Introduction: Ceramics is a refractory, inorganic, and nonmetallic material. Strength of ceramics is very dependent upon the flaw distribution in the materials. These "flaws" can be of a microscopic nature and may not be flaws from a normal perspective. Because small flaws can have a very large effect, the strength data of ceramics tends to be widely scattered. In many applications, brittle fracture limits the use of ceramic materials. Failure in applications is often caused by brittle fracture, which results from thermal expansion mismatch between ceramic and metallic parts of electronic packages. In order to understand the fracture behavior of ceramic materials, it is necessary to understand the mechanisms of fracture of materials that are entirely brittle. In these t hese materials plastic deformation by dislocation motion does not occur, or occurs to such a limited extent that cracks are sharp to the atomic level of the solid. Resistance to fracture is provided by the lattice itself, and not by the movement of dislocations. Grain boundaries and crystal anisotropy are shown to be especially important in establishing the fracture resistance of ceramic materials.
Ceramics are brittle: Ceramics have very low toughness values and also fracture toughness values. Recall that brittle fracture consists of formation and propagation of cracks through the cross section in a direction perpendicular to the applied load. Usually Usually the crack growth is trans-granular and along a long specific crystallographic directions. The properties of ceramic materials, like all materials, are dictated by the types of atoms present, the types of bonding between the atoms, and the way the atoms are packed together. The type of bonding and structure helps determine what type of properties a material will have. At room temperature, both crystalline and non crystalline ceramics almost always fracture before any plastic deformation can occur in response to an applied tensile load. Furthermore, the mechanics of brittle fracture and principles of fracture mechanics also apply to the fracture of this group of materials. It should be noted that stress raisers in brittle ceramics may be minute surface or interior cracks (micro cracks), internal pores, and grain corners, which are virtually impossible to eliminate or control. For example, even moisture and contaminants in the
atmosphere can introduce surface cracks in freshly drawn glass fibers; these cracks deleteriously affect the strength. In addition, plane strain fracture toughness values for ceramic materials are smaller than for metals. There is usually considerable variation and scatter in the fracture strength for many specimens of a specific brittle ceramic material. This phenomenon may be explained by the dependence of fracture strength on the probability of the existence of a flaw that is capable of initiating a crack. The atoms in ceramic materials are held together by a chemical bond. Ceramics are crystalline structures that are made of covalent or ionic bonds, and sometimes both. By definition, covalent bonds are unidirectional in nature. As a result, when the force is applied on these covalent bonds, they resist the force without yielding. This explains why ceramics act so tough and do not change shape under any circumstance. On exceeding the threshold limit, the ceramic articles break and thus happen to be very brittle. The reasons that ceramics are so brittle: 1.
Ceramic materials inherently have cracks, flaws, pores and inclusions. These act as stress risers and failure initiates at one of these and propagates quickly (because there is no energy absorbing mechanism as there is in metals,) causing brittle fracture. The covalent bond is directional and electrons are shared. Hence, bonds will not reform easily and so brittle fracture will occur. Since the crystal structures tend to be complex, there are limited slip systems and large Burgers vectors. (called complex dislocation structures) Potential slip planes may involve like charges moving over each other which will cause separation and so brittle fracture will occur. Pore Free Ceramics have binder exists instead of a potential defect and they are distributed minutely and uniformly. When a specimen is loaded, a lot of cracks occur in boundary between binder and ceramic grain and strain energy is released. Some clusters are formed by some cracks' uniting.
fracture in ceramics:
Brittle fracture takes place without any appreciable deformation, and by rapid crack propagation. The direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress and yields a relatively flat fracture surface, as indicated in Figure
Brittle fracture in amorphous materials, such as ceramic glasses, yields a relatively shiny and smooth surface. For most brittle crystalline materials, crack propagation corresponds to the successive and repeated breaking of ato mic bonds along specific crystallographic planes; such a process is termed cleavage . This type of fracture is said to be trans-granular, because the fracture cracks pass through the grains. Macroscopically, the fracture surface may have a grainy or faceted texture, As a result of changes in orientation of the cleavage planes from grain to grain. In some a, crack propagation is along grain boundaries; this fracture is termed intergranular.
Metals are ductile: Ductility of metals is due to the presence of electrons that are scattered and move around all through the metals entire structure. Due to this, the metal does not crack upon pulling or smashing because of the absence of similarly charged ions that are facing each other for repulsion. Electrons are present moving like a sea in the metal causing no repulsion and hence the charges are not lined up even when the metal is pulled. The metallic bonds present in the metals are also responsible for the elevated levels of ductility. The valence shell electrons in the metallic bond are delocalized which is among many atoms. These electrons that are delocalized from the valence shell
help in the sliding of the metals one over the other without having been subjected to very strong repulsive forces allowing shattering of other materials .The bonds across grain boundaries in the crystal space lattice of different metals varies, but they can undergo large plastic deformations without rupture. Malleability and ductility:
Metals are described as malleable (can be beaten into sheets) and ductile (can be pulled out into wires). This is because of the ability of the atoms to roll over each other into new positions without breaking the metallic bond. If a small stress is put onto the metal, the layers of atoms will start to roll over each other. If the stress is released again, they will fall back to their original positions. Under these circumstances, the metal is said to be elastic .
If a larger stress is put on, the atoms roll over each other into a new position, and the metal is permanently changed.
fracture in metals:
The most common type of tensile fracture profile for ductile metals is brittle. The fracture process normally occurs in several stages. First, after necking begins, small cavities, or micro voids, form in the interior of the cross section. Next, as deformation continues, these micro voids enlarge, come together, and coalesce to form an elliptical crack, which has its long axis perpendicular to the stress direction. The crack continues to grow in a direction parallel to its major axis by this micro void coalescence process. Finally fracture ensues by the rapid propagation of a crack around the outer perimeter of the neck by shear deformation at an angle of about 45º with the tensile axis. This is the angle at which the shear stress is a maximum.
Sometimes a fracture having this characteristic surface contour is termed a cup and cone fracture because one of the mating surfaces is in the form of a cup, the other like a cone. In this type of fractured specimen the central interior region of the surface has an irregular and fibrous appearance, which is indicative of plastic deformation.
Callister, W.D. Jr., Material Science and Engineering, an Introduction 6 Edition, Wiley, 2003. eramic Bull et in, Abe, H., "Mechanical Properties of Engineering Ceramics," C Vol. 64, No. 12, pp.1594-1596, 1985. www.ceramics.org. (American ceramic society) eramic Rolf, R.L. and Weyand, J.D., "Structural Design of Brittle Materials," C Bull et in, Vol. 64, No. 10, pp.11360-1363, 1985.