Account Info ▽

My Profile My Products My Orders My Catalog

My Account ▽

Login Logout Help

Email:

P/w:

Login

Signup

Forgot Password

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

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.



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. Unlike 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)

Successful grinding is also a function of the following parameters:

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

• Soft non-ferrous metals - Initial grinding is recommended with 320 grit SiC abrasive paper followed by 400, 600, 800 and 1200 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 maintaining 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, 600, 800 and 1200 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.

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

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).

• 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

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

• Plain-backed or non-adhesive backing
• PSA or pressure sensitive backing
• Foil or polyester plastic 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.

<a href="https://www.metallographic.com/Metallographic-Consumables/Metallography-SiC-Papers.htm">

Double Sided Adhesive (for non PSA paper)

About Us | Site Map | Web Disclaimer | Terms and Conditions | Privacy Policy | Contact Us Copyright 2006-2024 PACE TECHNOLOGIES® - 3601 E. 34th St., Tucson, AZ 85713, USA Telephone +1-520-882-6598 FAX +1-520-882-6599 All Rights Reserved