New Diamond Options

June 1, 2005
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Manufacturers of next-generation ceramic and glass products can choose from a number of advanced diamond types to optimize material removal rates and surface quality.

Synthetic monocrystalline diamond (SMD) is used to machine and finish a variety of surfaces, including ceramics, optics, semiconductor materials and media storage components. In most cases, SMD is more than adequate to meet the required specifications for surface roughness and removal rate. However, the inherent crystal structure of SMD will eventually limit its use for next-generation finishing.

Recently, abrasive manufacturers have made some morphology refinements to the base crystalline structure of diamond. Microcrystallites, graphitic shells and nanotechnology are significantly improving modern finishing operations and are offering new options for companies seeking to optimize their products.

Figure 1. Phase diagram of carbon showing the shaded region of synthetic diamond production.

Monocrystalline Diamond

To understand the advantages offered by the newest generations of diamond abrasives, it is helpful to review the features of conventional monocrystalline diamond. Monocrystalline diamond is a highly ordered crystalline solid. The carbon atoms are linked together in a regular structure; each atom shares one of its outer shell electrons with four other carbon atoms in an equally spaced tetrahedral environment.

To meet industry volume demands and emulate the production conditions of diamonds found in nature, artificial synthesis requires the use of high temperatures and

pressures-typically around 2000 K and 60 kilobars (kbar), respectively, using modern diamond presses and nickel catalysts. Mechanically, presses differ slightly throughout the world; however, all SMD production falls within the shaded region of the phase diagram shown in Figure 1. SMD production factories often use hundreds of presses since the reaction chamber is quite small; one synthesis might produce fewer than 100 carats to specification per run. The raw diamond must then be crushed, milled, cleaned and graded.1

In finishing operations, SMD provides a good material removal rate (MRR), which increases with increased particle size distribution. However, workpieces finished with SMD are rough in comparison to workpieces finished with more advanced forms of diamond. SMD cutting edges are large in comparison to the total crystal size. They are also sharp and not friable. It is this toughness that enables micron and submicron SMD to retain some of the shape characteristics (such as pyramids and trigons) of the parent crystals after the crushing and milling process. SMD is resistant to acids and hot caustic solution, so the surface can be cleaned to very low impurity levels. Batch-to-batch quality from SMD suppliers is very consistent, and the cost per carat of SMD is the lowest of all synthetic diamond types. (Prices are currently $0.25-0.75 per carat, depending on purity and standard deviation.)

Figure 2. Diamond-workpiece interactions during the lapping or polishing process. a) Synthetic monocrystalline diamond (SMD) slivers that interact perpendicularly with the surface can lead to sub-surface damage from excessive pressures at the point. b) If the direction is parallel to the workpiece, the removal rate will decrease. c) Polycrystalline diamond (SPD) of the same size as monocrystalline diamond will contact the workpiece with "microcrystallites" instead of flat planes or sharp points. The result is an increase in material removal rate compared to SMD. d) Heat-treated monocrystalline diamond (HTSMD) has an excellent removal rate with less scratching, since sharp points are lubricated with a graphite shell. e) Ultra dispersed diamond (UDD) does not contain any large, irregularly shaped pieces and is used for ultrafine finishes.
During the crushing and milling stages of micron diamond production, some elongated pieces with high aspect ratios are inherently produced from cleavage along one of the lower energy crystal planes (see Figure 2a). When the orientation of these elongated structures is perpendicular to the workpiece during finishing, subsurface damage can occur from the excessive pressure at that point. Irregular finishing patterns and lower removal rates can also occur if a plate glides horizontally across the workpiece (see Figure 2b).

Reducing excessive high-aspect-ratio pieces in a SMD feed batch presents a considerable challenge to diamond suppliers because these structures are hydrodynamically equivalent to smaller, more regularly shaped pieces. During elutriation, elongated pieces gather momentum from the added frictional component and carry over into the graded batch. This phenomenon is similar to dropping two pieces of paper, one crumpled and one flat, from shoulder height to the ground. The crumpled paper will hit the ground faster since the frictional force of air will cause the flat paper to float. Elongated pieces can be separated in milling if the energy selected is above the cracking energy of the thin pieces and below the cleaving energy of regular-shaped pieces. The smaller fragments can then be removed in the grading process. However, extra-tight distributions add significant time and cost to SMD powders.

