Some important ceramics

We provide a summary of the characteristics of some important ceramic materials that have been converted into a fibrous form. 

Ceramics are primarily compounds. Ceramics other than glasses generally have a crystalline structure, while silica-based glasses, a subclass of ceramic materials, are noncrystalline. In crystalline ceramic compounds, stoichiometry dictates the ratio of one element to another. Nonstoichiometric ceramic compounds, however, occur frequently. Some important ceramic materials are listed in Table 

Physical and mechanical characteristics of some ceramic materials are given in Table 6.2. It should be noted that the values shown in Table 6.2 are more indicative than absolute. 

In terms of bonding, ceramics have mostly ionic bonding and some covalent 

Generally, metallic cations are smaller than nonmetallic anions. Thus, in crys-talUne ceramics, the metallic cations occupy interstitial positions in an array of nonmetallic ions. Common crystal structures in ceramics, shown in Fig. 6.1, are  

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Research and development in the field ate stiU continuing at a fast pace, particularly in the area of absorption and emission characteristics of the polymers. Several reasons account for this interest. First, the intractable polydimethyl silane [30107-43-8] was found to be a precursor to the important ceramic, siUcon carbide (86—89). Secondly, a number of soluble polysdanes were prepared, which allowed these polymers to be studied in detail (90—93). As a result of studies with soluble polymers it became cleat that polysdanes are unusual in their backbone CJ-conjugation, which leads to some very interesting electronic properties. 

Vapor—vapor reactions (14,16,17) are responsible for the majority of ceramic powders produced by vapor-phase synthesis. This process iavolves heating two or more vapor species which react to form the desired product powder. Reactant gases can be heated ia a resistance furnace, ia a glow discharge plasma at reduced pressure, or by a laser beam. Titania [13463-67-7] Ti02, siUca, siUcon carbide, and siUcon nitride, Si N, are among some of the technologically important ceramic powders produced by vapor—vapor reactions. 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

Since it measures the susceptibility of materials to plastic deformation (as contrasted with elastic deformation), hardness is very important for diagnosing the mechanical state of a material, in particular toughness. Purely elastic materials are brittle. Plasticity, by blunting cracks and other defects, allows metals and, to some extent ceramics, to tolerate small flaws and thereby become malleable and tough. 

Table 2 shows a compilation of different crucible materials, the working temperatures, atmospheres and some important physical data. Metal crucibles are used more for the investigation of clays, oxides, ceramics, glasses, inorganic materials as... 

The name fine ceramics is based on the grain size distribution of the hard components in the ceramic mass. This rather differs from the distribution as it is seen in the ceramic branch of industry which produces for instance bricks, the coarse ceramic industry. Another difference is that all fine ceramic products are provided with a protective and in some cases also decorative coating, a so-called glaze. In this section much attention will be paid to glazes because this technique is rather unique for fine ceramics and because it offers the possibility to explore the subject glass and some important physical and chemical properties of materials. 

The important ceramic matrix materials are glass, silicon carbide, silicon nitride, alumina, glass-ceramics, sialons, intermetallics and some elemental materials. A list of some ceramic matrix materials is given in Table 3.5. 

Glass-matrix materials can be considered as a non-crystalline solid with the frozen-in structure of a liquid. Characteristics of some important varieties of glass are given in Table 3.7. Glass-matrix materials are polycrystalline materials having fine ceramic crystallites in a glass matrix. Important glass-ceramic matrix materials are as follows. 

Some important features of structural ceramics with respect to corrosion are given below ... 

One of the key issues for both ceramics and ceramic composites is the lack of suitable standards and standard reference materials.51 In principle, this issue can be rectified by development of materials containing known types and numbers of flaws. In practice, it is difficult because of our lack of knowledge about the numbers and types of flaws which are important. Techniques suitable for some monolithic ceramics have been developed which incorporate known internal and surface-connected defects. Using these types of specimens, our knowledge of aspects of NDE related to probability of detection of different kinds of flaws, and the procedures which must be followed to optimize detection, will be increased. 

In this chapter, we define some important terms and parameters that are commonly used with fibers and fiber products such as yams, fabrics, etc., and then describe some general features of fibers and their products. These definitions, parameters, and features serve to characterize a variety of fibers and products made from them, excluding items such as fiber reinforced composites. These definitions and features are generally independent of fiber type, i.e. polymeric, metallic, glass or ceramic fibers. They depend on the geometry rather than any material characteristics. 

The last quarter of the twentieth century saw tremendous advances in the processing of continuous, fine diameter ceramic fibers. Figure 6.4 provides a summary of some of the important synthetic ceramic fibers that are available commercially. We have included in Fig. 6.4 two elemental fibers, carbon and boron, while we have excluded the amorphous, silica-based glasses. Two main categories of synthetic ceramic fibers are oxide and nonoxides. A prime example of oxide fibers is alumina while that of nonoxide fibers is silicon carbide. An important subclass of oxide fibers are silica-based glass fibers and we devote a separate chapter to them because of their commercial importance (see chapter 7). There are also some borderline ceramic fibers such as the elemental boron and carbon fibers. Boron fiber is described in this chapter while carbon fiber is described separately, because of its commercial importance, in Chapter 8. 

Ceramic fibers of the nonoxide variety such as silioon carbide, silicon oxycarbide such as Nicalon, silicon nitride, boron carbide, etc. have become very important because of their attractive combination of high stiffiiess, high strength and low density. We give brief description of some important nonoxide fibers. 

Carbon fiber reinforced ceramic composites also find some important applications. Carbon is an excellent high temperature material when used in an inert or nonoxidizing atmosphere. In carbon fiber reinforced ceramics, the matrix may be carbon or some other glass or ceramic. Unlike other nonoxide ceramics, carbon powder is nonsinterable. Thus, the carbon matrix is generally obtained from pitch or phenolic resins. Heat treatment decomposes the pitch or phenolic to carbon. Many pores are formed during this conversion from a hydrocarbon to carbon. Thus, a dense and strong pore-free carbon/carbon composite is not easy to fabricate. 

Femoral ball heads of hip endoprostheses made from bioinert ceramics such as alumina or zirconia have to sustain high mechanical stresses, resorp-tion/corrosion by aggressive body fluid and abrasive wear over the lifetime of the implant in the human body of 15-20years. Some important properties of ceramic femoral ball heads are listed in Table 2.3 (Willmann, 1995). Mechanical properties of alumina and zirconia are discussed in Chapter 4.1. 

Building Materials and Ceramics 341 Tab. 6.7-5 Some important characteristics of finished ceramic or building materials... 

The nearly two dozen phase diagrams shown below present the reader with examples of some important types of single and multicomponent systems, especially for ceramics and metal alloys. This makes it possible to draw attention to certain features like the kinetic aspects of phase transitions (see Figure 22, which presents a time-temperature-transformation, or TTT, diagram for the precipitation of a-phase particles from the [5-phase in a Ti-Mo alloy Reference 1, pp. 358-360). The general references listed below and the references to individual figures contain phase diagrams for many additional systems. 

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