All materials - EBSD sample preparation

All materials - EBSD sample preparation

Material Name: All materials
Recipe No.: 10101
Primary Chemical Element in Material: No data
Sample Type: Bulk
Uses: General microstructure
Etchant Name: None
Type (Macro/Micro): Micro
Etching Method: EBSD sample preparation
Etchant (Electrolyte) Composition: No data.
Procedure (Condition): 1.) Mechanical Polishing: Probably needed on most samples to some extent, to remove relief. Very useful for polyphase samples, or for analysis of harder particles in a softer matrix. Ideal for ceramics and geological samples. Similar process for all materials - easier. Stage 1 - Mounting: Easiest for subsequent polishing if samples are mounted. No single mounting technique is best. Conductive mounting material can be useful to dissipate charge. BUT – beware - some hot mounting processes may cause some materials (such as many geological minerals) to expand and possibly fracture. Samples on thin sections can also be polished (with care). Stage 2 - Grinding: This is the first mechanical stage of preparation. Primarily removes the deformation layer produced in sectioning and produces a flat surface. Start with 120 or 240 grit SiC paper. Proceed to 1200 grit SiC, using water as a lubricant to remove and waste material. Stage 3- Polishing: This removes most of the damage caused by grinding. Can be performed with many types of abrasive and suspension mediums. Use 4 or 5 steps, with self lubricating diamond suspension or diamond paste. Work down from15 mm to 0.25 mm grit size on a general purpose cloth. Stage 4 – Colloidal Silica: This final stage of polishing involves both mechanical and chemical polishing together. The colloidal silica solution is generally an alkali solution, and slightly etches the sample during the mechanical polishing process. Ideally you should polish with colloidal silica on an automatic lap for anything from 10 minutes to several hours (depending on the material and the state of the polish). Colloidal silica has a number of commercial names – e.g. Syton fluid, OPS.

2.) Electrolytic polishing - general: Removes material from the surface of the sample by electrolytic action. Works very well on many metals. Removes any deformation layer on the surface, as well as most surface irregularities. Unfortunately there is no single electrolyte that will work with all materials Necessary to use the correct solution for any given material. Electrolytic polishing - tips: Use some of the commercially available packages – come complete with suitable recipes and solutions for different materials. Examples are Buehler and Struers: also look on their websites for more information. Look at www.metallography.com, often enquiries about ideal Electrolytic polishing recipes for specific materials. There are many variables that alter the polishing rate: the solution used, the operating voltage, the solution temperature, the sample size and the time of contact – vary the settings to find the best result for your materials.

3.) Chemical Etching: Chemical etching is a simpler preparation technique than Electrolytic polishing. Variations in the set-up are not so critical. Sample is immersed in an etchant for a few seconds, before being rinsed (usually with ethanol) and blow-dried. Chemical Etching - tips: It is important to use an etchant that leaves no residue, and to choose an etchant suited to your material. One of the most commonly used ones is Nital (5% Nitric acid, 95% Ethanol). It is usually necessary to go through at least some of the stages for grinding and fine polishing (maybe down to 1 mm or even 0.25 mm diamond). Note: any topography will be further enhanced by the chemical etching process.

4.)Ion beam milling / etching: Routinely used for the preparation of samples for TEM analysis. Relatively new preparation technique for EBSD. Works on almost all types of material – the ion beam removes material from the surface at a rate determined by the voltage, ion gun current, gun-sample geometry and the material itself. Can very accurately remove a given thickness of material from the surface, so this technique can also be used for serial sectioning. Note, however, that these instruments are generally expensive (EUR 30,000 +). Ion beam milling - tips: It is important that the ion beam energy is low, otherwise this can introduce damage to the crystal lattice – this often happens with focused ion beam (FIB) instrument. Therefore work at low voltages/currents. Work at high tilts (45° - 70°). Better to etch slowly for a long time, than quickly for a short time. Can also be used to remove layers on the surface, such as oxides or contamination. Usually limited to relatively small areas ( <1cm2), making it unsuitable for preparing coarse grained geological samples, for example.

Preventing charging: There are a number of ways to reduce or even eliminate charging on insulators: Ensure that there is no surface topography. Ensure that the polish is very good. Only turn the beam on once the sample has been tilted to 70°. Coat with gold before the final polishing stage – this will fill cracks and voids with a conducting network. Work in variable pressure / low vacuum mode (if your SEM has this capability): ideally in the pressure range of 10-50 Pa. Any higher and the signal of the diffraction pattern will be too weak. Work at higher speed so that the beam does not stay in one area for a long time. Work at lower probe currents and/or accelerating voltages. Prepare a conductive tract from the sample to the stub/holder – using some conductive paint or metallic tape. But sometimes you will have to apply a conductive coat. Coating – key points: Keep the coat very thin – typically in the range of 2-5 nm. Too thick - the signal to noise ratio will decrease significantly and very poor EBSPs will result. Too thin - there will not be sufficient conductive material to dissipate the charge. Ideally the coat should be carbon (either sputtered or evaporated onto the sample). But it is possible to use other coating materials such as gold or tungsten. In cases where the coat may be a little too thick, it may be possible to obtain good EBSPs by increasing the accelerating voltage of the electron beam in order to penetrate through the coat.

Sample Storage: As sample preparation is so critical, it make sense to store your samples so that you do not need to repolish them: The sample surface does not accumulate dust or other particles. The sample surface is not scratched or subjected to further deformation. The sample does not acquire an oxide layer (on some materials this may be impossible, such as Magnesium, and a quick repolish will be needed before further analyses). The sample does not acquire moisture. This is especially relevant for materials such as halite that are very sensitive to moisture. The sample is kept at a cool temperature. Note that even room temperature may cause recrystallisation over a long time in many metals. These considerations indicate that a suitable place to store samples is in a desiccator, or at the very least in a clean room in appropriate containers or drawers. If you need to move samples, avoid contact with the surface and use suitable sample containers. These considerations indicate that a suitable place to store samples is in a desiccator. Alternatively in a clean room in appropriate containers or drawers. If you need to move samples, avoid contact with the surface and use suitable sample containers.
Note: No data
Reference: EBSD Sample Preparation, website HKL Technology, 2005.