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Transformation Toughened Zirconia (TTZ)

Transformation-toughened zirconium oxide (TTZ) is an important high-strength, high toughness ceramic that has been developed during the past 20–25 years. Transformation toughening is a concept requires a bit of explanation. It is among of those properties that involve control of composition and manipulation of microstructure. Zirconia undergoes a change in the way its atoms are stacked at different temperatures (this phenomena is known as polymorphic transformation). Zirconia has the monoclinic crystal structure between room temperature and about 950oC. Above 950oC Zirconia converts to the tetragonal crystal structure. This transformation accompanies a greater than one percent shrinkage during heating and almost equivalent expansion during cooling. At higher temperatures, the Zirconia changes from tetragonal to a cubic structure. With properly controlled chemical additions and heat treatments, a microstructure can be achieved during cooling that consists of lens-shaped “precipitates” of tetragonal Zirconia in cubic grains of Zirconia.

Normally, the tetragonal material would transform to the monoclinic form during cooling, but it must expand to do so. The high strength of the surrounding cubic Zirconia prevents this expansion, so the Zirconia retains its tetragonal form down to room temperature. As a result, each tetragonal Zirconia precipitate is under stress and full of energy that wants to be released, sort of like a balloon that has been stuffed into a box that is too small. As soon as the box is opened, the balloon is allowed to expand to its equilibrium condition and protrude from the box. The same thing happens for each tetragonal precipitate if a crack tries to form for breaking the ceramic. The crack is analogous to opening the box. Tetragonal precipitates next to the crack are now able to expand and transform back to their stable monoclinic form. This expansion adjacent to the crack presses against the crack and stops it. This is the mechanism of transformation toughening. This is quite similar to the toughening mechanism in some forms of steel, so the TTZ has sometimes been called “ceramic steel.”

TTZ has been developed in a couple of different forms. The one described above is typically called partially stabilized Zirconia (PSZ). The second form consists of nearly every crystallite or grain in the material being retained in the tetragonal form to room temperature so that each grain can transform instead of only the precipitates. This material is referred to as tetragonal Zirconia polycrystal (TZP). Both types are mentioned because they have different properties, and one may be preferable for a specific application.

Transformation toughening is a landmark breakthrough in achieving high-strength, high toughness ceramic materials. TTZ has fracture toughness (resistance to crack propagation) 3–6 times higher than normal Zirconia and most other ceramics. TTZ is so tough that it can be struck with a hammer or even fabricated into a hammer for driving nails.

Here are some of the production applications for TTZ:

• Tooling for making Aluminum Cans
• Wire Drawing Capstans, Pulleys, Rollers and Guides
• Metal Extrusion Dies
• Knives for Cutting Paper
• Cutting Tools
• Pump Pistons and Plungers
• Grinding Media

This is the reason why we refer to this material as the "Material for the Future".

* This piece is intended to be used purely for informational purposes. This has been derived from several research sources. All copyrights exist with the original authors.


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