Reference cited in this section

2. K. Leithner, Basics of Quantitative Image Analysis, Adv. Mater. Proc., Nov 1993, p 18-23 Sample Preparation

Microstructural analysis requires a deformation- and damage-free sample for correct characterization. A typical preparation procedure includes sectioning, mounting, planar grinding (coarse grinding), sample integrity (rough polishing), polishing, etching, and examination. Ideally a specimen is prepared in such a way as to minimize the amount of damage produced at each stage. For example, the correct cutting blade can significantly reduce the amount of damage to the material, thus reducing the aggressiveness of the subsequent coarse grinding step (Ref 3, 4, 5).

To achieve the required surface finishes, combinations of grinding, lapping, polishing, and mechanochemical polishing techniques are used. For metallic samples, hardness is the critical feature for determining the preparation sequence, whereas ceramic preparation procedures depend on both hardness and fracture toughness. For metal samples, mechanical abrasives such as SiC, diamond, and alumina are commonly used in the preparation sequence. For ceramics, initial sectioning and grinding are accomplished with diamond abrasives; however, for the fine grinding steps, either diamond or softer mechanochemical abrasives (colloidal silica) are used. Nearly all ceramic materials that are final polished with colloidal silica produce damage-free samples. In the final-polished condition, surface features such as porosity, cracks, inclusions, and other defects can be observed.

In most cases, however, an etching step is used to reveal microstructural features such as grain boundaries or phase features. Common etching techniques include chemical, electrolytic, thermal, and plasma techniques (Ref 6). The commonly used techniques for metals are the chemical and electrolytic etching methods. For ceramics, thermal and plasma etching methods are very effective; however, chemical etching with very aggressive acids is also common. Another approach for etching both ceramics and composites is to relief polish the specimen. This technique requires a high-nap cloth and an alkaline-based colloidal silica suspension.

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