Brittle fracture ceramics

In ceramics, brittle fracture is controlled by the extension of small flaws which are dispersed in a material or component s surface, and which behave like cracks. Flaws can arise not only from the production process, but also from handling and service. Some examples of critical flaws are shown in Figures 12.1 and 12.7. 

So why aren t today s engines made of ceramics The short answer is that, unlike metals, ceramics cannot bend and deform to absorb impacts. Intense research is currently under way to solve the problem of ceramic brittleness, with some success. Improved resistance to fracturing, for example, can be attained by careful quality control of starting materials and processing. As we shall see in the next section, brittleness can also be combated by compositing ceramics with other materials. 

The program also addressed the need to develop lough ceramic-matrix composites (CMCs) with much greater resistance to brittle fracture. Early in the program, researchers round that the chemical structure that imparts superior thermal and mechanical properties to ceramics also results in negative altribuies. panicularly of brittleness, which easily can lead 10 catastrophic failure. 

Hilmas, G., Brady, A., and Halloran, J.W. (1995), SiC and Si3N4 fibrous monoliths non-brittle fracture from powder processed ceramics , Ceram. Trans., 51, 609-614. 

Brittle fracture is accompanied by very little plastic deformation and is predominant in ceramics and inorganic material. Mostly, a very rapid propagation of the crack is observed. 

Tn many applications of plastics, when products finally fail, they fail because of brittle fracture. Plastics are not unique in this respect products made of metals, ceramics, and wood also sometimes fail by brittle fracture. Thus the mechanism and understanding of brittle fracture are a major concern of materials scientists and materials engineers. 

Some substrates must be heated for deposition. For example, large ceramic (brittle) substrates must be evenly heated to avoid large temperature gradients, which could result in fracture, across their surface. Many coating materials and phases also require a warm or hot substrate to deposit. This characteristic is related to the surface diffusion and thermodynamics of the materials. A material s dependence on a heated substrate may be only for the deposition itself to achieve a dense, continuous film or for the deposition of a particular phase or morphology. For example, many materials require a hotter substrate to form a crystalline film as opposed to an amorphous film. 

Finally, another important ramification of the stochastic nature of brittle fracture is the effect of the stress distribution during testing on the results. When a batch of ceramics is tested in tension, the entire volume and surface are subjected to the stress. Thus a critical flaw anywhere in the sample will propagate with equal probability. In three- or four-point flexure tests, however, only one-half the sample is in tension, and the other one-half is in compression. In other words, the effective volume tested is, in essence, reduced. It can be shown that the ratio of the tensile to flexural strength for an equal probability of survival is... 

Mecholsky [86] has proposed an equation of this sort to represent the brittle fracture of ceramics it would be of interest to investigate its applicability to the fracture of adhesive bonds. 

Fracture has been of importance to humankind since the shaping of primitive tools was first introduced and yet the scientific understanding of this subject has only been developed during this century. Ceramics are prone to brittle fracture, which usually occurs in a rapid and catastrophic manner. Clearly, such behavior is unacceptable for many technological applications of these materials. In response to this challenge, there has been a substantial research effort in the last 25 years aimed at improving the reliability and safety of these materials. 

Brittle fracture is used for shaping and machining ceramics after they have been fired. Ceramics can be modified to make them machinable this is controlled fracture and is the approach we adopt with machinable glass-ceramics such as Macor (Chapter 26). Of course, many ceramics already are machinable and can be shaped into intricate and beautiful forms as illustrated in the carved marble sculpture shown in Figure 18.1. 

It is well known that the material removal mechanism in the grinding of ceramics is mainly a brittle fracturing process and grinding induced damage in terms of microcracks has been observed in various ceramics. Ceramic materials are very sensitive to cracks due to their low fracture toughness. The principal induced crack systems are the lateral cracks and the median cracks. The lateral cracks are parallel to the ground surface, and the median... 

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