Addition of Si₃N₄ Strengthens SiC Coatings via Heat Treatment with Nitrogen Gas onto Carbon/Carbon Composites

We report the synthesis of SiC/Si₃N₄ coatings on carbon/carbon composites via pack cementation and heat treatment with nitrogen gas, the latter of which improves the coating wear resistance. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) are used to analyse the microstructure, surface morphology, chemical states and elemental distribution, respectively, of SiC and SiC/Si₃N₄ coatings. In addition, we analyse the tribological behaviour of the SiC and SiC/Si₃N₄ coatings and the related microstructure and wear mechanisms. The results show that SiC/Si₃N₄ coatings are compact and contain the three phases: Si₃N₄, SiC and C. Additionally, specimens with the SiC/Si₃N₄ coating have smaller, more stable friction coefficients and less weight loss than specimens with only the SiC coating. Adhesive wear and abrasive wear are the main wear mechanisms contributing to the higher friction coefficient of the SiC coating. Furthermore, with the SiC/Si₃N₄ coating, adhesive wear is the main wear mechanism causing a high friction coefficient at the initial stage of frictional wear. In the subsequent stages of frictional wear, the graphite in the SiC/Si₃N₄ coating generates a thin lubricating film that decreases the friction coefficient.