Polycrystalline boron carbide

The ceramic, polycrystalline siHcon carbide [409-21-2], SiC, is processed using P-siHcon carbide and boron (9). The boron is a sintering aid used at... 

Several fiber types have been mentioned so far, and several other types have been neglected that have been worked on over the past few years. Some of those not discussed may become important fibers for reinforcement in the years ahead. To date though, they have not been available in sufficient quantity for thorough evaluation in composite specimens. Included in this group are boron carbide, spinel, polycrystalline alumina and silica, titanium diboride, and miscellaneous silicides and intermetallics. Ten years from now as we look back on the 70s we no doubt will have an entirely different view of some of these materials. 

Polycrystalline diamond has been used for aluminum-matrix composites reinforced with particulate silicon carbide, boron carbide, or alumina. It also shows promise as a tool material for welding titanium, although this work is only in a preliminary stage. 

Polycrystalline boron nitride films with a structure similar to ihombo-hedral boron carbide and a ratio of boron to nitrc en of 3 1 were produced by hot-filament CVD. This work indicates the possible existence of odier boron nitride structures. ... 

ESK (Elektroschmelzwerk Kempten GmbH) (1995) Method of making polycrystalline dense shaped bodies based on boron carbide by pressureless sintering. European Patent EP 19940109275. 

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... 

This includes single crystal silicon [15], germanium [22] and alumina [10] fibers. Polycrystalline fibers can grow either by a VLS or a VS phase transformation when the incident laser power (focal temperature) is intermediate, and supports the growth of a fiber with a semisolid tip. This includes polycrystalline silicon [15], boron [5] and silicon carbide fibers [23]. Amorphous fibers are obtained by a VS phase transformation when the incident laser (focal temperature) is low, and supports the growth of a fiber with a hot but solid tip. This includes amorphous silicon [15], boron [12], carbon [13] [16], silicon carbide [23], and silicon nitride [17] fibers. 

Boron is a well-known sintering aid for silicon carbide, but temperatures of at least 2000 C are required for sintering polycrystalline SiC powders with a grain size of the order of 1 pm. Its effectiveness in sintering SiC based fibers at lower temperatures (1600-1800 C) is probably related to the extremely small SiC grain size of PCS based materials. 

P-BN tools work satisfactorily in hardened steel up to contact temperatures of 1000°C, since there is no chemical reaction between boron nitride and iron. This, however, also depends on the binding phase of the polycrystalline materials and can lead to adhesive wear [24, 25]. In hard steel, the main wear mechanism on the tool is abrasion by hard alloy carbide particles [26]. In the case of Co-based super alloy (Vitallium), the results on hard-BN tool wear are somewhat incongruous [27, 28], while Inconel 718 can be machined under proper selection of the cutting conditions [29]. Apparently, austenitic steels containing a high percentage of Co are difficult to cut by hard-BN tools, due to the formation of cobalt nitrides which leads to high tool wear [8]. 

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... 

Materials that are classified as fibers are either polycrystalline or amorphous and have small diameters fibrous materials are generally either polymers or ceramics (e.g., the polymer aramids, glass, carbon, boron, aluminum oxide, and silicon carbide). Table 16.4 also presents some data on a few materials that are used in fiber form. 

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