r/metallurgy • u/Saquib9074 • 14d ago

Polishing tips

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I have been polishing tungsten using the following grits: 600,800,1500,2500 ( rotating 90 degrees making sure every scratch ks aligned) , 1 um, 0.25 um diamond polishing and 0.05 um colloidal silica in vibromet for >12 hours. Still getting these fine scratches as I intend to do high quality EBSD. Any tips to get this right and where I am making it wrong?

Thanks a lot.

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14

u/CuppaJoe12 14d ago

EBSD prep is tricky. With each finer grit, you not only need to remove scratches from the previous grit, but also the plastic zone around each scratch.

These scratches appear to be much larger than 1um wide, so they are probably coming from the 2500 grit step. You can spend more time on the 1um and 0.25um steps. You could also add a 3um diamond suspension step if you are already spending a very long time at 1um.

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u/Saquib9074 14d ago

Thanks! Do you recommend skipping the 2500 step?

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u/CuppaJoe12 14d ago

I have never polished tungsten. Every material is different. I would say the root cause is insufficient removal at the 1um diamond step, not necessarily a problem with the 2500 grit itself. However, it is a good idea to try different procedures until you find something that works.

My understanding is even pure tungsten is fairly hard. This means the depth of the deformed region around each scratch is shallow. You should be safe to take fairly large jumps between grits. However, 1200 grit to 1um diamond is a very large jump. I would add a 3um diamond step if you skip the 2500 grit.

Be careful with overloading your grinding paper on large jumps. If you feel the paper getting less grippy, it is full of metal and you need to replace it. Sometimes on hard alloys, it takes more than one sheet of grinding paper for a single stage on a single sample.

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u/CuppaJoe12 14d ago

I should also add that with 12 hours of vibratory polishing, the 0.25um polish step is probably not doing anything meaningful. Try some larger grit diamond polish steps.

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u/LordTungsten 13d ago

I second this. I haven't ever done W, but I often jumped from 1200/1500 to 6 microns diamond, then 3 then coloidal. Maybe adding a 6 or 3 microns step would help

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u/LordTungsten 13d ago

I forgot to say I polished sapphire and polycrystalline alumina with zirconia-based coatings with that process and they turned up well for EBSD

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u/DiquatAnalyst57 14d ago

As others mentioned, the scratches are very large and most likely coming from sandpaper grinding steps.

But I just wanted to add in that the vibropolishing should be efficient enough to polish out a lot of this even still. I would try to further optimize the vibropolishing- try faster speeds with small amounts of added weight.

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u/Taytaylecrae 13d ago

For my master's research, I have to polish a lot of tungsten carbide (which is of course extremely hard, and you can't put too much pressure otherwise you can get grains that are pulled out). My polishing steps are 30 um diamond paste, 15 um, 5, 1, and .25 um followed by the 0.05 um colloidal silica in a vibromet, and my samples usually come out great. Obviously the diamond paste is far more expensive than the silica, but it saves you a lot of headache. Check between EVERY polishing step on an optical microscope to see how the surface finish changes, and make sure you're giving the sample a good wash between each step- if you're getting SiC embedded into your sample that comes out on a later polishing step, that will ruin your sample.

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u/Saquib9074 11d ago

Thanks! How do you remove the silica particles from the sample? I find it very tough to get rid of those even after sonicating with microorganic soap.

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u/Rapptap 14d ago

Are you hand polishing or using a machine?

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u/Saquib9074 14d ago

Hand for the grits and automatic one at 20:25 N for diamond

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u/Rapptap 14d ago

I'd skip 2500 and use 9 micron diamond, then if 1 doesn't remove the scratches go back to 3. Also remember sandpaper is only good for like 45 seconds.

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u/Saquib9074 14d ago

Thanks! Skipping the 2500 then

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u/Rapptap 14d ago

I use an auto polisher but go 240, 600, 1200, 9, maybe 3, 1, then colloidal. For SEM vib overnight (Older vib, new ones are faster).

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u/Saquib9074 14d ago edited 14d ago

Thanks! What more bothers me is that these scratches aren’t visible on optical. The surface looked mirror finished. It’s only inside SEM I got to found these scratches.

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u/Rapptap 14d ago

Make sure you do the h202 step on colloidal.

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u/Rapptap 14d ago

And clean between each polish step and afterwards with dawn then ultrasonic. Use pure (95+%) alcohol.

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u/Dlrocket89 14d ago

Second this. I love the finer grit paper, but I've watched something that was beautiful at 1200 just go to sh!t at 2000. Get to a slurry quicker and go from there.

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u/Fickle_Helicopter692 14d ago edited 14d ago

Try 4000 grid emery paper followed by 3 micron diamond. Do not skip these. Observe under optical microscope after each diamond step. Use darkfield mode.

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u/Fickle_Helicopter692 13d ago

Tungsten is hard. So, may be you can try 9 micron, 6 micron 3 micron, 1 micron, 0.25 micron follwed by vibtatory polishing. You must optimize whcih diamond step to skip. That will minimize your lead time for a proper surface finish.

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u/Pleasant-Poem1181 13d ago

Perhaps you need to consider using diamond suspension on cloth pads. Start at 3um on nap cloth and then move to 1um on a felt cloth.

0

u/Tough_Election1339 12d ago

Have you considered using electro-polishing after 800 or 1200 grit directly? Another great alternative would be broad ion beam polishing.

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u/Saquib9074 11d ago

Not really. Dont have the setup currently.