Influence of depth of information and of resolution in stereologic evaluation of surface electron micrographs

SUMMARY Due to the difference in preparation technique, the submicroscopic structure of metals, minerals and ceramics is often easier to evaluate with surface electron micrographs of bulk specimens than with transmission electron micrographs of thin films. As in the latter case, a surface electron micrograph also contains information from a certain depth which for stereologic evaluation necessitates a correction. This correction depends on the relation of grain size, depth of information and resolution. Experimental studies lead to the following results: With 20 keV scanning electron micrographs showing a material contrast resolution between 50 and 200 nm, depending on the material, stereometric analyses are influenced by the depth of information for grain sizes between 1 and 25 μm. Furthermore, the results of analysis are influenced by ‘microscope distortion’ and by ‘distortion of single micrographs’. Depth of information and distortions have the effect that the grain distribution image of polished sections may differ by some percent from an ideal section image. An automatic microstructure analysis seems to be nearly impossible because of variations of photographic density obtained from different particles of one and the same phase. For photo-emission electron micrographs with a material resolution between 15 and 30 nm depending on the material, the influence of the depth of information, microscope distortion and distortions of single micrographs may be neglected. Therefore, such micrographs are well suited for manual or automatic stereometric analysis.