Kinetics and mechanism of thermal oxidation of sialon ceramic powders

X-ray diffraction, solid state MAS NMR and isothermal thermogravimetry were used to study the oxidation mechanism and kinetics of pure X-phase and O′-sialon powders prepared by silicothermal synthesis. X-phase sialon (Si₁₂Al₁₈O₃₉N₈) begins to oxidise at above 940°C, forming amorphous silica and progressively losing nitrogen from the SiO₂N₂ groups to form mullite at 1200-1300°C. O′-sialon is more oxidation-resistant; although significant formation of amorphous silica occurs above 940°C, the crystalline products (silica and a small amount of mullite) appear above 1235°C. At 1400-1600°C, oxidation is progressively hindered by the formation of a protective fused silica surface layer containing octahedral Al. The oxidation kinetics can be described by a parabolic rate law, with temperature coefficients of reaction rate (E_a) of 250 and 454 kJ mol^-1 for X-phase and O′-sialon, respectively. The kinetic results are discussed in the context of previous sialon powder oxidation studies.