The purpose of the grinding step is to remove damage from cutting, planarize the specimen(s), and to remove material approaching the area of interest.
The most common metallographic abrasive used is Silicon Carbide – SiC. It is an ideal abrasive for grinding because of its hardness and sharp edges. For metallographic preparation, SiC abrasives are used in coated abrasive grinding papers ranging from very coarse 60 grit to very fine 1200 grit sizes. Some of the application procedures are given below.
• Soft non-ferrous metals – Initial grinding is recommended with 320 grit SiC abrasive paper followed by 400, 600, 800 and 1200 grit SiC paper. Because these materials are relatively soft they do not easily break down the SiC paper. Thus initial grinding with 320 grit is generally sufficient for minimizing initial deformation and yet maintain adequate removal rates. For extremely soft materials such as tin, lead and zinc it is also recommended that the abrasive paper be lightly coated with a paraffin wax. The wax reduces the tendency of the SiC abrasive to embed into the soft specimen.
• Soft ferrous metals – are relatively easy to grind with the depth of deformation being a major consideration. 240 grit SiC abrasives provide a good initial start with subsequent use of 320, 400, 600, 800 and 1200 grit SiC.
• Hard ferrous metals – require more aggressive abrasives to achieve adequate material removal. Thus coarse SiC abrasives (120 or 180 grit) are recommended for stock removal requirements. Once planarity and the area of interest are obtained a standard 240, 320, 400 and 600 grit series is recommended.
• Super alloys – are generally of moderate hardness but have extremely stable elevated temperature characteristics and corrosion resistance. The procedures for preparing super alloys are very similar to that for most non-ferrous metals.
• Ceramics – are extremely hard, corrosion resistant and brittle materials. They fracture producing both surface and subsurface damage. Proper grinding minimizes both of these forms of damage. This requires the application of a semi-fixed abrasive which are held rigidly for grinding but can be dislodged under high stress in order to minimize subsurface damage. The abrasive size is also important because very coarse abrasives will remove material quickly but can seriously damage the specimen. For ceramics, consideration of the damage produced at each preparation step is critical to minimizing the overall preparation sequence.
• Composites – are perhaps the most difficult specimens to prepare because of the wide range of properties for the materials used. For example, a metal matrix composite (MMC) such as silicon carbide ceramic particles in an aluminum metal matrix is a difficult specimen to prepare. This composite contains extremely hard/brittle ceramic particles dispersed in a relatively soft/ductile metal matrix. As a rule of thumb, initial grinding should focus on metal planarization and grinding to the area of interest. The secondary grinding steps require focusing on the ceramic particles and typically require the use of diamond abrasives.
The machine parameters which affect the preparation of metallographic specimens are:
• Grinding/polishing pressure,
• Relative velocity distribution,
• The direction of grinding/polishing.
Grinding/polishing pressure is dependent upon the applied force (Newtons) and the area of the specimen and mounting material. Pressure is defined as the Force/Area (N/m2). For specimens significantly harder than the mounting compound, pressure is better defined as the force divided by the specimen surface area. Thus, for larger hard specimens higher grinding/polishing pressures increase stock removal rates, however higher pressure also increases the amount of surface and subsurface damage. Higher grinding/polishing pressures can also generate additional frictional heat which may actually be beneficial for the chemical mechanical polishing (CMP) of ceramics, minerals and composites. Likewise for extremely friable specimens such as nodular cast iron, higher pressures and lower relative velocity distributions can aid in retaining inclusions and secondary phases.
Rotation Velocity and Direction:
The disk speed of the grinder/polisher(Base unit) and the speed of the specimen holder of the Automatic Head(Head Unit) play an important role. This relative rotation allows for a variable velocity distribution depending upon the head speed relative to the base speed.
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
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.