composites, causing gaseous byproducts. At high temperatures, these gaseous byproducts quickly evaporate to cause the sudden weight loss of the coating. This de-gasification produces some micro-cracks and deep cavities in the coating surface, which are the principal contributors to weight reduction.

The lower weight loss in the MoSi2-Si3N4/SiC coating indicates that the addition of Si3N4 is beneficial for the coating's integrity at high temperatures. Additionally, the absence of debonding or spallation at 1773 K indicates that the Si3N4 in the coating yields better coating–substrate compatibility at high temperatures. This suggests that the MoSi2-Si3N4 coating is more resistant to high-temperature oxidation.
