Preliminary polishing follows the grinding steps and aims at removing deformations and scratches from the grinding process. The final polishing can be done using a mechanical or vibrational polishing step using colloidal silica, however there are also other methods which can be used for the final polishing. The importance of the polishing should not be underestimated as it is vital for the preparation of good samples for Electron Backscatter Diffraction (EBSD) analysis.
To remove deformations and scratches from the Fine Grinding step 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 regarding 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||Risk of generating surface 'Relief' and edge rounding|
|Fast abrasion rates||Superior reflectivity|
|Best edge retention|
|Low risk of 'pull-out'|
Images courtesy of Struers.
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 regarding suitable polishing cloth, rotational speeds, direction, force and times used.
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. 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.
Images courtesy of Buehler.