Callister - An introduction - 8th edition

478 • Chapter 12 / Structures and Properties of Ceramics
Figure 12.25 The
magnesium oxide–
aluminum oxide
phase diagram;
ss denotes solid
solution. (Adapted
from B. Hallstedt,
“Thermodynamic
Assessment of the
System MgO–Al 2 O 3 ,”
J. Am. Ceram. Soc.,
75 [6] 1502 (1992).
Reprinted by
permission of the
American Ceramic
Society.)
Temperature (°C)
0
2800
2400
2000
1600
MgO
(ss)
MgO (ss)
+
Liquid
Composition (mol% Al 2 O 3 )
MgO (ss)
+
MgAl 2 O 4 (ss)
20 40 60 80
Liquid
MgAl 2 O 4 (ss)
+ Liquid
MgAl 2 O 4
(ss)
Al 2 O 3
+
Liquid
MgAl 2 O 4 (ss)
+
Al 2 O 3
5000
4500
4000
3500
3000
2500
Temperature (°F)
1200
2000
0
20 40 60 80 100
(MgO) Composition (wt% Al 2 O 3 )
(Al 2 O 3 )
Al 3 ions (0.072 versus 0.053 nm). For the same reasons, MgO is virtually insoluble
in Al 2 O 3 , as evidenced by a lack of a terminal solid solution on the right-hand side
of the phase diagram. Also, two eutectics are found, one on either side of the spinel
phase field, and stoichiometric spinel melts congruently at about 2100C (3800F).
The ZrO 2 –CaO System
Another important binary ceramic system is that for zirconium oxide (zirconia) and
calcium oxide (calcia); a portion of this phase diagram is shown in Figure 12.26. The
horizontal axis extends to only about 31 wt% CaO (50 mol% CaO), at which composition
the compound CaZrO 3 forms. It is worth noting that one eutectic (2250C
and 23 wt% CaO) and two eutectoid (1000C and 2.5 wt% CaO, and 850C and 7.5
wt% CaO) reactions are found for this system.
It may also be observed from Figure 12.26 that ZrO 2 phases having three different
crystal structures exist in this system—tetragonal, monoclinic, and cubic. Pure
ZrO 2 experiences a tetragonal-to-monoclinic phase transformation at about 1150C
(2102F). A relatively large volume change accompanies this transformation, resulting
in the formation of cracks that render a ceramic ware useless. This problem
is overcome by “stabilizing” the zirconia by adding between about 3 and 7 wt%
CaO. Over this composition range and at temperatures above about 1000C, both
cubic and tetragonal phases will be present. Upon cooling to room temperature under
normal cooling conditions, the monoclinic and CaZr 4 O 9 phases do not form (as
predicted from the phase diagram); consequently, the cubic and tetragonal phases
are retained, and crack formation is circumvented. A zirconia material having a calcia
content within the range cited is termed a partially stabilized zirconia, or PSZ.
Yttrium oxide (Y 2 O 3 ) and magnesium oxide are also used as stabilizing agents. Furthermore,
for higher stabilizer contents, only the cubic phase may be retained at
room temperature; such a material is fully stabilized.