31 Structural characterization

A metal foam is characterized structurally by its cell topology (open cells, closed cells), relative density, cell size and cell shape and anisotropy. Density is best measured by weighing a sample of known volume; the rest require microscopy.

Optical microscopy is helpful in characterizing metal foams provided that the foam is fully impregnated with opaque epoxy (or equivalent) before polishing. This requires that the foam sample be immersed in a low-viscosity thermoset containing a coloring agent (black or deep blue is best), placed in a vacuum chamber and degassed and then repressurized to force the polymer into the cells. The procedure may have to be repeated for closed-cell foams after coarse polishing, since this often opens a previously closed cell. Conventional polishing then gives reliable sections for optical microscopy (Figure 3.1(a)).

Scanning electron microscopy (SEM) is straightforward; the only necessary precaution is that relating to surface preparation (see Section 3.2). SEM is

(c)

Figure 3.1 (a) An optical micrograph of a polished section of an Alcan aluminum foam. (b) A SEM micrograph of an INCO nickel foam (Kriszt and Ashby, 1997). (c) An X-ray tomograph of an Alulight foam foam (B Kriszt, private communication, 1999)

most informative for open-cell foams (Figure 3.1(b)). Closed-cell foams often present a confusing picture from which reliable data for size and shape are not easily extracted. For these, optical microscopy is often better.

X-ray Computed Tomography (CT) gives low magnification images of planes within a foam which can be assembled into a three-dimensional image (Figure 3.1(c)). Medical CT scanners are limited in resolution to about 0.7 mm; industrial CT equipment can achieve 200 The method allows examination of the interior of a closed-cell foam, and is sufficiently rapid that cell distortion can be studied through successive imaging as the sample is deformed.

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