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PACE Technologies metallographic Abrasive Grinding consumables
Metallographic Abrasive Grinding Information



Introduction

Metallographic Abrasive Grinding Consumables

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 typical abrasives used for grinding include:

-Silicon Carbide (SiC)
-Alumina (ALO)
-Diamond
-Zirconia


Metallographic Abrasive Machining

The following are the most common metallographic abrasives:

Silicon Carbide
SiC is a manufactured abrasive produced by a high temperature reaction between silica and carbon. It has a hexagonal-rhombohedral crystal structure and has a hardness of approximately 2500 HV. It is an ideal abrasive for cutting and grinding because of its hardness and sharp edges. It is also somewhat brittle, and therefore it cleaves easily to produce sharp new edges (self sharpening). SiC is an excellent abrasive for maximizing cutting rates while minimizing surface and subsurface damage. For metallographic preparation, SiC abrasives are used in abrasive blades and for coated abrasive grinding papers ranging from very coarse 60 grit to very fine 1200 grit sizes.

Bonded or coated abrasive papers of SiC are designed so that the abrasive will have a large number of cutting points (negative abrasive rank angle). This is achieved by aligning the abrasive particles approximately normal to the backing. Note that coated abrasives are not quite coplanar, thus SiC papers produce the maximum efficiency (cut rate, stock removal and minimal damage) because new abrasive is exposed as the old abrasive breaks down.

Metallographic Abrasive Grinding Consumables

SEM micrograph of 600 grit SiC Abrasive Paper (original mag. 150x)

Alumina

Alumina is a naturally occurring material (Bauxite). It exits in either the softer gamma (mohs 8) or harder alpha (mohs 9, 2000 HV) phase. Alumina abrasives are used primarily as a final polishing abrasive because of their high hardness and durability. Uunlike SiC, alumina breaks down relatively easily to submicron or colloidal particles.

Note that larger coated or bonded grit sizes of alumina are commercially available, however they are not ideal for metallographic applications because they become dull, resulting in lower cut rates and higher surface and subsurface damage.

Diamond

Is the hardest material known to man (mohs 10, 8000 HV). It has a cubic crystal structure, and is available as a natural or an artificial product. Although diamond would be ideal for coarse grinding, its price makes it a very inefficient grinding material for anything except for hard ceramics. For metallographic applications, polycrystalline diamond is recommended as a rough polishing abrasive.

Zirconia

Zircon, or zirconium silicate, is another less common abrasive used for coarse grinding. It is a very tough abrasive, so it lasts longer, however it is generally not as hard or sharp, and thus requires higher pressures to be effective. Typically 60 or 120 grit sizes have been found to be the most useful grain sizes for metallographic grinding with zircon.

For metallographic abrasives the particle size is typically classified by grit size or average particle size in microns. Grit size would refer to the size of the particle if it were classified or sized with mesh screens. Roughly speaking, grit size represents the number of wires or mesh of wires per a specified area. Thus larger grits numbers would represent smaller or more openings in a mesh screen and thus would correlate to smaller sized particles. For example, a 120 grit particle is approxmiately 105 microns in size, whereas, a 1200 grit particle has a particle size of 2.5 microns. The difference between the European P-grading system (number has a P in front of the number) and the more common ANSI or CAMI standard is that the European number is based on the number of openings if the width of the wire mesh was eliminated from the calculation. Therefore for larger particles the P-grading is approximately the same as the ANSI or CAMI number (e.g. 120 grit = P120 grit), whereas, for finer particles the P-grading number can be much larger (e.g. 1200 grit = P4000 grit)

Comparison of standard US grit size to European grit and micron size
Standard grit 120 180 240 320 360 400 600 800 1200
European
(P-grade)
P120 P180 P220 P320 P500 P800 P1200 P2400 P4000


Grinding Parameters

Successful grinding is also a function of the following parameters:

  1. Grinding Pressure
  2. Relative Velocities
  3. Grinding Direction

PACE Technologies metallographic Abrasive Grinding consumables
Abrasive Grinding Selection Guidelines Charts

