Polishing Abrasives

Polishing usually involves the use of one or more of the following abrasives: diamond, aluminum oxide (Al2O3), and amorphous silicon dioxide (SiO2) in collidal suspension.

For certain materials, cerium oxide, chromium oxide, magnesium oxide or iron oxide may be used, although these the late 1920s, as Hoyt [7] mentions a visit to the Carboloy plant in West Lynn, Massachusetts, where he saw sapphire bearings being polished with diamond dust in an oil carrier. He used some of this material to prepare sintered carbides and published this work in 1930.

Diamond Abrasives

Diamond abrasives were first introduced in a carrier paste but later aerosol and slurry forms were introduced. Virgin natural diamond was used initially, and is still available as MetaDi diamond paste. Later, synthetic diamond was introduced, first of the monocrystalline form, similar in morphology to natural diamond, and then in polycrystalline form.

MetaDi II diamond pastes and MetaDi suspensions use synthetic monocrystalline diamond while MetaDi Supreme suspensions and MetaDi Ultra pastes use synthetic polycrystalline diamonds. Figure 3.13 shows the shape differences between monocrystalline and polycrystalline diamonds. Studies have shown that cutting rates are higher for many materials using polycrystalline diamond compared to monocrystalline diamond.

Colloidal Silica

Colloidal silica was first used for polishing wafers of single crystal silicon where all of the damage on the wafer surface must be eliminated before a device can be grown on it. The silica is amorphous and the solution has a basic pH of about ~10. The silica particles are actually nearly spherical in shape, (as seen in

Figure 3.14) the polishing action is slow, and is due to both chemical and mechanical action.

Damage free surfaces can be produced more easily when using colloidal silica than with other abrasives during final polishing. Etchants can respond differently to surfaces polished with colloidal silica. For example, an etchant that produces a grain contrast etch when polished with alumina may instead reveal the grain and twin boundaries with a “flat” etch when polished with colloidal silica.

Color etchants frequently respond better when colloidal silica is used producing a more pleasing range of colors and a crisper image. But, cleaning of the specimen is more difficult.

For manual work, use a tuft of cotton soaked in a detergent solution. For automated systems, stop adding suspension about 10-15 seconds before the cycle ends and. For the last 10 seconds, flush the cloth surface with running water. Then, cleaning is simpler. Amorphous silica will crystallize if allowed to evaporate. Crystalline silica will scratch specimens, so this must be avoided. When opening a bottle, clean off any crystallized particles that may have formed around the opening. The safest approach is to filter the suspension before use. Additives are used to minimize crystallization, as in MasterMet 2 Colloidal Silica, greatly retarding crystallization.

Final Polishing

For routine examinations, a fine diamond abrasive, such as 1µm, may be adequate as the last preparation step.

Traditionally, aqueous fine alumina powders and suspensions, such as the MicroPolish II deagglomerated alumina powders and suspensions, have been used for final polishing with medium nap cloths. Alpha alumina (0.3µm size) and gamma alumina (0.05µm size) slurries (or suspensions) are popular for final polishing, either in sequence or singularly. MasterPrep alumina suspension utilizes alumina made by the sol-gel process, and it produces better surface finishes than alumina abrasives made by the traditional calcination process.

Calcined alumina abrasives always exhibit some degree of agglomeration, regardless of the efforts to deagglomerate them, while sol-gel alumina is free of this problem. MasterMet colloidal silica suspensions (~10pH) are newer final polishing abrasives that produce a combination of mechanical and chemical action which is particularly beneficial for difficult to prepare materials. Vibratory polishers, Figure 3.15, are often used for final polishing, particularly with more difficult to prepare materials, for image analysis studies, or for publication of quality work.