Abrasive Grains 101 | Characterization of Abrasives

Abrasives are most commonly used to remove part of the surface of some material, called the substrate or workpiece. This removal is called abrading. The abrading occurs by rubbing the abrasive under some pressure against the surface to be abraded. To effectively abrade, the abrasive must be harder than the material being abraded. The rate at which the surface is removed, and the smoothness of the abraded surface, depend upon a variety of characteristics of the abrasive and the substrate. Of these, the most important is the size of the abrasive particles. When we speak of characterization of abrasives, we frequently mean describing the particle size distribution. If everything else is equal, larger particles abrade more rapidly, and leave a rougher surface, than smaller particles. Thus, it is important for the user to know approximately the size of the abrasive they are using. Abrasives with relatively large particles are called coarse; those with smaller particles are called fine. These are, of course, relative terms. It is more correct to call one abrasive coarser or finer than another.

All abrasives contain particles with a range of sizes. In general, the more uniform in size the abrasive, the more expensive and difficult it is to manufacture. Sizing or grading refers to making the particle sizes within an abrasive more uniform so that the majority of the particles fall within a given range of sizes.

There are three ways this can be done: The abrasive particles themselves can be made smaller until they are all the same very small size; the abrasive particles can be joined together to make larger particles of a desired size; or the particles can be sorted into different sizes. Most abrasive manufacturing uses some combination of these methods to obtain particles of the desired size. However, there is no such thing as an abrasive in which are all the particles are the same size, down to the number of molecules for example in each particle.

Luckily, there are few if any applications which require all the particles to be the same size. A few very demanding applications require all the particles to be so close in size it is difficult to distinguish one particle from another, even with very sophisticated equipment. Less demanding applications may find no difference in performance between an abrasive with a very narrow range of sizes and one with a much broader range.

We describe the range of particle sizes within an abrasive by means of a particle size distribution. Thus, we may say that all of the particles are larger than ten microns in diameter, half of the particles are larger (and half smaller) than 30 microns in diameter, and none of the particles are larger than 60 microns in diameter. But the user of the abrasive may want to know if, having used this abrasive, they later use an abrasive in which all of the particles are larger than 11 microns, half are larger than 29 microns, and none are larger than 58 microns, they can expect it to perform the same.

To help answer this question, within the various types of abrasives discussed above, various sizes or grades of abrasive are available. These sizes are standardized within the abrasives industry. For example, while the particle size distribution of individual batches of ANSI 100 grade will vary, they will all meet a set of particle size distribution criteria. There will be few if any particles over 212 microns in diameter, no more than 20% of the abrasive will be made up of particles over 150 microns in diameter, etc. Standard sizes covered by various national and international standards are shown in Exhibits 2 through 7.

Many individual abrasive manufacturers have also developed their own sets of size ranges; in general, the designations used have some connection to the size of the abrasive particles. The individual manufacturers can generally relate these sizes to those covered by one of the national or international standards.

Of course, in addition to size, other characteristics of the abrasive, such as the bulk density, capillarity, pH, friability, surface area, and free iron and other chemical content may be crucial to appropriate performance in various applications. Development and updating standardized ways to measure such characteristics is the main focus of the Standards Committee. These standards, supported by the UAMA, provide the basis for manufacturers to supply globally. For a list of grain characterization standards supported by the Standards Committee, see Appendix 2.

When we talk about the size of an individual particle, what do we actually mean? Consider the particle shown in Exhibit 1. (The top left view and right view are the same at different levels of magnification.) Viewed from the "top", it looks like the particle on the right. Viewed on edge, it looks like the particle at the lower left. And viewed on edge from one end, it might look similar to the particle at the upper left. If we define the size of a particle as the mean diameter of the largest surface, this particle would be the diameter of the larger circle shown. If we define it as the average diameter of all its sides, it would be considerably smaller. If we define it based on its volume, it would be another value still.

