Small specimens generally require mounting so that the specimen is supported in a stable medium for grinding and polishing. The medium chosen can be either a cold mounting system or a hot compression mounting compound.
Cold mounting resins
A wide range of products are available on the market. Generally faster setting products including acrylic resin types are less favourable, as these often exotherm excessively and can have poor edge retention and excessive shrinkage. Shrinkage is the term given when the resin shrinks away from the sample surface during curing. This is undesirable as the gap that forms harbours contaminants, grit from grinding and polishing stages to cause cross contamination of polishing surfaces. Further, unsupported edges are more prone to dameag during preparation and rounding during polishing stages. It is difficult to obtain a well polished, scratch free surface when gaps in the mounting material are present.
Epoxy resin types generally have the best characteristics with respect to higher hardness and lower viscosity, less heat generation during curing and better edge retention. Adequate time should be allowed to ensure that the material is fully cured before proceeding. Epoxies often take a considerable period of time after initial 'setting' to develop full hardness. It is often advisable to use low-temperature oven curing expoxies, as these cure more quickly and tend to be harder than room temperature cured materials.conductive fillers are available for cold mounting systems.
Characteristics of Cold Mounting:
- Suitable for heat sensitive, brittle and/ fragile samples.
- Epoxy mounting systems offer minimum shrinkage and good adhesion to the sample.
- Epoxy mounting systems are suitable for vacuum impregnation.
- Mounts of any shape can be made.
Hot Sample Mounting
Hot mounting uses thermosetting or thermoplastic mounting compounds, hardened in a mounting press which exerts both heat and high pressure. This mounting method produces hard mounts in a short space of time. However the heating (generally in the order of 180 °C) and considerable pressure applied may be unsuitable for delicate, soft or low melting point specimens. Techniques may be used to protect a delicate sample from the effects of pressure, such as placing the sample under a supporting structure within the moulding cavity. Such a supporting structure can protect the sample from the initial pressure applied when the mounting material is in a granular form, and most likely to inflict damage. When the mounting material becomes fluid, infiltration should occur to encapsulate the sample which will then be subject to hydrostatic pressure. Hydrostatic pressure can be applied to all but the most delicate of samples without problem. In the case of very soft or thermally sensitive materials, hot mounting is not appropriate. Please note that a proper curing is important: Insufficient time and temperature can lead to partially cured specimen mounts. Under these conditions the properties of the mounting material are not properly developed and the material may be loose and powdery. Generally, if the material is improperly cured, the hardness and abrasion characteristics are poor and the material is adversely affected by etches and solvents. Further, the characteristics under vacuum are very poor with out-gassing a major problem. If the mounting stage is suspected to be at fault, it is best to break the sample out and start again.
Conductive mounting resins are available, which are good for SEM examination, although the adhesion and hardness characteristics are not necessaily as good as those of epoxy mounting compounds. Conductive mounting compounds contrain either a copper or graphite filler. If the edges of the specimen are not of interest, then non-conductive mounting materials can be used. In general, hot mounting is preferable to cold mounting, when the sample is not affected by temperature and pressure (180ºC & 290bar). However, not all specimens can tolerate this.
Non-conductive mounts must be covered with adhesive conductive tape or coated with a conductive medium (the sample area can be masked if sputter coating, or using an evaporator. Aluminium foil or glass cover slips are useful for this purpose. Note: many adhesive metal tapes have non-conductive adhesive, so the use of carbon/silver conductive paint may be required at seams. Whilst very thin films of carbon can be tolerated on the sample, the ideal is that the sample surface should be bare.
Characteristics of Hot Mounting:
- The quality and hardness of the mount is superior to those obtained with cold mounting.
- Good abrasion characteristics and sufficient hardness such that the edges of the sample are protected, i.e., the rate at which abrasion takes place should be even across the face of the mount and the specimen.
- Stable and adherent to sample. This is important. If the mounting material has poor adhesion or poor edge retention, gaps may open up between the mounting material and the sample surface. When this happens, it is very difficult to prevent cross-contamination of one abrasive to another, causing heavy scratching in the finished section. Also any friable surface layers (oxide layers etc.) should be held adhered to the surface and not pulled off.
- Fragile, brittle and heat sensitive materials cannot be hot mounted as the hot mounting process occurs under high temperatures and pressures.
- Fast method for one single mount.
- Respects the exact tolerances for the diameter of the mount.
- Hot mounting provides parallel top and bottom surfaces which makes it easier to scan across large specimens.
- Most Hot Mounting Materials are stable in vacuum - no out-gassing or vapour to cause contamination. This is particularly important for high magnification work, long map acquisition times and microscopes with high vacuum requirement (FEGSEMs).
- If a non-conductive mounting material is used, the specimen must be made conductive by use of conductive paint, metal strips or tape.
Summarise sample Mounting:
- Hot Mounting may be unacceptable, if the effect of temperature and pressure are expected to be inappropriate for the sample under investigation.
- Generally, the materials employed for cold Mounting cannot match the hardness of materials traditionally used in Hot Mounting. This may lead to compromises in the degree of edge retention and support that the mount provides for the sample. Further, the abrasion characteristics may need to be taken into account during the preparation.
- Conductive Mounting Materials, suitable for SEM, are only available for Hot Mounting.
- The mounting material should be stable under vacuum. Out-gassing can be a major problem leading to high contamination rates on the sample, and even microscope parts.
- If a non-conductive mounting material is used, the sample must be made conductive by use of conductive paint, metal strips or tape.
Images courtesy of Struers.