In this chapter, we provide a description of the processing, structure, and properties of high temperature ceramic fibers, excluding glass and carbon, which are dealt with in separate chapters because of their greater commercial importance. Before we do that, however, we review, ever so briefly, some fundamental characteristics of ceramics (crystalline and noncrystalline). Once again, readers already familiar with this basic information may choose to go directly to Section 7.3.

Some important ceramics

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

Bonding and crystalline structure

Ceramics are primarily compounds. Ceramics (excluding 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 7.1. Physical and mechanical characteristics of some ceramic materials are given in Table 7.2. It should be noted that the values shown in Table 7.2 are more indicative than absolute.

In terms of bonding, ceramics have mostly ionic bonding and some covalent bonding. Ionic bonding involves a transfer of electrons between atoms that make the compound. Generally, positively charged ions balance the negatively charged ions to give an electrically neutral compound, for example, NaCl. In covalent bonding, the electrons are shared between atoms. The characteristic high strength as well as brittleness of ceramic materials can be traced to this type of bonding which make the Peierls–Nabarro potential very high, i.e., inherent lattice resistance to dislocation motion is very high. Thus, crystalline ceramics have crystal imperfections such as dislocations but, unlike in metals, they are not very mobile. Also, the number of slip systems available in ceramics is fewer than that in metals. Thus, unlike metals, the stress concentration at a crack tip in a crystalline ceramic cannot be relieved by plastic deformation, at least not at low and moderate temperatures. This has led to attempts at toughening ceramics by means other than large scale dislocation motion, for example, by incorporating fibers or second phases (Chawla, 2003).