The term superabrasives was used to describe new options for cutting tools based on a polycrystalline diamond (PCD) material developed by the General Electric (GE) Company in the US in the 1970s. These very hard, brittle compounds were an improvement in the grinding and cutting tools available, though they required new techniques for proper usage. The materials are so hard, only diamond cutting wheels can be used to shape the cutting tools made from these compounds.
For one material to polish another, the first material must be harder than the second. Hardness is a semi-quantitative value and refers to the overall ability of a material to avoid breakage upon the application of force. Hardness is usually associated with ruggedness in use and long-life. The materials used in critical parts have grown harder and more abrasion-resistant as improvements in metal alloys and bimetallic structures have been developed. These parts must be machined into specification to meet the highest tolerances, forming alone is insufficient.
As metal parts became stronger, harder cutting-tool materials were developed, including carbon tool steel, high-speed steel and cast alloys. Ceramic-based materials, as in cemented carbides and cermets, were also employed. The introduction of superabrasives widened the options available to the tool maker immensely.
The GE PCD superabrasives consist of a micron-sized — 3.3 × 10-6 feet or 1 × 10-6 meters — diamond layer bonded to a carbide substrate. The PCD is a synthetic diamond consisting of multiple crystals. During grinding, these crystals break off and leave more sharp edges to continue the polishing. The clean edges of the broken particle clear the grinding surface more readily, avoiding plugging the surface. This self-clearing attribute allows the grinding wheel to be operated at higher speeds, reduces the electrical load, and produces less heat.
The polycrystalline cubic boron nitride (PCBN) is a similar class of superabrasives that substitutes cubic boron crystals for the diamond crystals. PCD is used for non-ferrous applications, such as aluminum and copper, and for organic materials, such as plastics, rubber compounds, and wood. PCBN materials are used for ferrous applications, such as gray cast iron. In both types of superabrasives, the means by which the agent is bonded to the substrate is important to performance as well. A vitrified bond appears to be superior to metal or resin bonds in many applications, exhibiting the superior edge finish of the metal bonded agents and the aggressive cutting of the resin agents.
To use the superabrasives cutting tools, new techniques had to be learned by toolmakers. Formerly successful combinations of feed rates and wheel speeds would not produce quality products. As a result, the grinding profession has developed new machinery and processes to work with the new materials.
PCD and PCBN cutting tools can themselves only be ground by harder materials. Diamond wheels alone are able to grind the superabrasives. The wheels must be carefully specified as to diamond type, crystal size, and friability to efficiently grind these super hard materials.