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|Head Speed (rpm)||Base Speed (rpm)||Relative Velocity Distribution||Characteristic||Application|
|150||300 to 600||High||Aggressive stock removal Differential grinding across the specimen surface||Useful for gross removal on hard specimens|
|150||150||Minimal||Matching head and base speed in the same direction eliminates relative velocity distributions Uniform stock removal Low stock removal Produces minimal damage||Provides superior flatness over the specimen Useful for retaining inclusions and brittle phases|
For high stock removal, a slower head speed relative to a higher base speed produces the most aggressive grinding/ polishing operation. The drawback to high velocity distributions is that the abrasive (especially SiC papers) may not breakdown uniformly, this can result in non-uniform removal across the specimen surface. Another disadvantage is that the high velocity distributions can create substantially more specimen damage, especially in brittle phases. In all cases, it is not recommended to have the head rotating contra direction to the base because of the non-uniform removal and abrasive break-down which occurs.
Minimal relative velocity distributions can be obtained by rotating the head specimen disk at the same rpm and same direction as the base platen. This condition is best for retaining inclusions and brittle phases as well as for obtaining a uniform finish across the entire specimen. The disadvantage to low relative velocity distributions is that stock removal rates can be quite low.
In practice, a combination of a high velocity distribution (150 rpm head speed/ 300 - 600 rpm base speed) for the initial planarization or stock removal step, followed by a moderate speed and low velocity distribution (120-150 rpm head speed/ 150 rpm base speed) step is recommended for producing relatively flat specimens. For final polishing under chemical mechanical polishing (CMP) conditions where frictional heat can enhance the chemical process, high speeds and high relative velocity distributions can be useful as long as brittle phases are not present (e.g. monolithic ceramics such as silicon nitride and alumina).
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.
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, 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 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. They also exist in either the softer gamma (mohs 8) or harder alpha (mohs 9) phases.