Material Objective Recommendation
Soft non-ferrous metals (aluminum, copper, brass, tin, zinc, lead, etc.) Minimize abrasive embedding and deformation on the surface P120 grit ALO paper
P220 grit ALO paper
P500 grit ALO paper
P1200 grit ALO paper
Soft ferrous metals (low carbon steels, stainless steel, case hardened steel, etc.) Minimize deformation and secondary phase damage 240 grit SiC paper
320 grit SiC paper
400 grit SiC paper
600 grit SiC paper
800 grit SiC paper
1200 grit SiC paper
Hard ferrous metals (high carbon steel, tool steel, case hardened steel, etc.) Planarize specimen and minimize specimen deformation 180 grit SiC paper
240 grit SiC paper
320 grit SiC paper
400 grit SiC paper
600 grit SiC paper
Super alloys (high nickel and cobalt alloys, titanium alloys) Minimize deformation 70 micron diamond disk
9 micron SIRIUS composite disk
3 micron ORION composite disk
Ceramics, ceramic matrix composites, and minerals Minimize surface and subsurface damage (fracturing and chipping) 30 um polycrystalline diamond on a Metal Mesh cloth
Metal matrix composites, polymer matrix composites Minimize composite fracturing and minimizing matrix deformation 240 or 320 grit SiC paper


PACE Technologies metallographic Abrasive Grinding consumables
Abrasive Grinding Process Description

The machine parameters which affect the preparation of metallographic specimens include: 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.

NOTE: Matching the head and base speed, while running in the same direction produces better flatness and lower surface damage because this minimizes the velocity distribution across the sample as it rotates.

High relative grinding velocity Low relative grinding velocity
High relative velocity distribution for non-matching head and base speeds. Constant relative velocity distribution for-matching head and base speeds.

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
200 200 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, the best condition to run the polishing machine is by matching the head and base speeds at 200 rpm and in the same direction. For polishing it is recommended that the speed be reduce to 100/100 rpm base/head speed. 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).


PACE Technologies metallographic Abrasive Grinding consumables

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.

PACE Technologies metallographic Abrasive Grinding consumables


Abrasive Grinding Recommend Procedures Application
(Fixed-abrasive Grinding)
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

PACE Technologies metallographic Abrasive Grinding consumables

Grinding Surface Backing

Grinding papers come with different types of backing

Plain-backed papers are the least expensive, however, they require either a double sided adhesive or a paper holding ring. The disadvanatage to using a paper holiding ring is that the mounted specimen cannot track over the edge of the grinding paper. Thus the paper does not break down uniformly and thus can produce over grinding of the mount away from the sample. This artifact results in the polished mount exhibiting a crescent moon which can affect the edge of the speciment. This artifact has also been known as "mooning" or MRD (material removal differential).

PSA-backed of adhesive backed papers are removal grinding papers. They are more expensive than the plain-backed papers and the foil papers. They are very easy to use and avoid the problems with "mooning" and MRD if the sample tracks over both the center and edge of the grinding papers.

Foil-backed or polyester backed grinding papers are easy to use and are more economical than adhesive backed papers. The system uses a receiver adhesive disk that can be washed in order to extend their lifes.

Magnetic polishing base Low relative grinding velocity
Foil-backed polisher grinding papers on a washable reusable adhesive disk.
PACE Technologies metallographic Abrasive Grinding consumables
Abrasive Grinding Trouble Shooting

Symptom Cause Action
Uneven grinding across the specimen and mount -Improper tracking of specimen over abrasive paper


-Unequal head / base speeds
-Orient specimen in holder so that the hardest portion
of the specimen/mount tracks over the entire abrasive paper
(uniform degradation of paper)
-Match the head and base speed and rotate in the same direction.
-Grinding at 200/200 rpm and polishing at 100/100 rpm

