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 characteriz ed with respect to their high-temperature micromechanical deformation behav ior employing both transmission electron microscopy and the internal fricti on technique. The latter method was utilized to gain a direct measure of th e high-temperature response of the respective material, i.e., the effect of the interfacial glass phase commonly observed in liquid-phase sintered cer amics on externally applied shear stress. Transmission electron microscopy provides complementary information about the structure and chemistry of int ernal grain boundaries, which are known to dominate the high-temperature me chanical behavior of the bulk ceramic polycrystal. In addition, the presenc e and distribution of amorphous or crystalline secondary phases were charac terized by electron microscopy. It is shown that, apart from the overall mi crostructure, the interface structure and/or the local chemical composition is the main parameter affecting the internal friction behavior. As a conse quence, this technique allows one to determine the effective interface visc osity of ceramic polycrystals and to reveal as to whether a bimodal grain-b oundary structure has developed, e.g., if both wetted and non-wetted interf aces are present, as is shown for the SiC ceramic.