Strength of ceramics

Tensile testing of ceramics is time-consuming and expensive because of the difficulty in machining test specimens. Instead, the simpler transverse bending or flexure test is used, where the specimen is loaded to failure in either 

The flaws in ceramics can be either internal or surface flaws generated during processing or surface flaws introduced later, during machining or service. 

Pores are usually quite deleterious to the strength of ceramics not only because they reduce the cross-sectional area over which the load is applied, but more importantly because they act as stress concentrators. Typically the strength and porosity have been related by the following empirical relationship  

Usually, the stress intensities associated with the pores themselves are insufficient to cause failure, and as such the role of pores is indirect. Fracture from pores is typically dictated by the presence of other defects in their immediate vicinity. If the pore is much larger than the surrounding grains, atomically sharp cusps around the surface of the former can result. The critical flaw thus becomes comparable to the dimension of the pores. If the 

Data taken from O. Kamigaito, in Fine Ceramics, S. Saito, ed., Elsevier, New York, 1988. 

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Fig. 17.2. Tests which measure the fracture strengths of ceramics, (a) The tensile test measures the tensile strength, CTj. (b) The bend test measures the modulus of rupture, o , typically 1.7 x CTj. (<) The compression test measures the crushing strength, a, typically 15 x...

As we shall see, there are two ways of improving the strength of ceramics decreasing by careful quality control, and increasing Ki - by alloying, or by making the ceramic into a composite. But first, we must examine how strength is measured. 

Describe briefly how the tensile strength of ceramic materials is determined by their microstructures. How may the tensile strength of ceramics be improved ... 

Based on these results, one may conclude that the theoretical strength of a solid should be roughly one-tenth of its Young s modulus. Experience has shown, however, that the actual strengths of ceramics are much lower and are closer to T/100 to 7/1000. The reason for this state of affairs is discussed in greater detail in Chap. 11, and reflects the fact that real solids are not perfect, as assumed here, but contain many flaws and defects that tend to locally concentrate the applied stress, which in turn significantly weaken the material. 

The theoretical strength of a bond is on the order of T/10. The actual strengths of ceramics, however, are much lower for reasons to be discussed in Chap. 11. 

Flaws, their shape, and their propagation are the central themes of this chapter. The various aspects of brittle failure are discussed from several viewpoints. The concepts of fracture toughness and flaw sensitivity are discussed first. The factors influencing the strengths of ceramics are dealt with in Sec. 11.3. Toughening mechanisms are dealt with in Sec. 11.4. Section 11.5 introduces the statistics of brittle failure and a methodology for design. 

Typically, the strength of ceramics shows an inverse correlation to the average grain size G. A schematic of the dependence is shown in... 

The effect of temperature on the strength of ceramics depends on many factors, the most important of which is whether the atmosphere in which the testing is being carried out heals or exacerbates preexisting surface flaws in the material. In general, when a ceramic is exposed to a corrosive... 

With the increase of FGM layer porosity P, the change of stress at the bonding interface of the three-layered plate increases, and the stress curves appear peak values. The tensile stress on the surface of ceramics reaches the maximum. The tensile stress is unfavorable to the strength of ceramics. 

Figure 1.12 summarizes the tremendous improvements that have been made in the strength of ceramics over the last 25 years. This is a direct result of improved processing, the detection of new toughening mechanisms, and the refined understanding of the relationships between microstructure and mechanical properties. In the remainder of this book, the various mechanical properties of ceramics will be discussed, both in terms of a formal description and the way... 

In all the above testing approaches, it is critical for the details of the testing procedure to be reported. As shown later, the strengths of ceramics are dependent on the loading geometry and specimen dimensions. It is particularly important to understand these effects, when comparing strength data from various sources. 

The strength of ceramics is affected by many factors, and this complexity is illustrated in Figure 16.3. The composition and microstructure are particularly significant and mechanical properties depend strongly on these characteristics. Figure 16.4 shows two specific examples that... 

Explain briefly why there is a size dependence for the strength of ceramics but not for metals. 

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