Ceramic slip systems

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts... 

Table 5.5 Slip Systems in Some Ceramic Crystals...

Mono- and polycrystalline natural and synthetic materials are not subject to plastic strain and have no independent slip system. Stress concentration occurs in them at.crack tips and at flaws in the material, affecting the maximum strength which originates from the chemical or physical cohesion forces present. Non-plastic materials (crystals, rocks, ceramics, glass) show brittle cracks—forming at very low plastic strain—usually originating from surface flaws. 

This is an important point. A sublattice phase with the FCC structure should not, generally speaking, be considered CCP with regards to slip. The atoms or ions on one sublattice may very well be in a CCP-hke arrangement, but they can be kept apart by large atoms or ions residing on the other sublattice (the interstitial sites). Slip is easiest along tmly close-packed layers of identically sized spheres that are in contact and, preferably, without obstacles such as interstitials. Thus, another reason for low ductility in intermetallics and ceramics is the lack of a sufficient number of active slip systems to allow plastic deformation. 

Table 6.2 Primary and secondary slip systems of ceramics... 

The stress-strain behavior of ceramic polycrystals is substantially different from single crystals. The same dislocation processes proceed within the individual grains but these must be constrained by the deformation of the adjacent grains. This constraint increases the difficulty of plastic deformation in polycrystals compared to the respective single crystals. As seen in Chapter 2, a general strain must involve six components, but only five will be independent at constant volume (e,=constant). This implies that a material must have at least five independent slip systems before it can undergo an arbitrary strain. A slip system is independent if the same strain cannot be obtained from a combination of slip on other systems. The lack of a sufficient number of independent slip systems is the reason why ceramics that are ductile when stressed in certain orientations as single crystals are often brittle as polycrystals. This scarcity of slip systems also leads to the formation of stress concentrations and subsequent crack formation. Various mechanisms have been postulated for crack nucleation by the pile-up of dislocations, as shown in Fig. 6.24. In these examples, the dislocation pile-up at a boundary or slip-band intersection leads to a stress concentration that is sufficient to nucleate a crack. 

For NaCl, the activation of the secondary slip systems at temperatures >200 C is required before ductility in polycrystals is obtained. A similar brittle to ductile transition occurs in KCl at 250 °C. For MgO, this transition occurs 1700 C. Some cubic materials, such as TiC, p-SiC and MgO.AljOj have sufficient independent primary systems but, unfortunately, the dislocations tend to be immobile in these materials. Thus, overall it is found that most ceramic polycrystals lack sufficient slip systems or have such a high Peierls stress that they are brittle except under extreme conditions of stress and temperature. 

TABLE 17.4 Independent Slip Systems for Some Ceramics ... 

However, in sharp contradistinction, the mechanical properties of the MAX phases cannot be more different than those of their binary cousins. The mechanical properties of the MAX phases are dominated by the fact that basal-plane dislocations multiply and are mobile at temperatures as low as 77 K and higher. The presence of basal slip is thus crucial to understanding their response to stress. This is true despite the fact that the number of independent slip systems is less than the five needed for ductility. In typical ceramics at room temperature, the number of independent slip systems is essentially zero. The MAX phases, thus occupy an interesting middle ground, in which in constraineddeformationmodes,highly oriented microstructures, and/or at higher temperatures they are pseudo-ductile. In unconstrained deformation, and especially in tension at lower temperatures, they behave in a brittle fashion. 

Low-temperature ductility is rarely observed in ceramics, which are inherently brittle, but some bulk ceramics show plasticity at ambient temperatures. One example of low-temperature plasticity in MgO is considered here. First, consider a single crystal, where i orientation-dependent properties are of interest. Orientation is one of the factors that influence mechanical properties. It was observed (by etch-pit technique) that the flow in MgO occurs on the 110 (110) slip system. However, it was also found [28] that the 110 (110) slip system contributes to deformation above 600 °C. Details on Plastic deformation in MgO single crystals were presented in Sect. 2.2, Figs. 2.33 and 2.38. Consequently, some information on deformation in polycrystalline ceramics may be of interest. 

