Oxidation mechanism of Si₃N₄-bonded SiC ceramics by CO, CO₂ and steam

This paper presents a theoretical and experimental investigation into the oxidation reactions of Si₃N₄-bonded SiC ceramics. Such ceramics which contain a small amount of silicon offer increased oxidation and wear resistance and are widely used as lining refractories in blast furnaces. The thermodynamics of oxidation reactions were studied using the JANAF tables. The weight gain was measured using a thermogravimetric analysis technique to study the kinetics. The temperature range of oxidation measurements is from 1073 to 1573 K and the oxidation atmosphere is water vapour, pure CO and CO-CO₂ gas mixtures with various CO-to-CO₂ ratios. Thermodynamic simulations showed that the oxidation mechanism of Si₃N₄-bonded SiC ceramics is passive oxidation and all components contribute to the formation of a silica film. The activated energies of the reactions follow the sequence Si₃N₄ > SiC > Si. The kinetic study revealed that the oxidation of Si₃N₄-bonded SiC ceramics occurred in a mixed regime controlled by both interface reaction and diffusion through the silica film. Under the atmosphere conditions prevailing in the blast furnace, this ceramic is predicted to be passively oxidized with the chemical reaction rate becoming more dominant as the CO concentration increases.