How to Mount, Polish, and Etch a Sample for Optical Microscopy.

Introduction: How to Mount, Polish, and Etch a Sample for Optical Microscopy.

In order to view a metal specimen under an optical microscope, the sample must be prepared using several steps. It must be mounted onto bakelite puck, polished using finer and finer grits as well as polishing slurry, and finally it needs to be etched. These instruction are intended for the Leco mounting press and Leco polishing stations. Figure 1 above shows an example of what a sample should look like after it has been mounted.

Useful Terms:
Grit - The measure of the fineness of an material to grind or polish a surface.
- A diamond paste used to polish metal.
Mounting - The process of compressing and heating bakelite powder around a piece of metal in order to form a solid disk or puck that can be used to handle the sample easier.
Etching - The process of using nital to reveal the grain structure in a sample for viewing under an optical microscope.
Ram - The upper and lower rods of the mounting press that are used to compress the sample. The upper one is attached by turning the levers on it and screwing it into place. The lower one is raised and lowered using a dial/knob on the machine.
Microstructure - The microscopic structure of a material.

Required Materials:
Gloves, Apron, and Safety Glasses
Sink with running water
Fume Hood
Blow Hose Gun
Bakelite (polyoxybenzylmethylenglycolanhydride)
Leco Mounting Press
Polishing Machines with 500, 400, 300, 200 and 100 grit
Diamond Slurry of 5, 1, and 0.5 micrometers
Nital Solution
A Metal Specimen (usually a steel or an alloy of steel)

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Step 1: Preparing the Mounting Press

Turn the handles on the upper ram until it is fully loose. Raise the lower ram in the mounting press using the knob until the upper ram sticks out. Remove the upper ram. Clean the surface of the lower ram as shown in figure 2 and remove any leftover Bakelite.

Step 2: Setting the Sample

Set sample on the lower ram with the surface you want to view under the microscope facing down as shown in figure 3.

Step 3: Preparing the Bakelite

Lower the lower ram until you can pour 20 to 30 ml of Bakelite into the mounting press without it overspilling. Scoop up enough Bakelite (figure 4) so that when it is done being pressed the sample will not stick out of the other side.

Step 4: Pouring the Bakelite

Pour the Bakelite into the mounting press as shown in figure 5. Be careful not to know the sample down as you pour Bakelite over it. The sample should be completely covered with Bakelite.

Step 5: Lower the Ram

Lower the lower ram all the way down using the knob shown in figure 6 and insert the upper ram into the mounting press.

Step 6: Securing the Upper Ram

Screw the upper ram back on (figure 7) and turn it completely until it will turn no further. Once the levers on the upper ram will turn no further turn it back 90 degrees in order to avoid the upper ram getting stuck after the sample is done being mounted.

Step 7: Mount the Sample

Turn the mounting press on. The machine should now begin to heat and press the Bakelite to mount the sample. The pressure should increase so that the pressure gauge moves up to the red dot on the scale that corresponds to the size of the mold as shown in figure 8.

Step 8: Wait

Wait for the Leco mounting press to finish heating and pressing the sample. After the sample has been mounted the heater light (figure 9) will turn off. The machine will now cool down, this can take about ten minutes. Do not begin removing the mounted sample when the machine is cooling down.

Step 9: Turn the Upper Ram

Turn the upper ram until it is loose. Do not remove the upper ram from the mounting press yet.

Step 10: Raise the Lower Ram

Raise the lower ram by turning the knob until the bottom section of the upper ram sticks out. Remove the upper ram out of the machine. Raise the lower ram again until it sticks out and the mounted sample is completely out of the machine as shown in figure 10 and from a closer view in figure 11.

Step 11: Label and Clean Sample

Label the mounted sample on the back side. Rinse the mounted sample under water to clean it. Dry the mounted sample using an air hose gun.

Step 12: Turn on the Polishing Machine

Turn on the polishing machine. Turn the dial that controls the water flow on so that a steady stream of water begins pouring out onto the polishing disk. Turn the dial that controls the spin of the polishing disk so that it begins to spin on medium settings.

Step 13: Polish the Sample

Set the mounted sample face down on the polishing disk of the highest grit. Apply only enough force to keep the sample from moving. Be careful not to apply to much force but make sure enough force so that you have a secure grip on the sample. This will prevent the sample from flying out of the machine. When the polishing is done all lines from the grit rubbing against the sample should be going in one direction. See embedded video for more information on how to polish the sample.

Step 14: Continue Polishing the Sample

Continue polishing the sample using finer and finer grits. When switching between grits, turn the sample 90 degrees from the previous grit so that new lines form perpendicular to the lines from the previous grit. Dry the sample using the blow hose gun between grits.

Step 15: Set Up Diamond Slurry Polishing Machine

When you are done using the finest grit, polish the sample using the diamond slurry. Turn on the machine. Pour about 5 to 10 ml of diamond slurry onto the polishing disk. Do not run water over the polishing disk used for the diamond slurry. Turn the dial to get the polishing disk to rotate.

Step 16: Polishing Using Diamond Slurry

Set the sample face down on the polishing disk and apply a gentle amount of force. Start from the largest slurry and work your way down to smaller and smaller sized slurries.Clean the disk between slurries so you do not contaminate other polishing disks. Clean the sample by running it under water and then blow drying it completely. Make sure a separate diamond slurry is used for each polishing disk.

Step 17: Clean and Check the Sample

When the sample is done being polished with the diamond slurries the metal surface should have a mirror finish. You should be able to see your reflection perfectly on the metal surface. Clean the sample by running it under water and then drying it with the blow hose gun.

Step 18: Prepare to Etch the Sample

Put on gloves, an apron, and safety glasses. Put the bottle of nital in the fume hood. Make sure the light and fan in the fume hood are turned on. Fill a beaker with water and set it into the fume hood as shown in figure 13. DANGER: nital can be explosive when mixed with ethanol. Handle the nital with extreme caution.

Step 19: Etch the Sample

Using a pair of tongs, pick up the sample. Pick up the bottle of nital with your free hand. Hold the sample over the beaker of water. Pour a few drops of nital on the sample. Tilt the sample so that about half of the metal surface is exposed to the nital. Keep half of the metal surface unexposed so if the sample becomes over-etched you will be able to try again on the unexposed half. Wait 3 seconds then drop the sample into the beaker. Wait a few seconds then use the tongs to pick the sample out of the beaker.

Step 20: After Etching

Clean the sample immediately after taking it out of the beaker by running it under water. Dry the sample completely. The sample should now be ready to be viewed under an optical microscope. Etching the sample should reveal the microstructure. If the sample looks burned under the microscope, than it has been over-etched. Repeat the etching process on the half of the metal surface that has not been etched. Drop the sample into the water sooner than the previous attempt.

Step 21: Clean Up

Turn off all the machines when you are done. Make sure the nital is stored properly and labeled. Turn off the fume hood fan and lights once the nital has been stored properly. Make sure none of the machines are leaking and no water is running on the polishing stations.

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