Polycrystalline Cubic Boron Nitride

As referred to in Section 1.2.7, cubic boron nitride (cBN) was first synthesized in 

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. 

Commercially, pcBN tool materials are produced using sintering additives and/or binding materials at pressures of 4-6 GPa and at temperatures of 1200-1500°C. The sintered pcBN has a higher thermal stability than pcD. 

The first polycrystalline cBN material was sintered by Wentorf and Rocco [160] in 1971 on a cemented carbide substrate using alloys of nickel, cobalt and iron with aluminum as binders. cBN to cBN bonding is believed to be achieved by a liquid phase sintering process involving these binder alloys. 

In February, 1980, Sumitomo from Japan filed the patent Sintered compact for a machining tool and a method of producing the compact [161]. This patent basically covers any compact with 10-80 vol% cBN and a balance of binder material that can comprise any carbides, nitrides, borides, or silicides of metals of groups IVa, Va, or Via. Specifically mentioned are titanium, zirconium, hafnium, vanadium, niobium. 

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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... 

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 (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... 

FIGURE 9.10 Polycrystalline cubic boron nitride (rcHN) blanks and workpiece materials. 

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