Transformational toughening

The transformation toughening mechanism has been most successfully exploited in materials where the phase transformation of interest is... 

Transformation toughening ceramics, 5 621-622 Transformed composition space, 22 330-331 Transforming particles ceramic—matrix composite reinforcement, 5 571—572 Transgene expression, controllable, 12 453 Transgenes, 12 452, 453... 

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. 

Honeyman-Colvin, P., Lange, F.F., Infiltration of porous alumina bodies with solution precursors strengthening via compositional grading, grain size control, and transformation toughening, J. Am. Ceram. Soc., 79(7), 1810-1814, 1996. 

N. Claussen, Transformation Toughening , in Concise Encyclopedia of Advanced Ceramic Materials (R. J, Brook, ed.), Pergamon Press, Oxford, Great Britain (1991). 

As mentioned previously, the main body of research on whisker-reinforced composites was concerned with alumina, mullite, and silicon nitride matrix materials. None the less, selected work examined zirconia, cordierite, and spinel as matrix materials.16-18 The high temperature strength behavior reported for these composites is summarized in Table 2.5. As shown, the zirconia matrix composites exhibited decreases in room temperature strength with the addition of SiC whiskers. However, the retained strength at 1000°C, was significantly improved for the whisker composites over the monolithic. Claussen and co-workers attributed this behavior to loss of transformation toughening at elevated temperatures for the zirconia monolith, whereas the whisker-reinforcement contribution did not decrease at the higher temperature.17,18... 

The followers of a phase transformation toughening (PTT) state that the change from a less densely packed crystalline structure (f)) to a more packed one (a) (i) promotes microvoiding in the earliest stages of the deformation and (ii) facilitates plasticity due to its exothermic character [72,78,193]. 

Zirconia Toughened Ceramics. Zirconia particles can be embedded in host matrices to form a variety7 of transformation-toughened... 

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