Observing the microscopic structure of materials reveals characteristics that have a tremendous influence on their technological utility. Some of the features that contribute to the strength of materials, and virtually all of the features that initiate mechanical failure, are resolved by optical microscopy. Thus, preparation of optical microscopy specimens, their observation using optical microscopes, and interpretation of photographs taken with optical microscopes (micrographs) continue to play a vital role in our attempts to understand the origin of material properties.

An important question to consider right away is: What size scale are the features revealed by optical microscopy? It might seem that the answer lies in the highest magnification that can be achieved by an optical microscope. However reasonable it might appear, this approach cannot answer the question. Magnification describes the enlargement of an image, but does not describe the resolution of objects in the image. The features revealed in an optical microscope image must be resolved into distinct objects to provide information about the microstructure. The resolution limit has more to do with the wavelength of the illuminating radiation than with the magnifying power of the optics. The wavelength of visible light is about 0.5 µm, so the resolution limit of an optical microscope is on the order of 0.5 µm.

Metallographic study, or metallography, is the imaging of topographical or microstructural features on prepared surfaces of materials. The structures studied by metallography are indicative of the properties and performance of materials studied. In this technique, planar surfaces are prepared to obtain a polished finish. Chemical or other etching methods are often used to delineate macrostructure and microstructure features. Once prepared, samples are examined by the unaided eye, light microscopy, and/or electron microscopy. Samples for microstructure evaluation are typically encapsulated in a plastic mount for handling during sample preparation. Sample preparation consists of grinding and then polishing using successively finer abrasives to obtain the desired surface finish. For microstructure examination, a mirror finish is needed, but a finely-ground finish is adequate for macrostructure evaluation. Etchants are specially formulated for the specific sample material and evaluation objectives.

Modern optical microscope.

Microstructure of steel D2.