Al2O3: Relationship between grain size and density? 1

[johnrh]

Colleagues,

I'm new to the ceramics industry, but I have been asked to come up with a relationship between the FIRED DENSITY and the FIRED GRAIN SIZE of alumina pieces. Is there a rule of thumb for that sort of thing that is known in the industry?

A few more questions the answers of which are probably well known outside of me and my small number of teammates:

1. What are the methods normally used to determine grain size?

2. I know grain size can be affected by sintering. Is this a DRASTIC EFFECT or SLIGHT ENLARGEMENT? Or maybe this is a better question: If you had pictures of the grain structure before and after sintering, would the grains be fused unrecognizably in the second picture? Or would that be the case only if we had too much time at temperature?

3. Our production people never seem to worry about "exaggerated grain growth" caused by being too hot for too long. I find mention of this concern only in books. Is this something that industry professionals need to watch out for or is it a worry mostly of the academic theorists?

With thanks in advance,
JohnRH

 

[genesis]

Dear Johnrh,

Question (1):

A representative sample must be obtained of the part, then polished, and etched to reveal grain structure.

"fined grain" alumina ceramics need a SEM(scanning Electron Microscope) to image the grain size. You would take a picture of a representative area on sample and measure at least 700 grains for size. The easiest way is to use a histogram. There are also some digital bitpads avalable to help count grain size(ie. Zeiss).

Question (2):

That depends on the time and temperature. Generally, in heat treating of metals that are one solid piece one can talk about a TTT curves, which are Time-Temperature-Transformation curves. However, ceramics I have seen afew curves in the literature but it depends on your exact process(your raw materials, formulation, equipment, and procedure). There are also grain growth additives that can be used to help control exaggerated grain growth.

Basically, the most practical way to handle this issue is to press some round discs and fired them for different times and temperatures. The measure the ultrasonic velocity through each disc, along with its diametral strength. Perform a microstructure of a few samples, then you have your working TTT curve for your ceramic material with your process.

Question (3):

It depends on how this ceramic component will be used. If the application is critical, then be careful. For example areas of exaggerated grain growth in ceramics(particularly in alumina ceramics) can have lower strength values than areas where in the grain size is smaller and more homogeneous. Generally, you can detect these grain size changes in the ceramic component using a simple ultrasonic thickness gage, which also measures sonic velocity.

If the grain are big enough, then an optical microscope can be used.

Exaggerated grain growth with cause reduction in mechanical strength, which can lead to problems at customer sites. For example: "This group of parts we received last year lasted longer than what we have received this year from you folks" Sound familar!!!! Yes, to answer this it will take some effort but if you are involved with critical components(ie.: aerospace), then this TTT study can be also used to show waht can be termed as "prudence" in a court of law. See your lawyer about product liability.


Good Luck,

Genesis



Genesis

 

[ugwuegbuchima]

Question(1) From what I know about firing of a ceramic body, there is a relationship between fired density and initial particle/grain size of the ceramic body. The initial particle/grain size will determine whether or not the fired density of the ceramic body will increase .
The rule of thumb guiding the fired density and initial grain size of a ceramic body is that the sintering or firing causes shrinkage of a ceramic body as the pore size between the particles is reduced. That is ,during sintering and firing ,atoms diffuse away from the grain boundary (between adjacent particles) to the pore.In effect, the pore is “filled in” by diffusing material which hence leads to the shrinkage of the ceramic body.During this process,the density of the fired ceramic body increases,but the final fired density depends on the degree of shrinkage of the ceramic body.The more the shrinkage, the more the increase in the fired density.Shrinkage on the other hand depends on the initial grain size of the ceramic body and temperature.Small/fine grain sizes accelerate the rate of pore shrinkage and hence increases the fired density of the ceramic body.Coarse grain size retards the rate of pore shrinkage and hence reduces the fired density of the ceramic body. Practicalize this and see if it will work for you.
Question(2)
Methods normally used to determine grain size are :
a. Sieving method,
b. Elutriation method,
c. Sedimentation method.
Sieving Method
Sieving method is a statistical process ,that is,there is always an element of chance as to whether a particle will pass through a given aperture or not.In sieving method,particles of different sizes are passed through a series of sieves after which the sieved particle sizes are graded in different sieve sizes and compared with standards given.Wet sieving is mainly used when fine particles are needed.
Elutriation and sedimentation methods depend on the fact that particles of the same specific gravity but different size fall through water at different rates. This is expressed by Stokes’law.
Elutriation Method
In this method ,as particles of different size fall through still water at different velocities,application of an upward velocity of water makes it possible to reverse the motion of the slower and therefore smaller particles,and carry them out with the water stream .Careful selection of the velocity of water should make it possible to remove all particles smaller than a given known size.Elutriation method finds considerable application in large-scale preparation of raw materials.(Note, Air Elutriation is also into existence ).
Sedimentation Method
The progress of sedimentation method is followed either by removing sample portions at a known height below the surface,or by following the density changes. The simplest method of sampling is with a tall cylinder with a side arm and tap.This has now been superseded by withdrawal with a pipette.
Question(4)

Industry professionals need to watch out for exaggerated grain growth. This is because exaggerated grain growth results to some pores being “trapped” within grains during sintering and firing process of a ceramic body. This result might lead to many consequences for instance, during the sintering and firing of Al2O3 powder exaggerated grain growth of Al2O3 powder leads to porosity of the Al2O3 powder which inturn leads to loss of visible light transmission (i.e,transparency) by providing a light scattering mechanism. Each Al2O3 -air interface at a pore surface is a source of light refraction. Only about 0.3% porosity can cause Al2O3 to be translucent ,and 3% porosity can cause the material to be completely opaque.
The solution to exaggerated grain growth is to add a small amount of MgO (about 0.1wt%),which retards grain-growth and allows pore shrinkage to go to completion.

 

[jtem]

Hi Johnrh,
Grain size of sintered ceramics can be determined by SEM. From my experience better to use thermal etching just 50-100C below those used for your sintering. Time should be no more than 1h, it is just removing grain boundary stress. Use as possible more grains and take average.

 

[JimMetalsCeramics]

Sintered (fired) density will tend to be greater for finer powders. Ideally you promote a higher initial "green" density. This comes from mixing fine particles with even finer ones. The mixture will ideally result in increased unfired "packing density". In order to improve fired density you can apply pressure to the powder as it sinters. This also inhibits grain growth while still allowing for full densification of a ceramic part.

If the powder is not initially well compacted and contains an uneven mixture of particle sizes, the densification will be compromised. Packing density is not a function of particle size, but of particle size distribution. However, finer particles will provide for faster densification and better final microstructure.

 

[grayfox82]

hi am in my final year of university. Could you tell me the consequences for the microstructure and the properties of a ceramic if grain growth occurs during sintering?

Thanks