As an abrasive family, synthetic diamond and cubic boron nitride are commonly referred to as superabrasives because their abrasive particles rarely wear more rapidly than the bond holding the abrasive in place. Within the diamond family, different species include monocrystalline, polycrystalline, natural and ultra-detonated diamond (UDD).

Because monocrystalline diamond's single crystal morphology and high degree of carbon-to-carbon bonds enable it to hold an edge for long periods of processing time, it is often the superabrasive of choice for applications such as the fluting of high-quality crystal stemware and edge grinding automobile glass.  A polycrystalline diamond is often a good choice for carbide die polishing. The unique microstructure of this species of diamond has many crystallites contained in the particle, and these micro-crystals provide many points of contact at the crystal surface. The multitude of diamond points of angstrom size can produce a mirror-like finish on many carbide surfaces and reduce friction by using less energy to draw either ductile or conducting wire while producing a superior draw on the wire.

Natural diamond is often selected in specialized operations, such as processing ceramic seals or prosthesis polishing, because its cubic orientation can be more beneficial in comparison to synthesized diamond's cubic octahedron structure.   

Ultra-detonated or nano-diamond is currently being tried in a number of operations ranging from texturing glass storage disks to polishing hard alumina and silicon substrates used in the electronics industry. UDD is essentially pure synthesized polycrystalline diamond. Because of its unique micro-structure (spherical) and functional hybrid carbon cover, it has become a popular diamond species when super finishes and purity are required.

Diamond is the world's hardest known material, though it does have some limitations in grinding applications. In conditions where temperatures exceed 600°C, the graphitization effect on diamond weakens the carbon atom bonding, returning the crystal form of diamond to graphite, and eventually, to amorphous carbon. (On the Mohs hardness scale, diamond is 10, graphite is 0.5 and amorphous carbon is 0.) In addition, if diamond is exposed to carbon steels, the resultant chemical reaction between the two forms of carbon lessens the diamond's superabrasive property.

Typical Applications

Various abrasives are available for many different applications. One abrasive type will not fit all, but some abrasives will work in many applications. The proper selection of the correct abrasive will provide for the following:

• Correct stock removal
• Required accuracy of the grind
• Required finish
• Correct relationships between wheel speed, work piece, power requirement and cost
• Desired productivity level

The unique combination of application specifics determines what diamond and wheel type is best suited for a specific job. For example, synthetic diamond is used in fluting for decorative crystal because precise finishes are required for the beauty of the finished part. These finishes can usually only be obtained by a wheel with a hard bond and a non-wearing abrasive like synthetic diamond.

In addition, in automotive window edging, a precise finish is needed on the window edge to maintain a good seal between the door gasket and window edge. The edge requires precision to keep wind, noise, water and dirt out, and it must be aesthetically pleasing. Other parts that are processed with synthetic diamond include hip and ball replacements, dental prosthesis, carbides, and glass.

Tungsten Carbide Tooling

Polished using diamond abrasives, these silicon nitride balls are used in stationary turbines and missile systems.

Additional applications for diamond abrasives include the peripheral grinding of tungsten carbide tool inserts (8-15% WC-Co) and the fluting of tungsten carbide end mills. Resin-bond diamond, a monocrystalline synthesized diamond with non-specific crystallographic morphology, is the abrasive of choice for these applications. The engineered crystal structure contains a metallic catalyst that enables quick removal rates while creating new cutting surfaces at the work piece. This type of diamond is also ideal for lapidary and other low-temperature applications where rapid removal is desired.

Wheel specifications, shapes and bonds are all important considerations. In this application, a standard shape was used with the intention of using automatic feed and coolant. No industry-wide standard exists to specify abrasive content; as a result, the amount of diamond varies greatly (along with the resulting performance). Consistent diamond content should be used to ensure that the diamond is suited to the bond and task. It is recommended that 4.4 carats/cm³ be used as a standard concentration for consistent performance in most superabrasive applications.

It's important to keep in mind that differences in machines, coolant, work piece and wheel geometry affect productivity. High material removal rates can show productivity gains, but these gains are sometimes reduced by increased grinding wheel costs. It is necessary for grinding wheels to wear in order to work effectively; choosing the correct diamond, bond and coolant is essential to control the wear rate. Wheel swarf (chips) from the work piece affect the bond of the wheel, so the proper wheel profile can minimize the swarf cut and prolong wheel life.

In one example, a 30-degree helix was fluted on a 1-in. diameter tungsten carbide end mill using Saint-Gobain Abrasives' T2 vitrified bond, a high-quality, hard glass bond.  A 35-hp Walter machine was used at 3600 rpm, the coolant was filtered TTS Sinto Grind, and the wheel was a Univel DC. The first pass achieved a material removal rate (MRR) of 0.112 in. at 10 inches per minute (IPM); the second pass, 0.06 in. at 10 IPM; and the third, 0.003 in. at 10 IPM. The entire fluting operation time took 178 seconds, and the wheel was dressed after 100 parts. The carbide was removed at a rate of 0.67 cu in./min, and the cycle time was reduced by 40% over a competitive stock abrasive as a result of the Saint-Gobain abrasive's precise tight particle size distribution and good particle morphology.

Superior Abrasives

Diamond abrasives are recommended for processing ceramics, glass or carbides because of diamond's unique hardness, heat transfer and thermal stability properties. Because they are generally of the same hardness as the work piece, traditional abrasives cannot compete with diamond for the required productivity or finishes. In addition, synthetic diamond can be tailored through the manipulation of surface chemistry and particle size to meet a multitude of end users' requirements.

For additional information regarding diamond abrasives, contact Warren/Amplex Superabrasives, 1401 East Lackawanna St., Mid-Valley Industrial Park, Olyphant, PA 18447; (508) 795-5908 or (800) 368-5155; e-mail ron.a.abramshe@saint-gobain.com; or visit www.warrenamplex.com.