Page 225 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

20 Chapter 8 Ceramics, Graphite, Diamond, and Nanomaterials: Structure, General Properties, and Applications
fine-grained polycrystalline aluminum oxide is a translucent gray.) Porosity influ-
ences the optical properties of ceramics in much the same way as air trapped in ice
cubes, making the material less transparent and giving it a white appearance.
Although ceramics are basically resistors, they can be made electrically conducting
by alloying them with certain elements in order to make the ceramic act like a semi-
conductor or even like a superconductor.

8.3.3 Applications

Ceramics have numerous consumer and industrial applications. Various types of
ceramics are used in the electrical and electronics industries, because they have high
electrical resistivity, high dielectric strength (voltage required for electrical break-
down per unit thickness), and magnetic properties suitable for such applications as
magnets for speakers.
The capability of ceramics to maintain their strength and stiffness at elevated
temperatures makes them very attractive for high-temperature applications. The
higher operating temperatures made possible by the use of ceramic components
mean more efficient combustion of fuel and reduction of emissions in automobiles.
Currently, internal combustion engines are only about 30% efficient, but with the
use of ceramic components, the operating performance can be improved by at least
30%.
Ceramics that are being used successfully, especially in automotive gas-turbine
engine components (such as rotors), are silicon nitride, silicon carbide, and partially
stabilized zirconia. Other attractive properties of ceramics are their low density and
high elastic modulus. They enable product weight to be reduced and allow the iner-
tial forces generated by moving parts to be lower. Ceramic turbochargers, for exam-
ple, are about 40% lighter than conventional ones. High-speed components for
machine tools also are candidates for ceramics (Section 253). Furthermore, the high
elastic modulus of ceramics makes them attractive for improving the stiffness of
machines, while reducing the weight. Their high resistance to wear makes them suit-
able for applications such as cylinder liners, bushings, seals, bearings, and liners
for gun barrels. Coating metal with ceramics is another application, often done to
reduce wear, prevent corrosion, or provide a thermal barrier.

EXAMPLE 8.2 Ceramic Gun Barrels
The wear resistance and low density of ceramics have important to the performance of the barrel and the bullet.
led to research into their use as liners for gun barrels. The separate ceramic segments are then
Their limited success has led to more recent develop- joined, and the barrel is wrapped with a carbon-
ments in making composite ceramic gun barrels, which fiber/polymer-matrix composite that subjects the
have improved performance over traditional steel bar- ceramic barrel to a compressive stress of 690 MPa,
rels. The 50-caliber zirconia ceramic barrel is formed thus greatly improving its capacity to withstand
in several separate segments, each l50~200 mm long tensile stresses developed during firing. Finally, the
and with a wall thickness of 3.75 mm, by the shaping inside of the barrel is rifled (cut to produce internal
and sintering processes described in Chapter 17. spiral grooves that give rotation to the exiting bullet
The segments subsequently are machined to the for gyroscopic stability) and fitted to a breech.
required dimensions and surface finish. Zirconia has
been chosen for its high toughness, flexural strength,
specific heat, operating temperature, and very low Source: Courtesy of KH. Kohnken, Surface Conversion
thermal conductivity. The thermal properties are Technologies, Inc., Cumming, Georgia.