Figure 3. TEM of polycrystalline diamond showing the microcrystallite structure of the parent piece. The bar represents 100 nm.

Polycrystalline Diamond

Unlike SMD, synthetic polycrystalline diamond (SPD) is produced through explosive shock synthesis. Several tons of explosives are used to generate about 250 kbar (equivalent to about 3 million psi) of pressure on the graphite feed. Each SPD piece contains smaller diamond "microcrystallites." The microcrystallites are oriented in different crystallographic directions every 10-50 nm, regardless of the particle size distribution of the parent pieces. The hardness of an individual microcrystallite is comparable to that of SMD. While the difference between SPD and SMD is discernable through scanning electron microscopy (SEM), a transmission electron microscope (TEM) is required to visualize the microcrystal structure (see Figure 3).

The MRR of SPD is up to 10 times higher than SMD because SPD has more cutting edges and a higher surface area (see Figure 2c). Multiple edges and corners are in contact with the workpiece simultaneously, thus reducing the probability of subsurface damage from excessive pressures. SPD also costs an average of 10 times more than SMD ($2.10-8.00 per carat). However, the upfront costs can be easily recovered in the finishing operation because less diamond is required to achieve equivalent or better results, and less labor is needed due to the shorter material removal times required.

Compared to SMD, SPD has greater source variability. Slight variations in explosion energy and geometry can produce a transitional-type polycrystalline diamond with microcrystallites oriented in one direction. Milling variability can also affect the edge sharpness. Less aggressively milled SPD produces a sharper crystal for higher removal rate, while more aggressive milling dulls edges for a smoother finish. Most end users finish with a single source of polycrystalline diamond to eliminate structure-source variability.

Figure 4. Color comparison between SMD (white) and HTSMD (black).

Heat-Treated Synthetic Monocrystalline Diamond

Heat-treated synthetic monocrystalline diamond (HTSMD) is manufactured by heating well graded and cleaned SMD in an inert atmosphere to about 1200°C. Under these conditions, the powder darkens from light gray to black (see Figure 4), which represents a reorganization of the surface into a more disordered, graphite-like layer. The particle size distribution of HTSMD is often tighter than SMD and SPD, since unwanted pieces on the fine end tend to fuse into larger clusters or disappear completely in the heat-treating process.

The graphite sheets provide a lubricating region between a sharp diamond edge and the workpiece (see Figure 2d). This shell coating lowers the scratch and defect rate compared to SMD. As noted previously, elongated pieces in SMD are very difficult to remove completely from a batch since they are hydrodynamically equivalent to well-shaped pieces. However, since the entire HTSMD batch is engulfed in a high-temperature oven during processing, all elongated pieces that would otherwise scratch or damage a surface are shaped and lubricated. The MRR of HTSMD is comparable to SPD, but one distinct advantage of HTSMD is reduced diamond embedding in the workpiece. Machining operators have indicated that workpieces have up to 33% less embedding when using HTSMD, compared to SMD and SPD.

Great care must be taken when preparing dispersions of HTSMD. The heat treating process decreases the surface polarity, resulting in a tighter stability region. In fact, the heat-treated diamond must undergo a proprietary surfactant soak to add polarity to the surface; otherwise, the material will settle clear in an aqueous environment in a matter of minutes. The cost of HTSMD is about 10% less than comparable SPD, but the benefits, once again, can more than compensate for the price in the right application.

Figure 5. Size comparison of Bermuda beach sand with ultra dispersed diamond (UDD). One carat of UDD has as many pieces as the surface of a beach 550 miles long.

Ultra Dispersed Diamond

In recent years, researchers have discovered that the functional utility of diamond increases with decreasing particle size. Nanodiamond (defined as diamond having an average diameter of 0.100 micron or less) has more pieces per carat than traditional micron diamond used in finishing (see Table 1). In fact, just one carat (200 mg) of 100 nanometer (nm) diamond abrasive contains slightly over 100 trillion pieces. Since the number of pieces per carat is proportional to the cube of the radius, the quantity of 3-micron diamond equivalent to the number of pieces in 100 nm diamond jumps to 100,000 carats, or roughly 44 lbs. The equivalent mass of 30-micron diamond, a size typically used in cutting wheels, is 22 tons.

