Frequently asked
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questions
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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.
Go to Page 2 of Frequently Asked Questions
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.
Go to Page 2 of Frequently Asked Questions
Chris at his desk after a hard day of
making ceramic parts.
making ceramic parts.
Christopher A. Holland
C.E.O. & Founder
INCERCO TECHNICAL CERAMICS INC.
C.E.O. & Founder
INCERCO TECHNICAL CERAMICS INC.
FAQ Page 3