Page 93 - Mechanical Behavior of Materials

94 Chapter 3 A Survey of Engineering Materials

a thermosetting plastic. For example, fiberglass contains glass fibers in the form of mats or woven
cloth, and these are embedded in a matrix of unsaturated polyester. Such a combination is a com-
posite material, which topic is considered further in a separate section near the end of this chapter.

3.6 CERAMICS AND GLASSES

Ceramics and glasses are solids that are neither metallic nor organic (carbon-chain based) materials.
Ceramics thus include clay products, such as porcelain, china, and brick, and also natural stone and
concrete. Ceramics used in high-stress applications, called engineering ceramics, are often relatively
simple compounds of metals, or the metalloids silicon or boron, with nonmetals such as oxygen,
carbon, or nitrogen. Carbon in its graphite or diamond forms is also considered to be a ceramic.
Ceramics are predominantly crystalline, whereas glasses are amorphous. Most glass is produced by
melting silica (SiO 2 ), which is ordinary sand, along with other metal oxides, such as CaO, Na 2 O,
B 2 O 3 , and PbO. In contrast, ceramics are usually processed not by melting, but by some other means
of binding the particles of a fine powder into a solid. Specific examples of ceramics and glasses and
some of their properties are given in Table 3.10. The microstructure of a polycrystalline ceramic is
shown in Fig. 3.20.
Engineering ceramics have a number of important advantages compared with metals. They are
highly resistant to corrosion and wear, and melting temperatures are typically quite high. These
characteristics all arise from the strong covalent or ionic–covalent chemical bonding of these
compounds. Ceramics are also relatively stiff (high E) and light in weight. In addition, they are
often inexpensive, as the ingredients for their manufacture are typically abundant in nature.
As discussed in the previous chapter in connection with plastic deformation, slip of crystal
planes does not occur readily in ceramics, due to the strength and directional nature of even
partially covalent bonding and the relatively complex crystal structures. This results in ceramics
being inherently brittle, and glasses are similarly affected by covalent bonding. In ceramics, the
brittleness is further enhanced by the fact that grain boundaries in these crystalline compounds are
relatively weaker than in metals. This arises from disrupted chemical bonds, where the lattice planes
are discontinuous at grain boundaries, and also from the existence of regions where ions of the same
charge are in proximity. In addition, there is often an appreciable degree of porosity in ceramics,
and both ceramics and glasses usually contain microscopic cracks. These discontinuities promote
macroscopic cracking and thus also contribute to brittle behavior.
The processing and uses of ceramics are strongly influenced by their brittleness. As a
consequence, recent efforts aimed at developing improved ceramics for engineering use involve
various means of reducing brittleness. Noting the advantages of ceramics, as just listed, success in
this area would be of major importance, as it would allow increased use of ceramics in applications
such as automobile and jet engines, where lighter weights and operation at higher temperatures both
result in greater fuel efficiency.
Various classes of ceramics will now be discussed separately as to their processing and uses.

3.6.1 Clay Products, Natural Stone, and Concrete

Clays consist of various silicate minerals that have a sheetlike crystal structure, an important
example being kaolin, Al 2 O 3 –2SiO 2 –2H 2 O. In processing, the clay is first mixed with water to