The newest type of nanodiamond, ultra dispersed diamond (UDD), contains about 100,000 trillion primary particles per carat. If we compare this number to 300-micron Bermuda beach sand at 4000 pieces per 200 mg, the equivalent mass now becomes 22 million tons (see Figure 5)-enough to fill a 50-yard beach 1 ft deep with dry-packed sand 550 miles long, or roughly 14 times around the entire Bermuda perimeter.

Figure 6. TEM of UDD. The size of the primary particles ranges from 3-7 nanometers. The bar represents 20 nm.
The diameter of the pieces generally falls within a 2- to 10-nm range, with the predominant size being 3-7 nm (see Figure 6). This incredibly small size range has led to some nomenclature inconsistencies. UDD is often referred to as "nanodiamond;" however, nanodiamond is a much broader term that can also be used to identify nanosized SMD, SPD and HTSMD types. UDD is a particular class of nanodiamond.

Another term related to UDD that is gaining popularity is "cluster diamond." This description is unique to UDD, as long as the terminology reflects a clustered nanodiamond and not a nanocluster diamond. The reason for the distinction is that UDD is composed of primary and secondary particles. The primary particle is the smallest identifiable unit of UDD. The surface energy of such small pieces prevents complete dispersion of UDD into a suspension or powder of primary particles. Average secondary cluster sizes used in the finishing industry currently range from 100-800 nm.

UDD is produced in the diamond stability region at temperatures above 3000 K and pressures above 100 kbar. However, one critical process difference distinguishes UDD from other diamond types-the feed, explosives and UDD product are contained within the explosion chamber. Both the chamber and explosives must be oxygen-deficient (<6%) to reduce unwanted byproducts and the conversion of diamond back to the more thermodynamically stable graphite. Unlike other diamond types, UDD contains many surface-functional groups, such as methyl, nitrile, COH, COOH, CO and C6H6, and it is composed primarily of carbon (87%), oxygen (10%), nitrogen (2%) and hydrogen (1%). The surface groups reduce UDD density to about 2.8, although the core retains the hardness of monocrystalline diamond.

The characteristic advantage of UDD particles is that the edges are not subject to the attrition variability inherent in all other diamond types. Unlike micron SMD, which must be milled from parent pieces, UDD particles are produced directly in the explosion process and do not contain any rogue elongated pieces. As a result, finishes obtained from UDD are relatively scratch-free. Initial results indicate that the MRR is relatively low, but this is expected when finishing to angstrom or sub-angstrom roughness with nanoparticles. Since UDD is relatively new on the open market, performance data compared to other diamond types are still being collected. However, UDD is expected to be the most tested diamond type in 2005, so the data gap will quickly close.

Selecting the Right Type

All diamond types offer advantages and challenges, and a fine line often exists between optimum performance and cost effectiveness. Diamond performance can also be controlled with the slurry chemistry; often a slight change in dispersion or slurry viscosity can reduce the scratch rate and/or increase MRR. Whether a company is considering process improvements or new state-of-the-art finishing applications, it is best to gauge the performance comparison by assuming that all other conditions-including machine, work pressures and slurry-are constant. The performance summary in Table 2 can be used as a guide when selecting diamond for current or next-generation finishing processes.

About the Author

Nicholas Tumavitch, Ph.D., has been the manufacturing and development chemist at Warren/Amplex Superabrasives in Olyphant, Pa., for the past eight years. He received his undergraduate degree in chemistry from the University of Scranton and earned a doctorate in physical chemistry, specializing in interfacial thermodynamics, from the State University of New York at Buffalo. He can be reached by e-mail at nicholas.j.tumavitch@saint-gobain.com.

For more information, contact Warren/Amplex Superabrasives, 1401 East Lackawanna St., Mid-Valley Industrial Park, Olyphant, PA 18447; (800) 368-5155 or (570) 383-3261; fax (570) 383-3218; e-mail info@warrendiamond.com; or visit http://www.warrenamplex.com.

Reference

1. Abramshe, Ron, "Grading Technique, Analysis and Standardization of Micron Materials for Application in Computers, Optics, Lasers, Medical Devices and Other Applications Where Exact Tolerances Are Required," Grinding and Abrasives Magazine, January 2004, pp. 10-14.

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