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After metallographic sectioning the sampled material is usually encapsulated with a plastic shell. This process step is called metallographic mounting. It results in the classic sample ready for metallographic grinding and polishing. In many cases, mounting leads to a simplified sample preparation and thus to better results.
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QATM offers metallographic mounting devices for any requirement
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ADVANTAGES OF METALLOGRAPHIC MOUNTING
METALLOGRAPHIC COLD AND HOT MOUNTINGBasically, a distinction is made between hot and cold metallographic mounting, depending on whether heat is required for the polymerization process during mounting. It must be noted that during cold mounting polymerization temperatures of up to 130 °C may arise when using, e. g., methyl acrylates. Today, the term cold mounting is generally used for all metallographic mounting methods where no or small pressures (<5 bar) are applied.
When it comes to the selection of metallographic mounting methods, arguments for or against a certain method can be found. The below overview presents the process differences between metallographic hot and cold mounting.
REQUIREMENTS FOR METALLOGRAPHIC MOUNTING COMPOUNDS
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Metallographic hot and cold mounting are not in direct competition, but there is a certain overlap in application ranges.The most important criteria for metallographic mounting compounds are hardness, abrasion resistance, shrinkage, and chemical resistance. Low shrinkage during solidification and good adhesion to the sample are important. Without these, a gap will form between the sample and the mounting material. This causes edge rounding, accumulation, and carryover of grinding and polishing media or the rupture of surface coatings. |
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The following points need to be observed as well:
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METALLOGRAPHIC HOT MOUNTINGHot mounting could also be called hot biaxial pressing. It is a process in which a granulated polymeric material is softened, compressed, and cooled down sequentially. The process is carried out in a metallographic hot mounting press, designed for this application. Of course, this method may only be applied on sufficiently pressure- and temperature-resistant samples with simple geometries. The process is carried out at temperatures of 150 to 200°C, while the pressure depends on the mold diameter and ranges from 100 to 300 bar. After placing the sample on the lower ram, the mounting material is added and the metallographic mounting process is started. |
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Two types of materials are used for metallographic hot mounting:
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Thermosets are usually cured between 150°C and 180°C, while the processing window of thermoplastics is slightly bigger. Because they are hardened during the cooling process, their cooling times, depending on the mold diameter, are longer than those of thermosets. In this case, the cooling rate, which is usually lower, must be considered. |
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Due to the required process parameters, hot mounting of metallographic samples is a limited application. These limitations apply to electronic assemblies (solders/composites) or pressure-sensitive materials, such as wires or sheets with small cross-sections. In modern metallographic mounting presses, this fact is taken into account by shifting the pressure onset to the point where the target temperature is reached. This extends the application range of the process, but complex network structures or porous rock cannot be hot mounted.
Four samples mounted using different compounds METALLOGRAPHIC COLD MOUNTINGThe technical requirements of cold mounting are negligible compared to the metallographic hot mounting process. Only a mounting mold and the cold mounting material are required. Besides hardness and abrasion resistance, shrinkage, curing (pot life), and exothermic heat development are the main selection criteria. The metallographic cold mounting process is carried out as follows:
Four classes of metallographic cold mounting materials are available:
Acrylic resins are easy-to-use synthetic resins with short curing time. The shrinkage is negligible, especially in mineral-filled systems. They consist of self-polymerizing components that cure by adding a catalyst. After curing, the resin has thermoplastic properties and is chemically resistant. Inorganic fillers are often used to ensure better grindability and hardness. A characteristic feature is the application of the "hardener" component to a powdery solid. These are usually fine PMMA beads with functionalized surfaces.
Like acrylic resins, polyester resins belong to the catalytic polymerizing systems. The curing time is relatively short, and the cured material is duroplastic. Polyester resins tend to show an exothermic effect between acrylics and epoxy resins and a low reaction shrinkage. Their chemical resistance is lower than that of epoxy resins.
