Polished section preparation
Metallic materials are opaque, i. e. they are impermeable for incoming visible light waves. Therefore a structural examination with an optical microscope is only possible with incident light mode.
The contrast is based on the different reflecting capacity of individual microconstituents – for example due to differences in light absorption or surface roughness. However, clear differentiation is only possible with a reflection difference of at least 10 %.
Structural examinations with an optical microscope therefore require carefully prepared surfaces. Specimen preparation (specimen collection, identification and embedding) is initially followed by profiling and smoothing the polished section (multi-stage grinding and polishing). However, contrasting the specimens is usually required in addition to make the structure visible.
Specimen collection according to the test objective is systematic – e. g. in quality control – or specific – e. g. for the clarification of defects. The specimen location (longitudinal, transversal or flat grinding, edge or centre of the component) often needs to be considered. Especially for the investigation of defects, taking overview photos of the component is recommended before collecting the specimens (see Figure 1).