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Metallographic Final Polishing Abrasives

Metallographic alumina and collodial silica abrasives

Final Polishing Abrasives


Selection Guidelines ] Nanometer Alumina ] Colloidal Silica ] Trouble Shooting ] Product Description ] [ Home ]



FINE POLISHING ABRASIVES

Metallographic alumina Alumina
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.
Metallographic colloidal silica
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.

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NANOMETER 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 polishing

Calcined alumina are typically very pure (>99%), whereas the purity of fused alumina can vary from 96% to 99+%.. 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

Metallographic 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
Application of Nanometer Alumina
  • 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.
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COLLODIAL SILICA AND CMP SLURRIES DESCRIPTION

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.
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TROUBLE SHOOTING GUIDE 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

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    TROUBLE SHOOTING GUIDE 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


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    FINE ABRASIVES PRODUCT SELECTION GUIDE

    ALUMINA Abrasive Slurry (pH 10)

    Description

    Quantity

    Catalog Number

     

    Nanometer 0.05 micron
    Alpha alumina

    16 oz
    (475 ml)

    NA-1005-16

    Metallographic acidic alumina

    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


    ALUMINA Abrasive Slurry (Acidic-pH 4)

    Description

    Quantity

    Catalog Number

     

    Nanometer Acid Alumina Slurry
    (0.05 micron)

    16 oz
    (475 ml)

    NA-1020-16

    Metallographic nanometer alumina

    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


    ALUMINA Abrasive Slurry

    Description

    Quantity

    Catalog Number

     

    NANOFINISH 0.05 micron slurry

    6 oz (0.475 ml)
    1 gallon

    NANO-1005-06
    NANO-1005-128

    Metallogarphic alumina slurry

    NANOFINISH 0.30 micron slurry

    6 oz (0.475 ml)
    1 gallon

    NANO-1003-06
    NANO-1003-128

    NANOFINISH 0.50 micron slurry

    6 oz (0.475 ml)
    1 gallon

    NANO-1105-06
    NANO-1105-128

    NANOFINISH 1 micron slurry

    6 oz (0.475 ml)
    1 gallon

    NANO-1010-06
    NANO-1010-128

    NANOFINISH 5 micron slurry

    6 oz (0.475 ml)
    1 gallon

    NANO-1050-06
    NANO-1050-128


    Fine ALUMINA Polishing Powders

    Description

    Quantity

    Catalog Number

     

    0.05 micron Alumina Powder (Deagglomerated)

    1 lb
    5 lbs

    ALR-1005-01
    ALR-1005-05

    Metallographic fine alumina abrasives

    0.3 micron Alumina Powder (high density)

    1 lb
    5 lbs

    ALR-0103-01
    ALR-0103-05

    0.3 micron Alumina Powder (Deagglomerated)

    1 lb
    5 lbs

    ALD-0103B-01
    ALD-0103B-05

    0.5 micron Alumina Powder

    1 lb
    5 lbs

    ALR-0150-01
    ALR-0150-05

    Metallographic medium grit alumina abrasive

    1 micron Alumina Powder

    1 lb
    5 lbs

    ALR-0110-01
    ALR-0110-05

    3 micron Alumina Powder

    5 lbs

    ALR-0130-05

    5 micron Alumina Powder

    5 lbs

    ALR-0150-05

    Metallographic medium coarse alumina abrasive
    9 micron Alumina Powder
    5 lbs
    ALR-0190-05
    12 micron Alumian Powder
    5 lbs
    ALR-1200-05

    Coarse ALUMINA Powders

    Description

    Quantity

    Catalog Number

     

    240 grit Alumina Suspension

    5 lbs

    ALR-0240-05

    Metallographic coarse alumina abrasive

    400 grit Alumina Suspension

    5 lbs

    ALR-0400-05

    600 grit Alumina Suspension

    5 lbs

    ALR-0600-05


    Colloidal Silica (0.05 micron)

    Description

    Quantity

    Catalog Number

     

    SIAMAT Colloidal Silica

    16 oz
    (475 ml)

    SI-100A-16

    Metallographic SIAMAT colloidal silica

    SIAMAT Colloidal Silica

    1/2 gallon
    (1.9 l)

    SI-100A-64

    SIAMAT Colloidal Silica

    1 gallon
    (3.8 l)

    SI-100A-128


    Colloidal Silica - Blue (0.05 micron)

    Description

    Quantity

    Catalog Number

     

    SIAMAT Blue Colloidal Silica

    16 oz
    (475 ml)

    SI-100B-16

    Metallographic SIAMAT colloidal silica

    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


    Colloidal Silica (0.05 micron)

    Description

    Quantity

    Catalog Number

     

    SIAMAT Colloidal Silica

    16 oz
    (475 ml)

    SI-100A-16

    Metallographic SIAMAT colloidal silica

    SIAMAT Colloidal Silica

    1/2 gallon
    (1.9 l)

    SI-100A-64

    SIAMAT Colloidal Silica

    1 gallon
    (3.8 l)

    SI-100A-128


    Chemcial Mechanical CMP Slurry

    Description

    Quantity

    Catalog Number

     

    Chemical Mechanical Alumina-Silica Slurry (pH 10)

    16 oz
    (475 ml)

    CMP-1005-16

    Metallographic SIAMAT colloidal silica

    Checmial 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


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