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In order to cut, grind and polish softer materials, abrasives are usually used. It is a natural or synthetic mineral, which can be widely used for shaping or finishing workpieces according to predetermined specifications.

Abrasive materials usually come from mines or synthetic production. Naturally mined abrasives include corundum, diamond, emery, garnet, and quartz. Synthetic abrasives include alumina, boron nitride, boron carbide, silicon carbide and synthetic diamond.

Generally speaking, abrasive materials for industrial applications are synthetic rather than natural to ensure the supply and consistency of their performance. Diamonds occur naturally, but they are usually produced industrially. Similarly, corundum is naturally occurring, but today it is more made from bauxite.

Because of the ability to shape the surface through friction and friction, abrasives are used in a wide range of household, industrial and technical applications, resulting in great changes in the particle size and shape of the abrasive.

Abrasives are used in many scenarios, from sandpaper used by carpenters to make wood smoother to abrasive polishes used to make expensive shiny finishes for cars.

Even the abrasive used in toothpaste for cleaning and polishing teeth must be coarse enough to polish but not corrode over a period of time: more sensitive teeth require less wear.

All grinding powders are produced to remove material. When choosing a grinding powder for a specific application, you need to consider the specific characteristics required.

There are many factors that can significantly affect the behavior of abrasive materials, including particle size, shape, hardness, and self-sharpening ability. The hardness is determined by the values ​​of the Mohs, Vickers or Knoop scales. These values ​​will determine which material can scratch another material during the measurement.

The abrasive must be harder than the material being processed. Diamond is considered the hardest material with a value of 10, which is the maximum value of Mohs' hardness. At this level, most minerals are rated as Grade 7 or higher.

When abrasives are used for grinding wheels, self-sharpening elements are the key.

Table 1. Mohs hardness values ​​of several common abrasives. Source: Bettersize Instruments Ltd

Figure 1. Self-sharpening characteristics of abrasives. Image source: Bettersize Instruments Ltd

This is because when the abrasive grains eventually become dull after grinding, the grinding force increases. This increase in grinding force will cause the abrasive grains to break. At this time, new angular abrasive grains come out to restore the grinding material to its original shape. Its sharpness.

Generally, the primary abrasive material provides the best resistance to wear points, but will split before significant passivation occurs, thereby maintaining cutting and finishing requirements.

Particle size is one of the most critical parameters related to the performance of abrasive products. The abrasive is sorted or crushed into specific sizes, usually between 10 microns and 2 mm, depending on the application.

Historically, screens have been carefully manufactured from metal wires of precise size and number per square inch, and have been used to measure abrasives. In each sieve, the number of sand particles represents the number of openings per linear inch to classify the particle size.

However, they only measure the second longest dimension, which is suitable for the polishing stage when the particles are close to spherical, but not for ideal abrasive irregularities.

Choosing the best material with the right size grade will enable users to obtain professional results in finishing and manufacturing, whether it is applied using metal, wood or plastic.

The following table shows the standard granularity level at each stage of the application process. The grade is inversely proportional to the granularity. Therefore, fine abrasives constitute a higher grade.

Although there is no one type of abrasive that can be used in all applications, as shown in Table 2, there are many grades and types of abrasives to choose from to meet application standards. In addition, there are different standards at hand to classify the size range of abrasives.

Table 2. Typical abrasive particle size grades for each application stage. Source: Bettersize Instruments Ltd

One of the most commonly used standards is the FEPA classification system, which is the European particle size standard. There are several other classification systems in the world, including the coated abrasive system called CAMI and F220 from the United States-JIS, the American system ANSI and the Japanese standard. All FEPA particle sizes begin with the letter "F", for example, F220.

Traditionally, techniques using sieves and precipitation have been applied to characterize the size of abrasive particles. Abrasives are divided into two categories: coarse abrasive grains based on screen sizes F4 to F220; and fine sand grains (F230 to F2000) that depend on settlement.

In addition to particle size, particle shape is another key performance factor for abrasive materials.

In the past, abrasive powders were finally used in different applications after being analyzed by a particle size measurement system only to maintain control of the powder quality.

In the case where the particles are assumed to be round, the size only system will display the size. They do not convey shape information, but only provide spherical volume equivalents that are only suitable for low aspect ratio powders.

This is not an appropriate application where the shape of the particle is directly related to its performance. Figures 2 and 3 show SEM images of zirconia (a common abrasive) with sharp edges.

Figures 2 and 3. SEM images of common abrasive (zirconia) with sharp edges. Image source: Bettersize Instruments Ltd

The shape of the abrasive can vary, and it can be spherical, blocky, semi-circular or angular. Angular abrasives can ensure the fastest cleaning speed when removing tightly adhered materials or contaminants from the substrate due to their sharp edges.

