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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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