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Have a question about technical ceramics?  That’s OK, a lot of people do.  This forum is for people to have those frequently asked questions answered.  Over the years, I’ve seen people having a lot of the same questions.  The purpose of this forum is to do away with the “smoke and mirrors” of the high-tech ceramic
industry’s way of marketing and promote an understanding of  technical ceramics and ceramic technology as a whole.

Q – What exactly are “technical ceramics”?

A – “Technical Ceramics”  ”Engineered Ceramics” or “Advanced Ceramics” can arguably be defined by function or by their material make-up.  For example:  you take a piece of clay (a non-”technical” material) and use it in a “technical” application and thus you have a ”technical ceramic”.  However, most would argue that a “technical” ceramic is defined by its material composition, more so than its application.  So if you are going to go the “material composition” route, a technical ceramic is a ceramic material that is derived from the same minerals that you can derive metals from.  Example:  If you take Bauxite, which is a mineral you dig up out of the ground, and make a metal out of it…you get aluminum metal. If you take Bauxite and derive a ceramic from it…you get aluminum oxide ceramic.  If you take zircon sand, you get zirconium metal.  If you derive a ceramic from zircon sand…you get zirconia ceramic.    So, a technical ceramic can be defined by its application, but usually is defined by its chemical make-up.

In simplistic terms for our forum here,  a technical ceramic is a ceramic material that is derived from the same minerals that you can get metals from.  While there are endless combinations of ceramic materials that can be derived, 99.9% of the technical ceramics that you will encounter are based upon the oxides, nitrides and carbides of various minerals, but primarily aluminum, zirconium and silicon.

Q – What are machinable ceramics?

A – Technical ceramics, like the ones that we manufacture are very, very hard.  In fact the only thing harder than (technical) ceramics is diamond.  When we machine our ceramics after they have been fired, we have to use diamond grinding wheels and run a lot of coolant.  Most businesses do not have these type capabilities, equipment or personnel to accomplish this.  By this reality, they are not “machinable” by most people or businesses.  A machinable ceramic is a ceramic material that you can machine with a single point tool (such as turning a piece of metal on a lathe or milling metal on a mill with an end mill).  Most of the machinable ceramics are glass mica type materials, or they can be “bisque” fired ”technical” ceramics that are not fully fired.  It should also be noted (because I am always amused by this when I see this), that if you are using a machinable ceramic in a wear application, you are usually wasting your money.  Usually, machinable ceramics are best used for non-wear type applications because they have no wear resistance (such as an electrical or thermal type application).

Q – My company gets an alumina ceramic part…it is white.  When I see higher purity aluminas they are not white, they are cream colored.  Shouldn’t they be white if they are higher purity?

A – While there are always exceptions to the rule…in a nut shell,  ”No”.  First of all, people assume that alumina should be white.  That’s wrong, get it out of your head.   The alumina is going to take on the color of the additives in it.  As you can imagine, the manufacturer puts in other “ingredients” into their mix.  Usually in lower purity aluminas, they will put in calcium oxide.  Thus, this has an “whitening” affect on the ceramic and thus turns it white.  In higher purity aluminas, you obviously have less “ingredients” that you can put in.  If the material manufacturer is going for physical properties, they will have put in some type of additive to act as a sintering aid.  Usually in higher purity aluminas, MgO (magnesium oxide) is added into the mix as a sintering aid.  Usually, in higher purity materials, you are going for better performance and better material properties.  One of the  ways to get this is to control what is called “grain growth”.  In oxide based materials like alumina (aluminum oxide), when you get above 2,000 degrees F in your sintering cycle, you start getting grain growth.

Grain growth in ceramics is bad when you are going for better physical properties.  MgO is added to the ceramic to slow grain growth during the firing cycle.  Because there is no Calcium Oxide (in any quantity to speak of) in the material that will off-set the “yellowing” nature of the MgO; the trace elements of MgO give the ceramic an ivory or cream color.  If you are wanting good physical properties like flexural strength, wear resistance and chemical resistance, you want an ivory (or cream) shaded alumina material.  Now, this is not to say that an ivory colored part can’t be over-fired and have poor properties; and this is not to say that you can’t make high purity aluminas with great properties that are white.  This is just saying that an ivory color part should work better for you.  In some cases however, you may want a larger grain size; such if you are metalizing the part for it to be brazed.  Thus, a larger grain in the ceramic would be beneficial for better adherence of the metalization.  Again, there are exceptions to the rule; but the reason the ”higher” purity alumina material is ivory (or cream) colored and not “white” is because MgO has been added as a sintering aid to help control grain growth.

Q-What is the difference between Hot Pressing and HIPing?

A-This bewilders a lot of lay-people (and a lot of ceramics people who don’t know what they are talking about).  HOT PRESSING is where you take ceramic powder and put it in a die in a press, and extreme heat (enough to fire the part) is applied to the die while an upper and lower punch press extreme tonnage onto the part.  In very simple terms, you are pressing and firing the ceramic in one operation.  So, with HOT PRESSING, you put your powder in and out pops a fired part.   HOT PRESSING is a slow and expensive process.   This is usually done on plates and tiles that require great strength, such as armor.  With HOT PRESSING, you are limited greatly on the geometry of part that you can make.

HIPing is HOT ISOSTATIC PRESSING.  HIPing is always done on a part after it has been pressed and then fired.  A part has to be fully fired (or at least around 92% to 94% dense), before it can be HIPed.  In HIPing, you put your part in a pressure vessel that has heating elements in it.  An inert gas, such as argone is then pumped in.  The part gets refired while the gas acts to re-press the part (while it is being re-fired).  The word “ISOSTATIC” means ”equal pressure from all sides”.  Essentially, you kind of “burp” any voids out of the material.  Thus, a HIPed part will have great physical properties because the part is free from voids and is very homogeneous.    Because you are using the inert gas isostatically to apply pressure, you can HIP any shape or size of part.  The reason that the part has to be fired before you HIP it is because if the part is porus (un-fired), the gas simply permeates into the porosity (because of the high pressure of the gas) and the part will not densify.

So, HOT PRESSING is used to produce a part.  HIPing is used on a part to make it better.

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
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)

You will see that companies list their materials in various ways.  Most Japanese and a few American firms (like Holland Technical Ceramics) 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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