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Specimen preparation for EBSD is critical, because diffracted electrons escape from within only a few tens of nanometres of the specimen surface.

If material near the surface is deformed, contaminated, or oxidized, then EBSD pattern formation may be suppressed.

For many materials, standard preparation methods may be employed successfully with care. As a general rule standard preparation methods can be progressed to the final polishing stage without any deviation from the normal route employed. Thereafter, an additional polishing stage using colloidal silica is all that is required to achieve a finish suitable for EBSD.

However, different materials respond differently to common preparation methods and composite materials are even more challenging. Therefore the material under investigation should be considered on an individual basis and prepared appropriately. The manufacturers of preparation equipment should be consulted for the applicability of a given approach for a given material.

Specimen holder with mounted samples

(a) Specimen holder with mounted samples.

figure 2b

(b) Specimen holder with un-mounted samples.

Images courtesy of Struers.

The first thing to remember when cutting samples for EBSD, is to preserve the sample axes orientation. Cut the sample in such a manner that important sample directions, such as the Rolling Direction, the Transverse Direction and Sample Normal are not lost. The second thing to remember that the cutting process must not damage or change the sample as this would lead to erroneous results.

Avoid aggressive cutting methods that generate heat or cause deformation at the cut surface. Severe damage induced at this stage may extend so deep into the material that it is not removed by subsequent grinding and polishing. Heating caused during cutting may cause changes to the microstructure - phase transformations or precipitation/diffusions mechanisms may become active. Therefore heating must be avoided at all costs.

Sectioning Damage-zinc

Sectioning Damage-zinc

Images courtesy of Buehler.

Wet abrasive cutting machines are common in the metallurgical industry and are suited to cutting larger sections of material. The sample may be subjected to considerable force and heat if used incorrectly, good cuts can be performed with minimal damage. using excessive force with an inappropriate wheel can cause sample to overheat locally. Manufacturers of cutting equipment and cut-off wheels publish tables and diagrams to help to make the choice of wheel easier. The importance of observing the manufacturer’s recommendations cannot be over-stressed. If over-heating of the sample occurs, it is often due to using a wheel that is too ‘hard’ for the material being cut. The wheel does not wear properly which causes the abrasive to become blunt. Another possibility is that the abrasive becomes clogged. Friction then causes excessive heating and damage to the sample. These processes can result in altered surface structures, which subsequent grinding and polishing do not remove. EBSD is very sensitive to damage caused in this manner and greater care than usual is warranted to avoid generating misleading results, or compromising pattern quality. Therefore, the selection fo an appropriate cut-off wheel is important to avoid introducing unnecessary levels of damage when cutting materials.

Precision, Low Deformation Cutting Machines

There are many examples of wet abrasive cutting machines on the market designed for precision and low damage cutting. Such machines normally employ CBN (Cubic Boron Nitride) and diamond type cutting wheels, although abrasive cut-off wheels  may also be available, depending on the machine. Generally, low deformation cutting machines are particularly well suited to cutting sections for EBSD.

Cutting Machines.

Large Wet Abrasive Cutting Machine

(a) Large Wet Abrasive Cutting Machine.

Precision Cutting Machine

(b) Precision Cutting Machine.

Cut Surface.

Heat Damaged Surface

(a) Heat Damaged Surface.

Good Cut Surface

(b) Good Cut Surface.

Images courtesy of Struers.

Coarse Grinding / Plane Grinding

In many cases, when preparing samples for EBSD, the cutting techniques used limit sectioning damage effectively. The initial grinding stage selected should minimise aggression, and cause less damage than the sectioning. For this reason, surfaces such as grinding stones and other aggressive grinding surfaces are not normally recommended.

Plane Grinding can be achieved in a variety of ways, using a variety of abrasives. Fixed abrasive surfaces are available using diamond, aluminium oxide or cubic boron nitride (CBN) abrasives. The method used to bind the abrasives to the wheel largely defines the grinding characteristics - the harder or more rigid the bonding medium, the more aggressive the grinding action of the surface. the type of grinding surface used to make the specimen plane will depend on the material being prepared.

