Solid-state sintering is a process in which granular or powdery starting materials are mixed and then combined with one another by heating. The sintering temperature is below the melting temperature of all phases involved. The sintering itself takes place only through diffusion processes and evaporation and condensation mechanisms; under appropriate external pressure, it also takes place through crystal-plastic flow. During the sintering process, the free energy of the entire system is reduced due to surface and interboundary energy reduction. However, it must be considered that in real systems the specific surface and interboundary energies depend on the crystallographic direction.