The FESEM analysis objective is to observe the grains and grain boundaries, i.e., the microstructure. I had conducted for one set of samples whose surfaces I had polished progressively with sandpaper gritness from 400 to 2000 followed by velvet cloth polishing with diamond paste and etching with Nital solution. Unfortunately no grains were visible. On the contrary the unpolished surface showed grains and grain boundaries but with low clarity and great difficulty. So I want to know what's the actual polishing and etching procedure (if any) to be done for such ceria-based samples for the purpose of FESEM analysis.
When googling for etchants, use additional word "metallography".
You can find some etchants here:
https://www.metallographic.com/Metallographic-Etchants/Metallography-Ceramic-etchants.htm
There may be a thin layer of polishing residuals.
Besides the mentioned etching You should play around with beam voltage (higher voltage for greater depth but risk of charging? --> coating) and detectors: BSE has greater information depth than ETD or inBeam/InLens.
The Nital solution you used for etching works for metals like Steel and not for oxides like ceria. Make use of thermal etching instead.
*Materials Needed*
• SiC sandpapers (320, 800, 1200, 2000 grit)
• Diamond suspensions/pastes (6 μm, 3 μm, 1 μm, 0.25 μm)
• Polishing cloths (hard woven, velvet, chem-cloth)
• Colloidal silica (0.05 μm, optional)
• Acetone, ethanol, distilled water
• High-temperature furnace (up to 1400°C)
• Sputter coater (Au-Pd or carbon, if needed)
*Protocol*
• Grinding (10–15 min total)
• Use water-lubricated SiC papers on a 200–300 rpm wheel.
• Grind progressively: 320 grit (2–3 min), 800 grit (2–3 min), 1200 grit (2–3 min), 2000 grit (2–3 min).
• Rotate sample 90° between grits. Rinse with water after each step.
• Polishing (20–30 min total or to satisfaction)
• On a 150–250 rpm wheel, polish with:
• 6 μm diamond suspension (5–7 min, hard woven cloth).
• 3 μm diamond (5–7 min, hard woven cloth).
• 1 μm diamond (5–7 min, velvet cloth).
• 0.25 μm diamond paste (5–7 min, velvet cloth).
• Optional: 0.05 μm colloidal silica on chem-cloth (10–15 min) for mirror finish.
Note: If you don't have the series of polishing suspension, just use either 0.25 um diamond suspension or 0.05 um colloidal silica.
• Check under optical microscope to ensure scratch-free surface.
• Cleaning (10 min)
• Ultrasonically clean in acetone (5 min), then ethanol (5 min).
• Rinse with distilled water and dry with compressed air/nitrogen.
• Thermal Etching (1–1.5 hr total)
• Place sample in furnace (air or oxygen atmosphere) on an alumina setter plate.
• Heat at 10°C/min to 1250–1350°C (100–200°C below sintering temperature).
• Hold for 15–20 min to form grain boundary grooves.
• Cool at 10°C/min to room temperature.
*FESEM Preparation*
• If sample charges under FESEM, apply 5–10 nm Au-Pd or carbon coating.
• Image at 1–5 kV for high-resolution microstructure.
*Notes*
• Avoid chemical etching (e.g., Nital) as it’s ineffective for ceria.
• If grains remain unclear, adjust etching to 10 min or increase temperature by 50°C (test on scrap sample).
• Unpolished surfaces may show grains but lack clarity due to roughness.
It seems like you have many good answers already.
I was going to ask more details about the grinding and polishing.
* I use a 1200-grit silicon carbide that seems to be cushioned or otherwise prepared so that it gives me a smoother surface than 2500-grit SiC. So, not all abrasives are the same.
* I use 3-um diamond with a low-nap followed by 1-um diamond on a softer (e.g., velvet) cloth. A couple minutes at each stage is sometimes enough. Check for scratches with an optical scope at 400x.
* Sometimes I can see grain orientation in my segmented BSE detector. If you can see grain orientation, that will reveal what you want. Other times, more polishing is needed. Many of our researchers resort to vibratory polishing with colloidal silica. If so, be sure to clean the sample well immediately afterward or you will find a very resistant residue.
* I defer to the others about etching. That is a skill/art that I have never learned. I am concerned about its preferential nature. It attacks one phase or structure more than another. I don't want (much of) that if I am trying to do chemistry. If all you need is grain size, then etching might be helpful.
Gideon C. Irogbele Thank you very much for the detailed answer. I have followed some of the procedures you mentioned. Next time, I will adopt your points for the preparation. However, I am concerned that the thermal etching at such high temperatures may lead to cracks in the pellet.
Warren Straszheim Thank you for the additional insights. I think I will stay away from chemical etching because, as pointed out by others, it is ineffective in my case. I actually couldn't observe the microstructure properly.
@Adarsh Shetty You may have to reduce the temperature a bit depending on your sample if you are concerned about cracking. But like I mentioned you first try with scrap samples to see if you get a promising result.
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