In addition, a relatively large number of shear deformation data for ceramics and metals is listed in Table 4.2, indicating the common slip systems. The results... 

In metals, slip and the common slip systems are usually discussed in terms of the various structures of single crystals BCC, FCC and HCP. There is no basic difference between the slip systems in ceramics (or other crystalline materials) and metals, since all the aforementioned structures appear in both materials. Thus, NaCl, MgO, CaO, KBr, etc. have an FCC structure with a primary 110 (110) slip system. jS-SiaNq is HCP with a (1120) (0001) slip system. a-SisNq with (0001) (llIO) and (0001) (1120)and fl-SiC cubic (zincblende) with a 111 (110) slip system. However, unlike pure metallic systems (see Fig. 4.22a), ceramic FCC... 

Trss, to the critical stress of a twin system must be greater than that of any slip system, namely trss/ Tc > CRSS for slip (b) the trss should be greater than the threshold (namely greater than some minimum) stress necessary for twinning to occur and (c) the Trss must satisfy the character of a twin shear. It is not currently known if such a concept exists also for ceramics. 

The anisotropic behavior of cubic crystals with OOlKHO) slip systems is the converse of those in the above category. This effect is shown in Figure 3.7 where results on a mechanically polished and chemically etched (001) plane of zinc blende type InP, indented at room temperature with a load of 0.49 N, are replotted from work reported by Brazen. Here the (100) directions are hardest and the (HO) directions are softest with a degree of anisotropy amounting to some 15%. Other ceramic crystals of this type include the fluorite structure materials UO2 and Cap2. 

Ceramics with cubic F structures and lllKHO) slip systems exhibit the same hardness anisotropy as fluorite structure solids with 001 (110) slip systems in the sense that the hardest directions on 100 are (100) and the softest are (110). Thus in order to determine which system is operative, a combination of the analysis given in Section 3.6.1 and other techniques, such as slip line analysis, is necessary. Ceramics with the diamond cubic structure have this slip system, and the parallel of their hardness anisotropy with that of fluorites can be seen by comparing the results for cubic boron nitride, BN, with the InP data in Figure 3.7. 

The degree of anisotropy of a property may be negligible, but this is not usually the case in indentation hardness measurements on ceramic crystals. Later we will consider the phenomenological aspect of hardness anisotropy to demonstrate that, whatever the ramifications of the theoretical models, the nature of anisotropy is consistent and reproducible for a wide range of ceramics. Then we shall consider the models based on a resolved shear stress analysis and discuss their implications in terms of the role of plastic deformation and indentification of active dislocation slip systems. 

The discussion of dislocations so far shows that the combination of a ceramic single crystal, a micro- or low-load indenter, and a suitable chemical etchant can prove valuable in identifying operative slip systems in the complex stress patterns generated by the indentation process. Observation of hardness anisotropy on the crystal faces, allied to a satisfactory theory, will also aid in identification of the active slip systems as Section 3.6 shows. 

That the method can reveal hardness anisotropy in nonmetallic materials, even if neither example might be considered to be a ceramic in the accepted sense, is shown in Figure 3.18. In order to achieve these results the temperature had to be elevated so that untorn grooves were produced. The two materials used, LiF and Cap2, are examples of the two types of slip system mainly encountered in cubic ceramics, as Table 3.1 shows, and clearly they are the converse of each other, as Fig. 3.18 clearly shows. This method is worthy of development. 

The most striking feature of the collected data in the tables in this chapter and in Chapter 6 on anisotropic indentation hardness values for crystalline ceramics is its dependence on the relevant active slip systems. This has been extended by observation to encompass materials beyond ceramics. Thus, the nature of anisotropy for a soft, face-centered cubic metal may be the same as for hard, covalent cubic crystals like diamond, since they both have lll (lTo) slip systems. Consequently it is natural that, in order to develop a universal model, we should first look for explanations based on mechanisms of plastic deformation. 

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