High-Performance Pellets

October 2, 2002
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Improved diamond abrasive pellets are providing manufacturers with increased efficiency and quality in cutting and lapping hard materials.

The new diamond-impregnated lapping pellets.
Diamond-impregnated lapping pellets (also known as plugs or buttons) have been successfully used for a number of years to cut and lap glass materials. However, many lapping machine operators have been disappointed with the results obtained when using these pellets on extremely hard materials, such as quartz, tungsten carbide, alumina or sapphire. Conventional diamond pellets were typically supplied as a “one-size-fits-all” solution, with little consideration for the requirements of the application. As a result, they often added significant costs while providing few, if any, benefits. Additionally, some of the pellets used poor-quality diamond powders that scratched the product’s surface or wore down quickly. For these reasons, many manufacturers of hard materials have continued using loose abrasives in their cutting and polishing operations.

Recently, however, new diamond pellets have been developed that incorporate higher quality diamond crystals and stronger bond formulations in carefully balanced compositions. With the availability of these custom-engineered, high-performance diamond pellets, manufacturers of hard materials can significantly increase their cutting and lapping efficiency and quality.

High-quality diamond crystals are essential when cutting or lapping hard materials. Photo courtesy of Lands Superabrasives Co.

Improving the Diamond Composition

High-quality, uniform diamond crystals are essential when cutting or lapping hard materials—especially when using diamond-impregnated lapping pellets. A lower-quality diamond powder contaminated with undesirable shapes (slivers, shales, windows or splinters) and a wide range of sizes can either scratch the work piece or break down inconsistently, diminishing the life of the powder and making frequent redressing or replacement necessary.

In the past, high-quality industrial diamond powders weren’t widely available to pellet suppliers, and the quality of the pellets suffered as a result. However, within the past several years, a proprietary production milling system was developed* to increase the quality of industrial diamond crystals. The process breaks off all extruding parts of the crystal and leaves a very solid core. The result is a premium diamond powder composed of uniform, block-shaped crystals with strong, defect-free edges. The powder is graded to severe standards, ensuring narrow size distribution and no oversize particles.

The quality of the bond, or coating, has also been significantly improved. Early diamond-impregnated pellets used a metal bond that worked well for cutting and lapping glass but failed in harder materials applications, causing the crystals to be pulled out of the coating. As a result, the pellets dulled rapidly. The improved bond formulations use a new metal-to-diamond adhesion process to help ensure that the diamond powders remain in place for a much longer period of time, even when cutting or lapping extremely hard materials, such as sapphire.

Developing New Pellets

To improve the performance of diamond-impregnated pellets, researchers evaluated many different diamond and bond compositions on a variety of materials and made extensive measurements in five different areas: the surface roughness (Ra), the material removal rate (mrr), the pressure required for operation, the amount of loading or dulling incurred over a period of time, and the lifetime or grinding ratio (G-ratio, defined as the volume of product removed divided by the volume of pellet lost) of the pellets. They found that these five characteristics are frequently interrelated. For example, increasing the operating pressure typically increased the mrr. Pellet compositions engineered to have a larger mrr also showed a higher Ra, while pellets designed to have a larger G-ratio generally exhibited lower mrr, a lower Ra and a shorter loading time. Most changes in composition to enhance one characteristic degraded another. However, researchers discovered that they could improve the overall performance of the pellets by using the newer high-quality diamond powders with high-strength bonds, and by carefully adjusting the composition and concentration of the separate components.

The test data also showed that the mrr generally declined over time, with the decline becoming progressively worse after just a few minutes of lapping. This decline could be quantified by a decay constant (D), which was unique to each combination of pellet and lapped material. Frequently, the mrr approached zero as the lapping process continued, at which point the pellets had to be redressed to restore the original mrr. When the lapping process was resumed, the decay of the mrr was repeated. Again, researchers discovered that by using the newer high-quality diamond powders with high-strength bonds, and by carefully adjusting the composition and concentration of the separate components, they could slow the speed of decay.

The high-performance fixed abrasives developed through these tests range from very aggressive lapping to fine cutting. Using the refined pellets, glass and quartz can be lapped up to 80 microns per minute on existing lapping machines—significantly faster than with conventional pellets or loose abrasives. Additionally, while the pellets have been successfully tested in pressures as high as 45 psi, they also work well with the lower pressure constraints often found in glass and ceramic lapping. Unlike conventional pellets, which required pressures of 4-6 psi and higher, the new pellets can operate at 1-3 psi—and even lower, in some cases. For example, glass lapping pellets are available that operate successfully down to 3⁄4 psi.

