Automatic Grinding and Polishing

An automatic grinder can produce flat specimens using wet SiC papers of 220, 500, and 1200 mesh, 30 N (6.7 lbf) force per specimen, 300 mm wheel, 125 rpm, counterrevolving the specimen. Grind no more than 30 s on a paper, for six 30 mm (14 in.) mounts done at the same time. Before polishing, ultrasonically clean for about 5 min. The ultrasonic cleaner used for washing P/M specimens consists of a power supply and a small tank, which holds a water bath containing a small amount of surfactant such as liquid soap. The surfactant provides the energy coupling needed between the water bath and a beaker of solvent, such as alcohol, is suspended in the bath. The water bath and beaker together should fill the ultrasonic tank to within 25 mm (1 in.) of the top, (to provide protection to heating elements that the ultrasonic cleaner may contain). The specimen is placed inside the beaker and rests face down on cotton wadding or a paper towel. The power source produces high-frequency waves in the room-temperature water bath. The waves strike the beaker and force the alcohol into the pores of the specimen, removing foreign substances. Because the specimen is placed in the alcohol bath, the washing takes place in what soon becomes a contaminated solution. Debris accumulates in the cotton or on the paper towel, and the solution must be changed periodically. In addition to removing polishing grit when changing to successively finer media, it has also been reported that the ultrasonic action may help to open pores that were smeared at grinding (Ref 4).

Rough polishing consists of 10 min on short-nap cloth with a3 /'m diamond paste at 30 N (6.7 lbf) force on each specimen and 125 rpm on a 300 mm (11.8 in.) wheel. Final polishing for 1 to 2 min with 0.05 /'m alumina on long-nap microcloth at 125 rpm, 1 N (4.4 lbf) force results in a mount free of scratches and flat to the edge. If the pores were well filled with epoxy resin, they will be seen in their true area fraction and shape. If the interior pores are not well filled, the above procedure must be supplemented by several repetitions of light etching (15 s immersion in 2% nital) and repolishing 1 to 2 min with the 0.05 /'m alumina. To learn if the pores are still smeared, examine the mount at about 1000/ and look for thin gray boundaries of the pore edges, for example, Fig. 9.

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