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. It is important to note that it is possible to create more damage in grinding than in sectioning. In other words, it is better to cut as close as possible to the area of interest using the correct abrasive or wafering blade. The following table gives several examples of the grinding procedures recommended for a variety of materials.

• Soft non-ferrous metals - Initial grinding is recommended with 320 grit SiC abrasive paper followed by 400 and 600 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 relatvely 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 and 600 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 is 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 use of a metal mesh cloth (CERMESH cloth) with an applied abrasive accomplishes both of these goals. 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 requires the use of diamond abrasives.

The machine parameters which effect the preparation of metallographic specimens includes: grinding/polishing pressure, relative velocity distribution, and the direction of grinding/polishing.

Grinding Pressure

Grinding/polishing pressure is dependent upon the applied force (pounds or Newtons) and the area of the specimen and mounting material. Pressure is defined as the Force/Area (psi, N/m² or Pa). 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. Note for SiC grinding papers, as the abrasive grains dull and cut rates decrease, increasing grinding pressures can extend the life of the SiC paper.

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.

Relative Velocity

Current grinding/polishing machines are designed with the specimens mounted in a disk holder and machined on a disk platen surface. This disk on disk rotation allows for a variable velocity distribution depending upon the head speed relative to the base speed.

For high stock removal, either a slow and/or contra head direction head with a high 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.

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 (30rpm head speed/ 240 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/ 120-150 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).

Grinding Direction

The orientation of the specimen can have a significant impact on the preparation results, especially for specimens with coatings. In general, when grinding and polishing materials with coatings the brittle component should be kept in compression. In other words, for brittle coatings the direction of the abrasive should be through the coating and into the substrate. Conversely, for brittle substrates with ductile coatings, the direction of the abrasive should be through the brittle substrate into the ductile coating.

Application (Fixed-abrasive Grinding)

• Start with the finest abrasive possible (typically 240 or 320 grit abrasive paper) Note: Only use coarser grit for very heavy stock removal
• Apply lubricant to abrasive surface
  • Most commonly water
  • Water soluble oils for water sensitive specimens
• Rinse specimens thoroughly before proceeding to the next finer abrasive

Application (CERMESH metal mesh cloth)

1. Apply CERMESH metal mesh cloth to flat base surface (Suggestion - peel back protective paper at one corner and align and place on base surface. Pull protective paper with one hand while guiding metal mesh cloth with other hand)
2. Pre-charge CERMESH metal mesh cloth with DIAMAT polycrystalline diamond
3. To avoid tearing the cloth, begin initial grinding at 50% force to set specimen(s) to metal mesh cloth
4. Ramp-up force gradually
5. Add abrasive as required
6. Rinse CERMESH metal mesh cloth with water at the end of the grinding cycle to remove swarf debris