Tungsten fiber

Boltzmann s constant, and T is tempeiatuie in kelvin. In general, the creep resistance of metal is improved by the incorporation of ceramic reinforcements. The steady-state creep rate as a function of appHed stress for silver matrix and tungsten fiber—silver matrix composites at 600°C is an example (Fig. 18) (52). The modeling of creep behavior of MMCs is compHcated because in the temperature regime where the metal matrix may be creeping, the ceramic reinforcement is likely to be deforming elastically. 

Fig. 18. Steady-state creep rate as a function of appHed stress for silver matrix (0) and tungsten fiber—silver matrix composites (A) at 600°C. To convert...

Finally, metallic fibers find some limited applications as reinforcement in composites. They are generally not desirable due to their inherently high densities and because they present difficulties in coupling to the matrix. Nonetheless, tungsten fibers are used in metal-matrix composites, as are steel fibers in cement composites. There is increasing interest in shape memory alloy filaments, such as Ti-Ni (Nitanol) for use in piezoelectric composites. We will discuss shape-memory alloys and nonstructural composites in later chapters of the text. 

An important application field for stainless steel fibers is the textile sector, in which 0., i to 6% of these fibers are incorporated to endow carpets, protective clothing etc. with an antistatic finish. A further application is protection against electromagnetic pulses, interference and charging. Tungsten fibers with a diameter of 12 pm are used for boron or SiC deposition and as light bulb filaments. Furthermore, metal fibers are used in the filtration of polymer melts and corrosive liquids, as well as for electrodes with high surface areas. 

Tungsten fiber composites can be produced either by infiltration of the fiber bundle by the molten matrix, or by bringing together the fibers and matrix in the solid state and... 

Ion propulsion jet (porous W). Emitters of electrostatic ion propulsion systems. Construction parts in space nuclear reactors (W and tungsten fiber reinforced Nb matrix alloys). 

Diamond /tungsten fiber endless solid one exper. vapor phase 2.4.6... 

Figure 8 contrasts a boron/tungsten fiber with a pure boron fiber. Aside from the >10x difference in diameter, the differences in surface texture are noteworthy. The surface of the pure boron fiber made by high pressure LCVD is smooth. Its strength is 7.5 GPa and its modulus is 400 GPa. In contrast, the surface texture of the boron/tungsten fiber is "nubby". Its strength is 3.6 GPa and its modulus is 400 GPa. In summary, the tensile strength of boron fibers is related to their surface uniformity. 

Figure 7. Schematic diagram of the CVD boron/tungsten fiber process. Redrawn from M. L. Dorf, Product bulletin, Textron Specialty Materials. Lowell, MA.

Figure 8. Boron fibers made by hot filament and by laser assisted CVD. This illustration compares the fiber diameter and surface character of a sheath/core boron/tungsten fiber (A,C) with that of a pure boron fiber (B,D). Reproduced from F. T. Wallenberger and P. C. Nordine, Strong, Small Diameter Boron Fibers by Laser Assisted Chemical Vapor Deposition, Materials Letters, 14 [4] 198-202 (1992). With permission from Elsevier Publishers (1992).

Table V. Polished and unpolished boron/tungsten fibers [after 35]...

Commercial boron/tungsten fibers are, in practical terms, limited to fiber diameters of 100-140 pirn, and strength levels up to 4.8 GPa. Pure boron fibers can be made with diameters of >6 Ijm and a strength levels 7.6 GPa, i.e., with 1.6x the maximum strength at 0.06-0.04x the diameter of the former. High specific properties (strength or modulus divided by density) are... 

Boron/tungsten fiber applications include the use of filaments and of boron/tungsten fiber reinforced prepreg tape, aluminum matrix composites, and boron/graphite structures. The major applications for these structures are found in the aerospace market and about 25% in sporting goods markets [36]. SiC/carbon fiber reinforced products include aluminum, titanium, and ceramic matrix composites. Major applications for these structures are also found in the aerospace market, minor uses in the industrial market [37]. 

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