Metallographic Grinding and Polishing Guide

Grinding should start with the finest grit size that will establish a flat surface and remove the effects of sectioning within a few minutes. An abrasive grit size of 180-240 (P180-P280) is usually coarse enough to use on samples sectioned by an abrasive wheel on a metallography saw – finer for very soft materials. For very hard materials such as ceramics and sintered carbides, choose diamond grinding disks with grit sizes from 125 to 75µm.

The table below shows recommended starting grit sizes for grinding different materials.

In metallography preparation, the cutting step and each grinding step produces damage itself. The depth of damage decreases with abrasive size but so does the metal removal rate. Large gaps in abrasive size would require excessive time to remove the damage from the prior step and is inefficient. 3-5 steps from initial grind to final polish are typically sufficient for most materials. Note that for a given abrasive size, the depth of damage introduced is greater for soft materials than for hard materials, but removal rate is controlled by the harder material. This means that samples with combined softer and harder components often require more preparation steps.

Many factors influence the surface finish, such as:

• Abrasive size and type
• Cloth characteristics (weave, flatness and resilience)
• Polishing time
• Specimen load
• Relative rotational direction
• Rotational speed

If help is needed to determine the correct polishing recipe, please contact our applications specialists. Buehler Solution Centers provide materials preparation and analysis training to our customers worldwide. Our mission is to deliver valuable application solutions by employing Buehler methodologies.

Polishing consists of two or more main stages, using successively finer abrasives.

• Coarse polishing follows grinding and removes the bulk of the deformation created in the grinding process.
• Intermediate polishing stages may be required to further reduce surface deformation and leave smaller scratches.
• Fine polishing perfects the surface finish by removing any trace of deformation.

A specimen is ready to move to the next step once all the scratches are uniform and evidence of the previous step is gone. Final polishing on soft cloths should be performed for the minimum amount of time required to achieve the desired results, as over-polishing can damage the specimen.

Often, a cloth may get contaminated from improper care or gouged before showing significant signs of wear. End of life for a cloth is typically indicated by unusually high polish time, a degradation in result or signs of visible damage such as fraying.

The grit size numbering systems differ above 180 grit [P180], but equivalent sizes can be determined using Table 3.1.

The chart shows the midpoints for the size ranges for ANSI/CAMI graded paper according to ANSI standard B74.18-1996 and for FEPA graded paper according to FEPA standard 43-GB-1984 (R1993). The ANSI/CAMI standard lists SiC particles sizes ranges up to 600 grit paper. For finer grit ANSI/CAMI papers, the particles sizes come from the CAMI booklet, Coated Abrasive (1996). *FEPA grades finer than P2500 are not standardized and are graded at the discretion of the manufacturer. In practice, the above standard values are only guidelines and individual manufacturers may work to a different size range and mean value.

There is a philosophical difference in the two systems. See the table below for a comparison.

Surface defects and artifacts can occur during metallography polishing resulting in inaccurate results by impeding the ability to see a material’s true microstructure and damaging the material.

• Likely Cause: Cross-contamination from larger abrasive particles carried over from earlier stages; insufficient cleaning between steps; debris trapped in porous or cracked specimens.
• Solution: Thoroughly rinse the specimen, holder, and platen between steps. Use ultrasonic cleaning for cracked/porous samples (keep duration <2mins to avoid damage). In final polishing, stop adding suspension in the last ~30 seconds and use water to flush the cloth and specimen.

• Likely Cause: Over-polishing, excess lubrication or use of cloths with excessive nap, causing uneven material removal between hard and soft phases.
• Solution: Reduce polishing time, use shorter-nap cloths, and apply diamond paste instead of suspension for better control of lubrication level.

• Likely Cause: Hard abrasive particles becoming lodged in soft materials, cracks, or voids due to loose abrasive or inadequate cleaning.
• Solution: Use a more fixed abrasive system (e.g., diamond paste instead of suspension) and incorporate ultrasonic cleaning between preparation stages.

• Likely Cause: Plastic deformation of soft materials due to poor lubrication, inappropriate cloth selection, or insufficient polishing control.
• Solution: Use short-napped cloths, improve lubrication conditions, apply vibratory polishing, or etch the specimen and repeat the final polish.

• Likely Cause: Poorly bonded hard phases in a soft matrix, porous structures, or excessive lubrication leading to particle drag effects.
• Solution: Use napless cloths and reduce applied pressure. For porous materials, impregnate pores with epoxy or wax before polishing.

• Issue: Surface defects and artifacts during polishing
• Likely Cause: Contamination, improper abrasive selection, poor lubrication, or material characteristics
• Solution: Optimize cleaning, abrasive type, cloth selection, and polishing parameters to minimize damage and preserve true microstructure

Proper storage and maintenance will enhance cloth life. Store the cloth such that it can dry out, and it is protected from contamination. Chamfering the edges of your mount will reduce the likelihood of damaging your cloth with a sharp edge and improve overall lifetime. Rinsing after use will also extend cloth life, but requires extra abrasive to recharge.

When a method that is known to work starts leaving large scratches, first check for cross-contamination. Rinsing the specimen, specimen holder, and platen between steps can help clean out the larger diamond, reducing cross-contamination. If a specimen is cracked or porous, rinse in an ultrasonic bath for the minimum time required to rinse it clean. Extended ultrasonic cleaning time can damage the specimen. During the last 30 seconds of final polishing, stop applying additional suspension, replacing it with water to flush the cloth surface and rinse the specimen. If process contamination is ruled out, replace the polishing surfaces if they are aged or if contaminant has become embedded.

Embedding occurs when harder particles become engrained in softer material or cracks and voids. Using a more fixed abrasive such as MetaDi Diamond Paste, or a surface that holds the abrasive more effectively such as TexMet C can address most problems. Ultrasonic cleaning specimens with cracks or pores between stages can reduce the likelihood of embedding.