Properties

Unlike most metals, nearly all ceramics are brittle at room temperature; i.e., when subjected to tension, they fail suddenly, with little or no plastic deformation prior to fracture. Metals, on the other hand, are ductile (that is, they deform and bend when subjected to stress), and they possess this extremely useful property owing to imperfections called dislocations within their crystal lattices. There are many kinds of dislocations. In one kind, known as an edge dislocation, an extra plane of atoms can be generated in a crystal structure, straining to the breaking point the bonds that hold the atoms together. If stress were applied to this structure, it might shear along a plane where the bonds were weakest, and the dislocation might slip to the next atomic position, where the bonds would be re-established. This slipping to a new position is at the heart of plastic deformation. Metals are usually ductile because dislocations are common and are normally easy to move.

In ceramics, however, dislocations are not common - though they do exist - and they are difficult to move to a new position. The reasons for this lie in the nature of the bonds holding the crystal structure together. In ionically bonded ceramics some planes contain only one kind of ion and are therefore unbalanced in their distribution of charges. Attempting to insert such a half plane into a ceramic would not favour a stable bond unless a half plane of the oppositely charged ion was also inserted. Even in the case of planes that were charge-balanced--for instance, the plane created by a vertical slice down the middle of the rock salt crystal structure, as shown in Figure 3, bottom--slip induced along the middle would bring identically charged ions into proximity. The identical charges would repel each other, and dislocation motion would be impeded. Instead, the material would tend to fracture in the manner commonly associated with brittleness.

In order for polycrystalline materials to be ductile, they must possess more than a minimum number of independent slip systems; i.e., planes or directions along which slip can occur. The presence of slip systems allows the transfer of crystal deformations from one grain to the next. Metals typically have the required number of slip systems, even at room temperature. Ceramics, however, do not, and as a result they are notoriously brittle.

Glasses, which lack a long-range periodic crystal structure altogether, are even more susceptible to brittle fracture than ceramics. Because of their similar physical attributes (including brittleness) and similar chemical constituents (e.g., oxides), inorganic glasses are considered to be ceramics in many countries of the world. Indeed, partial melting during the processing of many ceramics results in a significant glassy portion in the final makeup of many ceramic bodies (for instance, porcelains), and this portion is responsible for many desirable properties (e.g., liquid impermeability).

Unlike metals and glasses, which can be cast from the melt and subsequently rolled, drawn, or pressed into shape, ceramics must be made from powders. As pointed out above, ceramics are seldom deformable, especially at room temperature, and the microstructural modifications achieved by cold-working and recrystallizing metals are impossible with most ceramics. Instead, ceramics are usually made from powders, which are consolidated and densified by sintering. Sintering is a process whereby particles bond and coalesce under the influence of heat, leading to shrinkage and reduction in porosity. A similar process in metal manufacturing is referred to as powder metallurgy.