Figure 1. Test results on fused quartz (mrr = microns/min; Ra = micro-in.).
The new pellets also produced satisfactory results on hard materials. Figure 1 shows the test results on fused quartz using a test plate with new pellets engineered specifically for quartz lapping. In this instance, quartz straight from a high-quality saw was lapped in preparation for polishing. The saw left an Ra of 150 µ-in, and the final Ra after lapping with the medium grade quartz pellet test plate was 9 to 10 µ-in. A total of 0.020 in. of material was removed in this run. In this example, several hours of lapping with loose abrasives of three different grits was replaced with pellet lapping for only a few minutes. Because of the excellent Ra created by the pellets, the subsequent polish time was also reduced from about two hours to about 45 minutes. In addition, the abrasive waste (swarf) was reduced from several pounds to a few ounces.

Figure 2. Test results on c-axis sapphire (mrr = microns/min; Ra = micro-in.).
Figure 2 shows the results obtained from a new medium-grade sapphire pellet on c-axis sapphire. In this test, three 2-in. wafers were run with 33 lbs of dead weight at 80 rpm on a 12-in. SpeedFam lapping machine. The average pressure (always measured on the surface of the pellet) was 5.4 psi, and the test plate contained 202 pellets. The resulting population density (the area of the pellet to the area of the plate) was 64%. As can be seen from Figure 2, the average mrr was about 3.5 microns per minute, and the final Ra after 35 minutes of lapping was 13 µ-in. This operation to remove 0.005 in. of material replaced a 12-hour lap with loose abrasives. The same pellets exhibited similar results on hard alumina.

In yet another test, a more aggressive sapphire pellet yielded an mrr of 7 microns per minute with an Ra of 20+ on c-axis sapphire and much faster lapping on a-axis sapphire and alumina.

Efficient Lapping

Considerable progress has been made in the last few months in pellet performance on hard materials. With the availability of improved diamond powders and stronger bonds, pellets have been developed that work well with glass, quartz, silicon, ceramics, sapphire (including c-axis) and many metals. Advantages of the new pellets in these applications include:
  • A lapped surface with a low Ra that will polish quickly
  • A high material removal rate (mrr)
  • Operation at low pressure, resulting in reduced subsurface damage
  • Minimal loading or dulling over an extended period of time (at least several runs)
  • A high G-ratio (anywhere from 50 to 1000, depending on the pellet composition and product being lapped)
Pellets are also under development for extremely hard ceramic materials, such as silicon carbide and tungsten carbide.

With today’s high-performance fixed abrasives, ceramic manufacturers can reduce their polishing and lapping times and increase their manufacturing efficiency.

For More Information

For more information about diamond-impregnated lapping pellets, contact Gator Diamond, Inc., 1270 Belle Ave., Suite 106, Winter Springs, FL 32708; (407) 696-7450; fax 407-696-7465; e-mail sales@gatordiamond.com; or visit http://www.gatordiamond.com.

For more information about loose abrasives and other diamond abrasive products, contact Lands Superabrasives Co., a division of Lieber & Solow Ltd., 1140 Avenue of the Americas, New York, NY 10036; (212) 354-4060 or (888) 335-7600; fax (212) 354-4622; or visit http://www.landssuperabrasives.com.

*The proprietary milling and bond systems were developed by Lands Superabrasives Co., New York, N.Y. Lands Superabrasives produces high-quality industrial diamond powders for both fixed and loose abrasive applications.

SIDEBAR: Maximizing Pellet Life

While the availability of the new diamond-impregnated lapping pellets provides manufacturers with new options for lapping and cutting, the pellets must be carefully selected to match both the material and process to ensure success. For instance, “self-dressing” pellets are available, in which the rate of decay (D) has been reduced to zero. However, in most cases, zero-D pellets can make the G-ratio too low, causing the pellets to wear out too quickly.

It is also important that the right coolant and dressing techniques be used. If the diamond points are not flushed with the proper coolant at the right intervals, they will quickly become dull. The pellet supplier should be able to recommend the proper coolant and dressing technique for the pellets.

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