Mechanics of Ceramics and Glasses

In the previous sections, we saw how materials can behave in elastic and ductile manners and how plastic flow past the yield point could be considered a form of destructive failure due to permanent deformation. In this section, we will see that tliere 

We also know that to a first approximation, Hooke s Law applies to the elastic region, which for brittle materials is effectively up to the point of cleavage, so that the stress at cleavage, Oc, is given by Hooke s Law as 

Use values of modulus from Appendix 7, along with surface energy from Appendix 4, to estimate the following quantities. In both cases, take an interplanar distance of 10 cm as an order-of-magnitude estimate. 

Compare your answers. Based on these results, what would you expect the theoretical 

The crack detection limit for a device that inspects steel structural beams is 3 mm. A structural steel with a fracture toughness of 60 MPa m° has no detectable surface cracks. Assume a geometric factor of unity to determine  

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Material Removal Mechanisms in Grinding of Ceramics and Glasses... 

Specimen size, in general, has an effect on mechanical tests, and ceramics or glasses are no exception. The shapes of ceramic and glass test specimens are often a matter of convenience, but, in most cases, the specimen s form dictates the test conditions. Various other techniques have also been suggested to improve the reliability of the strength result. One such technique, used for the tensile testing of ceramic fibers, is video extensometry [25]. Figure 1.8 illustrates a schematic video set up for the evaluation of the results of SiC monofilaments. 

The subject of sol-gel science is cova-ed in great depth in Ref. 1. We outlined the basic steps in sol-gel processing in ChaptCT 1 when we surveyed the common methods used for the production of ca-amics. This chapter provides a more detailed examination of the science and practice of the process for the fabrication of ceramics and glasses. We shall pay particular attention to the sol-gel processing of silica glass not only because of its practical interest but also because of the heightened understanding of the process mechanisms and structural evolution developed from numerous studies. 

As with thermal conductivity, we see in this section that disorder can greatly affect the mechanism of diffusion and the magnitude of diffusivities, so that crystalline ceramics and oxide glasses will be treated separately. Finally, we will briefly describe an important topic relevant to all material classes, but especially appropriate for ceramics such as catalyst supports—namely, diffusion in porous solids. 

Heat resistance of organic polymers is far lower than that of metals, ceramics, and glass. There have been major improvements, based on aromatic and heterocyclic resonance, ladder structures, and other mechanisms, and we may see further improvement in the future. Perhaps more serious limitations are the high cost of synthesis and the difficulty of processing these polymers into the desired final products. This is an area where the polymer chemist could use more help from the plastics engineer. 

In the solid state, metals are crystalline, i.e. the atoms are arranged in a regular three-dimensional pattern with cubic structures being the most common. This accounts for the excellent mechanical properties of metals such as ductility and toughness. Ceramics and glasses have extremely complicated crystal shapes and, as a result, are very hard and brittle at room temperature. Due to their crystal structure, it is possible to form alloys of two or more metals and this can result in a considerable improvement in certain mechanical properties such as strength and hardness. 

On cooling, plastic materials tend to contract or shrink considerably more than other materials such as metals, ceramics and glass. For example, a copper pipe will shrink by 0.01% if the temperature is reduced by 10°C. Under the same conditions, a high-density polyethylene pipe would shrink by 0.07%, and polypropylene and hard PVC pipes by 0.04%. In addition, surfaces of plastic materials cool before their cores. Such a situation leads to the initial contraction of plastic materials at surfaces, before significant change in dimension occurs in the bulk. The skins of moulded plastics tend to be stiffer than the bulk, so are more prone to degradation by mechanical action, e.g. flexing. 

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