Page 220 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 8.2 The Structure of Ceramics |99

TABLE 8.l
Types and General Characteristics of Ceramics
Type General characteristics
Oxide ceramics
Alumina High hardness and moderate strength; most widely used ceramic; cutting tools; abrasives;
electrical and thermal insulation.
Zirconia High strength and toughness; thermal expansion close to cast iron; suitable for high-
temperature applications.
Carbides
Tungsten carbide Hardness, strength, and wear resistance depend on cobalt binder content; commonly used for
dies and cutting tools.
Titanium carbide Not as tough as tungsten carbide; has nickel and molybdenum as the binder; used as cutting
tools.
Silicon carbide High-temperature strength and wear resistance; used for heat engines and as abrasives.
Nitrides
Cubic boron nitride Second-hardest substance known, after diamond; used as abrasives and cutting tools.
Titanium nitride Gold in color; used as coatings because of low frictional characteristics.
Silicon nitride High resistance to creep and thermal shock; used in high-temperature applications.
Sialon Consists of silicon nitrides and other oxides and carbides; used as cutting tools.
Cermets Consist of oxides, carbides, and nitrides; used in high-temperature applications.
Silica High-temperature resistance; quartz exhibits piezoelectric effect; silicates containing various
oxides are used in high-temperature nonstructural applications.
Glasses Contain at least 50 percent silica; amorphous structures; several types available with a wide
range of mechanical and physical properties.
Glass ceramics Have a high crystalline component to their structure; good thermal-shock resistance and
strong.
Graphite Crystalline form of carbon; high electrical and thermal conductivity; good thermal~shock
resistance.
Diamond Hardest substance known; available as single crystal or in polycrystalline form; used as cutting
tools and abrasives and as dies for fine wire drawing.
Carbon nanotubes Unique crystalline form of graphite, with high electrical and thermal conductivity; under
investigation for MEMS and microelectronics applications and in composite materials.

Two important characteristics of PSZ are its coefficient of thermal expansion
(which is only about 20% lower than that of cast iron) and its thermal conductivity
(which is about one-third that of other ceramics). Consequently, PSZ is very suitable
for heat-engine components, such as cylinder liners and valve bushings, to help keep
the cast-iron engine assembly intact. Transformation-toughened zirconia (TTZ) has
higher toughness because of dispersed tough phases in the ceramic matrix.

EXAMPLE 8.1 Ceramic Knives

The use of ceramics now is being extended to knives, they are ground and polished on a diamond wheel to
generally made of zirconium oxide. Ceramic knives form a sharp edge, and the handle is attached. The
are produced by a process (described in Section 182) Mohs hardness (Section 2.6) of the zirconium oxide
that starts with a ceramic powder mixed with various ceramic is 8.2, as compared with 6 for hardened steel
binders and compacted (molded) into blanks under and a maximum of 10 for diamond.
high pressure. The blanks are then fired (sintered) at Among the advantages ceramic knives have over
temperatures above 1000°C for several days. Next, steel knives are the following: (a) Because of their very