In many cases, when preparing samples for Electron Backscatter Diffraction (EBSD), the used cutting techniques are chosen to limit the sectioning damage effectively. The initial grinding stage should minimise aggression and cause less damage than the sectioning step. For this reason, surfaces such as grinding stones and other aggressive grinding surfaces are not normally recommended.
The fine grinding less damage than the course grinding step. In this way each step in the preparation process will remove damage introduced in the previous step and gradually generate a surface with a very thin damage layer which can be removed during the final polishing steps.
Plane Grinding can be achieved in a variety of ways, using a variety of abrasives. Fixed abrasive surfaces are available using diamond, aluminium oxide or cubic boron nitride (CBN) abrasives. The method used to bind the abrasives to the wheel largely defines the grinding characteristics - the harder or more rigid the bonding medium, the more aggressive the grinding action of the surface. The type of grinding surface used to make the specimen plane will depend on the material being prepared.
For softer materials coarse grit of grinding paper with Silicon Carbide or Alumina abrasives may be used, but the durability or characteristics of such materials may be inappropriate for certain materials. Generally, to maintain sharp abrasive particles, grinding papers need frequent changing. For harder and mixed materials, diamond grinding discs are often the best choice. Follow the manufacturer’s recommendations and advice.
Planar grinding can also be achieved using pastes or slurries applied to a suitable surface. Grinding in this way rather than with fixed abrasives can be significantly less aggressive, retain better flatness, and limit brittle fracture and plastic deformation often associated with fixed abrasives.
Characteristics of Coarse Grinding:
Planer Ground sample of nodular cast iron.
Fine Grinding can be achieved in a variety of ways, using a variety of abrasives, mainly Silicon Carbides and Diamonds. Different preparation surfaces are available for Fine Grinding that can be differentiated according to either the abrasive particles being fixed or being added onto a rigid grinding surface. SiC-Grinding paper is traditionally used for grinding, but it has some disadvantages; Due to a certain resilience of the paper, the samples are not kept very flat, and it requires several preparation steps, 3-4 papers of different grits are needed for fine grinding. SiC-Grinding paper blunts quickly and therefore should be discarded after a short period of grinding to maintain efficient 'stock' removal. Grinding on a surface that has blunt abrasives causes a great deal of surface damage by smearing, 'burnishing' and local heating.
By using diamonds as an abrasive on coarse polishing cloths or on rigid preparation surfaces, Fine Grinding can be reduced to one step, which is more efficient and produces flatter sample surfaces than grinding with SiC-Grinding paper. Ensure that sharp abrasives are used and follow the manufacturers' instructions regarding grinding rotational speeds, direction, force, time and lubricants used. Damage injected during grinding may be invisible in the polished surface but serve to distort the EBSD result or even completely suppress pattern formation.
Grinding Discs: Image of rigid fine grinding discs.
Grinding Paper: Image of SiC grinding papers used for different step in the sample preparation process.
Remember that different materials have different abrasion characteristics. The selection of grinding material and conditions can therefore be specific to a given sample. After every grinding stage it is advisable to inspect the ground surface using a light microscope to ensure that all damage from the previous stage, whether that is a cutting or grinding stage, is completely removed. Advance in this manner to the finest abrasive size required, ready for polishing. Care at this stage will greatly reduce the amount of polishing required to achieve a good surface.
Characteristics of Fine Grinding:
Fine Ground sample of nodular cast iron.