Metal Powder Particles

The powders, if mounted in epoxy resin as noted above, present a planar surface suitable for fine grinding. It is usually possible to begin using wet 600 grit SiC on revolving wheels or fixed sheets of paper. Approximately 30 s of grinding exposes enough particles to be viewed. The specimen is washed and then polished by hand for 2 min using 1 /Jm diamond on a short-nap cloth (MOL) at 250 rpm on a 200 mm (8 in.) diam lap. The use of an alcohol-base, water-soluble lubricant allows easy specimen cleaning in water.

The powders must not be overpolished, or the epoxy resin will be polished away between them and the particle edges will become rounded. Final polishing should be performed using deagglomerated 0.05 ,i,!m AfO;, on a long-nap microcloth for 30 s with light hand pressure on a 200 mm (8 in.) diam, 125 rpm lap. Prolonged polishing or heavy hand pressure during final polishing rounds the particle edges.

Because some particles have internal pores that may have been smeared, it is important to examine some of the particles, unetched at 500 or 1 000/. for the thin, gray lines that are the edges of smeared pores (Fig. 9). If such undisclosed pores are noted, 15 s etching and 1 /'m diamond polish must be repeated. Etching the mounted powders removes enough surface material to lower the particles below the surface of the epoxy resin, which then provides an opportunity to repolish them. The repolishing should always be performed using 1 J^m diamond followed by 0.05 /''m A1:0;, for 30 s on a long-nap cloth.

Aluminum powders and alloys must be polished using 1 /,!m diamond, rather than A1203, which reacts with aluminum powders. Final polishing is best done with colloidal silica on microcloth for 30 to 60 s. Hard powders, such as tungsten, may be examined as-polished using 1 diamond. Final polishing of very soft materials, such as pure copper, may be carried out using a fixed, long-nap cloth, as described previously for full-size specimens. The colloidal silica noted previously also works well on pure copper. Pure iron powder may require two or three 30 s fine polishings and light 15 s etching in 2% nital to remove fine scratches. The procedure also opens porosity in fine-porosity sponge iron.

Fig. 9 Intermediate stage of polishing showing the edges of smeared pores, (see arrows), which have not yet been opened by the polishing. The material is pressed and sintered Fe-2%Cu-0.8%C, 6.8 g/cm3. Magnifications of 500 to 1000x are required to see these features. These smeared pore edges must be removed by polishing and etching so that the original particle boundaries characteristic of undersintering can be seen, in the unetched condition, e.g., Fig. 27, 28, and 29. Unetched. 960x

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