HIGH-TEMPERATURE OXIDATION OF Si_3N_4-TiN COMPOSITES: MECHANISMS AND KINETICS

摘要

The oxidation in air of Si_3N_4-based ceramics containing 35vol.% of TiN particles and various additive contents has been studied in a wide range of temperature in dry and humid air. Oxidation scales were characterized by X-ray diffraction, scanning and transmission electron microscopy. At temperatures between 600 and 950℃, only TiN particles at the outer surface are locally oxidized to nanocrystalline rutile. With increasing temperature, such rutile islands laterally spread over the entire surface and eventually form a dense surface scale. At 1000℃, matter transport within the intergranular glass starts to control the oxidation behaviour, and a double scale forms with a rutile top layer and a silicon nitride subscale, in which TiN particles are oxidized to agglomerates of rutile, glass and pores. At 1200℃, silicon nitride is also oxidized; more glass phase forms, and oxidation scales are less porous and more protective.