These sharp edges of hard, angular abrasives create steep peaks and valleys in the anchor profile, thereby expanding the surface area and providing an excellent finish for the mechanical bonding of the coating.

These sharp edges can only be seen in SEM images and dynamic image analysis (DIA), but not through sieving, sedimentation or traditional laser diffraction analysis.

If these particles are smooth and round, then the removal effect is not enough.

Round particles without cutting edges are usually used to strike the surface, while particles with sharp edges remove surface material by reducing the surface area in contact with the workpiece and then increasing the local contact pressure.

Sieve analysis is a conventional measurement method for abrasives, which is suppressed due to its ability to measure very small sizes and is prone to shape deviations. Currently, laser diffraction is becoming more and more popular as a measurement technique.

This is because it can cover almost all abrasive particle size ranges, especially in the particle size range. Bettersizer S3 Plus can measure the precise particle size of fine sand and coarse sand.

In addition, due to the combination of laser diffraction and dynamic image analysis technologies, the size range of the Bettersizer S3 Plus measurement system is 0.01 to 3500 μm. Figure 4 shows the laser diffraction and dynamic image analysis technology built on the Bettersizer S3 Plus.

Figure 4 Schematic diagram of Bettersizer S3 Plus combination technology. Image source: Bettersize Instruments Ltd

Bettersizer S3 Plus is not only a laser diffraction particle size analyzer, but also because it is equipped with two high-speed CCD cameras (0.5X and 10X magnification), it has the ability to capture images of the measured sample.

During the whole measurement process, the particles dispersed in the medium are fed through two measurement sampling units. In the first sampling cell, a short-wave laser (532 nm) is pumped through the sample and dispersed according to particle size, which helps to measure very small particles as low as 10 nm.

In this process, the CCD camera takes pictures through the second sampling unit to provide particle image analysis in the range of 2 to 3500 µm, easily covering the macro-particle size range.

The two technologies of laser diffraction and dynamic image analysis for particle size distribution measurement have different characteristics and advantages.

Laser diffraction has obvious advantages in measuring widely distributed samples and small particles. As the particle size increases above 1 mm, accurate quantitative measurements of these large particles may become questionable.

When the number of large particles is found to be trace or small, there will not be enough particles to provide reliable statistical results. In addition to measuring all particles as equivalent spheres, this is the only other shortcoming of laser diffraction technology.

This is where the Bettersizer S3 Plus comes into play, because it does not rely on laser diffraction to record a small number of large particles, and, most importantly, it measures shapes of various sizes through dynamic image analysis.

Dynamic image analysis can calculate the shape value, and can clearly see and measure the parameters of each particle.

For large particles, in addition to size and morphology, the absolute number of particles can also be provided, so the test accuracy when measuring large particles is extremely high.

Due to the limitation of the resolution of the CCD camera and the existing capacity, the accuracy of dynamic image analysis decreases as the particle size decreases.

Therefore, Bettersizer S3 Plus innovatively combines the advantages of the two technologies to measure the entire size range as small as 10 nm micrometers, and measure shapes as small as 2 μm without any compromise.

By combining the two methods into a system that covers a wide range of dimensions and measurement shape determination, the overall advantage of improving measurement accuracy can be achieved, which is becoming more and more important.

The following example demonstrates the use of laser diffraction measurement and parallel dynamic image analysis to characterize the study of abrasives.

Bettersizer S3 Plus measured three corundum abrasive powder samples-fine, medium and coarse-to characterize the size and shape of the particles. It has been demonstrated how the size of these powders changes and how their shape parameters contrast.

As shown in Figure 5, after the characterization of three different abrasive powder samples, the particle size results are arranged in the correct expected order.

Figure 5. Particle size cumulative volume curve of fine, medium, and coarse corundum powder using combined technology. Image source: Bettersize Instruments Ltd

It can be inferred that the coarser particles will produce a greater impact force, and therefore will quickly remove the surface of the material, resulting in a heavier texture.

Interestingly, although the sizes of the three types of grinding powders are different, their circularity is quite uniform, thus verifying the manufacturer's good control over the grinding process of these grinding powders.

In addition, because the dynamic image analysis technology is based on counting, Bettersizer S3 Plus can easily identify individual particles that are too large or too spherical, or in fact, by observing all the size and shape of all particles, the number of parameters generated is based on distribution and many other parameters.

