Self-propagating high-temperature synthesis of a highly dispersed Si3N4–SiC ceramic powders composition using sodium azide and polytetrafluoroethylene

Refractory ceramic composite materials of the silicon nitride–silicon carbide (Si 3 N 4 –SiC) system possess a wide range of valuab­le properties and are used across various industrial fields as excellent refractories, structural heat-resistant materials capable of withstanding high mechanical loads at elevated temperatures, and lightweight functional materials for microwave radia­tion shielding in aviation and aerospace applications. The performance of Si 3 N 4 –SiC composite ceramics can be significantly enhanced by increasing the dispersion of the component powders, transitioning from micron-sized particles to highly dispersed powders (<1 µm). This study focuses on improving a simple, energy-efficient method of azide self-propagating high-temperature synthesis (SHS) for obtaining such highly dispersed powder compositions, using mixtures of sodium azide (NaN 3 ) with elemental silicon and carbon powders, activated and modified by carbiding addition of powdered polytetrafluoroethylene (PTFE). These charge compositions, in both bulk and pressed forms, were combusted in a nitrogen atmosphere at 3 MPa. The maximum pressure and solid product yield were measured. The phase composition and microstructure of the combustion products were examined using X -ray diffraction (XRD) and scanning electron microscopy (SEM). The introduction of PTFE as a reactive carbiding and activating additive effectively overcame the limitations of conventional azide SHS processes that use halide salts such as NH 4 F, Na 2 SiF 6 , and (NH 4 ) 2 SiF 6 . In addition to ensuring a high dispersion of the synthesized powders, the phase composition – particularly for the pressed charges – became significantly closer to the target theoretical composition. Notably, the silicon carbide content in the Si 3 N 4 –SiC product increased substantially, while the amounts of free silicon and carbon impurities decreased.