Fig. 1. (from left): Salt-bath nitrided 1215 carbon steel; borided 42CrMo4 heat-treated alloy steel; CVD coatings on a WC-8% Co (plus Ta, Ti and Nb) cutting tool. The magnification bars are 10-µm long; etchants are nital for left and center and Murakami’s at right.

For maximum edge retention, it is necessary to mount specimens before commencing the grinding and polishing steps. A free edge will always be rounded to some extent, regardless of the procedures used. But mounting resins vary in their ability to provide edge protection. Polymeric resins will tend to shrink away from the encapsulated piece, forming a small gap between specimen edge and mount. This gap must be avoided or minimized. A shrinkage gap creates a free edge where rounding will occur. It also leads to seepage problems that may cause contamination of the polishing steps or bleed out of solvents or the etchant, which will obscure the edge structure. Protective platings, such as electroless nickel, are helpful when done properly but are not a guarantee for edge retention. The plating must be compatible with its substrate, must adhere well and must not interfere with the etching reaction or exhibit the same contrast as the coating.  

Grinding may be done with a variety of products, such as traditional SiC paper, alumina paper, metal- or resin-bonded diamond disks, or rigid grinding discs. In general, one grinding step is adequate. The cut surface should be made with an abrasive cut-off or precision saw. Both produce high-quality cut surfaces with minimal damage depths. Consequently, coarse-grinding abrasives are not needed (and should be avoided), as they produce excessive damage. Generally, one can commence grinding with a SiC paper with an abrasive size of 240-grit (P220 or P280) after abrasive cutting or 320-grit (P400) after use of a precision saw.

Fig. 2. (from left): Cu-30% Pb babbit metal on carbon steel; gas nitrided H13 tool steel; Cr-plated 18Ni250 maraging steel (note cracks at arrows). The magnification bars are 50, 50 and 20 µm; the etchants are nital, nital and Modified Fry’s (left to right).

Polishing should be conducted using napless cloths. For the first rough-polishing step, a silk cloth (such as the Ultra-Pol™ cloth) with 9- or 6-µm diamond abrasive produces excellent results with a good removal rate and an excellent surface finish. Alternatively, the first polishing step can also be performed using a rigid grinding disk, such as the BuehlerHercules™ H or S disks, which produce exceptional flatness. The RGD is more aggressive (for the same diamond abrasive size), but the surface finish is not as good. However, the next step will improve the surface finish. The second polishing step often uses 3-µm diamond, and a napless cloth is chosen, such as the synthetic chemotextile pads (like Texmet® 1000). or a fine woven cloth, such as a Trident™ cloth. Depending on the material being prepared, a 1-µm diamond step may be employed, or the next (and last) step may use either colloidal silica or alumina abrasives on a napless synthetic cloth, such as a Chemomet® cloth, or on a medium nap cloth, such as a Microcloth® pad.