Metallographic Mounting for simplified sample preparation

In metallography, mounting is typically the second process step after sectioning. Mounting encapsulates the sampled material with a plastic shell and prepares the sample for the following metallographic grinding and polishing step. In many cases it leads to a simplified sample preparation and thus to better results.

QATM is a leading manufacturer and supplier of high-quality consumables as well as a range of innovative machines, from hot mounting presses to UV mounting devices. The QATM application experts combine decades of experience with thousands of processed materialographic samples and will be happy to assist with your application.

The difference between cold and hot mounting in metallography

In metallography, a distinction is made between hot and cold mounting, depending on whether heat is required for the polymerization process. 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 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

Metallographic hot and cold mounting are not in direct competition, but there is a certain overlap in application ranges. The most important criteria for compounds are hardness, abrasion resistance, shrinkage, and chemical resistance.

Low shrinkage during solidification and good adhesion to the specimen are important. Without these, a gap will form between the specimen and the mounting material. This causes edge rounding, accumulation, and carryover of grinding and polishing media or the rupture of surface coatings. 

The following points need to be observed as well:

• Bubble-free condition – ground air bubbles act as pores and promote the carryover of grinding and polishing media
• Correct viscosity during casting. All irregularities in the sample such as cracks, shrink holes or pores must be filled.
• Identical metallographic grinding or polishing behaviour as the mounted material. Soft materials should be mounted in soft and hard materials in hard mounting media
• Inert behaviour of the material during metallographic sample preparation and analysis
• No reaction with the sample
• The material must not be changed by high temperatures or pressure during the process; undesirable deformations and transformations could be the result
• If necessary, good electrical conductivity for further processingi n metallographic electropolishing or electron microscopic applications

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 mould diameter, are longer than those of thermosets. In this case, the cooling rate, which is usually lower, must be considered.

Therefore, metallographic hot mounting presses have pulsed cooling modes, which contribute to the release of the plastics’ internal stresses during curing. This prevents crack formation and guarantees a clear sample.

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 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.

The plane-parallelism of the samples and the easy handling of the process are of advantage, especially in hardness testing.

It is possible to use several compounds in layers. This allows the specimen to be mounted in hard a medium. Afterwards, a cheaper filling material is used, which is covered by a transparent material to enclose a sample identification.

_{Four samples mounted using different compounds}

Four classes of materials are available:

• Acrylic resin

• Polyester resin

• Epoxy resin

• Light curing materials

Selection of the appropriate mould

The moulds used for metallographic cold mounting are reusable. Here only the most commonly used moulds are described. Various constructions, for example based on polymer-coated metal parts or different plastics, may be observed in laboratory practice.

• Polyolefin based moulds (PE and PP)
  In case of older moulds, the removable base is often no longer flat. If several small parts are mounted in one sample, different grinding planes can be created depending on the position of the individual sample.
  
  
• Teflon (PTFE) based moulds
  Due to their high dimensional accuracy, ideally suited for automatic grinding and polishing, expensive.
  
  
• Silicone rubber-based moulds
  After repeated use, silicone moulds lose their roundness, which can be a disadvantage for automatic preparation in single pressure. When polyester resins are used, sticky spots may arise as a reaction between the mounting compound and the silicone mold. These moulds are also suitable for the hardening of UV-initiated compounds. 

_{Different moulds used in metallography}

How to avoid marginal gaps

Even when high-quality products are used, the formation of marginal gaps cannot always be avoided and occurs especially during cold mounting. This is often due to inadequate preparation of the sample or its geometry. To avoid the formation of marginal gaps, attention should be given to the following:

• Sample geometry
  Simple rectangular cross-sections are unproblematic. Dovetail connections or drill holes place high demands on the metallographic mounting compound.Especially with inner radii, a non-existent and/or negative shrinkage wouldbe required to mount with minimal gaps.

simple

              difficult

• Arrangement of samples in the mould
  Very little distance between individual samples or to the edge of the mould can cause gaps to form and promote crack formation (approximately 2-3 mm distance should be maintained). Breakouts can occur especially at the edge of the specimen which complicates the preparation and may lead to artefacts.

             incorrect

                                                optimum

• Cleanliness of the sample

• Thermal conductivity
  The risk of gap formation increases the faster the mounting material is cooled down (special caution with methacrylates!).

• Hardness of sample and mounting material

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 is not required. This reduces the shrinkage of the material.

_{Different hardness for sample and mounting material. 
Poor transition of mounting material to sample}

_{Identical hardness of sample and mounting material. 
Ideal transition of mounting material to sample} 

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 micrographs due to after-etching or discoloration in areas close to the gaps.

_{Shrinkage gap - poor transition of mounting material to sample}