Excessive vibration in machine

-Too high a load or too low a speed
-Inadequate machine design
-Improper lubricant
-Unequal head / base speeds
-Reduce initial grinding force or increase grinding speed
-Check with equipment vendor for equipment upgrades
-Increase lubricant flow and/or use a water-soluble lubricant
-Match the head and base speed and rotate in the same direction.
- Grinding at 200/200 rpm and polishing at 100/100 rpm
Embedding of fractured abrasive grains -Common in the grinding of very soft materials
-Unequal head / base speeds

-Use alumina grinding papers vs. SiC papers
-Match the head and base speed and rotate in the same direction.
Grinding at 200/200 rpm and polishing at 100/100 rpm


PACE Technologies metallographic Abrasive Grinding consumables

SiC Abrasive Paper (w/PSA)

SiC Abrasive Paper (PSA-pressure sensative adhesive) - 100/pkg
Description 8-inch
Diameter
10-inch
Diameter
12-inch
Diameter
14-inch
Diameter
60 grit SiC paper w/ PSA adhesive SIC-060P8-100 SIC-060P10-100 SIC-060P12-100 SIC-060P14-100
80 grit SiC paper w/ PSA adhesive SIC-080P8-100 SIC-080P10-100 SIC-080P12-100 SIC-080P14-100
120 grit SiC paper w/ PSA adhesive SIC-120P8-100 SIC-120P10-100 SIC-120P12-100 SIC-120P14-100
180 grit SiC paper w/ PSA adhesive SIC-180P8-100 SIC-180P10-100 SIC-180P12-100 SIC-180P14-100
240 grit SiC paper w/ PSA adhesive SIC-240P8-100 SIC-240P10-100 SIC-240P12-100 SIC-240P14-100
320 grit SiC paper w/ PSA adhesive SIC-320P8-100 SIC-320P10-100 SIC-320P12-100 SIC-320P14-100
360 grit SiC paper w/ PSA adhesive SIC-360P8-100 SIC-360P10-100 SIC-360P12-100 SIC-360P14-100
400 grit SiC paper w/ PSA adhesive SIC-400P8-100 SIC-400P10-100 SIC-400P12-100 SIC-400P14-100
600 grit SiC paper w/ PSA adhesive SIC-600P8-100 SIC-600P10-100 SIC-600P12-100 SIC-600P14-100
800 grit SiC paper w/ PSA adhesive SIC-800P8-100 SIC-800P10-100 SIC-800P12-100 SIC-800P14-100
1000 grit SiC paper w/ PSA adhesive SIC-1000P8-100 SIC-1000P10-100 SIC-1000P12-100 SIC-1000P14-100
1200 grit SiC paper w/ PSA adhesive SIC-1200P8-100 SIC-1200P10-100 SIC-1200P12-100 SIC-1200P14-100

PACE Technologies metallographic Abrasive Grinding consumables
Alumina Abrasive Paper (w/PSA)

ALO Abrasive Paper (PSA-pressure sensative adhesive) - 100/pkg
Description 8-inch
Diameter
10-inch
Diameter
12-inch
Diameter
P120-grit Alumina (PSA-backed) ALO-120P8-100 ALO-120P10-100 ALO-120P12-100
P220-grit Alumina (PSA-backed) ALO-220P8-100 ALO-220P10-100 ALO-220P12-100
P500-grit Alumina (PSA-backed) ALO-500P8-100 ALO-500P10-100 ALO-500P12-100
P1200-grit Alumina (PSA-backed) ALO-1200P8-100 ALO-1200P10-100 ALO-1200P12-100


PACE Technologies metallographic Abrasive Grinding consumables
SiC Abrasive Paper (Plain backed)

