Sample Preparation for EBSD

Etching

Directly polished surfaces can be inspected using EBSD,  but in many cases the pattern quality is improved by etching.  Additionally, etching delineates the grain structure, which is of obvious benefit.  However, etching may attack a second phase preferentially, or attack grain boundaries excessively.  Caution should be exercised when choosing and using etches.  Inspect the sample surface using a light microscope before and after etching to assess the effect.  Materials that are difficult to polish may benefit from repeated etching and repolishing.  This method can expose an undamaged surface suitable for EBSD when conventional polishing and etching fails to achieve an adequate surface. Using special acid or alkali resistant cloths,  it is also possible to add dilute etchants to the polishing wheel during polishing.  This can be effective,  but can be difficult to control.  Some experience is required.  Any etchant that is used must dissolve the specimen surface in an even manner, and not leave behind any oxide or reaction product layers.  Such layers can completely suppress diffraction.  Many etchants listed in metallographic text books are 'contrast etches' which rely on the formation of different thickness oxide layers to generate colors visible using a light microscope.  Therefore such etches are generally not suitable for EBSD.

Electrolytic Polishing and etching

Electrolytic preparation uses an electrolytic reaction cell containing a liquid electrolyte with two electrodes: an anode and cathode. The sample to be polished/etched forms the anode. The electrodes are connected to an external power supply and voltage applied to cause reaction within the cell.

Shown above is the characteristic curve for an electrolytic cell.  This curve is dependant on the electrolyte used and will vary for different electrolytes.  Control of the voltage and current density at the anode, plus electrolyte composition, temperature and agitation are all critical in achieving the desired etching/polishing characteristics.   Establishing adequate control of these parameters can be difficult and further, many of the electrolytes are hazardous or even explosive.  In the case of the latter, temperature control is critical.   Do not attempt electro polishing or etching without the necessary experience and safety measures in place.

 Factors controlling etching/polishing characteristics include:

• Electrolyte composition

• Electrolyte temperature

• Electrolyte stirring

• Area to be polished/etched (current density)

• Voltage

Plasma Etching

Plasma etching is a process similar to sputter coating - commonly employed for depositing conductive films for SEM inspection.  However the process is working in reverse with the specimen forming the cathode in the vacuum reaction cell.  High voltage is applied between the anode and cathode which oppose each other with a small gap between.  The cathode is the 'target' on which the sample is placed.  Gas, usually an inert gas such as argon is leaked under controlled conditions into the vacuum.  The gas atoms become positively ionized in the high electrostatic field between the electrodes.  The positively charged gas ions accelerate toward the cathode and bombard the sample, eroding the surface in the process.  There are many different suppliers and models of plasma etching equipment on the market.  Those with low power rating are generally referred to as cleaners, whereas those with higher power or wattage are termed etchers. Plasma etching should generally be used as a means of cleaning or improving a mechanically or electrolytically prepared surface.