For softer materials coarse grit of grinding paper with Silicon Carbide or Alumina abrasives may be used, but the durability or characteristics of such materials may be inappropriate for certain materials. Generally, in order to maintain sharp abrasive particles, grinding papers need frequent changing. for harder and mixed materials, diamond  grinding discs are often the best choice. Follow the manufacturer’s recommendations and advices.

Grinding surfaces

Grinding surfaces.

Planar grinding can also be achieved using pastes or slurries applied to a suitable surface. Grinding in this way rather than with fixed abrasives can be significantly less aggressive, retain better flatness, and limit brittle fracture and plastic deformation often associated with fixed abrasives.

Characteristics of Coarse Grinding:

  • The abrasives used for grinding are fixed or bonded, either glued onto a paper or mixed with resin and made into a grinding stone or grinding discs.
  • During the mechanical rotation of the grinding disc or paper, the individual abrasive particles act like tools that stick up and take out chips from the surface.
  • The first grinding step, Plane Grinding removes damage introduced by cutting, and levels specimens clamped in a holder for automatic preparation.
  • The choice of surface depends on the physical characteristics of the material to be ground and is critical.
  • Rigidly fixed abrasives are more aggressive than those in a shock absorbing backing and also ensure a better flatness.
  • In general maintaining sharp abrasives promotes good grinding characteristics with minimal damage.
Plane Ground sample of nodular cast iron

Plane Ground sample of nodular cast iron.

Fine Grinding

Fine Grinding can be achieved in a variety of ways, using a variety of abrasives, mainly Silicon Carbides and Diamonds. Different preparation surfaces are available for Fine Grinding that can be differentiated according to either the abrasive particles being fixed or being added onto a rigid grinding surface. SiC-Grinding Paper is traditionally used for grinding, but it has some disadvantages; Due to a certain resilience of the paper, the samples are not kept very flat and it requires several preparation steps, 3-4 papers of different grits are needed for fine grinding. SiC-Grinding Paper blunts quickly and therefore should be discarded after a short period of grinding in order to maintain efficient 'stock' removal. Grinding on a surface that has blunt abrasives causes a great deal of surface damage by smearing, 'burnishing' and local heating.

By using diamonds as an abrasive on coarse polishing cloths or on rigid preparation surfaces, Fine Grinding can be reduced to one step, which is more efficient and also produces flatter sample surfaces than grinding with SiC-Grinding Paper. Ensure that sharp abrasives are used and follow the manufacturers' instructions with regard to grinding rotational speeds, direction, force, times and lubricants used. Damage injected during grinding may be invisible in the polished surface, but serve to distort the EBSD result or even completely suppress pattern formation. Remember that different materials have different abrasion characteristics. The selection of grinding material and conditions can therefore be specific to a given sample. After every grinding stage it is advisable to inspect the ground surface using a light microscope in order to ensure that all damage from the previous stage, whether that is a cutting or grinding stage, is completely removed. Advance in this manner to the finest abrasive size required, ready for polishing. Care at this stage will greatly reduce the amount of polishing required to achieve a good surface.

Grinding paper and discs.

SiC-Grinding Paper

SiC-Grinding Paper.

Fine Grinding rigid discs

Fine Grinding rigid discs.

Characteristics of Fine Grinding:

  • The abrasives used for Fine Grinding are fixed or bonded, either glued onto a paper or mixed with resin or are added onto a rigid grinding surface.
  • The goal of Fine Grinding is to reduce damage and surface roughness of samples to a degree that is suitable for polishing.
  • The choice of surface depends on the physical characteristics of the material to be ground and is critical.
  • Rigidly fixed abrasives gives a flatter preparation result than when using surfaces with a shock absorbing backing.
  • In general maintaining sharp abrasives promotes god grinding characteristics with minimal damage.
  • The fine ground surface should be inspected using a light microscope to ensure that all damage from the previous stage is completely removed.
Fine Ground sample of nodular cast iron

Fine Ground sample of nodular cast iron.

Images courtesy of Struers.

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.

Cold Mounting.

Principle of Cold Mounting

(a) Principle of Cold Mounting.

Samples mounted in Epoxy under vacuum impregnation

(b) Samples mounted in Epoxy under vacuum impregnation.

Samples mounted in different Cold Mounting Materials

(c) Samples mounted in different Cold Mounting Materials.

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.

Hot Mounting.

