Akasel Insights

Common artefacts in metallography are pull-outs. This happens when hard and brittle non-metallic inclusions or carbides are unintentionally removed from the softer metal matrix during the grinding or polishing process.

Because the particle is gone, you are left with a black void. To minimise the risk of getting these pull-outs, the polishing times should be kept as short as possible.

• Use co-rotation: Running the head and the disc in the same direction provides a more gentle preparation (e.g., lower relative velocity between the sample and the abrasive) than counter-rotation. This helps to keep inclusions in place and is generally recommended for both grinding and diamond polishing steps.

General instructions for preparing metals containing inclusions and carbides:

• Fine grinding: Ensure the step is long enough to remove all damage from the plane grinding step. It is common to use a rigid grinding disc to which diamond suspension is added.
  • Surface selection: If you get pull-outs using an Aka-Allegran disc, switch to a softer rigid disc like Aka-Largan for a less aggressive removal. For exceptionally large or brittle inclusions that still pull out or fracture on rigid discs, use a hard polishing cloth like Plaran instead.
• Diamond polishing: Soft cloths with a nap tend to drag out inclusions. Prepare the material on a hard, napless cloth like Silk or Daran (the cloth should always be moist, not too wet or too dry).
• Cleaning: Use ultrasonic cleaning with caution. While it removes residue, it can loosen brittle inclusions. If pull-outs persist, it is advisable not to use ultrasonic cleaning to see if the inclusions remain intact.
• Number of steps: Add an extra step. Instead of going directly from 9 µm (micrometer) to 3 µm, use three steps: 9 µm, 6 µm, and 3 µm (shorter times can be used on the 6 µm and 3 µm steps here).
• Suspension: Use a suspension with a low removal rate and high viscosity, such as DiaDoublo.
• Water-sensitive inclusions: When dealing with water-sensitive inclusions, using water-based consumables or improper cleaning can cause pull-outs.

To prevent these water-induced pull-outs:

• Use water-free consumables: Change to a water-free diamond suspension and lubricant during grinding and polishing.
• Optimise cleaning: Wash the samples immediately with water and neutral soap, keeping the washing time as brief as possible. Follow immediately with an alcohol rinse and dry thoroughly. If the water-sensitive inclusions are in ferrous materials, use cold water.
• Water-free cleaning: Perform cleaning using only alcohol if the material is highly water-sensitive.