Transmission electron microscopy in conjunction with internal friction measurements - A powerful tool for characterization of ceramic interfaces

Three different non-oxide ceramics, Si3N4, SiAlON, and SiC mere characterized with respect to their high-temperature micromechanical deformation behavior employing both transmission electron microscopy and the internal friction technique. The latter method was utilized to gain a direct measure of the high-temperature response of the respective material, i.e., the effect of the interfacial glass phase commonly observed in liquid-phase sintered ceramics on externally applied shear stress. Transmission electron microscopy provides complementary information about the structure and chemistry of internal grain boundaries, which are known to dominate the high-temperature mechanical behavior of the bulk ceramic polycrystal. In addition, the presence and distribution of amorphous or crystalline secondary phases were characterized by electron microscopy. It is shown that, apart from the overall microstructure, the interface structure and/or the local chemical composition is the main parameter affecting the internal friction behavior. As a consequence, this technique allows one to determine the effective interface viscosity of ceramic polycrystals and to reveal as to whether a bimodal grain-boundary structure has developed, e.g., if both wetted and non-wetted interfaces are present, as is shown for the SiC ceramic.