Fiber boron

Boron fibers possess good mechanical properties at low densities, which accounts for their use in composites for lightweight structures. Commercially obtainable boron fibers exhibit an elasticity modulus at room temperature of 400 Gpa, a tensile strength of 3-4 GPa and a thermal expansion coefficient from room temperature to 327°C of 4.9 10 /K. The maximum use temperature is 367°C, the elasticity modulus having dropped to 240 GPa at 627°C. 

Boron fibers exhibit good chemical compatibility with polymer matrices, hence their utili/.ation in composites therewith. At the production temperature of 577 - 677°C, boron reacts with the surface of metal matrices to form 

Boron fibers have at their core a tungsten (ca. 12 to 15 pm in diameter) or carbon fiber, which serves as a substrate during manufacture. Due to the high density of tungsten (19.3 Mg/m- ), a fiber thickness of 100 to 200 pm is necessary to achieve a low overall density for the fiber (ca. 2.6 Mg/m ). Therefore, latterly deposition on carbon fibers (density 1.8 Mg/m, diameter 8 to 10 pm) has been favored. This development has been driven by their commercial availability of carbon fibers. In addition to their low density (ca. 2.0 to 2.3 Mg/m- ) these fibers exhibit a low surface roughness and low internal stress. 

Whereas other high performance fibers have exhibited annual growth rates of 15 - 25% per year in the period 1987 to 1992, boron fibers have not yet achieved major economic importance. The worldwide production decreased from 40 t in 1987 to ca. 20 t in 1996. The price of 1 kg boron prepreg has increased from 1300 DM/kg in 1979 to 3200 DM in 1995. The worldwide capacity is estimated to be less than 50 t/a. 

Attempts to produce boron fibers in a pure state have been frustrated by the brittleness of the material, which has hindered the manufacture of continuous filaments. Currently boron fibers are produced by chemical vapor deposition (CVD) in which boron is deposited by reducing BCI3 in a hydrogen-containing atmosphere at 1127 to 1177°C, according to the overall reaction  

Properties SiC (SCS-6) monotilament SiC (Niealon) liber Boron (B W) fiber  

The other process is the transformation of an organic precursor into a continuous thin ceramic fiber. In the spinning process, polycarbosilane, a high molecular weight polymer containing Si and C, is obtained by thermal decomposition and polymerization of polydimethylsilane. The fiber thus produced consists of a mixture of P-SiC, carbon crystallite and SiO. The presence of carbon crystallite suppresses the growth of SiC crystals. Yajima and coworkers (Yajima et al., 1976, 1978, 1979) were the first to produce fine (10-30 pm in diameter), continuous and flexible fibers, which are commercialized with the trade name of Nicalon (Nippon Carbon Co.). 

SiC monofilaments produced by the CVD process is generally superior to Nicalon SiC fibers in mechanical properties because of its almost 100% 6-SiC purity while Nicalon is a mixture of SiC, Si02 and free carbon. Representative properties of SiC monofilaments and Nicalon fibers are given in Table 5.15. 

There are four types of SCS fibers depending on the thickness of the final SiC coating designed for different metal matrices. They are the standard SCS, SCS-2, SCS-6 and SCS-8. Fig. 5.30 illustrates schematically the cross sections of two commercially produced SiC fibers, the standard SCS and SCS-6 fibers, according to DiCarlo (1988). Both types of fibers consist of a carbon core of 37 pm in diameter, a SiC sheath of varying thickness and a carbon-rich surface coating of 0-4 pm in 

Additional surface modifications on vapor deposited SiC fibers, including WC. TaC, TiN, B4C, Al, Ni and Fe, have been applied with varying degree of success (Wawner and Nutt, 1980 DeBolt, 1982 Wawner, 1988). After exhaustive trial and error, TiB is selected as an additional coating material to further prevent the diffusion-induced reactions between the SCS-6 fibers and matrix materials, including Ti alloys and Ti Al intermetallic alloys (e.g. Ti Al, TiAl and TiAl ) (Donncllan and Frazier, 1991 James et al., 1991). When the coated fiber is subjected to tensile 

As previously mentioned, the tungsten wire that acts as substrate, remains entrapped and ends in the fiber. Therefore due to the dense preexisting wire of tungsten, the boron fibers produced by CVD exhibit  

Unidirectional laminate longitudinal (0 ) properties Tensile strength Tensile modulus of Ksi(MPa) 218(1502) 208(1433) 197.7 (1362) 141.7 (976) 

RC = resin content by weight VC = void content by volume 

Graphite-polyimide RC=35% MC=0q6 Graphite-polyimide RC=27.5-31% vc=o% Graphite-polysulfone RC= 33-34% VC 0-1.9%  

Material composition Phenolic (33%), carbon fabric (67%) Phenolic (31%), graphite fabric (69%) Epoxy- novoloc (37%), carbon fabric (63%) Epoxy novoloc(35%) graphite fabric (65%) 

All properties are parallel to the fabric warp unless otherwise noted. 

