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0 Chapter 8 Ceramics, Graphite, Diamond, and Nanomaterials: Structure, General Properties, and Applications

improved by modifying their composition and by heat-treatment techniques. First
developed in 1957, glass ceramics are suitable for cookware, heat exchangers in gas-
turbine engines, radomes (housings for radar antennas), and electrical and elec-
tronics applications.

8.6 Graphite

Graphite is a crystalline form of carbon having a layered structure with basal planes
or sheets of close-packed carbon atoms (see Fig. 1.4). Consequently, graphite is weak
when sheared along the layers. This characteristic, in turn, gives graphite its lovv fric-
tional properties as a solid lubricant. However, its frictional properties are low only
in an environment of air or moisture; in a vacuum, graphite is abrasive and a poor
lubricant. Unlike in other materials, strength and stiffness of graphite increase with
temperature. Amorphous graphite is known as lampblack (black soot) and is used as
a pigment.
Although brittle, graphite has high electrical and thermal conductivity and
good resistance to thermal shock and to high temperature (although it begins to ox-
idize at 500°C). It is, therefore, an important material for applications such as elec-
trodes, heating elements, brushes for motors, high-temperature fixtures and furnace
parts, mold materials (such as crucibles for the melting and casting of metals), and
seals (Fig. 8.3). A characteristic of graphite is its resistance to chemicals; thus, it is
used in filters for corrosive fluids. Also, its low absorption cross section and high
scattering cross section for thermal neutrons make graphite suitable for nuclear ap-
plications. Ordinary pencil “lead” is a mixture of graphite and clay.
Graphite is available commercially in square, rectangular, and round shapes of
various sizes and generally is graded in decreasing order of grain size: industrial, #ne
grain, and micrograin. As in ceramics, the mechanical properties of graphite improve
with decreasing grain size. Micrograin graphite can be impregnated with copper. In
this form, it is used for electrodes in electrical-discharge machining (Section 27.5 ) and
for furnace fixtures. Graphite usually is processed first by molding or forming, then
by oven baking, and finally by machining to the final shape.

Graphite Fibers. An important use of graphite is as fibers in reinforced plastics
and composite materials, as described Section 9.2.

(H) (D)
FIGURE 8.3 (a) Various engineering components made of graphite. Source: Courtesy of
Poco Graphite, Inc., a Unocal Co. (b) Examples of graphite electrodes for electrical discharge
machining. Source: Courtesy of Unicor, Inc.