Polishing by Metallurgical Supplies

POLISHING in Metallography

Polishing is the most important step in preparing a specimen for microstructural analysis. It is the step which is required to completely eliminate previous damage.

Ideally the amount of damage produced during cutting and grinding was minimized through proper blade and abrasive grinding so that polishing can be minimized.

To remove deformation from fine grinding and obtain a surface that is highly reflective, the specimens must be polished before they can be examined under the microscope. Polishing is a complex activity in which factors such as quality and suitability for the cloth, abrasive, polishing pressure, polishing speed and duration need to be taken into account. The quality of the surface obtained after the final polishing depends on all these factors and the finish of the surface on completion of each of the previous stages.

Polishing Cloths

There are three types of polishing clothes; Woven, Non-Woven and Flocked.

•  Woven cloths offer ‘hard surface’ polishing properties and guarantee flat pre-polishing, without deterioration of the edges.

•  Non-woven cloths, are used on very hard materials for high precision surface finishing such as glass, quartz, sapphire and semi-conductors.

•  The Flocked cloths, guarantee a super-polished finish. The polishing duration must be as short as possible, to avoid inclusions from being extracted.

Diamond products

Diamond, due to its exceptional hardness and cutting capacity, has become the first choice abrasive in metallographic polishing. Diamonds for metallographic grinding and polishing are available in two different crystalline shapes: Polycrystalline (P) and monocrystalline (M). Polycrystalline diamonds provide vast numbers of small cutting edges. In the metallographic preparation process these edges result in high material removal, while producing only a shallow scratch depth.

Monocrystalline diamonds are more block-shaped and provide few cutting edges. These diamonds give high material removal with a more variable scratch pattern. For high requirements, the (P)-type diamonds are chosen. The (M) type diamonds are best suited for all-purpose polishing. Diamond products are usually available in three forms; diamond paste, diamond suspension and diamond spray.

Polycrystalline diamond as compared to monocrystalline diamond provides better surface finishes and higher removal rates for metallographic specimen preparation. The features and advantages of polycrystalline diamond include the following:

•  Higher cutting rates

•  Very uniform surface finish

•  More uniform particle size distribution

•  Higher removal rates (self sharpening abrasives)

•  Harder/tougher particles

•  Blocky shaped

•  Hexagonal microcrystallites (equally hard in all directions)

•  Extremely rough surface (more cutting points)

•  Surface area 300% greater than monocrystalline diamond

•  No abrasion-resistant directionality (abrasion independent of particle orientation) 

Final Polishing Abrasives

Final polishing abrasives are selected based upon specimen hardness and chemical reactivity. The most common polishing abrasives is alumina. Alumina abrasives are primarily used as mechanical abrasives because of their high hardness and durability.