Superplasticity of Ceramics

Abstract

Ceramics possess a number of unique properties, such as high-temperature strength, wear resistance and hardness, enhanced resistance to corrosion, and other species-specific physical and mechanical properties. These are determined by their electronic, atomic, micro- and macrostructure. In the electronic structure two types of bonds dominate — covalent and ionic. Such compounds can be semiconductors and dielectrics, have enhanced hardness and high elasticity moduli, stable mechanical properties in a broad range of temperatures, and low coefficient of thermal expansion. However, it is the nature of the bonding that also determines the principal limitation of ceramics, i.e., their absolute brittleness. When a load is applied, the bonding force hinders the motion of dislocations because of the high value of the potential Peierls-Nabarro barriers. Hence, under conventional conditions, SP deformation in ceramics is practically impossible. The brittleness of ceramics is caused by microscopic defects such as voids and inclusions that, being strong concentrators of stresses, become sources of cracking. Since the plasticity of ceramics is zero, no relaxation of stresses occurs in such places and under a load exceeding a certain threshold value the material immediately fails. Although ceramic materials possess many useful properties, the lack of plasticity considerably limits their application as construction materials.