NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 75

unique strength and thermal properties initiated the rapid development of
advanced ceramics, such as zirconias, aluminum oxides, silicon carbides,
and silicon nitrates. These materials have high strength, stiffness, and wear
resistance, and are corrosion resistant. The high-performance or engineered
ceramics have found many applications in machine and tool design. Although
the availability of high-performance ceramics has grown rapidly, the civil and
construction engineering applications of these materials have been generally
limited due to the cost of the sophisticated ceramics and their lack of frac-
ture toughness that is needed for structural design.
Five classes of ceramic materials have been defined (Ashby and Jones
1986):

1. glasses—based on silica
2. vitreous ceramics—clay products used for pottery, bricks, etc.
3. high-performance ceramics—highly refined inorganic solids used for
specialty applications in which properties, not available from other
materials, compensate for the high cost
4. cement and concrete—a multiphase material widely used in civil engi-
neering applications
5. rocks and minerals

Ceramics can also be classified by the predominant type of their atomic
bonding. Materials composed of a combination of nonmetallic and metallic
elements have predominantly ionic bonds. Materials composed of two non-
metals have predominantly covalent bonds. The type of bond dictates the
crystal structure of the compound. As with metals, inorganic solids have a
well-defined, although more complicated, unit cell structure. This structure
is repeated for the formation of the crystals.
In a simple ionic compound, the nonmetallic element will form either
a face centered cubic or hexagonal close pack structure, as shown in
Figure 2.18. Octahedral holes fit in the middle of the octahedron that is
formed by connecting all the face atoms of the FCC unit cell. Tetrahedral
holes are in the middle of the tetrahedron formed by connecting a corner
atom with the adjacent face atoms. In the FCC structure, there is one octa-
hedral hole and six tetrahedral holes. The number of atoms and the way

Corner atom

Face atom

Octahedral or
tetrahedral hole

(a) (b)

FIGURE 2.18 Lattice structure for simple ionic bonded
ceramic materials: (a) octahedral hole and (b) tetrahedral hole.