Fibers nonoxide

FIGURE 1-6 Single filament of boron nitride-coated Nippon Carbon Nicalon non-oxide ceramic fiber. Nonoxide fibers discussed in this report include polycrystalline SiC fibers and multiphase (amorphous or crystalline) fibers consisting of B,C,N,Ti, or Si. Current manufacturers include Bayer, Dow Corning, Nippon Carbon, Textron, Tonen, and Ube. Source Dow Coming Corporation. 

Stress-cracking resistance is substantially enhanced in the presence of reinforcement such as glass fibers. Nonoxidizing. 

Fibrous materials may be naturally occurring or synthetically manufactured by thermal or chemical processes (Fig. 1) (see Fibers, survey). Refractory fibers are generally used in industrial appHcations at temperatures between 1000°C and 2800°C. These fibers may be oxides or nonoxides, vitreous or polycrystalline, and may be produced as whiskers, continuous filaments, or loose wool products. 

Fiber chemistry determines whether the material is an oxide or nonoxide and can also influence its vitreous or polycrystalline physical form. Refractory fibers generally have diameters ranging from submicrometer to 10 )J.m, and lengths, as manufactured, may range from millimeters to continuous filaments. 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

Refractory fibers are most often used in applications above 1000°C. Table 1 shows the maximum long-term use temperatures in both oxidising and nonoxidizing atmospheres. For short exposures, however, some of these fibers can be used with Htde degradation at temperatures within 100°C of their melting points. 

Table 2. Mechanical Properties of Oxide and Nonoxide Fibers...

The growth and commercializa tion of the nonoxide fiber market parallels the high strength composite industry. If prices for nonoxide fibers with lengths of 2—10 cm reach the 10—20/kg range, a large potential market should develop. 

Oxide and nonoxide refractory fibers have become essential materials for use in modem high temperature industrial processes and advanced commercial appHcations. Future process improvements, cost reductions, and performance enhancements are expected to expand the uses and markets for these specialized fibrous materials. 

In the last 10 years, significant advances in fibrous monolithic ceramics have been achieved. A variety of materials in the form of either oxide or nonoxide ceramic for cell and cell boundary have been investigated [1], As a result of these efforts, FMs are now commercially available from the ACR company [28], These FMs are fabricated by a coextrusion process. In addition, the green fiber composite can then be wound, woven, or braided into the shape of the desired component. The applications of these FMs involve solid hot gas containment tubes, rocket nozzles, body armor plates, and so forth. Such commercialization of FMs itself proves that these ceramic composites are the most promising structural components at elevated temperatures. 

When examined with a scanning electron microscope, none of the exposed chemically modified celluloses exhibited any significant changes in morphology beyond the appearance of fractured fiber ends and fibrillar cracks. Such damage was also detected in the exposed nonoxidized control. 

SiC is an excellent nonoxide ceramic with high-temperature stability and suitable mechanical properties. Since silicon-containing polymers are generally used for preparing nonoxide ceramics, various polymeric precursors with different structures have been designed. Preceramic polycarbosilane (PCS), used for preparing commercial Nicalon fiber,... 

This analysis demonstrates that oxygen diffusion can occur rapidly through even extremely small cracks in a matrix and will oxidize nonoxide fibers and/or coatings unless oxygen is gettered by the formation of a protective oxide within the crack. 

The last quarter of the twentieth century saw tremendous advances in the processing of continuous, fine diameter ceramic fibers. Figure 6.4 provides a summary of some of the important synthetic ceramic fibers that are available commercially. We have included in Fig. 6.4 two elemental fibers, carbon and boron, while we have excluded the amorphous, silica-based glasses. Two main categories of synthetic ceramic fibers are oxide and nonoxides. A prime example of oxide fibers is alumina while that of nonoxide fibers is silicon carbide. An important subclass of oxide fibers are silica-based glass fibers and we devote a separate chapter to them because of their commercial importance (see chapter 7). There are also some borderline ceramic fibers such as the elemental boron and carbon fibers. Boron fiber is described in this chapter while carbon fiber is described separately, because of its commercial importance, in Chapter 8. 

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