Because we have minimized surface damage in cutting, grinding can commence with a relatively fine particle-size abrasive, depending on the hardness of the metal/alloy and/or the metal-removal rate in grinding. For some metals or alloys, one grinding step is adequate before going to diamond polishing abrasives. For others, two steps might be needed. Polishing is generally accomplished with two diamond steps and one final polishing abrasive, either colloidal silica or alumina. But the method must be properly planned and executed. During polishing, the cloth surface must be uniformly covered with abrasive and lubricant. If it dries, the surface will be smeared and deformed, and color etch results will be poor. To achieve this, set the platen to rotate counterclockwise and the head to rotate clockwise at a speed between 30 and 90 RPM, depending on the metal/alloy. The new Struers Tegramin grinder/polisher is ideal for such work.
Examples of natural color in metals are rare. Gold and copper exhibit yellow color under bright-field illumination. Color can be produced using optical methods such as polarized light and differential interference contrast illumination. The microstructure of metals with non-cubic crystal structures can be examined without etching using polarized light, but color is not always observed. The specimen must be prepared completely free of residual damage for color to be observed, and even then some non-cubic metals still exhibit little color. However, many metals and alloys can be etched with reagents that deposit an interference film on the surface that creates color in bright-field illumination. If it is difficult to grow such a film to the point where the color response is excellent, the color can be enhanced by examination with polarized light, perhaps aided with a sensitive tint filter (also called a lambda plate or first-order red filter).
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