Toughness transformation toughening

In addition to improved mechanical strength and fracture toughness, transformation-toughened PSZ exhibits R-curve behavior that is, the resistance against... 

Despite the fact that ceramics are inherently brittle, a variety of approaches have been used to enhance their fracture toughness and resistance to fracture. The essential idea behind all toughening mechanisms is to increase the energy needed to extend a crack, that is, in Eq. (11.11). The basic approaches are crack deflection, crack bridging, and transformation toughening. 

Transformation-toughened materials owe their very large toughness to the stress-induced transformation of a metastable phase in the vicinity of a... 

It is unfortunate that the reason transformation toughening works so well at ambient temperatures — mainly the metastability of the tetragonal phase — is the same reason it is ineffective at elevated temperatures. Increasing the temperature reduces the driving force for transformation and consequently the extent of the transformed zone, leading to less tough materials. 

F. F. Lange, Transformation toughening. Part 4 Fabrication, fracture toughness and strength of AljOj/ZrOj composites, J. Mater. ScL, 17 (1982) 247 54. 

Figure 8.63 The fracture toughness of transformation-toughened ceramics will be limited by the transformation stress. In the shaded region, however, strength will be dependent on flaw size and the shape of the R curve.

It is important to note that in contrast to ZTA toughened by microcracking, transformation toughening can lead to an improvement in both toughness and strength and, consequently, is the preferred toughening mechanism. 

Apart from temperature dependent transformation toughening (Zr02 additives), high values of the fracture toughness are obtained first of all by anisotropic crystal shapes (whisker reinforcement, rod shaped crystals with high aspect ratio in Si3N4, platelets) generally new mechanism are not likely to be found in the near future. 

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