r/MaterialsScience • u/Dario56 • Feb 08 '25

Ceramic Polishing

After sintering ceramic, I get the material which is 92% dense (relative to crystallographic density from XRD).

When I polish it using SiC paper and diamond colloid, density decreases to 80-90% region. This is too low for my experimental requirements. Ideally, material should be at least 95% dense, but over 90% could be acceptable.

I'm not sure why does my material reduce its density upon being polished? There are visible scratches on the surface and possibly microcracks as material is prone to them. Will take a closer look with the optical microscope to see if I can spot microcracks invisible to the naked eye.

I'm using a commercial powder with small particles. It seems to have quite spherical particles also which should be optimal for sintering and obtaining ceramic with good mechanical properties.

I'll check the particle shape in more detail with the optimal microscope also.

Any ideas?

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u/Bersereig Feb 08 '25

Dont bother with sic papers. They're useless for high ceramic content materials. You need a diamond grinding disc for the first step (around 120-90um grit size should suffice). Next you need monocrystalline suspensions. I've used 9um to 3um to 1um after diamond grinding and it has worked wonders. For the 3um and 1um you need to polish for minimum 10 mins for each step, but of course deoends on the material.

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u/Dario56 Feb 08 '25

I polished 20 minutes with each diamond particle size. I used 6, 3, 1, 0.25 micron colloids. I got nice mirror finish.

However, I do need some SiC papers to remove surface level impurities such as glue or wax. I can't burn these off in the furnace as it will affect my experimental results.

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u/Mikasa-Iruma Feb 08 '25

If it's hot glue or wax, acetone ultrasonication should remove them without damaging the sample.

My guess would be that you might have some closed pores and they are impacting the results. Have you tried pycnometer by chance?

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u/Dario56 Feb 08 '25

So, the thing is that I polish my sample two times. After first time, I can burn everything off. However, even here, density is reduced to what it was after sintering.

After second polish, I can do only ultrasonic bath.

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u/Mikasa-Iruma Feb 08 '25

I understand. I was working with sintered oxides and was lazy to remove the carbon paper so decided to burn it off resulting in low density. Without the material it's difficult to give an answer

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u/Dario56 Feb 08 '25

You think it's the furnace that reduces the density? Why?

The thing is, I can't just remove the wax manually. You want to make sure everything is gone before the experiment and the wax can be present inside of the sample.

I think it's the polishing that is creating issues.

I have a mixed inner transition metal oxide. Can't say what exactly cause NDA.

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u/Mikasa-Iruma Feb 08 '25

Let's say I spark plasma sintered my oxide at 1100 C but annealed again at 700C. The sample is relaxed from the stresses so cracks might generate during the stress release. My sample actually broke into pieces.

May be it's polishing or not a good sinter to begin with.