Diffusion in Si₃N₄

Abstract

The interest in Si₃N₄ is associated with the desire to find and develop a suitable ceramic for high-temperature applications, particularly for gas turbines. It is a predominantly covalent bonded compound which decomposes at 1877 °C. Therefore, it is impossible to densify Si₃N₄ without sintering additives. The presence of a large degree of porosity was solved by the hot-pressing of previously formed silicon nitride with various sintering additives, after which it was observed that the self-diffusivity became quite low. The low self diffusivity is important because of the intended application at high temperature for gas turbines. The additives not only improve the mechanical properties but also the processing of Si₃N₄. One immediately realizes the importance not only of self-diffusion studies, but also of the diffusion of solutes. Therefore in this chapter the subject is self-diffusion and solute diffusion, which have been the topics of the other ceramics as well. Despite of the tremendous amount of research and the important role that Si₃N₄ plays as diffusion barriers in silicon device technology, not enough experimental emphasis has been devoted to the provision of diffusion data. In particular, data on self- and solute diffusion in grain boundaries and dislocations are missing.