Frequently asked
questions page 2
questions page 2
Q-What is the difference between the different types of
Zirconias? How is Mg-Zirconia different from PSZ and YTZP
materials?
A-A lot of people have misunderstandings about the different
types of Zirconias...even people in the ceramics industry. It is
probably easiest to differentiate by why and how materials do
not fall into categories, than to make a definition.
It is probably best to start with an explanation of what PSZ is
and go from there.
"PSZ" means Partially Stabilized Zirconia. In very simple terms,
a partially stabilized Zirconia is made strong by stressing the
material internally by altering its grain structure. Essentially by
stressing the material...you make it stronger. For the majority
of you out there reading this for an overview, you will have
Zirconias? How is Mg-Zirconia different from PSZ and YTZP
materials?
A-A lot of people have misunderstandings about the different
types of Zirconias...even people in the ceramics industry. It is
probably easiest to differentiate by why and how materials do
not fall into categories, than to make a definition.
It is probably best to start with an explanation of what PSZ is
and go from there.
"PSZ" means Partially Stabilized Zirconia. In very simple terms,
a partially stabilized Zirconia is made strong by stressing the
material internally by altering its grain structure. Essentially by
stressing the material...you make it stronger. For the majority
of you out there reading this for an overview, you will have
two Zirconia materials to decide between...Yttria stabilized or Magnesia stabilized Zirconia.
In very few cases, you may consider a Ceria stabilized Zirconia.
There are many, many designations that companies use to describe Zirconia ceramics. It
can be very confusing. PSZ, TTZ, Y-PSZ, Mg-PSZ, YTZP, YZP are all designations that
you will see for Zirconia ceramics. Essentially, you need to look for a "Y" or a "Mg" (or "M")
in the material name. If the material is noted only as "PSZ" or "TTZ", you need to confirm
what type material it is because the definition can vary greatly.
OK, so how are Zirconias different from each other.
There are three (3) phases of Zirconia. Monoclinic, tetragonal and cubic. When we are in
the firing cycle, around the temperature of 800 to 900 degrees C on the way up, Zirconia
will go into a cubic phase. At the end of the firing cycle, we cool the material and lock it
into a tetragonal gain structure. This is how you make Zirconia parts. So at this point in
time, we have a part that is "Partially Stabilized" and in a tetragonal grain structure. For
the purpose of our discussion this is where the similarities end between Yttria and Mag
stabilized Zirconias.
With a Yttria stabilized material, we now have a material that is capable of reverting into a
low energy monoclinic phase to absorb stresses. This is key to understanding what makes
a Y-PSZ so special. Mag stabilized material does not have the ability to do this. This is
the primary difference between the materials. With a Yttria material, we have a material
that is truly toughened. What is "toughened" anyway. Y-PSZ is a toughened material.
Y-PSZ can absorb stresses and impacts because it has the ability to release the energy (of
a force or impact) by reverting into a low energy monoclinic phase. It releases heat and
goes monoclinic, thus absorbing the stress. Let's review. Three phases of Zirconia.
Monoclinic, tetragonal and cubic. At the end of the firing cycle, when we release heat, we
go into a tetragonal phase. The material (out of the kiln) is in a tetragonal phase. When it
gets impacted, it releases heat and goes into a low-energy monoclinic phase. The material
then takes heat back in and reverts back into tetragonal phase. This is how all of your
Yttria and Ceria stabilized materials work. The reason that they can absorb such high
impacts, is that they give the stress off in a thermal-chemical reaction. Now, if you are a
lay person, this is where you will get really confused. Many companies refer to Mg
Zirconias as PSZ or Transformation Toughened materials. Others do not. We do not. We
define a Mg Zirconia as a Partially Magnesia Stabilized Zirconia. Is a Mg-Zirconia a
PSZ or "transformation toughened"? Well maybe. If you have heard the expression..."a
little bit pregnant"...Well that could be said for Mg-Zirconia. It is in fact partially stabilized.
