Page 224 - 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 203

ceramic components subjects them to compressive stresses. The methods used include
the following:

° Heat treatment and chemical tempering
° Laser treatment of surfaces
° Coating with ceramics having different thermal-expansion coefficients
° Surface-finishing operations (such as grinding) in which compressive residual
stresses are induced on the surfaces.

Major advances have been made in improving the toughness and other proper-
ties of ceramics, including the development of machinable and grindable ceramics.
Among these advances are the proper selection and processing of raw materials,
the control of purity and structure, and the use of reinforcements-with particular
emphasis on advanced methods of stress analysis during the design of ceramic
components.

8.3.2 Physical Properties

Most ceramics have a relatively low specific gravity, ranging from about 3 to 5.8 for
oxide ceramics as compared to 7.86 for iron (Table 3.1). They have very high melt-
ing or decomposition temperatures.
Thermal conductivity in ceramics varies by as much as three orders of magni-
tude (depending on their composition), whereas in metals it varies by only one
order. Like that of other materials, the thermal conductivity of ceramics decreases
with increasing temperature and porosity, because air is a poor thermal conductor.
The thermal conductivity le is related to porosity by
k = /e,,(1 - P), (8.3)

where /eo is the thermal conductivity at zero porosity and P is the porosity as a frac-
tion of the total volume.
Thermal expansion and thermal conductivity induce internal stresses that can
lead to thermal shock or to thermal fatigue in ceramics. The tendency toward
thermal cracking (called spalling when a small piece or a layer from the surface
breaks off) is lower with the combination of low thermal expansion and high
thermal conductivity. For example, fused silica has high thermal-shock resistance
because of its virtually zero thermal expansion.
A familiar example that illustrates the importance of low thermal expansion is
heat-resistant ceramics for cookware and electric stove tops. (See also glass ceram-
ics, Section 8.5 .) They can sustain high thermal gradients, from hot to cold and vice
versa. Moreover, the similar thermal expansion of both ceramics and metals is an
important reason for the use of ceramic components in heat engines. The fact that
the thermal conductivity of partially stabilized zirconia components is close to
that of the cast iron in engine blocks is an additional advantage to the use of PSZ in
heat engines.
Another characteristic is the anisotropy of thermal expansion of oxide ceram-
ics (like that exhibited by hexagonal close-packed metals), wherein the thermal
expansion varies in different directions in the ceramic (by as much as 50% for
quartz). This behavior causes thermal stresses that can lead to cracking of the
ceramic component.
The optical properties of ceramics can be controlled by using various formula-
tions and by controlling the structure. These methods make possible the imparting
of different degrees of transparency and translucency and of different colors. (For
example, single-crystal sapphire is completely transparent, zirconia is white, and