Page 98 - Mechanical Behavior of Materials

98 Chapter 3 A Survey of Engineering Materials

Figure 3.21 Diamond cubic crystal structure of silica, SiO 2 , in its high-temperature cristobalite
form. The crystal structure at ambient temperatures is a more complex arrangement of the
basic tetrahedral unit shown on the right.

ranging from 3 to 25%. Other carbides are also used in the same manner, namely, TiC, TaC, and
Cr 3 C 2 , typically in combination with WC. The most frequent binder metal is cobalt, but nickel and
steel are also employed.
The metal matrix of cemented carbides provides useful toughness, but limits resistance to
temperature and oxidation. Ordinary ceramics, such as alumina (Al 2 O 3 ) and boron nitride (BN),
are also used for cutting tools and have advantages, compared with cemented carbides, of greater
hardness, lighter weight, and greater resistance to temperature and oxidation. But the extra care
needed in working with brittle ceramics leads to the prevalence of cemented carbides, except where
ceramics cannot be avoided. Some of the advantages of ceramics can be obtained by chemical vapor
deposition of a coating of a ceramic onto a cemented carbide tool. Ceramics used in this manner
include TiC, Al 2 O 3 , and TiN.

3.6.4 Glasses
Pure silica (SiO 2 ) in crystalline form is a quartz mineral, the crystal structure of one of which is
illustrated in Fig. 3.21. However, when silica is solidified from a molten state, an amorphous solid
results. This occurs because the molten glass has a high viscosity due to a chainlike molecular
structure, which limits the molecular mobility to the extent that perfect crystals do not form upon
solidification. The three-dimensional crystal structure in Fig. 3.21 is depicted in a simplified two-
dimensional form in Fig. 3.22. A perfect crystal, as formed from solution in nature, is represented
by (a). Glass formed from molten silica has a network structure that is similar, but highly imperfect,
as in (b).
In processing, glasses are sometimes heated until they melt and are then poured into molds
and cast into useful shapes. Alternatively, they may be heated only until soft and then formed by
rolling (as for plate glass) or by blowing (as for bottles). Forming is made easier by the fact that
the viscosity of glass varies gradually with temperature, so that the temperature can be adjusted
to obtain a consistency that is appropriate to the particular method of forming. However, for pure
silica, the temperatures involved are around 1800 C, which is inconveniently high. The temperature
◦
for forming can be lowered to around 800 to 1000 C by adding Na 2 O, K 2 O, or CaO. These oxides
◦
are called network modifiers, because the metal ions involved tend to form nondirectional ionic
bonds with oxygen atoms, resulting in terminal ends in the structure, as illustrated by Fig. 3.22(c).