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Electronic-Grade MMCs. Metal-matrix composites can be tailored to have optimal thermal and physical properties to meet requirements of electronic packaging systems, eg, cotes, substrates, carriers, and housings. A controUed thermal expansion space tmss, ie, one having a high precision dimensional tolerance in space environment, was developed from a carbon fiber (pitch-based)/Al composite. Continuous boron fiber-reinforced aluminum composites made by diffusion bonding have been used as heat sinks in chip carrier multilayer boards. 

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. 

Because of this continued emphasis on adhesive bonding technology development over the years, the airframes of modem front-line aircraft such as the B-2 bomber and the F-117 and F-22 fighters are largely structurally bonded advanced composites. They tend to be comprised of materials that are more advanced (expensive) than commercial aircraft such as carbon and boron fiber reinforcements with cyanate esters, bismaleimides, polyimides or other high-temperature resin matrices and adhesives. 

Boron fibers exhibit the highest stiffness and strength efficiencies in Figure 1-24. When placed in a lamina as unidirectional fibers, the... 

Fibers in which the basic chemical units have been formed by chemical synthesis, followed by fiber formation, are called synthetic fibers. Examples include nylon, carbon, boron fibers, organic fibers, ceramic fibers, and metallic fibers. Among all commercially available fibers, Kevlar fibers exhibit high strength and modulus. (Kevlar is a DuPont trademark for poly [p-phenylene diamine terephthalamide].) It is an aromatic polyamide (aramid) in which at least 85% of the... 

Fiber-reinforced plastics have been widely accepted as materials for structural and nonstructural applications in recent years. The main reasons for interest in FRPs for structural applications are their high specific modulus and strength of the reinforcing fibers. Glass, carbon, Kevlar, and boron fibers are commonly used for reinforcement. However, these are very expensive and, therefore, their use is limited to aerospace applications. 

CVD boron fibers which are extremely stiffand strong and are used as reinforcement in structural components in aerospace designs. 

Carlsson, J., and Lundstrom, T., Mechanical Properties and Surface Defects of Boron Fibers Prepared in a Closed CVD System, / Mater. ScL, 14(4) 966-974 (1979)... 

CVD is used in the industrial production of inorganic structural fibers such as boron and silicon carbide. Boron fibers are, in... 

Properties. CVD boron fibers have high strength, high modulus, and low density. Their properties are summarized and compared with SiC fibers and other inorganic fibers in Table 19.2 (data supplied by the manufacturers). 

Applications. Boron fibers are used as unidirectional reinforcement for epoxy composites in the form of preimpregnated tape. The material is used extensively, mostly in fixed and rotary wing military aircrafts for horizontal and vertical stabilizers, mdders, longerons, wing doublers, and rotors. They are also used in sporting goods. Another application is as reinforcement for metal matrix composites, in the form of an array of fibers pressed between metal foils, the metal being aluminum in most applications. 

CVD silicon carbide fibers are a recent development with prom-ising potential which may take over some of the applications of CVD boron fibers or other refractory fibers, providing that the production cost can be reduced. 

Properties. Properties of SiC fibers are shown in Table 19.2. They are similar to those of CVD boron fibers except that SiC is more refractory and less reactive than boron. CVD-SiC fibers retain much of their mechanical properties when exposed to high temperature in air up to 800°C for as long as one hour as shown in Fig. 19.3. [ 1 SiC reacts with some metals such as titanium in which case a diffusion barrier is applied to the fiber (see Sec. 2.5 below). 

Other than in polymer matrix composites, the chemical reaction between elements of constituents takes place in different ways. Reaction occurs to form a new compound(s) at the interface region in MMCs, particularly those manufactured by a molten metal infiltration process. Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled. Depending on the composite constituents, the atoms of the fiber surface diffuse through the reaction site, (for example, in the boron fiber-titanium matrix system, this causes a significant volume contraction due to void formation in the center of the fiber or at the fiber-compound interface (Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product. Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites. 

Fig. 5.2y, Surface of boron fiber on tungsten substrate showing a corn-cob" structure with noilulcs.

Carlsson, J.O. (1986). Boron fibers. In Encyclopedia of Materials Science and Engineering (M.B. Bever. ed.). Pergamon Press, Oxford, pp. 402-464,... 

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