Cubic boron nitrides

Cubic boron nitride (c-BN) is a different material altogether with a structure similar to that of diamond characterized by its extremely high hardness (second to diamond) and thermal conductivity as shown in Table 

It is a relatively new material (discovered in 1957 and available commercially since the 1970 s). Little has been published r arding its properties. 

Thermal Conductivity 300-600 W/m- C (theoretical value 1300) Thermal Expansion 4.9 x 10- / C 

Hardness 29.89-43.12 GPa (depending (m crystal orientation) Compressive Strength 2730 MPa Young s Modulus 650 GPa Fracture Toughness 6.4 MPa-m -  

Chemical Resistance essentially inert to all reagents at room temperature 

Cubic BN is less stable than h-BN, but the conversion rate between these forms is negligible at room temperature. c-BN is prepared by annealing h-BN powder at higher temperatures, under pressures above 5GPa. The cubic form has the sphalerite crystal structure (the B and N atoms are tetrahedrally coordinated), as in the diamond structure. Every boron atom is surrounded by four nitrogen atoms, and vice versa in such an arrangement the boron and nitrogen atoms have sp hybridization. c-BN is often also referred to as P-BN or sometimes z-BN (zinc-blende) [140, 141]. 

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Yoo C S, Akella J, Nicol M and Cynn H 1997 Direct elementary synthesis of hexagonal and cubic boron nitrides at high pressures and temperatures Phys. Rev. B 56 140... 

Diamond. Diamond [7782 0-3] is the hardest substance known (see Carbon, diamond, natural). It has a Knoop hardness of 78—80 kN/m (8000—8200 kgf/m ). The next hardest substance is cubic boron nitride with a Knoop value of 46 kN/m, and its inventor, Wentorf, beheves that no manufactured material will ever exceed diamond s hardness (17). In 1987 the world production of natural industrial diamonds (4) was about 110 t (1 g = 5 carats). It should be noted that whereas the United States was the leading consumer of industrial diamonds in 1987 (140 t) only 260 kg of natural industrial diamonds were consumed this is the lowest figure in 48 years (4), illustrating the impact that synthetic diamonds have made on the natural diamond abrasive market. 

Annual production of powdered BN is ca 180—200 metric tons per year and its cost is 50—250/kg, depending on purity and density. The price of cubic boron nitride is similar to that of synthetic diamond bort. Hot-pressed, dense BN parts are 3—10 times more expensive than reaction-sintered parts. 

A wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

HSS = high speed steel cBN = cubic boron nitride. DCL = depth of cut line. 

The greatest use of cubic boron nitride is as an abrasive under the name Bora2on, in the form of small crystals, 1—500 p.m in si2e. Usually these crystals are incorporated in abrasive wheels and used to grind hard ferrous and nickel-based alloys, ranging from high speed steel tools and chilled cast-iron to gas turbine parts. The extreme hardness of the crystals and their resistance to attack by air and hot metal make the wheels very durable, and close tolerances can be maintained on the workpieces. 

Cubic boron nitride (c-BN) is a different material altogether from h-BN, with a structure similar to that of diamond, which is characterized by extremely high hardness (second to diamond) and high thermal conductivity.As such, it is a material of great interest and a potential competitor to diamond, particularly for cutting and grinding applications. Its characteristics and properties are shown in Table 10.3... 

Summary of Characteristics and Properties of Cubic Boron Nitride... 

Cubic boron nitride is obtained from hexagonal boron nitride at high pressure and temperature in the presence of lithium nitride as a catalyst. It is almost as hard as diamond and has superior chemical resistance and a much higher oxidation threshold.Efforts to... 

Ca3(BN2)2 is readily formed when (distilled) calcium metal is melted in the presence of (layer-type) boron nitride. This reaction provides some insight on how alkaline-earth metals like calcium may act as a catalyst in the phase transformation of layered a-BN into its cubic modification. Instead of metals, nowadays alkaline-earth (Ca, Sr, Ba) nitridoborates can be used as a flux catalyst in high-pressure and high-temperature transformation reactions to produce cubic boron nitride [15]. 

Evolution did not use this element, only in certain plants is it important as a trace element. The element became well-known because of heat-resistant borosilicate glasses. Boranes are chemically interesting as B-H bonds react very specifically. Perborates are used in laundry detergents (Persil). The hardness of cubic boron nitride approaches that of diamond. Amorphous (brown) boron burns very quickly and gives off much heat and is therefore used in solid-propellant rockets and in igniters in airbags. 

D. He, Y. Zhao, L. Daemen, J. Qian, and T. D. Shen, Boron Suboxide As Hard as Cubic Boron Nitride, Appl. Phys. Lett., 81, 643 (2002). 

Boron nitride, 4 244. See also Cubic boron nitride... 

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,... 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

Single crystals of cubic boron nitride, with excess boron, have been studied20 by X-band and W-band EPR. Two types of defect were detected called D1 and D2. The D1 centre exhibits local axial < 111 > symmetry and ground spin state S = 1/2, with g = 2.0033 and gL = 2.0094 at T = 10 K. A broad line from D2 was observed only at X-band at high temperature with g = 2.0084. It was attributed to transitions inside the excited levels of another boron related defect. 

Table 3 summarizes the properties of the so-called nonmetallic hard materials, including diamond and the diamondlike carbides B4C, SiC, and Be2C. Also included in this category are comndum, A1203, cubic boron nitride, BN, aluminum nitride, AIN, silicon nitride, S N and silicon boride, SiB6 (12). 

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