SiC ABRASIVE PAPER (Plain Backed)
Description 8-inch
Diameter
10-inch
Diameter
12-inch
Diameter
14-inch
Diameter
60 grit SiC paper plain backed SIC-2108-060 SIC-2110-060 SIC-2112-060 SIC-2114-060
80 grit SiC paper plain backed SIC-2108-080 SIC-2110-080 SIC-2112-080 SIC-2114-080
120 grit SiC paper plain backed SIC-2108-120 SIC-2110-120 SIC-2112-120 SIC-2114-120
180 grit SiC paper plain backed SIC-2108-180 SIC-2110-180 SIC-2112-180 SIC-2114-180
240 grit SiC paper plain backed SIC-2108-240 SIC-2110-240 SIC-2112-240 SIC-2114-240
320 grit SiC paper plain backed SIC-2108-320 SIC-2110-320 SIC-2112-320 SIC-2114-320
360 grit SiC paper plain backed SIC-2108-360 SIC-2110-360 SIC-2112-360 SIC-2114-360
400 grit SiC paper plain backed SIC-2108-400 SIC-2110-400 SIC-2112-400 SIC-2114-400
600 grit SiC paper plain backed SIC-2108-600 SIC-2110-600 SIC-2112-600 SIC-2114-600
800 grit SiC paper plain backed SIC-2108-800 SIC-2110-800 SIC-2112-800 SIC-2114-800
1200 grit SiC paper plain backed SIC-2108-1200 SIC-2110-1200 SIC-2112-1200 SIC-2114-1200


PACE Technologies metallographic Abrasive Grinding consumables
Alumina Abrasive Paper (plain backed)

ALO Abrasive Paper (Plain backed) - 100/pkg
Description 8-inch
Diameter
10-inch
Diameter
12-inch
Diameter
P120-grit Alumina (Plain-backed) ALO-2108-P120 ALO-2110-P120 ALO-2112-P120
P120-grit Alumina (Plain-backed) ALO-2108-P220 ALO-2110-P220 ALO-2112-P220
P120-grit Alumina (Plain-backed) ALO-2108-P500 ALO-2110-P500 ALO-2112-P500
P120-grit Alumina (Plain-backed) ALO-2108-P1200 ALO-2110-P1200 ALO-2112-P1200


PACE Technologies metallographic Abrasive Grinding consumables
Double Sided Adhesive (for non PSA paper)

Description Diameter (inches) Quantity Catalog Number
Double Sided Adhesive 8 10/pkg DSA-08
Double Sided Adhesive 10 10/pkg DSA-10
Double Sided Adhesive 12 10/pkg DSA-12
Double Sided Adhesive 14 10/pkg DSA-14


PACE Technologies metallographic Abrasive Grinding consumables
Zirconia Abrasive Papers

Zirconia Coarse Grinding Disks
Description Units/pkg Catalog Number
8-inch 60 grit Zirconia paper (PSA) 25/box ZR-060P08-25
12-inch 60 grit Zirconia paper (PSA) 25/box ZR-060P12-25


PACE Technologies metallographic Abrasive Grinding consumables


Abrasive Belts (4-inch x 36-inch)
Description Units/pkg Catalog Number
60 grit Zirconia belt 5/box ZrO2-B060
120 grit Zirconia belt 5/box ZrO2-B120
120 grit Alumina belt 5/box ALO-B120
60 grit SiC belt 10/box SIC-B60
80 grit SiC belt 10/box SIC-B80
120 grit SiC belt 10/box SIC-B120
180 grit SiC belt 10/box SIC-B180
240 grit SiC belt 10/box SIC-B240
320 grit SiC belt 10/box SIC-B320
400 grit SiC belt 10/box SIC-B400


PACE Technologies metallographic Abrasive Grinding consumables


Abrasive Rolls (3-7/16" width, 60-feet length, 5/8" core)
Description Units/pkg Catalog Number
240 grit SiC roll each SIC-240R-60
320 grit SiC roll each SIC-320R-60
360 grit SiC roll each SIC-360R-60
400 grit SiC roll each SIC-400R-60
600 grit SiC roll each SIC-600R-60
800 grit SiC roll each SIC-800R-60
1200 grit SiC roll each SIC-1200R-60

PACE Technologies metallographic Abrasive Grinding consumables

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email: pace@metallographic.com
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