Book Description

Zirconium oxide or Zirconia has a melting point of about 27000, is resistant to chemical attack by acids and bases, is very stable at high temperatures in oxidizing atmospheres, and is inert when in contact with most metals at high temperatures. In addition, zirconia is relatively inexpensive and abundant. These characteristics of zirconia would make it a very satisfactory material for many high-temperature applications, were it not for the fact that pure zirconia undergoes an allotropic transformation from tetragonal to monoclinic on cooling through a temperature range in the neighborhood of 900 C. This transformation takes place with a volume increase of about 3 percent. During the reverse transformation near 11000 C on heating, zirconia shrinks by about the same amount. The large anisotropic volume changes associated with the transformation cause bodies made from pure zirconia to disintegrate during their manufacture or when in use. In practice, this difficulty is circumvented by adding small amounts of certain Oxides, such as calcia, magnesia, yttria, etc., to zirconia. Depending on the kind and amount of oxide added to the zirconia, the high-temperature crystal structure of the combination is totally or partially retained on cooling, and the allotropic transformation is also totally or partially suppressed. This so-called stabilized zirconia performs satisfactorily in many high-temperature applications, but the addition of stabilizing oxides also introduces some undesirable features, such as an increase in the thermal-expansion coefficient, a lowering of the melting point, and, for some types of stabilized zirconia, a tendency to disintegrate on prolonged thermal cycling. A zirconia-base material combining the high-temperature properties of pure zirconia without the disadvantages associated with the use of stabilizers would be highly desirable.