Depending on the analysis that is to be performed, sample preparation refers to the ways in which a sample is treated prior to its analysis. Preparation is a very important step in most analytical techniques in order to gain meaningful analytical results by preserving the nature of the analyte while enabling the analysis.
The key factor for preparing a good metallographic specimen is to produce a “defect-free flat” surface with the area of interest. To achieve this goal, a series of proper procedures including cutting, mounting, grinding, polishing and etching is required. The ASTM standard E3-11 could be used as guidance for preparation of metallographic specimens.
The cutting or sectioning is the first step to prepare a metallographic specimen. It is usually done with an abrasive cutter or a diamond precision saw. The abrasive cutter is generally used for metal specimens and the diamond precision saw is used for electronic materials, ceramics and biomaterials. An ease subsequent procedure depends on the quality of the cutting step. The proper equipment selection and technique, including the blade type, cutting speed, load and coolant, will minimize the deformation. Some metal specimens, such as tin or zinc alloys, have tendency to recrystallize at surface to form smaller grain size. These kinds of false metallographic characterizations may be introduced by the preparation process. It should be avoided as much as possible or be aware of its existence.
The mounting procedure is necessary for specimens that are hard to be held steady due to its small size or irregular shape or the specimen edge needs to be protected during the polishing procedure. Normally, such specimens are fixed inside a polymer block called metallurgical mount. The most common polymers used in mounting are epoxy resins, acrylic resins and polyester resins. They are typically two-part systems consisting of a resin and a catalyst (hardener). Several criteria for a suitable resin including good adhesion to the specimen, sufficient strength and chemical resistance to the following etching process are observed in selecting an appropriate resin for a specific application. Two common mounting techniques are cold mounting and hot mounting depending on the resin curing temperature and if conductive fillers will be used.
The mounted specimens then undergo grinding and polishing procedure. The grinding is performed with number of grades, 200-2000, of sand papers. One should start with the lowest grade or coarsest one and gradually go to the higher grade, in general. The specimen must be washed thoroughly before proceeding from one grinding grade to the next. For soft metal specimens, or the specimen has hard inclusions that could be easily pulled out, the grinding should start with higher grade of sand paper. The polishing is done with soft cloth impregnated with diamond or other polishing particles. The surface should be thoroughly cleaned with water followed by alcohol after the final polishing and dried with air.
The etching will enhance the microstructure features by the etchant attacking the grain boundary area and different phases at different degrees to reveal the crystal orientations, grain boundaries, precipitates and phases. There are numerous etchant formulations depending on the types of specimen. They usually consist of acids or bases with oxidizing or reducing agents. For example, the most commonly used etchant for ferrous alloys is the Nital solution which contains 2-5% nitric acid in balance amount of alcohol. The etching procedure varies from seconds to minutes. It is always a good practice to follow related safety regulations and precautions when handling chemicals during etching.
A satisfactory metallographic specimen must represent a flat surface and free from defects and/or artifacts caused by preparation such as recrystallization, smears, pull-outs and scratches. Decent sample preparation is the key to a successful microscopic structural study or an examination of parts for failure analysis.
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