Pace Technologies, NANO-1200T, 12" / 14" Single Wheel Bench Top Grinder / Polisher
Rough / Planar Grinding
Rough or planar grinding, is required to produce flat specimens and to reduce the damage created by sectioning. The planar grinding step is accomplished by decreasing the abrasive grit particle size sequentially to obtain surface finishes that are ready for polishing. Care must be taken to avoid being too abrasive in this step, and actually creating greater specimen damage than produced during cutting. This is especially true for very brittle materials such as ceramics and silicon.
The machine parameters which affect the preparation of metallographic specimens include: grinding / polishing pressure, grinding direction and the relative velocity distribution between the specimen and the polishing wheel.
Grinding Pressure
Grinding / polishing pressure is dependent upon the applied force (pounds or Newton's) and the area of the specimen and mounting material. Pressure is defined as the Force / Area (psi, N/m2 or Pa). For specimens significantly harder than the mounting compound, pressure is better defined as the force divided by the specimen surface area. Thus, for larger hard specimens, higher grinding / polishing pressures increase stock removal rates. However, higher pressure also increases the amount of surface and subsurface damage produced in the specimen.
Note regarding SiC grinding papers: as the abrasive grains dull and cut rates decrease, increasing grinding pressures can extend the life of the SiC paper.
Higher grinding / polishing pressures can also generate additional frictional heat which may be beneficial for the chemical mechanical polishing (CMP) of ceramics, minerals and composites. Likewise for extremely friable specimens (such as nodular cast iron), higher pressures and lower relative velocity distributions can aid in retaining inclusions and secondary phases.
Grinding Direction
The orientation of the specimen can have a significant impact on the preparation results, especially for specimens with coatings. In general, when grinding and polishing materials with coatings, the brittle component should be kept in compression. In other words, for brittle coatings, the direction of the abrasive should be through the coating and into the substrate. Conversely, for brittle substrates with ductile coatings, the direction of the abrasive should be through the brittle substrate and into the ductile coating.
Manual Preparation
In order to ensure that the previous rough grinding damage is removed when grinding by hand, the specimen should be rotated 90 degrees and continually ground until all of the scratches from the previous grinding direction are removed. When necessary, the abrasive paper should be replaced with a newer paper to maintain cutting rates.
Rough Polishing
The purpose of the rough polishing step is to remove the damage produced during cutting and planar grinding. Proper rough polishing will maintain specimen flatness and retain all inclusions or secondary phases. By eliminating the previous damage and maintaining the microstructural integrity of the specimen at this step, a minimal amount of time is required to remove the cosmetic damage at the final polishing step.
Rough polishing is accomplished primarily with dia-mond abrasives ranging from 9 micron to 1 micron. Polycrystalline dia-mond - because of its multiple and small cutting edges - produces high cut rates with minimal surface damage. Therefore, polycrystalline dia-mond abrasives are recommended for metallographic rough polishing on low-napped polishing cloths.
Final Polishing
The purpose of final polishing is to remove only the cosmetic surface damage. It should not be used to remove any damage remaining from cutting and planar grinding. If the damage from these steps is not completely removed, the rough polishing step should be repeated or continued.