Ceramic Single Crystals

Abstract

Ceramic single crystals span a very wide range of materials from ionic crystals like the alkali halides with ionic bonding over oxides with mixed bonding to the carbides or nitrides with strong covalent bonding. Correspondingly, the Peierls stress and the double-kink formation energies are very different. On the one hand, these quantities are low in the alkali halides so that the Peierls mechanism controls the dislocation mobility and the flow stress only at low temperatures. On the other hand, the ceramic crystals with strong covalent bonding become plastic only at high temperatures. Typical examples of ionic crystals and oxides are presented in this chapter. Heuer and Mitchell recently presented a review of the dislocation and deformation properties [387]. Structural applications of ceramic materials are mostly in the form of polycrystals. At low temperatures, their strength is limited by the brittleness, which is controlled by flaws in the polycrystalline structure acting as crack nuclei. At high temperatures, the materials creep. The creep rates are governed partly by the properties of the grain boundaries and partly by the plastic properties of the crystal grains, the latter of which are discussed in this chapter. A review of creep parameters and models of polycrystalline ceramics is given in [340].