Chemical Engineering @ Carnegie Mellon

Andrew J. Gellman Group S.O.P.

Polishing Metal Crystals

Precautions: It is very important to maintain a clean environment when polishing the sample to a mirror finish. Any small amount of grime or dirt can severely set back the cleaning process. Ensure all surfaces are kept clean before and between polishing of different grit sizes.

NOTE: These instructions are for polishing a curved sample. An addendum following these instructions will describe the procedure for polishing a sample flat. Read all addendum items before beginning this SOP.

WARNING: Some curved watch glasses are actually flat in the center. Check your watch glass carefully before polishing a curved sample.

Sample preparation
  1. Clean flat watch glass and glass ring form with isopropanol.
  2. Coat surfaces with release agent and allow to dry
  3. Place sample on flat watch glass inside the glass ring form such that the sample is centered in the ring form
  4. The epoxy should not have bubbles in it when poured. Bubbles will entrap coarse polishing grit which will reappear and scratch the sample when one is doing fine grade polishing.
  5. Hold the back of sample onto the watch glass with the cotton end of a long cotton swab.
  6. Pour epoxy mixture into the ring form and continue to hold sample with cotton swab for about 10 minutes until the epoxy has cured. The epoxy should be about 1 cm thick.
  7. Once epoxy has set and cured, remove the epoxied sample from the ring form.
Coarse polishing
  1. Obtain a clean, curved watch glass.
  2. Remove adhesive from the back of a Carbimet paper disc of appropriate grit and place on the watch glass. Begin with the lowest grit size (i.e. 120 grit) for the initial shaping of the sample. The disc may have to be cut such that it can lay in flat contact with the watch glass.
  3. Holding the epoxied sample firmly, rotate the sample on the Carbimet paper disc, following the contour of the curved watch glass, until the epoxy and sample have been shaped to approximate the curve of the watch glass.
  4. A small amount of deionized water should be used in the contact region.
  5. Once the sample has been curved, remove the disk and carefully wash all the grit off the sample and the epoxy holder. If any grit is left then it will produce large scratches during subsequent polishing with finer grit.
  6. Change to a Carbimet paper disc of a finer grit (i.e. 240 grit) and again rub the sample against the disc following the contour of the watch glass. Rub in one predominant direction, say left to right, until all the scratches appear to have been caused by the current grit size and all scratches from the larger grit have disappeared.
  7. Change the Carbimet paper disc again to the finest grit size (i.e. 400 grit) and rub in a direction perpendicular to the previous direction, say up and down, until all the scratches appear to have been caused by the current grit disc.
  8. Remove finest grit disc and proceed to the next section, fine polishing.
Fine polishing
  1. Clean all surfaces with isopropanol.
  2. Remove adhesive from a nylon polishing cloth disc and apply to curved watch glass. The disc may have to be cut such that it can lay in flat contact with the watch glass.
  3. Apply about inch of the highest grit diamond paste (i.e. 30 micron) to the center of the polishing cloth.
  4. Add a few drops of lapping oil to the diamond paste.
  5. Polish the sample, as before, predominantly in one direction to eliminate the scratches produced from the previous polishing step.
  6. Once all scratches are due to the current diamond paste, remove the nylon polishing cloth and discard.
  7. Thoroughly clean all surfaces with isopropanol and apply a new nylon polishing cloth disc to the watch glass. Be certain to remove all grit before proceeding to the use of finer grit.
  8. Continue fine polishing using the next diamond paste size (i.e. 6 micron) with a few drops of lapping oil and in a perpendicular direction to eliminate scratches from the previous polishing step.
  9. Continue to polish with new nylon polishing cloth discs and successively finer diamond pastes until the smallest size diamond paste has been used (i.e. micron)

  10. NOTE: If the surfaces become contaminated with dirt or dust, then scratches may be formed which are larger than the current paste size. These scratches will be noticed and polishing must be continued using higher grit or paste sizes to remove these larger scratches. This sets back the polishing process and is why cleanliness is of utmost importance.
  11. Remove nylon polishing cloth disc from watch glass and discard.
  12. Remove adhesive backing from microcloth polishing cloth discs and apply to curved watch glass. The disc may have to be cut such that it can lay in flat contact with the watch glass.
  13. Apply about 1mm thick film of 0.05 micron alumina paste to the center of the polishing cloth disc. Apply very little pressure during this final step. The sample does not really have to press against the microcloth.
  14. Polish the sample in this paste by rubbing in a circular motion until the sample has a mirror finish and no large scratches remain.
Removing the sample from epoxy
  1. Place epoxied sample in a beaker of methyl ethyl ketone (MEK) and allow it to soak for approximately 1 day.
  2. The epoxy should begin to dissolve where it is in contact with the sample.
  3. Once the sample has been freed, gently remove from the epoxy. It should not take a great effort to accomplish this and additional soaking may be required.
ADDENDUM for polishing a sample flat
  1. All polishing procedures should be followed as for a curved sample with the following modifications
  2. The sample should be mounted in the epoxy using the same procedure as above, but using the smaller brass ring form, such that the cured epoxied sample can fit inside the steel press used for polishing.
  3. A flat watch glass should be used onto which the Carbimet paper discs are applied.
  4. The steel press should be used to rub the sample against the polishing surface. This ensures that the edges do not get rounded and that even pressure is applied to the sample as it is rubbed against the polishing surface.


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