Chemical bonding of oxygen in intergranular amorphous layers in high-purity β-SiC ceramics

An amorphous intergranular layer, 0.4 nm thick, is found by high-resolution transmission electron microscopy of beta-SiC ceramics fabricated by the HIP glass-encapsulation technique without the use of additives. The intergranular layer is found to contain oxygen, which may also be present in the SIC grains near the grain boundary. The mean composition of the intergranular layer is determined to be SiO0.10 (+/-) C-0.01(0.93) (+/- 0.04), whose C/O ratio is close to a hypothetical Si-oxycarbide stoichiometry. Electron energy loss spectroscopy shows that both the Si-L-23 and O-K edges from the intergranular layer are distinctive compared to those of SiC and/or SiO2. This provides direct evidence of the presence of oxygen in a chemical environment different to that in glassy SiO2, that can also be found in the intergranular layer. First principle calculations by the DV-X alpha method using an interface model cluster, satisfactorily reproduce the chemical shifts of the energy loss near edge structure (ELNES) for both the Si-L-23 and O-K edges. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.