It does have a tetragonal grain structure (from cubic) at the end of the firing cycle. So, by
those descriptions, some argue a Mg-Zirconia is a PSZ or a "transformation toughened"
material. And by their logic, they are correct. By our definition though, a PSZ has to be
able to go through phase transformation to release stresses. Y-PSZ does this. Partially
Magnesia Stabilized Zirconia does not. The primary drawback to Yttria stabilized
Zirconias is that they will go through what is called "phase migration". What is "phase
migration"? Phase migration is a bad thing. Remember how our Y-PSZ material could
revert into a low-energy monoclinic phase to absorb stresses? Well, when Y-PSZ is
exposed to heat and especially heat and moisture, it will lose it's tetragonal grain structure
and go monoclinic...thus losing its physical strength. Thus, you end up with a really weak
material. Mag stabilized Zirconia does not do this. If you are above 300 degrees C in your
application, you should probably consider using a Mag stabilized Zirconia. Otherwise, over
time a Yttria material will lose its properties.
So, let's have an overview:
Impact resistance: Material to choose...Y-PSZ. Ceria stabilized Zirconia has higher impact
resistance (fracture toughness), but is much more expensive. It should also be noted that
some companies have Mg stabilized Zirconias with high impact resistance properties. The
way that they achieve this is to basically really over-fire the material. You can increase
fracture toughness doing this in a Mg Zirconia, but you decrease flexural strength as well.
Least expensive: Material to choose...Mg stabilized Zirconia.
Best material properties: Material to choose...Y-PSZ
Large wear parts: Material to choose...Mg stabilized Zirconia.
Part with thick cross sections: Material to choose...Mg stabilized Zirconia
Part with extremely fine detail such as a knife edge: Material to choose...Y-PSZ
Application over 300 degree C: Material to choose...Mg stabilized Zirconia
Application requiring flexural strength: Material to choose...Y-PSZ (or HIPed Y-PSZ)
In very few cases, you may consider a Ceria stabilized Zirconia.
There are many, many designations that companies use to describe Zirconia ceramics. It
can be very confusing. PSZ, TTZ, Y-PSZ, Mg-PSZ, YTZP, YZP are all designations that
you will see for Zirconia ceramics. Essentially, you need to look for a "Y" or a "Mg" (or "M")
in the material name. If the material is noted only as "PSZ" or "TTZ", you need to confirm
what type material it is because the definition can vary greatly.
OK, so how are Zirconias different from each other.
There are three (3) phases of Zirconia. Monoclinic, tetragonal and cubic. When we are in
the firing cycle, around the temperature of 800 to 900 degrees C on the way up, Zirconia
will go into a cubic phase. At the end of the firing cycle, we cool the material and lock it
into a tetragonal gain structure. This is how you make Zirconia parts. So at this point in
time, we have a part that is "Partially Stabilized" and in a tetragonal grain structure. For
the purpose of our discussion this is where the similarities end between Yttria and Mag
stabilized Zirconias.
With a Yttria stabilized material, we now have a material that is capable of reverting into a
low energy monoclinic phase to absorb stresses. This is key to understanding what makes
a Y-PSZ so special. Mag stabilized material does not have the ability to do this. This is
the primary difference between the materials. With a Yttria material, we have a material
that is truly toughened. What is "toughened" anyway. Y-PSZ is a toughened material.
Y-PSZ can absorb stresses and impacts because it has the ability to release the energy (of
a force or impact) by reverting into a low energy monoclinic phase. It releases heat and
goes monoclinic, thus absorbing the stress. Let's review. Three phases of Zirconia.
