Cutting tool material - Diamond
Of all known materials, the hardest substance is diamond. It has low friction, high wear resistance, and the ability to maintain a sharp cutting edge. Diamond is used when good surface finish and dimensional accuracy are required, particularly with soft nonferrous alloys and abrasive nonmetallic materials. Single-crystal diamond of various carats is used for special applications, such as machining copper-front precision optical mirrors for the Strategic Defense Initiative program.
Because diamond is brittle, tool shape and sharpness are important. Low rake angles (large included angles) are generally used to provide a strong cutting edge. Special attention should be given to proper mounting and crystal orientation in order to obtain optimum tool life. Wear may occur through microchipping (caused by thermal stresses and oxidation) and through transformation to carbon (caused by the heat generated during cutting).
Single-crystal diamond tools have been largely replaced by polycrystalline-diamond (PCD) tools, which are also used as wire-drawing dies for fine wire. These materials consist of very small synthetic crystals, fused by a high-pressure, high-temperature process to a thickness of about 0.5 to 1 mm and bonded to a carbide substrate; the result is similar to CBN tools. The random orientation of the diamond crystals prevents the propagation of cracks through the structure, significantly improving its toughness.
Diamond tools can be used satisfactorily at almost any speed, but are most suitable for light, uninterrupted finishing cuts. In order to minimize tool fracture, the single-crystal diamond must be resharpened as soon as it becomes dull. Because of its strong chemical affinity, diamond is not recommended for machining plain-carbon steels or titanium, nickel, and cobalt-based alloys. Diamond is also used as an abrasive in grinding and polishing operations and as coatings.
