Intro: How to Mount, Polish, and Etch a Metallographic Sample for Optical Microscopy
Are you someone who's interested in metals, likes to look at things under a microscope, and finds themselves in a lab with time to spare? Then learning how to mount, polish, and etch a metallographic sample for optical microscopy might just be for you! Chances are you're an engineering student who has access to the necessary equipment to do this, and that you'll have to do this at least one time in your coursework. But, for those of you who are curious, or just need a little refresher in the steps, this set of instructions will prove to be very valuable. The preparation process takes approximately 20-30 minutes from start to finish.
A little background on the subject:
The mounting and polishing of a sample is typically done to create a metal sample possessing the proper conditions for viewing its microstructure. These samples are widely used in the metallurgical engineering field to compare and contrast various types of metals.
Step 1: Safety and Materials
*As always in the laboratory setting make sure you:
-Always have lab supervision
-Are wearing safety glasses at all times
-Are wearing close-toed shoes (no sandals or flip flops)
-Are wearing safety gloves and apron as needed (for etching portion)
Equipment and Materials
-Approximately 30 mL of Bakelite
-A small metal sample you'd like to analyze (no longer than 3 cm, and no wider than 1.5 cm)
-An engraving tool
-Grinding wheels with sandpaper of varying grits (60, 240, 320, 400, 600, and 800 are used in this example)
-Polishing wheels with varying diamond suspensions (6, 3, and 1 microns are used here)
-Fume hood (necessary in the etching process, as hazardous chemicals are used)
-A beaker containing pure water
-Tongs to operate in the fume hood
-Etchant (a 5% nitric acid 95% ethanol solution is used here)
Step 2: Preparing for the Mounting Press
2.1. Choose the sample you'd like to analyze. This could be any metal of your choosing, so the possibilities are endless!
2.2. Measure out 30 mL of Bakelite. This is what your sample will be mounted in, creating a small puck which makes the polishing and etching steps much easier to carry out.
Step 3: Mounting the Sample
3.1. Place the sample onto the stage of the mounting press, lowering the ram via the lever on the lower left of the machine.
3.2. Pour in the appropriate amount of Bakelite.
3.3. Place the press on top, screwing it on tightly, then loosening it a quarter turn (since there is a buildup of heat and pressure during the process). then place the ram lever into the "up" position and leave it there.
3.4. Follow the directions on the machine to set the appropriate amount of pressure, and time.
3.5. Push start, and wait (about 9-10 minutes).
Step 4: Removing the Sample
4.1 Once the sample is finished pressing, the machine will beep to alert you that it's time to remove it.
4.2 First, push the ram lever into the "down" position for about 5 seconds, then place it into the "neutral" position.
4.3 Unscrew the press, so that it is no longer threaded onto the machine.
4.4 Place the ram into the "up" position. The sample will be pushed up to the top, so keep one hand on the press to keep it from falling off. Be careful as the both the sample and the bottom of the press will be warm to the touch.
4.5 Take the sample off of the stage, and clean off any excess Bakelite. This can be down with a wire brush or your fingers (if the sample is cooled down enough).
Step 5: Inspecting the Sample
It is important to make sure the sample was pressed properly. It should not be crumbled at all, and if it is that means there was no heat applied to the sample while it was pressing. If this happens, do not worry, just remove the metal sample and repeat steps 1-4 again. If the metal is partially covered in Bakelite, that is fine since it will be ground away during the polishing process.
Above are some examples of defective sample mounts compared to a correct mount. The far left shows a sample using too much bakelite, the middle two samples show a process performed without heat, and the far right shows what the sample should look like.
Step 6: Labeling the Sample
Often times, you will need to label the back of your sample with an engraving tool so you can keep track of what class/type of sample you are currently working with. This makes it easier to tell multiple samples apart, along with aiding in the polishing process as we'll see later on...
Step 7: First Step in Polishing
7.1 Locate the grinding wheel equipped with the 60 grit sandpaper. Start with the grit with the smallest number, as this is the coarsest which will do most of the grinding away of the top oxide layer of the metal revealing the shiny surface. You will gradually work your way through finer and finer grit.
7.2. Start the machine as prompted by the machine instructions, and make sure there is a constant stream of water running onto the wheel while polishing.
7.3 Place the sample face firmly down onto the spinning wheel, making sure to have a good grip on the piece so it does not fly out of your hand. Be careful not to press your fingers onto the wheel as the sandpaper is very abrasive. IMPORTANT: make sure you hold the sample in the same location throughout this process, as the direction of the grinding is crucial. You may lift up the piece from time to time to check the progress, but replace it to its position when finished.
