The rate-controlling processes in the oxidation of HIPped Si3N4 with and without sintering additives

Abstract Various possible rate-controlling processes in the oxidation of HIPped Si 3 N 4 with 4 wt% Y 2 O 3 and without sintering additives have been studied using TGA, XRD, FTIR, SEM and TEM/STEM methods. It is proposed that, in the temperature range 1000 to 1450°C, the oxidation kinetics of the studied Si 3 N 4 materials is mainly determined by O 2 diffusivity in the heterogeneous oxide scales and possibly also by the chemical reactivity of Si 3 N 4 materials with O 2 . It is shown that the effective O 2 diffusion coefficient in the glassy oxide scales decreases with an increasing volume of devitrified phases and changes with the structural form of the oxide scales. This may explain why experimental oxidation kinetic curves usually deviate from the conventional parabolic growth law.