Epoxy resins have the lowest shrinkage of all cold mounting resins. Another advantage is their excellent adhesion to almost all materials, which sometimes leads to difficulties when removing the cured sample from its mold. However, a rather long curing time must be taken into account. Another feature is the lower heat development compared to acrylate-based mounting compounds. The polymerization starts as soon as the components are brought together. The cured epoxy resin has duroplastic properties and is insensitive to moderate heat exposure (90-100 °C) and chemical attack. It is the only cold mounting material which allows vacuum impregnation and can be mixed with fluorescent dyes like uranin. This makes them well suited for fluorescence microscopy and enables the contrasting of cracks, pores, and other irregularities in a material. Filled epoxy resins are not available on the market, which limits their applicability in combination with very hard materials.
Light curing mounting materials are usually based on acrylates as well. Very few epoxy-based systems, that may be used for this purpose, are available on the market. All of these compounds are ready-to-use, single component solutions. These cure if they are irradiated with blue light or UV-radiation. The application of these materials in metallography is a quite recent development. Therefore, semiautomatic curing ovens have been developed which are used for curing the mounting material. Furthermore, UV-transparent molds, e.g., based on certain glasses, need to be used. Common curing temperatures range between 90 and 120°C and can be influenced by irradiance and irradiation time. Curing times of 1 to 15 minutes are common. The main drawbacks of such one-component mounting materials are their comparably large shrinkage, and high removal rates. These are related to the absence of a hard, inert filler. The resin consists solely of polymeric precursors and initiator. Furthermore, the curing in shaded areas or pores is irregular and limited. To reach a proper curing in spite of these drawbacks, thermal hardener systems are added. Of course, this makes a further tempering (e. g. at 60°C) necessary.
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SELECTION OF COLD MOUNTING MOLDSThe molds used for metallographic cold mounting are reusable. Here only the most commonly used molds are described. Various constructions, for example based on polymer-coated metal parts or different plastics, may be observed in laboratory practice.
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Cold mounting molds |
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SPECIAL METHODS OF METALLOGRAPHIC COLD MOUNTING
Vacuum Impregnation Porous materials such as ceramics, sintered materials or spray coatings mustbe mounted under vacuum. Only then can all open pores connected to the surface be filled with the mounting material. This is possible with epoxy resins since vapor pressure and viscosity are sufficiently low. Nevertheless, the vacuum must be limited to pressures below 0.8 bar, otherwise the low-boiling components of the epoxy system will release gas or start to boil. Infiltration of porous material or thin holes
Vacuum impregnation is used for infiltration of porous sample material and for optimum metallographic mounting of samples with thin holes, fine pores or micro cracks. Application of overpressure
Metallographic cold mounting under pressure only makes sense when using acrylates. A simple pressure device is required (compressed air connection 5-6 bar). Better transparency is achieved with unfilled methacrylates. The applied overpressure of 2 to 2.5 bar increases the boiling point of the metallographic mounting compound and suppresses the formation of gas bubbles during polymerization. This enables crystal-clear embedded samples. Pressure cannot replace vacuum as the gas volume cannot completely escape from the pore volume. Therefore, open pores remain partially unfilled and cause the formation of preparation artefacts. HOW TO AVOID MARGINAL GAPSDespite the high quality of the metallographic mounting compound used, the formation of marginal gaps cannot always be avoided and occurs especially during cold mounting. This is often due to inadequate metallographic preparation of the sample or its geometry. To avoid the formation of marginal gaps between sample material and mounting material, various parameters must be observed:
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For a sharp-edged preparation and protected boundary areas, it is crucial to observe the correct hardness of the metallographic mounting material. In general, a mounting material should be as hard and impact-resistant as possible to achieve a metal-like removal behavior. For this reason, high-filled systems are always used when transparency of the mounting material is not required. This reduces the shrinkage of the material.
The gaps between sample and metallographic mounting material should be as small as possible. Marginal gaps and edge rounding bear the risk of carrying over dirt and grinding or polishing particles. This leads to a deterioration of the metallographic preparation result. Leaking etchant or cleaning alcohol can then falsify the sample micrographs due to after-etching or discoloration in areas close to the gaps. ![]() Shrinkage gap - poor transition of mounting material to sample |
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Stránka 1 z 1 - 4 položek celkem
Stránka 1 z 1 - 4 položek celkem