2.9.2.1 Picture Analysis
Picture analysis is based on a two-dimensional projection of the grit, e. g. via back light microscopy or film scanners, or on a picture, e. g. by transmitted light
microscopy or scanning electron microscopy. Picture analysis has some restrictions. The equipment defines the minimum grit size that is measurable. In addition, the grit placement interferes with the measured results. For example, when the grits are dispersed on the scanner plate, the probability of grits to fall on certain grit planes varies.
Picture analyses work mostly on a two dimensional projection of the abrasive grits. Figure 2.25 gives an example for different measurement values from a two-dimensional picture, such as maximum and minimum grit diameter, grit circumference, and grit cross-sectional area in the projection plane. Some shape characteristics are derived in comparison with a coextensive circle, i. e. a circle with the same area as the projected grit (Fig. 2.25).
Equations 2.21 [LIEB96] and 2.22 [LIEB96, SCHI02] define shape factors indicating grit roundness. The ratio in Eq. 2.23 is another characteristic for the roundness or compactness [PIRA03, LIST06]. The grit ellipticity can be evaluated by Eq. 2.24 [GESU00, SCHI02]. Further characteristics are derived from the
Two methods to define the particle size are possible, such as the grit’s largest dimension or the average of the maximum grit dimension and the largest dimension rectangular to it [DAVI73, DAVI74, BENE10]. Taking a round grit, both values are similar. For an elongated grit, the largest dimension is much bigger than the average dimension calculated by the second method. Thus, the greater the deviations from the ideal blocky shape, the bigger the difference of both measurement methods. One
SEM picture of a diamond grit Picture analysis
Perimeter
400 pm
Min. grit diameter Max. grit diameter
Fig. 2.25 Example for picture analysis of a grit
method for grit size analysis of micron powder recommended by FEPA is to take the smallest circle, which circumscribes the particle [HERB81].