**1. Introduction**

For aggressive service environments in the nuclear and aerospace industry, carbon/carbon composites are candidate structural materials due to their attractive properties such as a high strength-to-weight ratio, thermal cyclic oxidation resistance, strength retention, and a low thermal expansion coefficient [1–5]. However, these composites exhibit limitations such as susceptibility to oxidation beyond 773 K and strength loss upon exposure to high temperatures. This makes them highly unfavorable for specific high-temperature applications [6–8]. It was shown by previous researchers that carbon/carbon composites could be protected against oxidation at high temperatures by the application of anti-oxidizing coatings [9].

In multi-layered coatings, MoSi2 is used as an outer layer, while SiC forms the inner buffer layer. The peripheral multi-layer coating containing MoSi2 exhibits a superior oxidation-resistant capability for carbon/carbon composites at 1500–1600 ◦C [10]. However, due to the mismatch of the thermal expansion coefficient between the SiC bonding layer and the MoSi2 outer coating, micro-cracks in the MoSi2 appear at extended high-temperature (i.e., 800–1000 ◦C) exposure [11–14]. Therefore, the protective temperature range of coatings containing MoSi2 is constricted; this limits the structural and high-temperature applications of such materials. The coefficient of thermal expansion (CTE) of MoSi2 (~8.1 <sup>×</sup> 10−6/K) is higher than that of carbon/carbon composites (~1.0 <sup>×</sup> 10−6/K). Therefore, the mismatch between the CTEs of the outer MoSi2 layer and the inner SiC-coated carbon/carbon matrix causes the degradation of the coating under thermal cycling and ultimately lowers the coating durability [15,16].

MoSi2 coating at a temperature range of 400–600 ◦C efficiently reacts with oxygen to form MoO3 and SiO2. These oxides result in the considerable volume expansion of the MoSi2 matrix, which results in the disintegration of bulk MoSi2 into powders and causes catastrophic failure of the coating [17]. This is called the "pest phenomenon" and is another primary reason for coating failure. In addition, MoSi2 is a low toughness material which limits its industrial application even at ambient temperature [18].

Earlier reports have suggested that the addition of Si3N4 to MoSi2 can minimize the coefficient of thermal expansion (CTE) mismatch between the outer MoSi2 and inner SiC coatings. Si3N4 exhibits high flexural strength and excellent compatibility with MoSi2 and SiC due to better mixing with these compounds. It also exhibits a reasonable resistance to creep, isothermal oxidation, and cyclic thermal oxidation [19]. The MoSi2-Si3N4 coating exhibits excellent oxidation resistance with minute weight loss. At high temperature, MoSi2-Si3N4 forms a dense glassy SiO2 film on the coating surface. Owing to its low oxygen-diffusion coefficient, the SiO2 film serves as an oxygen diffusion barrier and efficiently protects carbon/carbon composites from oxidation at 1773 K [20,21]. Due to good fluidity at high temperatures, SiO2 can seal all the micro-cracks formed during the volatilization of MoO3, CO2, and N2 [22]. Furthermore, the addition of Si3N4 provides better refractory and some oxidation-resistant properties, which decrease the possibility of pest disintegration of MoSi2. Huang et al. [23] prepared a MoSi2/Si3N4 coating on Mo substrate and described the effect of the Si3N4 content on the microstructure and antioxidant properties of multi-layer coatings.

In this study, coatings composed of a SiC bonding layer and a MoSi2-Si3N4 outer layer were prepared by a simple and low-cost slurry method. The phase composition, microstructure, and oxidation resistance of the above multi-layer coatings at 1773 K in the air were investigated in detail.
