Interfacial boundaries in Si3N4-based ceramic composites: constraints from matrix effects and stability of microstructure

The stability of the microstructure of Si3N4-based composites with discontinuous SiC reinforcement components (whiskers, platelets) is addressed, focusing on the role of interfacial boundaries. This approach basically explores four issues: (1) The evaluation of structural configuration and interfacial chemistry of internal interfaces by HREM imaging and parallel detection electron energy loss spectroscopy (PEELS) considering the effects of different oxygen sources in the SiC/Si3N4 system. (2) The influence of whisker-related impurities, such as excess carbon and oxygen, on the liquid phase formation process and microstructural development of the Si3N4 baseline material. (3) The variety of local microstructures along interfacial boundaries in Si3N4-based composites employing SiC whiskers, platelets and fibres due to thermodynamic constraints which emphasise the dynamic nature of internal interfaces in ceramic matrix composites. (4) The fundamental problem of correlating microscopic characteristics from grain and phase boundaries with the fracture resistance behaviour of composite materials, considering that macroscopic matrix-related parameters contribute to the mechanical response. 83 refs.