*2.1. Coating Preparation*

Small specimens (10 <sup>×</sup> 10 <sup>×</sup> 10 mm3) of carbon/carbon composite (Shandong Weiji Carbon Technology Co. Ltd., Jinan, China) with a density of 1.79g/cm3 used as substrates were cut from thin bulk sheet. The carbon/carbon composite used in this study was carbon fiber-strengthened graphite. The surface area of the samples was increased by abrading them with commercially available rough SiC papers. To remove any remaining debris and moisture, the samples were cleaned ultrasonically with distilled water and dried in an oven at 100 ◦C for 2 h.

The powder compositions used were Si 75 wt%, graphite 20 wt%, and Al2O3 5 wt% respectively. All the powders used were lab-grade and had a purity ≥ 99% with particle size ranging between 1–10 μm. SiC inner coatings were applied to the specimens by the pack-cementation method. The carbon/carbon composites and pack mixtures were put in a graphite crucible and heated at 1873 K for 2 h in an argon atmosphere to form the SiC coating. After the completion of inner coating, the SiC-coated specimens were cleaned ultrasonically and dried in an oven at 100 ◦C for 2 h.

The liquid slurry mixture method was adopted for applying the outer coating of MoSi2-Si3N4 over the already applied inner SiC coating. High-purity MoSi2 and Si3N4 powders supplied by Turnnano Ltd., Henan, China) were used. The MoSi2 and Si3N4 powders were mixed in 10mL aqueous solution of H-PSO, and a homogenous slurry solution was prepared. The SiC-coated carbon/carbon specimens were put in a slurry mixture of MoSi2 and Si3N4. The as-coated specimens were put in a

graphite crucible and heated up to 2073 K for two hours in the nitrogen atmosphere to apply the outer MoSi2-Si3N4 coating on the samples.

The slurry method was preferred over others due to its compositional flexibility, ease of processing, and low cost. Also, it is known that the coating deposited via the slurry method exhibits reasonable resistance to oxidation and thermal cyclic oxidation [11]. The MoSi2-SiC and MoSi2-Si3N4/SiC coatings were prepared using a similar approach to elucidate the effect of Si3N4 addition on the oxidation resistance of the multi-layer coatings.
