2 - Deformation and Fracture of Ceramic Materials

Abstract

When a load is applied to a material, deformation occurs because of a slight change in the atomic spacing. The load is defined as stress (σ), and it is typically measured in units of pounds per square inch (psi) or megapascals (Mpa). The deformation is defined as strain (ɛ): measured in inches (or centimeters) of deformation per inches (or centimeters) of the initial length or in percent. The mechanism of plastic deformation involves movement of dislocations. A dislocation is a defect in the way planes of atoms are stacked in a crystal structure. There are two types of dislocation: edge dislocation and screw dislocation. Under an applied load, these types of dislocations can form and multiply leading eventually to the fracture of the ceramic material. Ceramics are known as very brittle materials. The phenomenon of plastic deformation due to the dislocation activity is limited to very high temperatures especially for polycrystalline ceramics. At room temperatures, the random orientation of the grains severely inhibits dislocation motion (which terminates at the grain boundaries). The two mechanisms for plastic deformation are slip and twinning.