Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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10.16 Ceramic Phase Diagrams • 377

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 1000◦C 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 above is termed apartially stabilized zirconia,

orPSZ. 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.

The SiO 2 –Al 2 O 3 System

Commercially, the silica–alumina system is an important one since the principal

constituents of many ceramic refractories are these two materials. Figure 10.26 shows

the SiO 2 –Al 2 O 3 phase diagram. The polymorphic form of silica that is stable at these

temperatures is termedcristobalite, the unit cell for which is shown in Figure 3.11.

Silica and alumina are not mutually soluble in one another, which is evidenced by the

absence of terminal solid solutions at both extremities of the phase diagram. Also, it

may be noted that the intermediate compoundmullite,3Al 2 O 3 –2SiO 2 , exists, which

is represented as a narrow phase field in Figure 10.26; furthermore, mullite melts

incongruently at 1890◦C (3435◦F). A single eutectic exists at 1587◦C (2890◦F) and

7.7 wt% Al 2 O 3. In Section 13.7, refractory ceramic materials, the prime constituents

for which are silica and alumina, are discussed.

100

(Al 2 O 3 )

0 20

(SiO 2 )

Temperature (

°C)

Temperature (

°F)

2200

2000

1800

1600

1400

4000

3800

3600

3400

3200

3000

2800

2600

20 40 60 80

40 60 80

Composition (mol % Al 2 O 3 )

Composition (wt % Al 2 O 3 )

Liquid

Mullite (ss)

+

Liquid

Mullite (ss)

+

Cristobalite

Liquid

+

Alumina

1890 ± 10 °C

1587 ± 10 °C

Alumina

+

Mullite (ss)

Mullite

Cristobalite+ (ss)

Liquid

Figure 10.26 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.)