Metal surfaces are generally polished by a combination of two processes usually carried out simultaneously. The first is an aggressive cleansing operation which involves the removal of some of the metal and produces a very scratched surface with scratches fine or coarse depending on the size of the abrasive grain. A polishing compound which provokes such an effect to a high degree is said to have a good "cut." It has been well established as for instance by Tabor, Proc. Phys. Soc., London, Sect. B, 67, (1954) p. 249, and by Richarson, Wear, 11, (1968) p 245, that an abrasive must be significantly harder than a metal if it is to wear the metal to any extent. Those early attempts to quantify the differences required have been more recently extended by Torrance, Wear, 68, (1981) p 263, who, seeking to show that Moh's Scale of Hardness has a rational basis, applied a simple slip line field model to the abrasive metal contact and studied the relative shear yield stresses in the abrasive and in the metal. In another contribution Angus, Wear, 54, (1979) p 52, distinguished abrasive materials as "hard" or "soft" in relation to a metal using as his essential criterion the relative hardness of the abrasive (H.sub.a) and the maximum work-hardened hardness of the metal (H.sub.m). For a hard abrasive H.sub.a H.sub.m ; for a soft abrasive H.sub.a H.sub.m. From considerations like these it is clear that while "cutting" may be an essential precursor to "polishing" the influence of the metal is such as to preclude the use of generalizations related only to the abrasive powder.
The second process is the true polishing operation. This has a function very apparent to the observer, since it has produced a good "color" on the piece, but it is more difficult to describe in theoretical terms. To some, the polished surface is so "polished" because of a plurality of ultrafine microscratches and therefore the difference between abrasion and polishing is simply a matter of degree. This approach has been well summarized by Aghan and Samuels, Wear, 16, (1970) p 293, on the basis of evidence derived microphotographically. Another view has been advocated for perhaps sixty years by some workers in this field who have accepted that the deformation of a material at the rubbing interface is dominated by plastic deformation properties of the materials rather than by the brittle properties. The authorities usually quoted will include G. Beilby, "Aggregation and Flow of Solids," Macmillan: London, (1921) and P. W. Bridgman, "Studies in Large Plastic Flow and Fracture," McGraw-Hill: London, (1952). To them, the polished surface is a "fluid layer" or "Beilby layer" consisting of a thin film, perhaps only a few tens of Angstroms thick, of amorphous metallic matter. Yet another view is that the polishing process is "chemical" in nature and that specific chemical reactions take place between the polishing powder and the substrate usually by processes which involve the liquid medium. Rabinowicz, in the "Science Journal" (1970) Vol. 6, p 45, sees polishing as a process on the molecular scale, so "chemical" in that special sense, which is akin to dry burnishing.
Bowden, writing in 1953, emphasized the importance of another factor: the difference in the melting points of the polishing powder and of the metal substrate (Symposium on Properties of Metallic Surfaces, Institute of Metallurgy Monograph Ser. 13, p 335).
Since both processes, for "cut" and "color," are usually carried out simultaneously, the majority of commercial polishing compounds incorporate powder materials which provide for both functions. For instance, the cutting powder may be a relatively coarse "bunker alumina" (the final calcined product of the Bayer process) or a silicon carbide or corundum or quartz or other. The polishing component may be a product compositionally identical to that which is providing the cutting action except that it is very much finer in particle size. Alternatively, it may be a material specifically intended to contribute mainly the polishing action by means different to those which are held to polish by virtue of the production of microscratches.
It is normal commercial practice for both powder components to be combined together and mixed with a carrier which helps to retain the powder near the metal surface, or which lubricates the surface so preventing the appearance of grossly damaging major scratches, or which may act in some chemical manner on the metal, or which dicourages the oxidation of the newly cleansed surface. To some extent all the carriers used commercially will show some if not all of these properties. Such carrier compounds include oils, fats, waxes, tallows, wool stearines, and oleic or stearic acids. In the case of polishing compounds which are largely based upon alumina, the proportion of powder to carrier will usually be about 70%/30% although, with other powders, the precise ratio will depend on the consistency required, the specific gravity of the powder, and its surface area. Formulations like these can then be further converted to an emulsion form wherein the powders and the carrier are dispersed in an aqueous solution.