Transmission Electron Microscopy of Micro structures in Ceramic Materials

摘要

Based on selected examples from the area of Si_3N_4 ceramics, the value of utilizing transmission electron microscopy (TEM) as a technique to study ceramic microstructures as well as a characterization tool for the development of new materials is demonstrated. In the field of 'new ceramics', one Si_3N_4-based composite is discussed, which was processed via pyrolysis of liquid precursors (polysilazanes). Moreover, it is shown that TEM in general can helpfully accompany ceramic processing techniques. This applies to the characterization of ceramic starting powders as well as to the study of densified materials. The investigation of Si_3N_4 powders, in particular the influence of the addition of sintering aids via organometallic precursors, which leads to a homogeneous distribution of sintering additives in the powder compact, in contrast to the use of metal oxide powders, is shown. The variation of micro-structures during the densification process of liquid assisted sintering is also demonstrated. The most common application of the TEM technique is to characterize dense ceramic components in the as-processed (as-sintered) state. Post-sintering heat treatment can initiate secondary phase crystallization. However, the very important aspect of microstruc-ture integrity at elevated temperatures, e.g. the stability of microstructures under severe service conditions, is also addressed. Emphasis is placed on the fact that ceramic microstructures, which are typically thought to be rather stable, can undergo serious microstructural changes when temperature and stress is applied simultaneously, which strongly limits potential applications of these materials.