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Coarse Grinding / Plane Grinding

In many cases, when preparing samples for EBSD, the cutting techniques used limit sectioning damage effectively. The initial grinding stage selected should minimise aggression, and cause less damage than the sectioning. For this reason, surfaces such as grinding stones and other aggressive grinding surfaces are not normally recommended.

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, in order 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 advices.

Grinding surfaces

Grinding surfaces.

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:

  • The abrasives used for grinding are fixed or bonded, either glued onto a paper or mixed with resin and made into a grinding stone or grinding discs.
  • During the mechanical rotation of the grinding disc or paper, the individual abrasive particles act like tools that stick up and take out chips from the surface.
  • The first grinding step, Plane Grinding removes damage introduced by cutting, and levels specimens clamped in a holder for automatic preparation.
  • The choice of surface depends on the physical characteristics of the material to be ground and is critical.
  • Rigidly fixed abrasives are more aggressive than those in a shock absorbing backing and also ensure a better flatness.
  • In general maintaining sharp abrasives promotes good grinding characteristics with minimal damage.
Plane Ground sample of nodular cast iron

Plane Ground sample of nodular cast iron.

Fine Grinding

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 in order 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 also produces flatter sample surfaces than grinding with SiC-Grinding Paper. Ensure that sharp abrasives are used and follow the manufacturers' instructions with regard to grinding rotational speeds, direction, force, times 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. 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 in order 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.

Grinding paper and discs.

SiC-Grinding Paper

SiC-Grinding Paper.

Fine Grinding rigid discs

Fine Grinding rigid discs.

Characteristics of Fine Grinding:

  • The abrasives used for Fine Grinding are fixed or bonded, either glued onto a paper or mixed with resin or are added onto a rigid grinding surface.
  • The goal of Fine Grinding is to reduce damage and surface roughness of samples to a degree that is suitable for polishing.
  • The choice of surface depends on the physical characteristics of the material to be ground and is critical.
  • Rigidly fixed abrasives gives a flatter preparation result than when using surfaces with a shock absorbing backing.
  • In general maintaining sharp abrasives promotes god grinding characteristics with minimal damage.
  • The fine ground surface should be inspected using a light microscope to ensure that all damage from the previous stage is completely removed.
Fine Ground sample of nodular cast iron

Fine Ground sample of nodular cast iron.

Images courtesy of Struers.

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