Step 8: Finishing Up With the Sandpaper
8.1 Check to make sure that the oxide layer is removed from the metal sample, there are no cracks in the sample, and the lines lines from the sandpaper are all running in the same direction.
8.2 Rinse the sample off with water, and dry it off to make sure the finer grit wheels are not tainted with larger grit. This could scratch any samples and cause major problems for anyone using the wheels.
8.3 Move on to the 240 grit sandpaper grinding wheel, and repeat the process of turning the machine on with the water running constantly.
8.4 When you place the sample onto the moving wheel, make sure you turn it 1/4 turn relative to the last orientation so the new scratches run perpendicular to the first set.
8.5 As with the last wheel, keep the sample on the same location on the wheel throughout this process.
8.6 Stay on this wheel until all of the new scratches are running in the same direction.
8.7 Repeat rinsing, drying, and turning of the sample 1/4 turn throughout all of the grinding wheels.
Step 9: Polishing to a Mirror Finish
Once you have finished on the finest grit of sandpaper, you should notice that the scratches are appearing smaller and smaller. This is good! You're now ready to move onto the final polishing wheels. These wheels look different from the sandpaper grinding wheels as they have a softer surface and use a diamond suspension (a slurry containing micrometer sized diamonds) to complete the polishing process leaving the sample with a shiny, mirror finish.
9.1 Rinse and dry your sample one final time before transitioning from sandpaper to diamond suspension.
9.2 Start the first polishing wheel equipped for the 6 micron diamond suspension in accordance with the instructions on the machine. IMPORTANT: Water is to not be used on these wheels, only the diamond suspension.
9.3 Spray about 4-5 sprays of the suspension onto the wheel, moving from the inner circle outwards.
9.4 Firmly place the sample face down on the polishing wheel, and move your hand in clockwise circles. Orientation of the sample does not matter in this part of the polishing process.
9.5 Check to see that the lines are even more buffed out, and the surface appears shinier. This should take about 10 seconds.
9.6 Rinse and dry the sample.
Step 10: Finishing the Polishing
Your sample is starting to look pretty, isn't it?
10.1 After rinsing and drying off the sample from the 6 micron diamond suspension, prepare the 3 micron diamond suspension wheel by turning it on and spraying it with the proper suspension (exactly like you did in the last step).
10.2 Place the sample face down on the wheel and continue to move your hand in clockwise circles until the scratches appear to be buffed away even more (approximately 10 seconds).
10.3 Rinse, dry, and repeat this process with the 1 micron diamond suspension.
Your sample should be scratch free and shiny like a mirror! If it isn't quite there yet, no worries! Just continue polishing with the 1 micron suspension until it gets there.
Step 11: Etching the Sample to View Grain Boundaries
This portion is technically optional, if you do not plan on using a microscope to view the grain boundaries and phases of the metal, this step is not necessary. It is a very common process, however, and should be known how to do.
IMPORTANT: A partner, fume hood, apron, safety glasses and rubber gloves are required for this process.
11.1 After you and your partner have applied the proper safety gear, locate the tongs within the fume hood. The fume hood lid can be adjusted to whatever height you feel comfortable working with.
11.2 Make sure there is a beaker of water within the fume hood. That will be used to dunk the sample after the etchant is applied.
11.3 Use one hand to grasp the sample in the tongs, and the other hand will be used to apply the etchant.
11.4 Very carefully squeeze the bottle of etchant so that 1-2 drops lands on a small area of the sample's surface.
11.5 Let the etchant remain on the surface for 3-5 seconds, and then dunk it into the beaker of water, and swirl it around.
11.6 Hand the sample to your partner to rinse under running water, and dry off.
the portion that was etched will appear cloudy compared to the rest of the sample.
If under the microscope the sample appears to be either over-etched or under-etched, simply re-polish the sample using the polishing wheel, and etch again.
Congratulations! You now have a perfect metallographic sample, and are ready for some optical microscopy!
Step 12: Ready for Testing
Once all of the polishing, and etching is completed, you now have a perfect sample to test in any way you need! Mounting, polishing, and etching samples prove to be very important in the world of Materials Engineering to help better understand the microstructural relationships between phases of all sorts of metal, and also to compare and contrast these relationships. Not only are these samples used in optical microscopy, they can also prove to be valuable when testing the hardness of a certain metal. This is another important aspect of better understanding the properties of metals.
I hope that these instructions helped your curious mind gain a better understanding of the wonderful world of metals, or was able to serve as a refresher for those of you who just needed a little reminder on how to prepare samples for observation. No matter what your reason, I would like to thank you for taking the time to look over my instructions today!