Sample Preparation for EBSD

Polishing

Preliminary polishing

Diamond polishing compounds or slurries are good for preliminary stages for most materials. 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 cloths allow the greatest shock absorbency and therefore allow for gentle polishing with little damage associated.  However soft 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. Final polishing should not be prolonged, but just sufficient to achieve the desired surface finish without causing excessive relief.

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 wheel 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 wheel 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 wheel cannot occur. Meticulous attention to avoiding contamination of wheels is an important aspect to achieve the best results.