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 damage 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 a low-temperature oven for curing epoxies, as these cure more quickly and tend to be harder than room temperature cured materials. Notice that conductive fillers are available for cold mounting systems.
Characteristics of Cold Mounting:
Illustration of the cold mounting process using Epoxy
Cold mounted (Epoxy) samples under vacuum impregnation
Examples of samples mounted using different cold mounting materials
Images courtesy of Struers.
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 the considerable applied pressure 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 become 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 out-gassing being 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 necessarily as good as those of epoxy mounting compounds. Conductive mounting compounds contain 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 coated or coated 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:
Images courtesy of Struers.