Page 222 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 8.3 General Properties and Applications of Ceramics

Cermets. Cermets are combinations of a ceramic phase bonded with a metallic
phase. Introduced in the 1960s and also called black ceramics or hot-pressed ceram-
ics, they combine the high-temperature oxidation resistance of ceramics with the
toughness, thermal-shock resistance, and ductility of metals. A common application
of cermets is in cutting tools, with a typical composition being 70% Al2O3 and 30%
TiC. Other cermets contain various oxides, carbides, and nitrides. They have been
developed for high-temperature applications, such as nozzles for jet engines and
brakes for aircraft. Cermets can be regarded as composite materials and can be used
in various combinations of ceramics and metals bonded by powder-metallurgy tech-
niques (Chapter 17).

8.2.4 Silica
Abundant in nature, silica is a polymorphic material: that is, it can have different
crystal structures (e.g., the cubic structure is found in refractory bricks used for
high-temperature furnace applications). Most glasses contain more than 50% silica.
The most common form of silica is quartz, a hard, abrasive hexagonal crystal used
extensively in communications applications as an oscillating crystal of fixed fre-
quency because it exhibits the piezoelectric effect (Section 3.7).
Silicates are products of the reaction of silica with oxides of aluminum, mag-
nesium, calcium, potassium, sodium, and iron; examples are clay, asbestos, mica,
and silicate glasses. Lithium aluminum silicate has very low thermal expansion and
thermal conductivity and good thermal-shock resistance. However, it also has a very
low strength and fatigue life; thus, it is suitable only for nonstructural applications
(such as catalytic converters, regenerators, and heat-exchanger components).

8.2.5 Nanoceramics and Composites

In order to improve the ductility and manufacturing properties of ceramics, the par-
ticle size in ceramics has been reduced by means of various techniques, such as gas
condensation. Called nanoceramics or nanophase ceramics, these materials consist
of atomic clusters containing a few thousand atoms. Control of particle size, distri-
bution, and contamination are important in nanoceramics, which exhibit ductility
at significantly lower temperatures than do conventional ceramics and are stronger
and easier to fabricate and to machine with fewer flaws. Present applications are in
the automotive industry (such as valves, rocker arms, turbocharger rotors, and
cylinder liners) and in jet-engine components.
Nanocrystalline second-phase particles (on the order of 100 nm or less) and
fibers also are used as reinforcements in composites. These composites have enhanced
properties, such as better tensile strength and creep resistance. (See also ncmomaterials
in Section 6.16.)

8.3 General Properties and Applications of Ceramics

Compared with metals, ceramics typically have the following relative characteristics:
brittleness; high strength and hardness at elevated temperatures; a high elastic modu-
lus; low toughness, density, and thermal expansion; and low thermal and electrical
conductivity. However, because of the wide variety of material compositions and
grain sizes, the mechanical and physical properties of ceramics vary considerably.
Properties of ceramics can also vary widely because of their sensitivity to
flaws, defects, and surface or internal cracks; the presence of different types and lev-
els of impurities; and different methods of manufacturing. The general mechanical
and physical properties are described next.