Page 221 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

200 Chapter 8 Ceramics, Graphite, Diamond, and Nanomaterialsz Structure, General Properties, and Applications

high hardness and wear resistance, ceramic knives can edges of the knife can chip. Also, they should be used
last months and even years before sharpening. (b) They only for cutting (not for prying), and in cutting meat,
are chemically inert; consequently, they do not stain, contact with bones is not advisable. Knives have to be
food does not stick to them (hence, they are easy sharpened at the factory to a precise edge shape, using
to clean), and they leave no metallic taste or smell. diamond grinding wheels. Ceramic knives are more
(c) Because they are lightweight, they are easier to use. expensive than steel knives, typically ranging from
The knives should be stored in wooden knife $60 for a 75-mm paring knife to $250 for a 150-mm
blocks and handled carefully. Sharp impact against serrated knife.
other objects (such as dishes or dropping it on its edge
on a hard surface) should be avoided, as the sharp Source: Courtesy of Kyocera Corporation.

8.2.3 Other Ceramics
The major other types of ceramics may be classified as follows:

Carbides. Typical carbides are those made of tungsten and titanium (used as cut
ting tools and die materials) and of silicon (used as an abrasive, especially in grind-
ing wheels). The following are some examples of carbides:
° Tungsten carbide (WC) consists of tungsten-carbide particles with cobalt as a
binder. The amount of binder has a major influence on the material’s proper-
ties; toughness increases with cobalt content, whereas hardness, strength, and
wear resistance decrease.
° Titanium carbide (TiC) has nickel and molybdenum as the binder and is not as
tough as tungsten carbide.
° Silicon carbide (SiC) has good resistance to wear, thermal shock, and corrosion. It
has a low friction coefficient and retains strength at elevated temperatures. Thus,
it is suitable for high-temperature components in heat engines and also is used as
an abrasive. First produced in 1891, synthetic silicon carbide is made from silica
sand, coke, and small amounts of sodium chloride and sawdust. The process is
similar to that for making synthetic aluminum oxide.

Nitrides. Another class of ceramics is the nitrides, examples of which are as
follows:
° Cubic boron nitride (cBN) is the second-hardest known substance (after dia-
mond) and has special applications, such as in cutting tools and as abrasives in
grinding wheels. It does not exist in nature and was first made synthetically in
the 1970s by means of techniques similar to those used in making synthetic
diamond (see Section 8.7).
° Titanium nitride (TiN) is used widely as a coating on cutting tools; it improves
tool life by virtue of its low friction characteristics.
° Silicon nitride (Si3N4) has high resistance to creep at elevated temperatures, low
thermal expansion, and high thermal conductivity. Consequently, it resists ther-
mal shock. It is suitable for high-temperature structural applications, such as in
automotive-engine and gas-turbine components, cam-follower rollers, bearings,
sandblast nozzles, and components for the paper industry.
Sialon. Sialon (derived from silicon, aluminum, oxygen, and nitrogen) consists of
silicon nitride with various additions of aluminum oxide, yttrium oxide, and titanium
carbide. It has higher strength and thermal-shock resistance than silicon nitride.
Presently, it is used primarily as a cutting-tool material.