Callister - An introduction - 8th edition

480 • Chapter 12 / Structures and Properties of Ceramics
Figure 12.27 The
silica–alumina phase
diagram. (Adapted
from F. J. Klug,
S. Prochazka, and
R. H. Doremus,
“Alumina–Silica
Phase Diagram in
the Mullite Region,”
J. Am. Ceram. Soc.,
70 [10] 758 (1987).
Reprinted by
permission of the
American Ceramic
Society.)
Temperature (°C)
2200
2000
1800
1600
Cristobalite
+
Liquid
20 40 60 80
Liquid
Composition (mol% Al 2 O 3 )
1587 ± 10°C
Mullite (ss)
+
Liquid
Mullite
(ss)
Liquid
+
Alumina
1890 ± 10°C
Alumina
+
Mullite (ss)
4000
3800
3600
3400
3200
3000
Temperature (°F)
Mullite (ss)
+
Cristobalite
2800
1400
0
20
40 60 80
100
2600
(SiO 2 )
Composition (wt% Al 2 O 3 )
(Al 2 O 3 )
Mechanical Properties
Prior to the Bronze Age, human tools and vessels were primarily made of stone (a
ceramic). Between 3000 and 4000 years ago, metals came into widespread use
because of their toughness that is derived from their ductility. For most of that history,
ceramic materials were somewhat limited in applicability because of their brittle
nature. Their principal drawback has been a disposition to catastrophic fracture
in a brittle manner with very little energy absorption. Although many new composites
and other multiphase ceramics with useful toughness are being developed
(often mimicking naturally occurring composite ceramics such as seashells), the bulk
of ceramic materials currently in use are brittle.
12.8 BRITTLE FRACTURE OF CERAMICS
At room temperature, both crystalline and noncrystalline ceramics almost always
fracture before any plastic deformation can occur in response to an applied tensile
load. The topics of brittle fracture and fracture mechanics, as discussed previously
in Sections 8.4 and 8.5, also relate to the fracture of ceramic materials; they will be
reviewed briefly in this context.
The brittle fracture process consists of the formation and propagation of cracks
through the cross section of material in a direction perpendicular to the applied
load. Crack growth in crystalline ceramics may be either transgranular (i.e., through
the grains) or intergranular (i.e., along grain boundaries); for transgranular fracture,
cracks propagate along specific crystallographic (or cleavage) planes, planes of high
atomic density.