Figure 6 shows in detail a camera image of a large amount of coarse powder taken by the Bettersizer S3 Plus. The number above the image is the specific diameter selected by the user for each particle (Feret length or Feret width, etc.).

Figure 6. Camera image extracted from a single particle list of coarse abrasive powder (approximately 500 to 200 μm). Image source: Bettersize Instruments Ltd

The particle size distribution is important, but a single particle size measurement is not appropriate because the abrasive particles are not spherical. As shown in Table 4, the length and width are as necessary as the circularity.

Table 3. Key parameters that characterize the abrasive powder in Figure 5. Source: Bettersize Instruments Ltd

Table 4. Critical shape parameters of some large particles contained in coarse abrasive powders. Source: Bettersize Instruments Ltd

One of the main shape parameters produced by Bettersizer S3 Plus is roundness. The more spherical the particles are, the closer their circularity is to 1. The more elongated the particles, the lower their circularity.

All selected shape parameters can provide data related to abrasive morphology. The measurement of Bettersizer S3 Plus allows more accurate characterization of different particle shapes and identification of shape differences between other equivalent products.

Finally, for demonstration purposes, a new sample was generated from the mixture of fine, medium, and coarse samples. The particle size distribution of the four samples (including the mixture) is shown in Figure 7.

Figure 7. Comparison of particle size distribution of fine, medium, coarse and mixed abrasive powders. Image source: Bettersize Instruments Ltd

Analysis of the mixture indicated the presence of the three expected raw materials, thus confirming the high-resolution capabilities of the combined technology.

With the novel combination technology, Bettersizer S3 Plus is able to perform detailed statistical analysis on the size and shape of the particles, and allows the user to view any large particles that may exceed the specifications.

The unqualified powder products provided by an abrasive manufacturer were evaluated, and the results are shown in Figure 8. First, use another internationally renowned brand of laser diffractometer to measure the particle size distribution of the abrasive product.

Figure 8. Comparison of particle size distribution of different measurement methods. Image source: Bettersize Instruments Ltd

The results confirmed the presence of small particles of about 1 μm, thus proving that the abrasive was over-grinding.

Next, use the Bettersizer S3 Plus to evaluate the abrasive powder using only the laser diffraction method: the main peak near 14 μm and the small peak at 1 to 2 μm are shown in Figure 8.

Due to the characteristics of the double lens and oblique incidence optical system (DLOIOS), the laser diffraction method of Bettersizer S3 Plus is extremely sensitive to the detection of small particles.

Finally, the combined technology of Bettersizer S3 Plus (laser diffraction and dynamic image analysis) was used to evaluate the abrasive powder. Interestingly, there is an additional small peak next to the main peak at 120 μm, as shown in Figure 8.

The overall results show that there are unwanted ultrafine particles and large particles in this fine powder. In order to verify the existence of super large particles (underground), images of large particles were obtained, which can be seen in Table 5.

Table 5. Images and diameters of oversized particles present in unqualified abrasive products. Source: Bettersize Instruments Ltd

This proves that there are unwanted large particles in this kind of abrasive that does not meet specifications. This confirms that the abrasive powder does not meet the manufacturer's specifications because of the presence of ultra-fine particles and over-coarse particles.

The experiment shows that, with its novel design, Bettersizer S3 Plus has higher accuracy than traditional static light scattering equipment in a rough range, and allows quick identification of unqualified products.

The size alone is not enough to guarantee the consistency of the abrasive, and it is well known throughout the abrasive industry that the shape of the particles is an equally important control parameter.

Since Bettersizer S3 Plus uses a combination of laser diffraction and dynamic image analysis technology, it can measure the size and shape of all abrasive powders.

In the first experiment, key data related to the size and shape of different non-spherical abrasive powders were comprehensively displayed. In the second experiment, the size and shape information that is important for different non-spherical abrasive powders is extensively displayed.

In addition, in the second experiment, due to the wide measurement range and the superior sensitivity of the combined technology, the presence of ultra-fine particles and over-coarse particles in unqualified abrasives was detected.

Bettersizer S3 Plus has proven that it can simultaneously characterize the size and shape of abrasives and provide more information than traditional laser diffraction analyzers.

It can identify a small number of oversized gravel particles (a key factor from a QC point of view), distinguishing it from the capabilities of standard laser-based particle size measurement systems.

The dynamic image analysis technology of Bettersizer S3 Plus can detect a small amount of grit in the presence of high-quality products that cannot be achieved by other systems. This makes it a valuable tool in the 21st century abrasives industry.

This information is derived from materials provided by Bettersize Instruments Ltd. and has been reviewed and adapted.

For more information on this source, please visit Bettersize Instruments Ltd.

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