Metallographic Final Polishing Abrasives
Introduction
Alumina Powders and Suspensions
Final polishing abrasives are selected based upon specimen hardness and chemical reactivity. The most common polishing abrasives are alumina and colloidal silica. 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.
Colloidal Silica and CMP Slurries
Colloidal silica and CMP slurries are a relatively soft final polishing abrasives with a high chemical activity. They are ideal for chemical mechanical polishing (CMP). The chemical activity of colloidal silica results from the electrochemical balance (zeta potential) required to keep the fine particles from aggregating. This chemical balance produces a surface phenomena which makes the specimen surface more reactive. This reacted layer is consequently removed by the colloidal silica abrasives themselves or by a mechanical scrubbing of the surface with other abrasives and/or the polishing pad.
Alumina Polishing Abrasives
There are a number of different types of alumina polishing abrasives. These abrasive are classified by their manufacturing process, crystal structure (hardness) and their classification process. The main types of alumina’s produced for polishing are either calcined or polycrystalline alumina. Calcined powders can also be characterized as either fused, platey or levigated (air classified / sized) alumina depending upon the grain shape, purity and classification process. There is a major difference in the performance between calcined and polycrystalline alumina abrasives, especially for the submicron particle sizes. The two common crystal structures are the harder alpha alumina (mohs hardness 9) and the softer gamma crystal structure (mohs hardness 8).
| Alumina / property | Polycrystalline | Calcined | Fused | Levigated |
|---|---|---|---|---|
| Crystal structure | Polycrystalline alpha alumina | Monocrystalline alpha alumina | Alpha alumina | Monocrystalline gamma or alpha |
| Shape | Hexagonal platelets | Hexagonal platelets | ||
| Particle size | 0.05 microns | Submicron up to 30 microns | 5-70 micron | 3-5 micron (air classified) |
| pH | 4 and 10 | Range from 8-11 | 9.0-10.5 | |
| % solids | 20-30% | |||
| Specific gravity | 3.95 gms/cc | 3.95 gms/cc | 3.95 gms/cc | 3.95 gms/cc |
| Hardness | Knoop 2000 (Mohs 9) |
Knoop 2000 (Mohs 9) |
Knoop 2000 (Mohs 9) | Mohs 8,9 |
| Applications | pH 4 (acidic) – metal polishing pH 10 (basic)- ceramic polishing |
Linde A, Linde B and Linde C alumina polishes |
Calcined and Fused Alumina
Calcined alumina powders are typically used for metal lapping and polishing, as well as have been the traditional rough and final polishing abrasive for many years and is relatively inexpensive and readily available.
Calcined alumina grains consist of single platey crystals, whereas fused alumina is more blocky in shape. A perfect clacined alumina particle would be a six-sided disk, with a thickness of about 1/5 of its diameter.
Calcined alumina are typically very pure (>99%), whereas the purity of fused alumina can vary from 96% to 99+%https://www.metallographic.com The issue with the calcined powders is the agglomeration issue created by processing these type of alumina’s as dry powders creates agglomeration of the particles. For larger particles (>600 grit), the agglomeration is not too much of an issue, however for smaller particles the agglomeration issue can be significant. For example, the agglomerated or standard version of a 0.05 um aluimina crystal can form an agglomerate size as large as 40 um. When polishing hard samples these agglomerates can often be broken down, however for soft materials such as aluminum, copper, zinc, etc. these fine agglomerated abrasives can form large random scratches.
Agglomerated vs. Deagglomerated
The term agglomeration refers to the electrostatic attraction of small particles to each other. This arises from the fact that all surfaces have an unbalanced surface charge. When the surface area of small alumina particle becomes large compared to its volume, then it becomes nearly impossible to keep the particle from attracting each other in a dry processing state.
The term deagglomeration refers to the process by which a dry agglomerated particle is milled or broken down. The process for deagglomeration of dry powders is to blast the agglomerated powder at each other at high velocities, thus breaking down the agglomerate size. The process is only moderately successful.
Nanometer Polycrystalline Alumina
Nanometer alumina is a colloidal alumina processed by a proprietary process which produces a polycrystalline alumina crystal. This processing offers two significant improvements over conventional alumina calcining processes:
1. Tighter more controlled particle size distributions
2. Harder alpha alumina phase
A tighter more controlled particle size distribution is a result of less particle aggregation. For example, standard calcined 0.05 um gamma alumina products form aggregate sizes as large as 5 um. These aggregates can be broken down during polishing with harder specimens. However, these large aggregates have been known to cause significant scratching in soft metals such as aluminum, tin, lead, copper and soft steels.
