Boron/tungsten fibers

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]. 

Commercial boron/tungsten fibers are derived directly from the vapor phase. Commercial silicate glass fibers and most commercial silica glass fibers are derived from their melts, but some silica fibers, as discussed in Chapter 5, can be derived from viscous aqueous solutions. Glass fibers are therefore derived directly from a liquid phase. Commercial ceramic and carbon fibers are produced from solid precursor, or green, fibers which, in turn, are derived from a melt, dispersion, or viscous solution. This section of the book deals with fibers which are derived from solid precursor fibers. 

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. 

The hot fiber (wire) CVD process has been commercially used for 30 years to produce continuous sheath/core bicomponent boron/tungsten and silicon carbide/carbon fibers. Since they are continuous fibers, they are discussed in Chapter 3.3. More recently, this process was used to produce discontinuous, i.e., short, experimental sheath/core diamond/carbon fibers by depositing a thick diamond sheath on short pieces of a potentially carbon fiber. 

The technology of manufacturing sheath/core bicomponent boron/ tungsten, boron/carbon, silicon carbide/tungsten, and silicon carbide/ carbon fibers is 40 years old and the relationships between process variables, structures, and properties have been authoritatively described in important review articles. One article deals mainly with their preparation [33] another correlates process variables with structures [34], and one explores potential correlations between structures and properties [30]. 

Boron fiber is produced by passing a very thin filament of tungsten through a sealed chamber, during which the element boron is deposited onto the tungsten fiber by the process of chemical vapor deposition. 

Boron filaments are formed by the chemical vapor deposition of boron trichloride on tungsten wire. High performance reinforcing boron fibers are available from 10—20 mm in diameter. These are used mainly in epoxy resins and aluminum and titanium. Commercial uses include golf club shafts, tennis and squash racquets, and fishing rods. The primary use is in the aerospace industry. 

Two fibers are presently produced by CVD on a commercial scale boron and silicon carbide. The production of these two fibers requires a monofilament starter core capable of being heated resistively such as a tungsten or graphite fiber. I l The deposition apparatus is shown schematically in Fig. 19.1. 

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