Particle Shape Analysis of Diamond Abrasives
Finely cut diamond abrasive is utilized in the manufacture of industrial drills, grinding wheels and discs, dental instruments and lapidary equipment. An important aspect for producers of these abrasive products is the removal rate and grit of the materials used. The grinding tools are subsequently implemented in the production of steels, alloys, ceramics, glass, granite and other materials. Most in this industry resort to size-only measurements to control the quality of the abrasive powders used. Unfortunately, this does not capture the true aspects that deal with removal rates such as smoothness. In this case study, two sizes, or grits, of abrasive were examined. A raw image of particles from both samples are presented here.
The first sample analyzed contains very fine sizes. There are also a few spherical bubbles present. They are between about 30 and 40 microns in diameter. Dynamic image analysis is capable of ignoring the bubbles and obtaining 32+ shape measurements for each measured particle.
The second sample has considerably larger grains, with straight-faceted sides. Some of the sides are convex and some concave. Here we can obviously see that assuming that all particles are round, as many size-only instruments do, will not be able to properly differentiate between the two abrasive particle populations.
Dynamic Imaging is required to do a proper assessment of these samples. Dynamic imaging can differentiate particles based on shape and measure the truly important measures for this sample such as Circularity, Smoothness, Aspect Ratio, etc..
Why Diamond Abrasives need Dynamic Imaging for proper analysis.
As with many raw material processes the abrasive powder industry uses control equipment to ensure incoming and outgoing product is consistent and functioning as designed. Particle size instrumentation is a very common tool for this use. Size-only particle statistics are very common but will report size information assuming all particles are round in shape. Because abrasive grains are not spherical, a singular Size measurement is not enough to adequately qualify raw materials. Particle morphology measurements, comprising of Length and Width, as well as Circularity and Smoothness are required to perform a true measurement of these irregular particles. Particle morphology bears strongly on abrasive effectiveness. A polygon morphology characterized by sharp angles is the optimal performer.
For size calculation, the elliptical methodology is another effective model. Although the particles are not elliptical, an ellipse can be fitted, which will yield an effective measurement of Length, Width and Aspect ratio. In this case, the least bounding ellipse (BE) is utilized. In addition, the polygon model is implemented, as most particles possess a polygonal appearance, with straight sides.
The quantity of fitted polygon sides is measured as Polygon order, and the Polygon angles category describes the interior angles of the polygon. Convexity, another available shape measurement, indicates the degree to which a particle embodies a convex morphology, in other words how well it matches the fitted polygon. A particle characterized by indented sides would embody a lower convexity than one which filled the majority of the polygon.
Image analysis settings with the Pi Sentinel PRO comprised minimum pixel area: 5; maximum pixel area: 50000; binary threshold: 80%; focus rejection parameter: 50; pump speed: 40%, and the option to adjust dimensions for diffraction effects was allowed.
Results. Which ones to use and how to use them….
Abrasive Particle Sample #1
Below are statistical histograms for bounding ellipse width and length. Above we stated why an elliptical shape measure would be the optimal Shape measure to use for these abrasive particles. As can be seen by these statistical histograms, there is great uniformity both in the length and the width of these particles. This is shown by how uniform and Gaussian these curves are. A very good measurement to use is the aspect ratio. Elliptical aspect ratio here would be calculated as(BE Width – BE Length).
Convexity as shown below has a slightly broader statistical distribution. Circularity exhibits a spike at 1 as a result of the occurrence of the spheres, but for the most part, these particles are not circular as the statistical histogram shows..
Quantity of sides (Polygon order) averages between 7 and 8. Interior angles (Polygon angles) are distributed around 135 degrees.
Utilizing the post-processing filter, the capacity for reanalysis of the particles, without the spheres, is enabled, by imposing an upper limit of 0.99 on circularity.
The data exhibits only minimal change, since the spheres comprise under 10% of the sample (1430 out of 15114).
Sample 2
In the second sample, the Bounding ellipse width (BE Width) average increased to 28.7 microns, compared to 17.0 in the first sample. Bounding ellipse length (BE Length) likewise grew from 23.9 to 49.4. For these larger abrasives, both Circularity and Smoothness are smaller than the fine particles constituting the first sample.
Polygon order is roughly the same in this instance. Polygon angles exhibit a comparable average, but the histogram now describes a typical skewed normal distribution. A high quantity of the particles in the first sample is too fine to calculate angularity. The larger abrasives possess a higher convexity.
A third sample was generated from the first and second samples for demonstration reasons. Examination of the third mixture reveals the occurrence of the two original materials, in disparate quantities. It is possible to isolate the two components with the use of one of the Particle Insight separation techniques and correlate the results with the initial samples.