Nanometer alumina is specifically milled to produce a much smaller aggregate particle size distribution (<0.5 um). Nanometer alumina is also the harder alpha alumina particle, therefore making it a more efficient cutting abrasive. Thus nanometer alumina is a much more controlled polishing abrasive than calcined gamma alumina.
Advantages and Features of Nanometer Alumina- Improved surface finishes over gamma alumina
- Less random scratching
- Higher cutting rates than gamma alumina
- Much more uniform surface finish
- Tighter particle size distributions than gamma alumina
- Low viscosity for easy dispensing
- Semi-permanent suspensions
- The most efficient application is to initially wet the polishing cloth with Nanometer alumina and then to slowly drip or spray the suspension onto the polishing cloth
- It is recommended that water be continuously applied to the polishing cloth for several seconds prior to completion of the polishing step. This aids in removing the fine particles which may be electrostatically attracted to the specimen surface. This phenomena occurs because of the deagglomeration characteristics of the alumina.
Colloidal Silica and CMP Slurry Descriptions
Colloidal silica is a very unique polishing suspension because it provides a chemical mechanical polishing (CMP) action for materials such as ceramics, composites and soft metals. Colloidal silica has particle size distribution between 0.05 um and 0.07 um and is stabilized at alkaline pH values (typically >9.5).
Ceramics and Minerals - At higher pH values, colloidal silica is held in nearly perfect suspension by the electrochemical repulsive forces of the fine particles themselves. This chemical balance electrochemically attacks the surface of a ceramic or mineral surface to form a thin reacted layer on the specimen surface. This reacted layer can then be removed via the mechanical action of the polishing cloth or another abrasive (e.g. diamond). Polishing rates and surface finishes are significantly better for ceramics and minerals as compared with straight mechanical abrasives such as diamond. Thus the highest removal rates and the most reliable action for removing both surface and subsurface damage on ceramics and minerals is via chemical mechanical polishing with colloidal silica.
As with other exothermic chemical reactions, the chemical contribution can be enhanced by increasing the temperature of the polishing action. This can be accomplished by increasing the polishing pressure which increases the friction between the polishing cloth, colloidal silica and the specimen.
Cleaning of the surface is best accomplished by running water on the surface for the final 10-15 seconds of the polishing cycle and then immediately rinsing the specimen surface with water. This allows the cloth to mechanically remove the reacted layer and any residual colloidal silica.
Application of Colloidal Silica- The most efficient application is to initially wet the polishing cloth with SIAMAT Colloidal Silica and then to slowly drip or spray the suspension onto the polishing cloth
- It is recommended that water be applied continuously to the polishing cloth for 10-15 seconds prior to completion of the polishing step. This aids in removing the fine particles which may be electrostatically attracted to the specimen surface.
Trouble Shooting Guidelines for NANOMETER Alumina
| Symptom | Cause | Action |
|---|---|---|
| Agglomerate particles |
-Bacterial growth in suspending agent |
-Replace product |
| Specimen has a fine matte like finish | -Thin layer of particles on the surface |
-Clean surface with 100% cotton swab and alcohol -Clean specimen by flushing specimen and polishing pad with water for last 10-15 seconds of the polishing operation |
| Excessive relief in surface finish | -Overpolishing | -Repeat step prior to final polish and shorten final polishing time |
| Scratches in specimen |
-Improper selection of polishing pad -Contaminated pad -Removal of secondary phases |