Abrasive sizes are broadly broken into two groups, macrogrits (also called "screen sizes") and microgrits (also called "sedimentation sizes"). This division is due to the different methods of size measurement traditionally used. (Some modern methods of particle size measurement may be used on either type of material, as discussed below.)

Suppose we wished to make sure that none of the particles in our ANSI 100 grade abrasive were over 212 microns in diameter. We could look at a large volume under a microscope, checking to make sure there were no oversized particles; or, we could screen it through a sieve with openings 212 microns in size, hoping it all passed through. This latter method, using sieves with known opening sizes to see how much abrasive can pass through under given conditions, has long been the industry standard. These measurements are performed with specially produced and controlled test sieves.

Test sieves are woven wire or electroformed screens or perforated metal pans that are used for testing and sifting. Of these, woven wire sieves are most commonly used for testing materials to ensure they meet a designated particle size distribution. Woven wire test sieves are constructed by placing wire cloth between two suppressed die formed frames. Stainless steel or brass is generally used in the construction of both the frame and woven wire mesh that performs the sieving. These devices are widely used in various types of laboratory particle size analysis.

Test sieves are manufactured to standardized requirements; the specific standard used depends on where it is manufactured, and the type of sieve. In the United States, ASTM E11 covers the requirements for design and construction of woven wire cloth test sieves. European sieves are manufactured to ISO Standard 3310-1. Electroformed sieves are manufactured in the United States to ASTM E161, while perforated plate sieves are manufactured to ASTM E323, or British Standard BS140-1. All of these standards specify a number of properties to which any rated test sieves must adhere. These ratings include acceptable opening sizes, opening dimensions, maximum number of allowable openings in each test sieve, and in the case of woven wire sieves, nominal wire diameter.

Sieves are available in a number of quality levels, with the precise nomenclature used varying by manufacturer. Commonly used terms include certified, inspection, matched, calibrated, matched and calibrated, and midpoint. Certified or inspection sieves are the most widely used. They are manufactured to a national or international standard and come with a certificate of conformity. It is also possible to obtain pairs of sieves that have been manufactured and tested to match each other, and sieves with a test certificate which gives the range of tolerances and measurements taken.

Macrogrits (sizes 4 to 220 or 240, also called screen sizes or sieve sizes) are traditionally measured using test sieves. The particles in these sizes range from less than 45 microns to up to 8mm (8000 microns). A range of particles is allowed to be present in a given size, with a maximum coarse limit and a minimum fine percentage. (For most sizes, no more than 3% of the abrasive by weight is allowed to be finer than the fine limit.) To determine the particle size distribution of a material, a stack of sieves with known openings is prepared, with the sieve with the biggest openings on top, the smallest on the bottom. A known weight of the material to be tested is placed on the top sieve, and the stack is shaken or tapped to sift the material through the sieves. (The devices most commonly used to tap or shake the sieves during testing are the Rotap and the CAMI sifter.) Particles too large to pass through a sieve are retained on top of it. After a given time, the stack is disassembled and the material retained on each sieve is removed and weighed. So, if the top sieve on the stack has an opening of 200 microns, and all the material has passed through it, we know that the material contains no particles larger than 200 microns. If the next sieve has an opening of 170 microns, and 10% of the material is retained on it, we know that 10% of the material is from 170 to 200 microns in size. Thus, with the appropriate sieves, we can obtain a complete measurement of the distribution of particle sizes within it.

The problem of how to correctly calibrate a sieve lies in the high levels of precision required, combined with the relatively imprecise nature of the sieves themselves. The processes by which the sieves are made give rise to variations in both the average opening size and the range of sizes in a given sieve. For example, according to ISO 3310-1, the maximum permitted opening on a 63 micron sieve is 89 microns. While it is rare to find such discrepancies in practice, should they occur there could be serious consequences. It is therefore critical that the effective opening size of a sieve is known, and sieves should be calibrated on a regular basis.
Since it is not possible to adjust how a test sieve measures, calibration is a verification procedure based on sieving performance on a known material compared to a set of master sieves of known accuracy. The known material used is a standard sand (see Appendix 3). Standard sands are manufactured so that a set percentage (generally around 50%, but specified in the documentation accompanying the sand) is retained on a sieve of a given size, when measured under specified conditions (described in ANSI B.74-12-2001; see Appendix 2.) To be used for grading abrasives, the sieve must retain the required percentage, plus or minus a small margin of error, which may then be taken into account when evaluating materials.

