About Grinding
The purpose of metallographic sample preparation is to reveal the true structure of the specimen. This true structure allows precise imaging of the sample surface we want to analyse.
Ideally, the true structure should satisfy all of the following conditions:
- No deformation
- No scratches
- No pull-outs
- No introduction of foreign elements
- No smearing
- No relief or rounded edges
- No thermal damage
Mechanical preparation (grinding and polishing) is the most commonly used method for preparing materialographic specimens for microscopic examination. Abrasive particles are used in progressively finer steps to remove material from the surface until the required result is achieved. Mechanical preparation can be divided into two processes: grinding and polishing.
Proper grinding removes damaged or deformed surface material while introducing only a very limited amount of new deformation. The goal of grinding is to obtain a flat surface with only very fine scratches — scratches fine enough to be easily removed in the shortest possible time during polishing.
Grinding is divided into two stages: Plane Grinding (PG) and Fine Grinding (FG).
Plane Grinding (PG)
The first grinding step is commonly referred to as "Plane Grinding (PG)". Regardless of the specimen's original condition or any prior treatment, the surface after plane grinding should be consistent across all specimens. In addition, when multiple specimens held in a fixture must be prepared simultaneously, they must all be brought to the same level or plane before further preparation can proceed. Coarser fixed abrasive particles remove material rapidly. Depending on the nature of the material, different types of abrasives may be used:
- Silicon Carbide (SiC): The most commonly used abrasive for plane grinding soft materials (hardness < 150 HV), such as MD-Primo.
- Aluminium Oxide: Suitable for grinding ferrous metals.
- Diamond: Used for grinding materials harder than 150 HV — harder ferrous metals, ceramics, or sintered carbides — such as MD-Piano.
Fine Grinding (FG)
The surface produced by fine grinding has only minimal deformation that can be removed during the polishing step. For soft materials (hardness < 150 HV) or composites with a soft matrix, MD-Largo should be used; for harder materials (hardness > 150 HV), MD-Allegro is recommended.
MD-Largo and MD-Allegro are used in combination with diamond suspension. The unique surface of these "fine grinding discs" allows some of the diamond abrasive particles to be embedded in the surface. The precise positioning of the diamond particles allows material to be removed rapidly with minimal deformation. MD-Largo and MD-Allegro also ensure excellent flatness while keeping maintenance requirements to a minimum.
About Polishing
Like grinding, polishing must remove the deformation introduced by the preceding step. This is achieved by using progressively finer abrasive particles. Polishing is divided into two distinct stages:
- Diamond Polishing (DP)
- Oxide Polishing (OP)
Diamond Polishing (DP)
Diamond is used as an abrasive capable of removing material most rapidly while achieving the best possible flatness — no other abrasive can produce comparable results. Because of its hardness, diamond has excellent cutting performance.
Oxide Polishing (OP)
Certain materials — particularly soft and ductile ones — require a final polishing step to achieve the best possible quality. Colloidal silica with a particle size of approximately 0.04 μm and a pH of approximately 9.8 has shown outstanding results. The combination of chemical action and fine, gentle abrasion produces specimens that are absolutely scratch-free and deformation-free.
- OP-U: A universal polishing suspension that produces excellent results on a wide range of materials.
- OP-S: Can be used together with reagents to increase chemical reactivity, making it ideal for polishing highly ductile materials.
- OP-A: An acidic alumina suspension, used for the final polishing of low- and high-alloy steels, nickel-based alloys, and ceramics.
Polishing Cloth Selection
Polishing is performed on polishing cloths. For diamond polishing, a lubricant must be used. The choice of polishing cloth, diamond particle size, and lubricant depends on the material being polished. The initial polishing steps are typically carried out with low-nap cloths and low-viscosity lubricants. Final polishing uses higher-nap cloths and higher-viscosity lubricants (high-viscosity lubricants are preferred for soft materials).