-Review application guideline chart for polishing pads or select a softer pad -Replace polishing pad -Alternate between polishing and etching |
| Etching of specimen | -Alumina abrasive pH too high | -Use lower pH alternative abrasives |
Trouble Shooting Guidelines for Colloidal Silica
| Symptom | Cause | Action |
|---|---|---|
| Crystallized residue on specimens | Improper cleaning of specimen | Repolish specimens and apply rinse water to pad and specimen for last 10-15 seconds |
| Etching of specimen | Too high a polishing pH | Use an alternative polishing suspension |
| Excessive scratching on specimen | Crystallization of silica | Keep bottle closed when not in use and filter colloidal silica through a filtering cloth |
Final Polishing Abrasives Product Selection Guidelines
Polycrystalline Alumina Abrasive Slurries (pH 10)
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
|
Nanometer 0.05 micron
Alpha alumina |
16 oz (475 ml) | NA-1005-16 |  |
| Nanometer 0.05 micron Alpha alumina |
32 oz (950 ml) | NA-1005-32 | |
| Nanometer 0.05 micron
Alpha alumina |
1/2 gallon (1.9 l) | NA-1005-64 | |
| Nanometer 0.05 micron
Alpha alumina |
1 gallon (3.8 l) | NA-1005-128 |
Polycrystaline Alumina Abrasive Slurries (Acidic-pH 4)
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| Nanometer Acid Alumina Slurry (0.05 micron) |
16 oz (475 ml) |
NA-1020-16 |  |
| Nanometer Acid Alumina Slurry (0.05 micron) |
32 oz (950 ml) |
NA-1020-32 | |
| Nanometer Acid Alumina Slurry (0.05 micron) |
1/2 gallon (1.9 l) |
NA-1020-64 | |
| Nanometer Acid Alumina Slurry (0.05 micron) |
1 gallon (3.8 l) |
NA-1020-128 |
Deagglomerated Electronic Polishing Grade Alumina Suspensions
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| 0.05 micron Electronics Graded Deagglomerated Alumina Suspensions | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1005-06E NANO2-1005-16E NANO2-1005-32E NANO2-1005-128E |
 |
| 0.30 micron Electronics Graded Deagglomerated Alumina Suspensions | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1003-06E NANO2-1003-16E NANO2-1003-32E NANO2-1003-128E |
|
| 1 micron Electronics Graded Deagglomerated Alumina Suspensions | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1010-06E NANO2-1010-16E NANO2-1010-32E NANO2-1010-128E |
ALUMINA NANO Low Viscosity Abrasive Slurry
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| NANO 0.05 micron slurry | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO-1005-06 NANO-1005-16 NANO-1005-32 NANO-1005-128 |
 |
| NANO 0.30 micron slurry | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO-1003-06 NANO-1003-16 NANO-1003-32 NANO-1003-128 |
|
| NANO 0.50 micron slurry | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO-1105-06 NANO-1105-16 NANO-1105-32 NANO-1105-128 |
|
| NANO 1 micron slurry | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO-1010-06 NANO-1010-16 NANO-1010-32 NANO-1010-128 |
|
| NANO 3 micron slurry | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO-1030-06 NANO-1030-16 NANO-1030-32 NANO-1030-128 |
|
| NANO 5 micron slurry | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO-1050-06 NANO-1050-16 NANO-1050-32 NANO-1050-128 |
ALUMINA NANO2 High Viscosity Abrasive Susupension
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| NANO2 0.05 micron suspension | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1005-06 NANO2-1005-16 NANO2-1005-32 NANO2-1005-128 |
 |
| NANO2 0.30 micron suspension | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1003-06 NANO2-1003-16 NANO2-1003-32 NANO2-1003-128 |
|
| NANO2 0.50 micron suspension | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1105-06 NANO2-1105-16 NANO2-1105-32 NANO2-1105-128 |
|
| NANO2 1 micron suspension | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1010-06 NANO2-1010-16 NANO2-1010-32 NANO2-1010-128 |
|
| NANO2 3 micron suspension | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1030-06 NANO2-1030-16 NANO2-1030-32 NANO2-1030-128 |
|
| NANO2 5 micron suspension | 6 oz
(0.475 ml) 16 oz (475 ml) 32 oz (950 ml) 1 gallon (3.8 l) |
NANO2-1050-06 NANO2-1050-16 NANO2-1050-32 NANO2-1050-128 |
