*Article* **E**ff**ect of MoSi2-Si3N4**/**SiC Multi-Layer Coating on the Oxidation Resistance of Carbon**/**Carbon Composites above 1770 K**

**Imran Abbas 1, Yanxiang Wang 1, Hassan Elahi 2,\*, Muhammad Ali Siddiqui 3, Mudaser Ullah <sup>4</sup> and Faisal Qayyum <sup>5</sup>**


Received: 25 May 2020; Accepted: 30 June 2020; Published: 3 July 2020

**Abstract:** To improve the oxidation resistance of carbon/carbon composites at high temperatures (>1770 K), they were coated with MoSi2-Si3N4/SiC. The slurry and pack cementation methods were adopted to deposit the inner SiC layer and outer MoSi2-Si3N4 layer. The phase composition, microstructure, and elemental distributions in the coating were analyzed using SEM, XRD, EDS, and Raman spectroscopy. Oxidation tests show that the deposited multi-layer coating can protect the carbon/carbon matrix from oxidation at high temperatures (>1770 K) for 150h and that the coating can withstand 40 thermal cycles between 1773 and 300 K. It is observed that Si3N4 assists in the formation of a dense SiO2 layer at a high temperature, which plays a vital role in increasing the thermal cyclic and oxidation resistance of the coating itself. The weight loss of coated carbon/carbon composite is attributed to the formation of micro-cracks and diffusion of SiO2, MoO3, and N2 out of the material at high temperatures.

**Keywords:** carbon/carbon composites; multi-phase coatings; oxidation resistance; thermal cycling
