Metallography is the study of the microstructure of metals and alloys using a microscope. Proper preparation of a metallographic sample is critical to obtaining accurate and reliable results. Polishing is the final step in the preparation of a metallographic sample, and it is necessary to produce a flat, scratch-free, and mirror-like surface. In this article, we will discuss the process of polishing in metallography, including the equipment and materials used, and the different techniques and abrasive compounds.
Equipment and Materials Used in Polishing
The equipment used in polishing includes a polishing machine, polishing cloths, and abrasive compounds. The most commonly used polishing machine is a rotary polisher, which uses a rotating disc to apply the abrasive compound to the surface of the sample. The polishing cloths used in metallography are made from synthetic fibers and are designed to hold the abrasive compound.
The abrasive compounds used in metallography include aluminum oxide, magnesium oxide, chromic oxide, iron oxide, and diamond compounds. Aluminum oxide is the most widely used abrasive for general metallographic polishing. Magnesium oxide is recommended for final polishing, especially for the preparation of magnesium and aluminum and their alloys. Diamond compounds are becoming increasingly popular for preparing metallographic specimens, especially for materials such as boron carbide and sintered tungsten.
Techniques for Polishing
The polishing technique used depends on the type of material being polished and the desired surface finish. Typically, polishing is started on a hard cloth with a coarser abrasive and finished on a softer cloth with a finer abrasive. Final polishing should not be prolonged, but just sufficient to achieve the desired surface finish without causing excessive relief. Before final polishing is started, the surface condition should be at least as good that obtained by grinding with a 400-grit (25 microns) abrasive.
Polishing is a similar action to grinding, except that the supporting medium used to hold the abrasive is capable of far greater shock absorbency. Different polishing surface materials have differing characteristics, with soft cloths allowing the greatest shock absorbency and, therefore, allowing for gentle polishing with little damage associated. However, soft cloths allow the abrasive to abrade different areas at different rates, giving rise to relief, the term used to describe the undulations that form in a polished surface. Extreme undulations or relief in the polished surface are to be avoided, although a certain amount can be tolerated or even desirable because the SEM generally has a high depth of field.
Conclusion
Polishing is the final step in the production of a surface that is flat, scratch-free, and mirror-like in appearance. Such a surface is necessary for subsequent accurate metallographic interpretation, both qualitative and quantitative. The polishing technique used should not introduce extraneous structure such as disturbed metal, pitting, dragging out of inclusions, comet tails, and staining. High-quality preparation of most metallographic specimens often can be expedited by the use of automatic polishers. The ability to select the proper combination of cloth, abrasive, carrier, polishing speed, and pressure applied can be acquired only by experience.