Transformation toughened partially stabilized zirconias

It should be noted that it is possible to produce fully stabilized bodies with much higher fracture strengths than listed here but this requires the use of fine particle size, chemically prepared powders (3). The use of this type of material involves a number of penalties both in cost and processability that may be prohibitive for a high volume automotive application. In addition to the type of partially stabilized body described here, two other basic types of partially stabilized bodies have been reported (4, ). Both are classified as transformation toughened partially stabilized zirconias and involve different processing techniques to obtain a body with various amounts of a metastable tetragonal phase. While the mechanical properties of these materials have been studied extensively, little has been reported about their electrical properties or their stability under the thermal, mechanical and chemical conditions of an automotive exhaust system. 

Yttrium oxide partially stabilized zirconia (YPSZ) has been advocated as an alternative to alumina (Christel et al., 1989 Cales and Stefani, 1995). TTiis class of ceramic has a higher toughness than alumina since it can be transformation toughened and is used in bulk form or as a coating (Filiaggi et al., 1996a, 1996b). There are currently approximately 150,000 zirconia components in clinical use (Cales and Stefani, 1995). 

FIGURE 13.3 Schematic of micFOStructure in yttrium partially stabilized zirconia (YPSZ) bioceramic undergoing transformation toughening at a crack tip. (From Coles and Stefani, 1995, with permission.)... 

Biomedical-grade zirconia was introduced 20 years ago to solve the problem of alumina brittleness, and the consequent potential failure of implants. The reason for this is that biomedical-grade zirconia exhibits the best mechanical properties of oxide ceramics as a consequence of transformation toughening, which increases its resistance to crack propagation. Likewise, partially stabilized zirconia shows excellent biocompatibility, and it has therefore been applied to orthopedic uses such as hip and knee joints [255]. 

Partially stabilized zirconia (PSZ). In this kind of zirconia, the cubic phase is formed initially. This cubic phase is partially stabilized with the help of MgO, CaO, or Y2O3. Then, this material is heated to a temperature where the tetragonal phase is stable. By this heat treatment, tetragonal precipitates are formed from the partially stabilized cubic phase. The heat treatment is controlled in such a way that it results in fine precipitates. Only when a stress is applied, do these precipitates transform to develop a toughened zirconia matrix. 

It is partially stabilized zirconias (PSZ) that have justified the resounding article ( Ceramic steel ) published in 1975 by Garvie et al. [GAR 75], The title suggests that a ceramic can exhibit the high mechanical performances associated with steel, but also that toughening mechanisms recall those used by steel manufacturers. The t- m transformation of zirconia is a martensitic transformation, in analogy with the transformation used to obtain martensite in tempered steels, and the role of microstructural parameters inZr02 is similar to what is observed in metals. 

The zirconia toughening of various ceramics is of great interest and technological importance. It also undergoes Martensitic transformation. Figure 5.50 shows such a transformation in MgO-partially-stabilized Z1O2 [henceforth Mg-PSZ]. The Martensitic transformation upon cooling is a t -> m transition. 

Properties Units Monoclinic zirconia Zr02 (m)" Partially stabilized ZrOj (c) + 2.65 mol% MgO zirconia (PSZ) ZrOj (c) + 5.1 mol% Y2O3 Transformation Toughened ... 

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