Standard sizes have been developed both for the sieves and for the abrasives they measure. For the test sieves, these are given in the various standards mentioned above. For the abrasives, they are given in several of the standards listed in Appendix 2. These sizes are discussed in more detail below, and in Exhibit 1.

Test sieves have long been the single most important tool for grading abrasives and many other dry materials. This is not to say they are without their drawbacks. Uniform size testing of abrasives by Rotap has long been a problem for the industry. This is due to variability in the manufacture of test sieves and changes in the specifications of wire sieving cloth, as was discussed above. It was these problems that led to the development and implementation of standard sands for sieve calibration. As of now, test sieves remain the only standardized method for measuring macrogrit distribution. The standards committee is actively investigating alternative methods, including laser diffraction analysis and ,photosedimentation and image analysis (see below). However, no standards have been developed yet for macrogrits using these methods.

Microgrits (also called sedimentation sizes) are defined as sizes corresponding to 240 or 280 (approximately 60 microns in size) and finer. For many years, the standard method of measuring these sizes was through sedimentation using Stokes' Law. In lay terms, Stokes Law says that the bigger the particle, the faster it settles in a liquid. If you know the apparent specific gravity of the material, and the density and viscosity of the liquid, and the distance it settles, and the time it takes to settle, you can calculate how big the particle is. This is applied in practice through the use of a long column filled with alcohol (called the settling medium) at a known temperature, sitting inside a larger tube filled with water to maintain the alcohol at the correct temperature. At the bottom of the tube is a smaller graduated collecting tube. This apparatus is called a sedimentometer, or sedimentation tube. The material to be tested is pre-wet, then placed in the settling medium at the top of the tube, and the time recorded. When the first material reaches the collecting tube, the time is recorded. As the material reaches the various graduations in the settling tube, these times are recorded, until all the material has settled. Based on the total height of material in the tube, say 25mm, we know that the time required for the material to reach 12 mm represents 48% of the cumulative volume percentage. If it reached this height in 8 minutes, that means 48% of the material is 28.2 microns and coarser in size. If the 2mm height was reached in 4 minutes, that means 8% of the material is 39.8 microns and coarser. (These figures are taken from material in ANSI Standard B74.10, for aluminum oxide. Times for materials with a different density, such as silicon carbide, are different.)

Obviously, this is a very time-consuming method. Very fine materials may take 24 hours or more to settle completely. Only one test per sedimentation tube can be run at a time. During the test, the operator must constantly tap the settling tube to insure even packing and level settling. For these reasons, this method has largely been replaced over the past 30 years by other methods of measurement. However, it is still in use in some companies, and the standard governing its use remains in effect.

Beginning in the 1970's, some abrasives companies began using electrical resistance to measure microgrits. The principle of electrical resistance measurement is that a particle will cause a change in the strength of a current proportional to the volume of the particle. The standard apparatus used for electrical resistance measurement is the Coulter Counter, which has gone through a variety of model numbers over the years. (A competitive product using the same principle is the Elzone.) For ease of reference, electrical resistance methods will be referred to simply as "Coulter", recognizing that Coulter Corporation makes a variety of test and analytical equipment and that other companies manufacture electrical resistance measurement equipment.

