Metallography, materialography and specimen preparation

Metallography is the science and art of preparing a metal surface for analysis by grinding and polishing, and etching to reveal the structure of the specimen. Ceramic, sintered carbide or any other solid material may also be prepared using metallographic techniques, hence the collective term, materialography.

Metallographic and materialographic specimen preparation seeks to find the true structure of the material. Mechanical preparation is the most common method of preparing the specimens for examination. Abrasive particles are used in successively finer steps to remove material from the specimen surface until the needed metallographic surface quality is achieved. A large number of materialographic preparation machines for grinding and polishing are available, meeting different demands on preparation quality, capacity, and reproducibility.

In the Struers Application Notes about metallography you will find information about specific materials and their application, and recommendations for the metallographic preparation, including cutting, mounting, grinding and polishing, cleaning and etching.

Pure aluminium 99,985%, etched with 4% HF in distilled water, polarized light.

The true metallographic structure

Theoretically, in metallography we are interested in examining a specimen surface which shows us a precise image of the structure we are to analyze. This nearly perfect condition, with only superficial damage remaining, is commonly called the true structure (see Undisturbed Structure below).

1 Rough surface

2 Microcracks

3 Damaged layers with internal defects

4 Undisturbed Structure

A systematic preparation method is the easiest way to achieve the true materialographic structure. When the work routinely involves examining the same material, in the same condition, the metallographer wants to achieve the same result each time. This means that the preparation result must be reproducible. Different materials with similar properties (hardness and ductility) will respond alike and thus require the same consumables during preparation. Specimen preparation must therefore pursue rules which are suitable for most materials. The Metalog Guide will give you efficient and systematic guidelines for your materialographic work with sample preparation.

The metallographic preparation process

The specimen taken must represent the features of the parent piece from which it is cut. Depending on the size or shape of a piece of material, it may need to be sectioned. A plane surface, with as little deformation as possible, is required to facilitate and expedite further preparation. Consequently, the most appropriate sectioning method is abrasive wet cutting, which will introduce the least amount of damage in relation to the time needed. Struers offers a complete range of materialographic cut-off machines and cut-off wheels for cutting materialographic specimens.

Metallographic specimens are typically mounted using hot mounting or cold mounting. A typical hot mounting cycle will use hot mounting resins and a hot mounting press compressing the mounting media to 4,000 psi (28 MPa) and heat to a temperature of 350 °F (180 °C). When specimens are very sensitive to pressure or temperature, cold mounts may be made with cold mounting resins. Mounting a specimen provides a safe, standardized, and ergonomic way by which to hold a specimen during the grinding and polishing operations.

After mounting, the next steps are grinding and polishing to reveal the structure of the metal. The specimen is successively prepared with finer and finer abrasive media. Silicon carbide (SiC paper) was the first method of grinding and is still used today. Many metallographers, however, prefer to use a diamond suspension which is applied onto a composite disc throughout the grinding process. Diamonds in suspension for grinding might start at 15 micron and finish at 6 or 3 micron. Generally, grinding with diamond suspension gives finer results than using SiC paper, especially when revealing porosity, which silicon carbide paper sometimes smear over. After grinding the specimen, polishing is performed. Typically, a specimen is polished with diamonds, first using grain sizes of 6, 3 or 1 micron. For final polishing suspensions of silica or diamond can be used. A variety of polishing cloths from non-woven to woven to nap-type cloths are employed to produce a scratch-free mirror finish, free from smearing or pull-outs and with minimal deformation remaining from the metallographic preparation process.


	Hot mounting: The specimens are placed in the mounting press, and the resin is added. The specimens are mounted under heat and high pressure.

	Cold mounting: The specimens are placed in a mounting cup and mounting material is then poured over the specimens. A vacuum impregnation unit (photo) is used for mounting of porous materials.

Microscopy and analysis of the prepared specimens

After polishing, certain microstructural constituents can be seen in the microscope, e.g., inclusions and nitrides. If the crystal structure is non-cubic (e.g., a metal with a hexagonal-closed packed crystal structure, such as Ti or Zr) the microstructure can be revealed without etching using crossed polarized light (light microscopy). Otherwise, the microstructural constituents of the specimen are revealed by using a suitable chemical or electrolytic etchant for electrolytical preparation. A lab management system is recommended to acquire images, collect associated data, measure and analyse.