sample preparation

Proper preparation of metallographic specimens is a key step to determine the accurate microstructure of a material and often requires sectioning, mounting, course grinding, fine grinding, polishing, etching and microscopic examination.

SECTIONING

MES offers many types and sizes of diamond and abrasive sectioning saws to ensure we do not alter the microstructure or damage fracture features when cutting a specimen. Low-speed cut-off wheels are utilized in cases where the heat created by standard abrasive cutters must be avoided or to avoid cold working a material. We ensure ample coolant (when allowable) and proper speed controls during sectioning operations.

MOUNTING

Mounting samples in plastic provides convenience in handling and protection to the edges of the specimen being prepared during polishing operations. Specimens are generally mounted in epoxy in 1 to 1.5 inch diameter molds of a hard polymer. Compression molding materials are typically either thermosetting (Bakelite) or thermoplastic.

Thermoplastics are utilized when transparency and high edge retention are required, but require heat and pressure during the molding cycle. Cold molding (room temperature) is often used with epoxy to mount samples by simply mixing the epoxy and pouring it over a sample that is positioned face down in a cold-mounting ring.

GRINDING AND POLISHING

Coarse grinding generates the initial flat surface necessary for the subsequent grinding and polishing steps. Course grinding can be accomplished either wet or dry using 80 to 220 grit sandpaper on rotating disks. Care must be taken to avoid significant heating of the sample.

The final objective is to obtain a flat surface free from tool marks due to cutting. Fine grinding utilizes up to 1,200 grit sandpaper to produce a scratch free surface by employing a series of successively finer abrasives. Polishing involves the use of abrasives, suspended in a water solution, on a cloth-covered electrically powered wheel.

ETCHING

Following initial inspection in the unetched state, etching is often employed to highlight microstructural features or phases present in the material. Etching occurs when the acid or base placed on the specimen preferentially attacks specific phases present. Nital is an etchant commonly utilized with common irons and steels.