Boron cubic polycrystalline

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... 

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. 

M. Collier, R. Steel, T. Nelson, C. Sorensen, and S. Packer, Grade Development of Polycrystalline Cubic Boron Nitride for Friction Stir Processing of Ferrous Alloys, Proceedings of the Fourth International Conference on Friction Stir Welding, May 14-16, 2003 (Park City, UT), TWI, paper on CD... 

Until recently, there were no tool materials that would stand up to the high stresses and temperatures necessary for FSW of materials with higher melting points, such as steels, stainless steels, and nickel-base alloys. In 1998, tungsten alloys and polycrystalline cubic boron nitride (PCBN) were developed to create FSW tools for use in steel, stainless steel, titanium alloys, and nickel-base alloys. Properties of the resultant welds have been shown to be outstanding. Although some issues remain (primarily limited tool life with tungsten-base tools), FSW has been demonstrated as a technically and eco-... 

Polycrystalline cubic boron nitride tools produce an exceptionally smooth surface on the weld. This is thought to be due to the low coefficient of friction between PCBN and the weld metal. 

Fig. 5.1 Pin features produced on polycrystalline cubic boron nitride friction stir processing tools, including (a) flats, (b) helical threads, and (c) a combination of convex scrolled shoulder and helical threaded pin...

Table 6.1 Results of preliminary friction stir welding testing with polycrystalline cubic boron nitride tools...

Feng et al. (Ref 35), conducted a preliminary study to investigate the feasibility of FSSW of AHSS sheet metal. The objective was to weld 600 MPa (87 ksi) dual-phase steel and 1310 MPa (190 ksi) martensitic steel. A single tool, made of polycrystalline cubic boron nitride, survived over 100 welding trials without noticeable degradation and wear. The tool had a tapered pin,... 

Fig. 1 3.28 Photos of 6 mm (0.25 in.) tapered with flats (bottom left), 6 mm (0.25 in.) stepped-spiral (top left), and 1 2 mm (0.500 in.) stepped-spiral high-temperature polycrystalline cubic boron nitride friction stir weld pin tools. Courtesy of Mega...

Polycrystalline cubic boron nitride is preferable over single crystal cBN because it can be manufactured in bigger sizes and has higher fracture toughness. However, like its counterpart pcD, it can only be sintered at very high pressures and temperatures with the aid of binders because of its strong covalent bonds. 

Tools made from polycrystalline p-BN (and y-BN) [63, 64] can be machined by laser irradiation [65]. Dressing of hard boron nitride tools can be done with sintered alumina-metal (Fe, Cu) sticks [66], and burnishing cubic boron nitride sinters can be done against a smooth complementary p-BN surface [76]. On the other hand, AI2O3 grinding wheels can be dressed by P-BN sintered tools [68]. 

Polycrystalline cubic boron nitride (CBN) is a material with excellent hot hardness and can be used at very high cutting speeds. It also has good toughness and resistance to thermal shock. CBN consists of boron nitride with ceramic or titanium nitride binder and is brazed onto a cemented carbide carrier to form an insert. CBN grades are largely used for finish turning hardened steel... 

TABLE 9.6 Summary of Properties of Commercial GE Polycrystalline Diamond and Cubic Boron Nitride... 

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