Dr. Dmitri Kopeliovich

Preparation of a specimen for microscopic examination is carried out according to the following stages:

• Cutting
• Mounting
• Grinding
• Polishing

The large body of the investigated material is not necessarily homogeneous; therefore the specimen should be taken from the region, adequately representing the structure and other features of the large sample.

Sometimes the directional factor is essential: preferential direction of crystals formed in directional solidification, elongated grains and inclusions after rolling. In these cases direction of the cut surface is taken into account.

Cutting operation itself may change the specimen micro-structure, due to heating and mechanical damage. Cutting technique is selected in a way, preventing such effect.

• Soft non-ferrous alloys are cut by resin bonded SiC discs.

• Steels and hard non-ferrous alloys are cut by resin bonded alumina discs.

• Ceramics, minerals, composite and electronic materials are cut by low speed diamond wafer blades.

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Mounting metallographic specimen serves for protection of the specimen edges and improvement of its irregular shape, enabling both easy manual and automated grinding/polishing operation.

Compression mounting is the most popular method for metal specimens.

• Phenolic resin is the most widely used low cost mounting material.

• Acrylic resin is used when clarity of the mounting is important.

• Epoxy resin is used when hardness of the mount is required.

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Surface damages, created by cutting discs, are reduced by grinding.

Dry or wet silicon carbide abrasive paper is used for grinding operation.

Special grinding tables equipped with clamps for strips of grinding paper are used for manual grinding.

Rotary grinding/polishing machine is used for automated grinding. The rotation speed, which initially is about 300 RPM should be decreased to150 RPM in the course of grinding.

Grinding is carried out on a series of stages with increasing the fineness of the paper: 240, 400, 600, 1000.

The fineness number indicates a number of SiC grains on square inch.

Each grinding operation removes the marks, produced in the previous operation.

After each grinding stage the specimen should be thoroughly washed in order to prevent carry-over the coarser grit to the grinding paper.

The grinding direction of each stage should be changed by 90° relative to the direction of the previous stage. This makes easier indication of completion of the grinding stage (removal of the scratches from the previous stage).

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The fine scratches caused by final grinding operation are removed by polishing.

Flaky powders of different fineness are used as polishing abrasives. A mixture of powder and water is spread over a rotating disc covered with polishing velvet cloth.

Alumina, magnesia and diamond powders of various fineness (12-15mkm, 5-6mkm, 1mkm) are usually used for successive polishing stages.

The specimen is thoroughly washed after each stage in order to prevent contamination of the polishing cloth by coarse powder particles from the previous stage.

Rotary disc machines with adjustable rotation speed are used for polishing. The machine may be equipped with an appliance for the specimens holding and loading.

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• Metallographic examination - introduction
• Macro-examination of metallographic specimens
• Etching metallographic specimens
• Metallurgical Microscope
• Scanning Electron Microscope
• Grain structure
• Solidification
• Phase transformations and phase diagrams
• Basic principles of heat treatment
• Chemical Composition Evaluation

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to Metallographic examination