Metal Sample Preparation:Differences between Hot and Cold Inlays

Why do we need to inlay?

We often come across small samples during our experiments, so we often mount our samples to make them easier to grind and polish. To summarise the application of inlay, there are several aspects: To protect the operator: to facilitate the holding of thin, brittle, small, irregular samples to cover the sharp edges, to ensure the safety of manual sampling; Provide a uniform standard sample size: to facilitate automated batch sampling, easy sample preservation and management; Protecting samples: preventing edge chamfering, filling holes, enhancing fragile samples, pre-setting protection before cutting; Marking: lettering, embedding labels, observation.

Cold inlay vs. hot inlay

Metal materials and pressure and heat insensitive materials, such as metals, ceramics, etc., these materials are suitable for hot inlay; on the contrary, the more sensitive to heat and pressure materials that use cold inlay.

How to do inlay?

Hot setting

Hot inlay is heated and pressurised, the resin and sample inlay into a regular shape, fixed size samples. Prior to hot setting, the surface is first cleaned of oils, impurities, rust, etc. using acetone or ultrasound. Cleaning the sample helps to improve the adhesion of the sample to the inlay, and the sample needs to be wiped dry or blown dry by a hair dryer after cleaning. Raise the inlay table and place the dry and clean sample on the inlay barrel of the hot inlay machine, then lower the table.

Figure 1: Procedure of hot mounting

Add an appropriate amount of resin powder (the amount to be added should be determined by the height of the final mounted sample) and close the lid tightly. Maintain a temperature of about 180°C and a pressure of about 250 bar while mounting the sample. After adjusting the parameters, wait for about 10 minutes (heating-holding-cooling process), open the sealing cover, raise the test stand, and remove the specimen.

Figure 2: The stages of temperature change with time during the hot setting process

Precautions in the process of thermal inlaying.

The size should be adjusted according to the size of the heat-setting cylinder diameter, 3-5mm from the edge of the cylinder, the sample is set to a height of about 20mm (suitable for hand holding) Thin slices, plates, small cylindrical samples, and irregular samples should be held in the slicer clamp; Multiple samples should be mounted at the same time with gaps between them; Irregular samples that cannot be held upright should be smoothed or taped first.

Thermal Inlay Resin Selection:

PhenoPowder Phenolic resin, black/red, for low-hardness materials; economical, medium shrinkage, easy buffing, average edge protection; EpoPowder G epoxy resin (granular), black, used for high-hardness materials; high edge retention, easier clean-up; EpoPowder F epoxy resin (powder), black, for use on high hardness materials, more suitable for complex basic shapes; -TransPowder acrylic resin (powder), black, for use on high hardness materials, more suitable for complex basic shapes; -TransPowder acrylic resin, clear, for specimen visibility; -CopperPowder Copper conductive filler for SEM and TEM analyses, not analysed for Cu -GraphPowder Carbon conductive filler, used for SEM and TEM analysis, not analysed for carbon.

Cold Inlay

Cold inlay is the preferred method for mounting samples because of its high efficiency and low cost. As with copper hot mounting, the sample should be cleaned with dionized water, alcohol, or acetone and dried before cold mounting. This ensures that the glue is bonded to the sample. Selection of the right size mould cup. Usually the size of the sample to the casting edge is best around 5mm, too small is prone to transverse cracks, too large will have high heat, shrinkage, cost and other problems. Common injection moulding cups are Silicon, PE, PTFE, PP, and so on. Silicone cups are not suitable for casting epoxy resin, because epoxy resin reacts with silicone.

Figure 3: Metallographic silicone soft mould

Metallographic silicone soft mould Determine the sample mounting position, fine samples can be fixed with quick-drying glue, or can be fixed with specimen clips or tape. Weigh a fixed proportion of glue, and powder mixed thoroughly for pouring (specific conditions according to the product instructions for operation). During the process, it is necessary to ensure that the proportion is accurate, the mixing is adequate and the curing curve is strictly controlled.

Cold Inlay Resins

Currently there are two main types of cold inlay resins: Epoxy and Arcylic. Epoxy resin industry, also known as AB adhesive, is widely used, almost suitable for all materials, it has a low viscosity, low shrinkage, good adhesion, transparency and other characteristics. Epoxy resin system has two parts: resin and curing agent. Before use, it needs to be measured according to the weight ratio or volume ratio, and used after mixing; it is recommended to do de-foaming treatment before casting, which is conducive to improving the transparency and reducing the porosity. It should be noted that heat will be released during the curing process of epoxy resin (peak temperature can reach 150~200℃), therefore, for some cases (e.g. ultra-small size samples, heat sensitive samples, etc.), precise temperature control equipment is needed to control the curing process. For porous materials (e.g. ceramics, coatings, etc.), a vacuum impregnation and casting process is required: after fixing the specimen in the injection cup, place it in the vacuum chamber. Close the lid and open the valve to allow the epoxy resin to be drawn into the chamber. After the sample is completely covered with glue, close the valve and switch off the vacuum pump. The epoxy slowly seeps into the pores and cracks under negative pressure. Note that only epoxy resins are suitable for vacuum impregnation. Acrylic resins are commonly referred to in the industry as acrylic glue. Acrylic adhesive has a shorter curing time than epoxy and is therefore more suitable for rapid prototyping. Note: Acrylics also emit heat when curing.

Cold mounting precautions

1. Remember not to use acetone to clean the acrylic after cold inlay.
2. Cold inlay is too thick, it will seriously affect the length of vacuum during SEM test.
3. As the surface has been polished, any slight friction will lead to surface scratches, which are very obvious under the microscope. It is therefore important not to use the violent method of hammering, but to soften the metal specimen by heating and baking and then gently cocking it out.
4. It is best to use small specimens and separate the specimens one by one to avoid scratching the polished surface.