Oxidation kinetics of an amorphous silicon carbonitride ceramic

The oxidation kinetics of amorphous silicon carbonitride (SiCN) was measure d at 1350 degreesC in ambient air. Two types of specimens were studied: one in the form of thin disks, the other as a powder. Both specimens contained open nanoscale porosity. The disk specimens exhibited weight gain that sat urated exponentially with time, analogous to the oxidation behavior of reac tion-bonded Si3N4. The saturation value of the weight gain increased linear ly with specimen volume, suggesting the nanoscale pore surfaces oxidized un iformly throughout the specimen. This interpretation was confirmed by high- resolution electron microscopy and secondary ion mass spectroscopy. Experim ents with the powders (having a particle size much larger than the scale of the nanopores) were also consistent with measurements of the disks. Howeve r, the powder specimens, having a high surface-to-volume ratio, continued t o show measurable weight gain due to oxidation of the exterior surface. The wide range of values for the surface-to-volume ratio, which included all s pecimens, permitted a separation of the rate of oxidation of the free surfa ce and the oxidation of the internal surfaces of the nanopores. Surface oxi dation data were used to obtain the rate constant for parabolic growth of t he oxidation scale. The values for the rate constant obtained for SiCN lay at the lower end of the spectrum of oxidation rates reported in the literat ure for several Si3N4 and SiC materials. Convergence in the behavior of SiC N and CVD-SiC is ascribed to the purity of both materials. Conversely, it i s proposed that the high rates of oxidation of sintered polycrystalline sil icon carbides and nitrides, as well as the high degree of variability of th ese rates, might be related to the impurities introduced by the sintering a ids.