Transformation toughening process

Utilization of the transformation toughening process available via zirconia. Such processes may involve any or all of the following four mechanisms ... 

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. 

In room-temperature applications, phase transformation-toughened zirconia ceramics are some of the toughest stmctural ceramics available. However, one drawback of these materials is that the phase-transformation process ceases to occur as the temperature rises above a few hundred degrees. 

Prouse, A. V. Virkar and D. K. Shetty, "Processing and Characterization of Transformation Toughened Ceramics with Strength Retention to Elevated Temperatures," Bimonthly ORNL/CF-89/307 pp. 135-142 (ORNL, Oak Ridge, TN 1989). ... 

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