Alumina Polishing Powders
Electronics Graded Deagglomerated Fine ALUMINA Polishing Powders
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| 0.05 micron Electronics Graded Alumina Powder (Deagglomerated) | 1 lb 5 lbs |
ALR-1005-01E ALR-1005-05E |
 |
| 0.30 micron Electronics Graded Alumina Powder (Deagglomerated) | 1 lb 5 lbs |
ALR-1003-01E ALR-1003-05E |
 |
| 1 micron Electronics Graded Alumina Powder (Deagglomerated) | 1 lb 5 lbs |
ALR-1010-01E ALR-1010-05E |
 |
Fine Standard Deagglomerated Colloidal Alumina Polishing Powders (<1 micron)
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| 0.05 micron Alumina Powder (Deagglomerated) | 1 lb 5 lbs |
ALR-1005-01 ALR-1005-05 |
 |
| 0.30 micron Alumina Powder (Deagglomerated) | 1 lb 5 lbs |
ALR-1003-01 ALR-1003-05 |
 |
| 0.50 micron Alumina Powder (Deagglomerated) | 1 lb 5 lbs |
ALR-0105-01 ALR-0105-05 |
 |
Alumina Polishing Powders
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| 1 micron Alumina Powder | 1 lb 5 lbs |
ALR-1010-01 ALR-1010-05 |
 |
| 3 micron Alumina Powder | 5 lbs | ALR-0130-05 | |
| 5 micron Alumina Powder | 5 lbs | ALR-0150-05 |
Coarse Alumina Grinding Powders
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| 9 micron Alumina Powder | 5 lbs | ALR-0190-05 |  |
| 12 micron Alumian Powder | 5 lbs | ALR-1200-05 | |
| 15 micron Alumina Powder | 5 lbs | ALR-1500-05 | |
| 20 grit Alumina Powder | 5 lbs | ALR-2000-05 |  |
| 30 grit Alumina Powder | 5 lbs | ALR-3000-05 | |
| 40 grit Alumina Powderon | 5 lbs | ALR-4000-05 | |
| 240 grit Alumina Suspension | 5 lbs | ALR-0240-05 |  |
| 400 grit Alumina Suspension | 5 lbs | ALR-0400-05 | |
| 600 grit Alumina Suspension | 5 lbs | ALR-0600-05 |
Colloidal Silica Polishing Abrasives
Colloidal Silica (0.05 micron)
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| SIAMAT Colloidal Silica | 16 oz (475 ml) |
SI-100A-16 |  |
| SIAMAT Colloidal Silica | 32 oz (950 ml) |
SI-100A-32 | |
| SIAMAT Colloidal Silica | 1/2 gallon (1.9 l) |
SI-100A-64 | |
| SIAMAT Colloidal Silica | 1 gallon (3.8 l) |
SI-100A-128 | |
| SIAMAT Colloidal Silica | 5 gallons (19 l) |
SI-100A-640 |
Low-Crystallizing Colloidal Silica (0.02 micron)
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| SIAMAT2 Low Crystallizing Colloidal Silica | 16 oz (475 ml) |
SI2-100A-16 |  |
| SIAMAT2 Low Crystallizing Colloidal Silica | 32 oz (950 ml) |
S2I-100A-32 | |
| SIAMAT2 Low Crystallizing Colloidal Silica | 1/2 gallon (1.9 l) |
SI2-100A-64 | |
| SIAMAT2 Low Crystallizing Colloidal Silica | 1 gallon (3.8 l) |
SI2-100A-128 | |
| SIAMAT2 Low Crystallizing Colloidal Silica | 5 gallons (19 l) |
SI2-100A-640 |
Colloidal Silica - Blue (0.05 micron)
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| SIAMAT Blue Colloidal Silica | 16 oz (475 ml) |
SI-100B-16 |  |
| SIAMAT Blue Colloidal Silica | 32 oz (950 ml) |
SI-100B-32 | |
| SIAMAT Blue Colloidal Silica | 1/2 gallon (1.9 l) |
SI-100B-64 | |
| SIAMAT Blue Colloidal Silica | 1 gallon (3.8 l) |
SI-100BA-128 | |
| SIAMAT Blue Colloidal Silica | 5 gallons (19 l) |
SI-100B-640 |
Chemical Mechanical Polishing Suspensions
Chemcial Mechanical CMP Polycystalline Alumina / Colloidal Silica Suspension
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| Chemical Mechanical Alumina-Silica Slurry (pH 10) | 16 oz (475 ml) |
CMP-1005-16 |  |
| Chemcial Mechanicl Alumina-Silica Slurry (pH 10) | 32 oz (0.95 l) |
CMP-1005-32 | |
| Chemical Mechanical Alumina-Silica Slurry (pH 10) | 1/2 gallon (1.9 l) |
CMP-1005-64 | |
| Chemical Mechanical Alumina-Silica Slurry (pH 10) | 1 gallon (3.8 l) |
CMP-1005-128 |
CMP2 High Viscosity Chemical Mechanical Polishing Suspension
| Description | Quantity | Catalog Number | Product Image |
|---|---|---|---|
| High Viscosity CMP Suspension | 16 oz (475 ml) |
CMP2-1005-16 |  |
| High Viscosity CMP Suspension | 32 oz (0.95 l) |
CMP2-1005-32 | |
| High Viscosity CMP Suspension | 1/2 gallon (1.9 l) |
CMP2-1005-64 | |
| High Viscosity CMP Suspension | 1 gallon (3.8 l) |
CMP2-1005-128 |
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