Cubic boron nitride abrasive properties

Cubed compound, in PVC siding manufacture, 25 685 Cube lattice, 8 114t Cubic boron nitride, 1 8 4 654 grinding wheels, 1 21 hardness in various scales, l 3t physical properties of, 4 653t Cubic close-packed (CCP) structure, of spinel ferrites, 11 60 Cubic ferrites, 11 55-57 Cubic geometry, for metal coordination numbers, 7 574, 575t. See also Cubic structure Cubic symmetry Cubic silsesquioxanes (CSS), 13 539 Cubic structure, of ferroelectric crystals, 11 94-95, 96 Cubic symmetry, 8 114t Cubitron sol-gel abrasives, 1 7 Cucurbituril inclusion compounds,... 

Silicon carbide is noted for its extreme hardness [182-184], its high abrasive power, high modulus of elasticity (450 GPa), high temperature resistance up to above 1500°C, as well as high resistance to abrasion. The industrial importance of silicon carbide is mainly due to its extreme hardness of 9.5-9.75 on the Mohs scale. Only diamond, cubic boron nitride, and boron carbide are harder. The Knoop microhardness number HK-0.1, that is the hardness measured with a load of 0.1 kp (w0.98N), is 2600 (2000 for aAl203, 3000 for B4C, 4700 for cubic BN, and 7000-8000 for diamond). Silicon carbide is very brittle, and can therefore be crushed comparatively easily in spite of its great hardness. Table 8 summarizes some typical physical properties of the SiC ceramics. 

The major industrial applications of hexagonal boron nitride rely on its high thermal conductivity, excellent dielectric properties, self-lubrication, chemical inertness, nontoxicity, and ease of machining. These are, for instance, mold wash for releasing molds, high-temperature lubricants, insulating filler material in composite materials, as an additive in silicone oils and synthetic resins, as filler for tubular heaters, and in neutron absorbers. On the other hand, the industrial applications of cubic boron nitride rely on its high hardness and are mainly as abrasives. 

The properties of diamond, cubic boron nitride, and conventional hard materials are summarized in Table 9.1. In addition to being the hardest known substance, diamond is chemically inert to essentially aU environments below a temperature of about 500°C and is therefore uniquely qualified for many applications. Diamond has a cubic structure, with each carbon atom bonded to four nearest neighbors. Cleavage normally occurs on one of four (111) planes. In addition to its intrinsic brittleness, diamond has two important limitations. Diamond begins to oxidize and/or graphitize rapidly at temperatures above 600-700°C in air or an oxidizing atmosphere. Diamond readily dissolves in and can be graphitized by ferrous metals such as iron, steels, nickel, and nickel-based superaUoys, and therefore abrasion resistance with these metals is poor. 

The invention of cBN was actually part of an attempt by GE to develop a material harder than diamond. Of course, cubic boron nitride turned out to be the second hardest known material (Table 9.1). However, cBN is considerably superior to diamond in precisely the two areas where diamond s performance is poor its abrasion resistance with ferrous alloys, such as steels, and its oxidation resistance. The former property enables it to effectively grind steels, nickel-based superalloys, and cast iron, whereas diamond works best with non-metals and nonferrous metals. The oxidation resistance is important in high-temperatme applications in air or a similar oxidizing environment. 

---