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.
