Dislocations in ceramics

Miigge and others found that the only minerals that could easily be deformed under ambient conditions were the alkali halides and a few sulfides and carbonates. An exception to this was periclase (MgO), which deformed by 110 (110) dodecahedral glide in the same way as halite (NaCl). A more recently discovered exception is SrTiOs with the cubic perovskite structure, which can be deformed plastically at ambient and high temperatures but is brittle at intermediate temperatures (see Section 9.4.7). Other oxides and silicate minerals either cleaved or twinned when attempts were made to deform them at normal temperatures and pressures [1]. 

Ceramu Science and Technoit Volume 2 Properties. Edited by Ralf Riedel and I-Wei Chen Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31156-9 

In this chapter, attention will be focused on oxide ceramics. Related reviews are available on ionic crystals by Haasen [5] and by Sprackling [4], on covalent crystals by Hirsch [10] and by Alexander [11], and on silicate minerals by Paterson [12]. Previous reviews on oxides by Wachtman [13], Terwillinger and Radford [14], Bretheau et al. [15], and Mitchell and Heuer [16] are also available. 

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Why consider dislocations in ceramics Conventional wisdom says that dislocations are not nearly as important in the mechanical deformation of ceramics as they are for metals. The reason is that dislocations in ceramics do not move as easily as those in metals and they are usually not as numerous. So we should be asking why the last sentence is true. Dislocations in ceramics are extremely important because of what they do not do they do not glide easily. Si devices would not work for long and ceramics, in general, would not be brittle if dislocations could glide easily. Understanding dislocations also helps us understand other more complex defects, how they interact with point defects, and how they can cause planar defects. Dislocations become very important when we use thin crystalline ceramic films, particularly when grown on crystalline substrates. 

We have not reviewed all the properties of dislocations, but have concentrated on those you should know when considering dislocations in ceramics. 

Perhaps the more important question is why learn about dislocations in ceramics when ceramics do not deform plastically as easily as metals This question then leads us to question why this statement is true. Is it always true Can we change anything Modern materials often involve interfaces. Interfaces are directly related to dislocations. The growth of thin films often involves dislocations. Dislocations also play an important role in radiation damage of ceramics. So there are many reasons for understanding dislocations in ceramics. 

Not much is really known about the core of dislocations in ceramics. For the examples we will show, you should remember that we have usually chosen one Burgers vector (usually the most important one), one line direction, and thus one glide plane. Furthermore, we usually draw the edge dislocation because it is easiest to draw, not because it is the most important. In this section, we will assume that the dislocation core is compact. The examples are chosen to illustrate particular features. 

FIGURE 12.14 Dislocations in ceramics with low SFE. (a-c) Dissociation in graphite (d and e) dissociation... 

We know that as a general rule, it is more difficult to move dislocations in ceramics than in most metals. In fact, at low temperatures it is often easier to fracture the sample than it is to propagate dislocations. We must then ask the following questions ... 

The general need is to understand the response of a material to an applied stress. The stress may be applied externally or induced by altering other parameters such as temperature (which can cause a phase transformation). The fundamental idea is the link to bonding. In Chapter 4 we described how the Young s modulus is related directly to the bond-energy curve. In Chapter 12 we described the nature of dislocations in ceramics. 

Dislocations in ceramics can be pinned by solute atoms just as they can in metals as shown in Figure 17.14. The dislocations are impeded because of their interaction with the stress field around the impurity. This effect has long been used to strengthen metals. 

As pointed out in Section 9.1, Kronberg [7] and Hornstra [8] produced seminal reports on the dissociation of dislocations in ceramics, particularly sapphire and spinel. Kronberg suggested that basal dislocations in sapphire should dissociate according to... 

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