Polycrystal ceramic

However, when HRTEM was employed on the SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual intergranular films was not detected even at the triple junctions. Hence, Kleebe concluded that SEM imaging and Fresnel fringe TEM imaging alone do not enable a safe conclusion to be drawn about interface wetting in ceramic polycrystals. 

Tabk 3.5 Thermal expansion coefficients of selected ceramic polycrystals... 

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. 

The failure of ceramic polycrystals may generally be related to preexisting flaws (or in cases of high-temperature failure, to flaws generated during service). 

The first observation of superplasticity in a 3 mol% yttria-stabilized tetragonal zirconia polycrystal ceramic (YTZP) with a grain size of 0.4 pm was reported by Wakai et al in 1986 (Fig. 16.1). Since then, a large number... 

Although superplasticity is defined as the ability of a polycrystalline material to exhibit large elongations, in many ceramics-related materials and ceramic composites superplasticity is also said to occur even though the polycrystal is deformed in compression, or in three- or four-point bending conditions, as long as GBS is the primary deformation process.4-7... 

Yoshida, M., Shinoda, Y., Akatsu, T., and Wakai, F., Deformation of monoclinic Zr02 polycrystals and Y203-stabilized tetragonal Zr02 polycrystals below the monoclinic-tetragonal transition temperature , J. Am. Ceram. Soc., 2002, 85, 2834— 6. 

F. F. Lange, Non-Elastic Deformation of Polycrystals with a Liquid Boundary Phase, in Deformation of Ceramic Materials, eds., R. C. Bradt and R. E. Tressler, Plenum Press, New York, NY, 1972, pp. 361-381. 

The absence of a lattice-based mechanism, such as slip planes, does not necessarily preclude aU deformation in brittle materials. Plastic flow can proceed in other modes. For example, at temperatures of about 40 percent to 50 percent of their melting points, grain-boundary shding can become important. Grain-boundary sliding is beheved to be the major contributor to the superplasticity observed in some polycrystal-hne ceramics. 

How are ceramics and other nonductile polycrystals fabricated into bulk articles ... 

Because of their strong chemical bonds, bulk ceramics are most efficiently fabricated by means of densification of powders. The fabrication process involves two main stages (1) consolidation of the powder to form a porous, shaped article (the green body), also referred to as forming, and (2) heating of the shaped powder form to produce a dense article, referred to as firing or sintering. The final product commonly consists of a relatively dense polycrystal with some residual porosity (Fig. 1). The microstructure, which... 

The sodium ions in the /3-type aluminas can be replaced by a host of monovalent and divalent ionic species (Ag +, Cu, Li, K, Rb, Ba ", Sr, Cd ) " , In all cases the conductivity is decreased. These cation-substituted / -type aluminas may have some specific applications as selective ion sensors however, very little is known about the preparation of these materials in the form of polycrystalline ceramics, other than by the ion exchange of sodium /3 and P" single crystals (sometimes polycrystals) in various molten salts. These homologues, while interesting, have not been developed in polycrystalline form and are not discussed further. 

Moreno, R., Requena, J., and Moya, J.S., Slip casting yttria-stabilized tetragonal zirconia polycrystals, J. Am. Ceram. Soc., 71,1036, 1988. 

Cheikh, A. et al., Ionic conductivity of zirconia based ceramics from single crystals to nanostructured polycrystals, J. Europ. Ceram. Soc. 21 (2001) 1837-1841. Hartmanova, M. et al., Correlation between microscopic and macroscopic properties of yttria stabilized zirconia 1. Single crystals. Solid State Ionics 136—137 (2000) 107-113. 

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