Ceramics

Ceramics have some attractive properties compared to metals and polymers, which make them useful for specific applications. Their physical properties have been utilized for many applications. In other applications their mechanical properties are important. The main drawbacks of ceramics are their brittleness and the large scatter in the mechanical properties. In this introductory section a short overview of the most important ceramics and of their basic properties is given.

In this section data for some important physical properties are compiled: thermal expansion coefficient α, thermal conductivity λ, electrical resistivity ρ, specific heat C_p, density ρ, Young’s modulus E, and Poisson’s ratio ν. These properties are not only important for material selection for a specific physical application, they also characterize the thermal shock sensitivity of ceramics.

Ceramic components in most cases fail by unstable propagation of natural flaws which are present due to manufacturing or surface treatment. Such defects may be pores, cracks or inclusions. The brittleness of ceramic materials is caused by the low resistance against crack extension. The observed large scatter in strength is due to the scatter of flaw size in a component.

The experimental observation of a rising crack growth resistance curve was explained in Sect. 3.1.2. In this section experimental and computational methods as well as the responsible mechanisms will be outlined in more detail.

So far two types of crack extension have been considered: stable crack extension starting at K ₁ =K ₁₀ and unstable crack extension, if K ₁=K _1inst is reached (or at K ₁ =K _1c for a material with a flat crack growth resistance curve). A third type of crack extension is called subcritical crack extension. This is a time-dependent phenomenon, where a crack is growing at constant load below K ₁ =K _1inst. In this section the subcritical crack extension below the high temperature regime is considered. The creep crack growth is described in Sect. 12.2. The subcritical crack extension under cyclic loading is dealt with in Chap. 6.

The failure of ceramic components under cyclic loading — often designated as cyclic fatigue — was already mentioned in Sect. 5.2.3. It was shown that the lifetime can be predicted from the results obtained in tests with constant load under the assumption that the same mechanism is responsible for constant and cyclic loading. This assumption has been confirmed by cyclic experiments on glass and porcelain [6.1]. For many ceramic materials (especially for alumina and zirconia) a real cyclic effect could be proved (see e.g. [6.2-6.8]). The crack growth rate in these materials is larger and the lifetime is shorter than that predicted from constant load tests.

The ‘strength’ of ceramics is commonly defined as the material resistance against tensile stresses. The compressive strength is of minor importance.

The scatter in the strength of ceramic materials is significantly larger than that of metals. As already mentioned, failure starts from small defects existing in the material. The scatter of strength, therefore, is caused by the scatter of the flaw size. This is also the reason for a pronounced influence of the component size on strength. The lifetime of ceramics undergoing subcritical crack extension also shows a large amount of scatter, which can be related to the scatter of the strength.

Due to the large scatter in strength of ceramics, failure of components must be expected even under low loading. The possibility of excluding such failure events is given by the application of a proof-test procedure [9.1-9.3].

In the usual tests to determine strength or lifetime under tension, bending or compression loading a uniaxial stress state is present. In components very often multi-axial stresses occur. Also under uniaxial external loading multiaxial stresses are possible, for instance in notched components. Rotating structures and components under internal pressure exhibit multiaxial stresses. Also thermal stresses are in general multiaxial. In this section, failure criteria under multiaxial loading are presented.

Most ceramic materials are sensitive to thermal shock and thermal fatigue. Due to inhomogeneous temperature distributions in rapidly cooled or heated ceramic components, high thermal stresses are generated which are responsible for the extension of existing cracks. Whereas in metals the temperature gradients only cause small plastic deformations, in the case of ceramics with its linear elastic material behaviour high stresses are generated. As a consequence, thermal stresses have to be avoided or at least to be minimized by an appropriate design or an appropriate material selection.

Deformation and failure behaviour of ceramics at very high temperatures is predominantly governed by creep effects. In the case of noticeable creep, creep-induced deformation itself may lead to a design limit if the function of a component is affected by an excessive global deformation. Creep rupture consists in the formation and extension of creep cracks.

Plastic deformation of materials in the general sense can be defined as irreversible deformation, which means that in a tensile or compression test after loading and unloading a deformation remains. Different mechanisms can be responsible for such an irreversible or plastic deformation: dislocation motion, vacancy motion, twinning, phase transformation, and viscous flow of amorphous materials. In a polycrystalline or multiphase material the deformation can take place within the grains or predominantly along the grain or phase boundaries. In metals at room temperature dislocation motion is the most important deformation mechanism. In ceramics dislocation motion requires high shear stresses due to covalent bonds. Therefore under most loading conditions ceramics fail by the extension of flaws, whereas the competing failure mechanism by dislocation motion would require higher stresses. Nevertheless the plastic deformation and the formation of dislocations have been observed under specific loading conditions.