Mounting

Mounting specimens in a holding device is necessary when preparing irregular, small, very soft, porous, or fragile specimens, and in those cases where edge retention is required. Embedding is indispensable when multiple specimens are to be included in a single mount or when automatic equipment is to be used in the following preparation. In most cases, mounting follows sectioning, but in the handling of a great number of very small specimens, it may be advantageous to reverse this order. In general, the mounting procedure can be easily adapted to the special problem in question. The shape, size, and numbers as well as the hardness, brittleness, porosity, and heat and pressure sensitivity of the specimens have to be considered when mounting. Other considerations are: should a cross or a longitudinal section be prepared, is a controlled material removal required, is good edge retention needed, and should the preparation be carried out manually or with automatic equipment in specific sample holders. A suitable mounting media must meet several criteria: it must have good adhesion to the specimen, sufficient mechanical strength (hardness), and chemical resistance to etchants or solvents that are used during the preparation. For electrolytic polishing, scanning electron microscopy examination, or microprobe analysis, the mounting medium has to be electrically conductive. The mounting material should be easy to handle, economical, if necessary easy to remove, and it should not affect the specimens. For some investigations, a transparent mounting medium is more appropriate than an opaque material, and in cases where the specimens have to be analyzed with x-rays, a mounting material free of any interference reflexes should be selected. Because of these varied requirements, many different techniques were developed for the mounting and embedding of metallographic samples. They are summarized schematically in Fig. 1.2 and can be described as two basic types of mounting: clamping with a sample holder or clamp, and embedding the specimens in organic or inorganic materials.

Embedding or casting of plastic materials around the specimens is the most popular technique and can be divided into “cold” and “hot” mounting, depending on whether or not heat is needed for the polymerization process

Cold mounting (room-temperature curing) requires the mixing of two agents (a crude polymer and a catalyst); this mixture is then cast over the specimen within a mold, in which it reacts to form a solid part. A slightly higher pressure during curing improves the adhesion to the specimen. Special equipment is available to mount several samples simultaneously.

Hot Mounting (compression molding) requires a mounting press where the sample and the mounting compound can be heated and simultaneously compressed. Two essential types of mounting materials are available, thermosetting and thermoplastics. Both types are available as hot and cold mounting compounds, depending on whether the polymer reaction occurs with added heat or with the addition of a catalyst. The curing of thermosetting materials is irreversible, and they cannot be re-softened after curing; cured thermoplastic materials, however, can be re-melted again at elevated temperatures.

   

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