Principle of Hot Mounting

(a) Principle of Hot Mounting.

Samples mounted in different Hot Mounting Materials

(b) Samples mounted in different Hot Mounting Materials.

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.

Preliminary polishing

To remove deformations and scratches from Fine Grinding and obtain a surface that is highly reflective, samples must be polished before they can be examined under the microscope. To achieve efficient material removal and to cut consistently through all materials and phases, the hardest known abrasive is used - diamond. Diamond polishing can be carried out on many different preparation surfaces/polishing cloths and with different diamond grain sizes. Follow the manufacturers' instructions with regard to polishing rotational speeds, direction, force, times and lubricants used.

Polishing is a similar action to grinding, accept that the supporting medium used to hold the abrasive is capable far greater 'shock absorbency' i.e. the ability of the medium to allow the abrasive to move to some degree and conform to the surface aspirates of the specimen. Thus different polishing surface materials have differing characteristics: soft polishing cloths (high resilience polishing cloths; resilience being, the capability to give way and bounce back into shape) allow the greatest shock absorbency and therefore allow for gentle polishing with little damage associated. However soft polishing cloths allow the abrasive to abrade different areas at different rates, giving rise to 'relief'. 'Relief' is the term used to describe the undulations that form in a polished surface. Extreme undulations or relief in the polished surface is to be avoided, although a certain amount can be tolerated (or even desirable) because the SEM generally has high depth of field. Harder polishing surfaces or cloths, conversely, produce a flatter or 'plane' surface, but may leave polishing damage in the surface of the material, and promote superficial scratching.

Therefore, it is usually the case that polishing is started on a hard cloth with a coarser abrasive and finished on a softer cloth with a finer abrasive. Fine polishing should not be prolonged, but just sufficient to achieve the desired surface finish without causing excessive relief.

Hard Cloths (low resilience) Soft Cloths (high resilience)

High planarity
Fast abrasion rates
Best edge retention
Minimum relief
Low risk of 'pull-out'
Coarser scratches

Risk of generating surface 'Relief' and edge rounding
Superior reflectivity

Polishing cloths of different resilience

Polishing cloths of different resilience.

Diamond polishing 3 µm of nodular cast iron

Diamond polishing 3 µm of nodular cast iron.

Final Polishing

For EBSD, it is generally necessary to use an additional final polishing stage using colloidal silica. Final polishing should not be prolonged, but just sufficient to achieve the desired surface finish without causing excessive relief.

Colloidal Silica is a chemo-mechanical polish, i.e., it combines the effect of mechanical polishing with etching. This type of stock removal is ideal in many cases for EBSD, as a damage free surface can be obtained with little effort. Typical abrasive size is 0.05 micron. Note: Colloidal Silica crystallizes readily and will ruin polishing cloths if left to dry. Further, a film can form on the polished surface of the sample which must be removed. A convenient method to achieve this is to flush the polishing cloth with water during the last few seconds of polishing to clean the sample surface. Remove and dry the sample in the usual manner, using a solvent with low water content and not so volatile as to cause water condensation on the surface. Alcohol is ideal, whereas acetone is not. Flush the polishing cloth with water until all traces of colloidal silica is washed away, spin to drain and store in a suitable container such that contamination of the polishing cloth cannot occur. Meticulous attention to avoiding contamination of polishing cloths is an important aspect to achieve the best results. Follow the manufacturers' instructions with regard to suitable polishing cloth, rotational speeds, direction, force and times used.

Oxide polishing of nodular cast iron

Oxide polishing of nodular cast iron.

Oxide polishing

Oxide polishing.

Images courtesy of Struers.

Vibratory Polishing

Vibratory Polishing removes minor deformation remaining after mechanical preparation. It is designed to prepare high quality polished surfaces on a wide variety of materials and applications, including EBSD polishing preparation application. A horizontal vibratory motion of typically 7200 cycles per minute produces a very effective polishing action with superior quality results and exceptional flatness. The unique vibratory action produces less deformation, flatter surfaces and reduces edge rounding. It also yields a stress-free surface without the use of dangerous electrolytes associated with electro-polishing.

Use of EBSD for deformation analysis in steel

Use of EBSD for deformation analysis in steel.

Vibratory polisher

Vibratory polisher.

Images courtesy of Buehler.

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