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Mechanical Polishing

The term mechanical polishing is frequently used to describe the various final polishing procedures involving the use of cloth-covered laps and suitable polishing abrasives. The laps have either a rotating or a vibrating motion, and the specimen are held by hand, held mechanically, or merely confined within the polishing area.

Polishing be done in a relatively dust-free area, preferably removed from the area for sectioning, mounting and rough grinding. Any contamination of a polishing lap by abrasive particles carried over from a preceding operation or by dust, dirt or other foreign matter in the air cannot be tolerated. Carryover as a result of improper cleaning between final polishing steps is another prime source of contamination. It is just as important for the operator to wash his hand meticulously as is for him to remove all traces of polishing abrasive from the specimen before proceeding to the next polishing operation. The specimen can be cleaned ultrasonically or by careful washing under running water and swabbing with cotton. In automatic equipment in which holding fixtures for the specimens are also transferred through successive polishing steps, proper cleaning of the assembly can be accomplished only by using an ultrasonic cleaner.

If a polishing lap becomes contaminated, it is virtually impossible to remove all of the contaminant by washing the polishing cloth. Instead, the operator should replace the cloth and use fresh polishing solution. Cleanness cannot be overemphasized. It takes only one particle of grit on a final polishing lap to ruin all prior preparation.

Hand Polishing

Aside from the use of improved polishing cloths and abrasives, hand polishing techniques still follow the basic practice established years ago:

Specimen Movement

The specimen is held with one or both hands, depending on the operator's preference, and is rotated in a direction counter to the rotation of the polishing wheel. In addition, the specimen is continually moved back and forth between the center and the edge of the wheel, thereby ensuring even distribution of the abrasive and uniform wear of the polishing cloth. The main reason for rotating the specimen is to prevent the formation of "comet tails".

Polishing Pressure

The correct amount of applied pressure must be determined by experience; in general, firm hand pressure is applied to the specimen in the initial polishing step and is proportionally decreased with successively finer polishing steps. For very soft materials, pressure other than that from the weight of the specimen itself may be eliminated entirely in the last polishing operation.

Washing and Drying

The specimen is preferably washed and swabbed in worm running water, rinsed with methanol or any other alcohol that does not leave a residue, and dried in a stream of warm air. Alcohol can usually be employed for washing when the abrasive carrier is not soluble in water.

Cleanness

The precautions for cleanness, as previously mentioned, must be strickly followed.

Automatic Polishing

High quality preparation of most metallographic specimens often can be expedited by the use of automatic polishers. Automatic polishing equipment usually allows the preparation of several specimens simultaneously. Some methods of specimen preparation can be done only with automatic polishers, such as remote polishing of radioactive materials, chemical-mechanical polishing, and polishing in special atmospheres. There is no ideal automatic polisher; each has its merits and shortcomings and each metallographer must determine which is best for his particular requirements.

Polishing Cloths

The requirements of any good polishing cloth include the ability to hold an abrasive, long life, absence of any foreign material that may cause scratches, and absence of any processing chemicals that may react with the specimen.

A cloth without nap or with a very low nap is preferred for the preliminary or rough polishing operation. The absence of nap ensures maximum contact with the polishing abrasive, and results in fast cutting with minimum of relief. The cloths most frequently used are canvas, low-nap, cotton, nylon, silk and Pelon. These cloths are stretched tight on the laps and fastened securely, usually by a band-type clamp. Some cloths are available with a contact adhesive on the back, which greatly simplifies installation on the wheel. After installation, the cloths are charged with the appropriate abrasive (usually in sizes from 15 microns down to 1 microns) and carrier. Rough polishing is usually done with the laps rotating at 500 to 600 rpm. Cloths with a medium or high nap are ordinarily used on slow rotating laps (less than 300 rpm) for intermediate and final polishing. Felt or billiard cloths (100 % virgin wool), used with 0.3 micron aluminium oxide or other comparable abrasive, are excelent for intermediate polishing of soft metals (most nonferrous alloys and low carbon steels) and final polishing of hard materials (such as hardened alloy steels). One of the most popular cloths for final polishing of most metals is composed of densely packed, vertically aligned, synthetic fibres bonded to a suitable backing. For some metals or for particular types of polishing, other cloths, such as velvets, satins, cashmeres or cottons, may be required. The ability to select the proper combination of cloth, abrasive, carrier, polishing speed (rotational speed of the polishing wheel), and pressure applied can be acquired only by experience.

Polishing Abrasives

Polishing usually involves the use one or more of five types of abrasive: aluminium oxide (Al2O3), magnesium oxide (MgO), chromic oxide (Cr2O3), iron oxide (Fe2O3), and diamond compound. With the exception of diamond compound these abrasives are normally used in a distilled water suspension, but if the metal to be polished is not compatible with water, other suspensions, such as ethylene glycol, alcohol, kerosine or glycerin, may be required. The diamond compounds should be extended only with the carrier recommended by the manufacturer.

Aluminium oxide (aluminia) is the polishing abrasive most widely used for general metallographic polishing. The alpha grade aluminium oxide is used in a range of particle sizes from 15 microns to 0.3 micron. For some hard materials the 0.3 micron size is sufficient for a final polish. The gamma grade of aluminium oxide is available in a 0.05 micron particle size for final polishing.

Magnesium oxide (magnesia) is recommended for final polishing, especially for the preparation of magnesium and aluminium, and their alloys. Only the metallographic grades, which contain no water soluble alkalis, should be used; otherwise, any free alkalis present could stain and chemically attack the specimen. Magnesium oxide also reacts slowly with water to form magnesium hydroxide. This in turn reacts with carbon dioxide present in the atmosphere and in tap water to form magnesium carbonate, which can contaminate the polishing lap. If carbonate is present, the polishing cloth must be replaced or treated in acidified destilled water (2 % HCl solution) to convert the carbonate to water-soluble magnesium chloride. If the polishing cloth is to be retained for future polishing, it should be removed from the wheel immediately after use, washed in running water and stored in the 2% hydrochloric acid solution.

Chromic oxide, available in a range of particle size down to 0.05 micron, is used for rough and final polishing of steel and cast iron.

Iron oxide, known as jeweler's rouge is available as a powder of aproximately 3 micron particle size. It may be used for rough and final polishing of steel and cast iron, depending on the finish required.

Diamond polishing compounds are becoming increasingly popular for preparing metallographic specimen. Diamond is the only substance hard enough and with good enough cutting qualities to be used for mechanical polishing of materials such as boron carbide and sintered tungsten. Specimens that have both hard and soft constituents, such as graphite in cast iron and silicon in aluminium, can be polished without causing relief, with diamond compounds on an appropriate lap. These polishing compounds are available either in water soluble and oil soluble carriers or in the form of dry diamond powder in particle size down to 0.25 microns.

Rough Polishing - The purpose of the rough polishing step is to remove the damage produced during cutting and planar grinding. Proper rough polishing will maintain specimen flatness and retain all inclusions or secondary phases. By eliminating the previous damage and maintaining the microstructural integrity of the specimen at this step, a minimal amount of time should be required to remove the cosmetic damage at the final polishing step. Rough polishing is accomplished primarily with diamond abrasives ranging from 9 micron down to 1 micron diamond. Polycrystalline diamond because of its multiple and small cutting edges, produces high cut rates with minimal surface damage, therefore it is the recommended diamond abrasive for metallographic rough polishing on low napped polishing cloths.

Final Polishing - The purpose of final polishing is to remove only surface damage. It should not be used to remove any damage remaining from cutting and planar grinding. If the damage from these steps is not complete, the rough polishing step should be repeated or continued.