All "Coulter" instruments have the same basic design. (A diagram is provided in ANSI B74.10-2001.) A reservoir is filled with filtered electrolyte solution (generally water with 1%-4% salt content). A sample beaker is filled with the same solution in which the material to be tested is suspended. The solution is pumped into a hollow glass tube (called the aperture tube) with a small hole of known diameter (the aperture) in the end. The tube is immersed in the sample beaker. Inside the tube is an electrode, and outside the tube, in the sample beaker, is another electrode. During analysis, a vacuum pump pulls solution from the sample beaker through the aperture into the tube and eventually to a waste container. A current is passed between the electrodes. As the solution passes through the aperture, any particles present cause changes in the current proportional to the volume of the particle. The instrument records these changes as representative of particles of various sizes, based on a calibration value for the aperture tube in question. (The calibration value is obtained by measuring latex spheres of known size.)

The instrument is meant to be run with a variety of sizes of aperture tube, based on the particle sizes to be measured. This is because each aperture tube can measure only a small range of particles, nominally ranging in size from 2% to 60% of the aperture diameter. Thus, a tube with a 100 micron diameter aperture is capable of recording particles from 2 to 60 microns. While there is a method of running samples with a wider distribution, it is complicated and time-consuming.

The sizes registered also present a problem. The Coulter cannot take particle shape into account, and registers only particle volume. To translate this into diameter, the instrument assumes all particles are spheres, and gives a particle's diameter as the diameter of a sphere of the same volume (spherical equivalency). For blocky or cubic material, this represents a small distortion. For sharp or platelet material, the distortion is larger. In general, non-spherical material will be measured as finer on the Coulter than it is by sedimentation method. The degree of the offset will be roughly proportionate to the aspect ratio of the material. For example, a calcined alumina with an aspect ratio of 5:1, which has an average particle size of 12 microns on a sedimentometer, will have an average size of approximately 7 microns on a Coulter.

This method is widely used in Europe, less so in the United States for abrasives. It relies on Stokes Law (as sedimentation does) with the particles centifuged in a liquid and the size determined by the time it takes particles to move from the center of a disk to photodetectors on the outside. Used for microgrits down to and including sub-micron particles.

Varying methods use microscopy with detectors to measure particle sizes on a sample. The principle is the same as looking at particles in a microscope and sizing each to determine the distribution of the sample. This method is not in wide use yet for particle size distribution measurements of abrasives. It does have the potential to measure both macrogrits and microgrits simultaneously across a wide range of particle sizes.

Gaining in popularity, this method passes the particles in front of an optical detector on which a laser bean is projected. The particles diffract (bend) the laser in an amount proportionate to their size. Beginning to be more widely used by abrasives manufacturers, but not yet subject to any standards in the United States. The size results tend to lie between sedimentation and electrical resistance measurements for microgrits. A very versatile method, newer instruments are also capable of measuring most macrogrits. However, because of the wide size ranges involved, poorer discrimination of very narrow size ranges is provided compared to electrical resistance. Multiple companies manufacture these instruments, making potential standardization problematic.

Used primarily for fine powders, the particles are suspended in an aerosol beam. The analyzer measures the time of movement of the particles between two points of known distance apart.

As the preceding survey indicates, the variety of methods of particle size analysis available, not to mention possible variations within each method, make standardizing of methods and sizes an on-going challenge. ASTM E-1919-00 covers particle characterization methods, particularly particle sizing.

Regardless of the size of the abrasive being measured, the results obtained will depend not only on the general method being used but also on the particular instrument chosen. In some cases, very good agreement can be had between instruments using the same principle of measurement (such as two instruments using laser diffraction) IF all other parameters which can affect the measurement are kept as identical as possible. In other cases, wide variation in results can be obtained even by using the same method and instrument. For example, if two different Rotaps using two sets of test sieves are used to measure the same material, differences as large as five percentage points in the amount retained on an individual sieve are common. This is due to variability between the Rotaps, variation between the sieves, and sample to sample variation. If a non-Rotap sieve shaker is used, even greater differences are likely.

In the case of other types of measurement, where variations in sample preparation, instrument settings, ancillary equipment, and operator technique can have a substantial affect on the results, the differences can be substantial, raising the issue of whose results to trust. When two totally different methods are used (for example laser diffraction by a supplier and electrical resistance a customer), the results of course cannot be compared at all.

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