Monoclinic, tetragonal and cubic. At the end of the firing cycle, when we release heat, we
go into a tetragonal phase. The material (out of the kiln) is in a tetragonal phase. When it
gets impacted, it releases heat and goes into a low-energy monoclinic phase. The material
then takes heat back in and reverts back into tetragonal phase. This is how all of your
Yttria and Ceria stabilized materials work. The reason that they can absorb such high
impacts, is that they give the stress off in a thermal-chemical reaction. Now, if you are a
lay person, this is where you will get really confused. Many companies refer to Mg
Zirconias as PSZ or Transformation Toughened materials. Others do not. We do not. We
define a Mg Zirconia as a Partially Magnesia Stabilized Zirconia. Is a Mg-Zirconia a
PSZ or "transformation toughened"? Well maybe. If you have heard the expression..."a
little bit pregnant"...Well that could be said for Mg-Zirconia. It is in fact partially stabilized.
It does have a tetragonal grain structure (from cubic) at the end of the firing cycle. So, by
those descriptions, some argue a Mg-Zirconia is a PSZ or a "transformation toughened"
material. And by their logic, they are correct. By our definition though, a PSZ has to be
able to go through phase transformation to release stresses. Y-PSZ does this. Partially
Magnesia Stabilized Zirconia does not. The primary drawback to Yttria stabilized
Zirconias is that they will go through what is called "phase migration". What is "phase
migration"? Phase migration is a bad thing. Remember how our Y-PSZ material could
revert into a low-energy monoclinic phase to absorb stresses? Well, when Y-PSZ is
exposed to heat and especially heat and moisture, it will lose it's tetragonal grain structure
and go monoclinic...thus losing its physical strength. Thus, you end up with a really weak
material. Mag stabilized Zirconia does not do this. If you are above 300 degrees C in your
application, you should probably consider using a Mag stabilized Zirconia. Otherwise, over
time a Yttria material will lose its properties.
So, let's have an overview:
Impact resistance: Material to choose...Y-PSZ. Ceria stabilized Zirconia has higher impact
resistance (fracture toughness), but is much more expensive. It should also be noted that
some companies have Mg stabilized Zirconias with high impact resistance properties. The
way that they achieve this is to basically really over-fire the material. You can increase
fracture toughness doing this in a Mg Zirconia, but you decrease flexural strength as well.
Least expensive: Material to choose...Mg stabilized Zirconia.
Best material properties: Material to choose...Y-PSZ
Large wear parts: Material to choose...Mg stabilized Zirconia.
Part with thick cross sections: Material to choose...Mg stabilized Zirconia
Part with extremely fine detail such as a knife edge: Material to choose...Y-PSZ
Application over 300 degree C: Material to choose...Mg stabilized Zirconia
Application requiring flexural strength: Material to choose...Y-PSZ (or HIPed Y-PSZ)
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You will see that companies list their materials in various ways. Most Japanese and a few
American firms (like INCERCO) do not consider Mag stabilized Zirconias to be "PSZ" or
"Toughened" materials. Mag stabilized Zirconias are defined as "Partially Magnesia
Stabilized Zirconias". We define Yttria (or Ceria) stabilized Zirconias as "PSZ" or
"Toughened" Zirconia materials.
Most (not all) American firms define Mag stabilized Zirconias as "PSZ" or "Toughened"
Zirconias.
It is irrelevant in this forum to debate who is right, and who is wrong. Just be aware that
there are differences in how companies define what a "PSZ" or a "Toughened" Zirconia is.
American firms (like INCERCO) do not consider Mag stabilized Zirconias to be "PSZ" or
"Toughened" materials. Mag stabilized Zirconias are defined as "Partially Magnesia
Stabilized Zirconias". We define Yttria (or Ceria) stabilized Zirconias as "PSZ" or
"Toughened" Zirconia materials.
Most (not all) American firms define Mag stabilized Zirconias as "PSZ" or "Toughened"
Zirconias.
It is irrelevant in this forum to debate who is right, and who is wrong. Just be aware that
there are differences in how companies define what a "PSZ" or a "Toughened" Zirconia is.
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