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By properly documenting the initial specimen condition and the subsequent microstructural analysis, metallography enables powerful quality control and serves as an invaluable investigative tool.
Metallography is the study of a material microstructure. It helps to determine if the material has been processed correctly, and is therefore a critical step for establishing product reliability and for resolving why a component failed. This article outlines the basic steps for proper metallographic specimen preparation: documentation, sectioning and cutting, mounting, planar grinding, rough polishing, final polishing, and etching.
The first steps
Following proper documentation, most metallographic samples need to be sectioned to the area of interest and for easier handling. Depending on the material, the specimen may be sectioned by abrasive cutting (metals and metal matrix composites), diamond wafer cutting (ceramics, electronics, biomaterials, minerals), or thin sectioning with a microtome (plastics).
Proper sectioning is required to minimize damage, which may alter the microstructure and produce false metallographic characterization.
Proper cutting requires the correct selection of abrasive type, bonding, and size; as well as proper cutting speed, load, and coolant. Table 1 lists the most common type of abrasive blades for metallographic sectioning, and Table 2 lists cutting parameters for diamond wafer cutting.
Mounting procedure
The mounting operation provides three important functions: It protects the specimen edges and maintains the integrity of surface features; it fills voids in porous materials; and it improves handling of irregular-shape samples, especially for automated specimen preparation.
The majority of metallographic specimens are mounted by encapsulating into a compression mounting compound. These include thermosets such as phenolics, epoxies, and diallyl phthalates; and thermoplastics such as acrylics.
Some samples are cast into ambient castable mounting resins, such as acrylic, epoxy, and polyester. Other specimens are glued with thermoplastic adhesives.
Compression mounting: Metal specimens are usually compression mounted. Phenolics are preferred because they are low in cost, whereas the diallyl phthalates and epoxy resins are chosen if edge retention and harder mounts are required. Acrylic compression mounting compounds are chosen when excellent clarity is required. Table 3 lists the applications of the most common compression mounting resins.
Castable mounting: Electronic and ceramic materials are frequently mounted in castable resins, which are recommended for brittle and porous materials. These mounting compounds are typically two-component systems (resin...