Metal boron fibers

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. 

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... 

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. 

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. 

Crystalline boron is a strong, hard, high-melting substance (mp 2075°C) that is chemically inert at room temperature, except for reaction with fluorine. These properties make boron fibers a desirable component in high-strength composite materials used in making sports equipment and military aircraft (see Section 21.8). Unlike Al, Ga, In, and Tl, which are metallic conductors, boron is a semiconductor. 

Inorganics Oxides (glass, MgO, Si02, A1203) Hydroxides (Al(OH)3) Salts (CaC03, BaS04, CaS04, phosphates) Silicates (talc, mica, kaolin, woolastonite) Metals (boron, steel fibers)... 

It is assumed that the tube is made of a composite material which is composed of stiff duetile fibers arranged in a parallel uniform array in a ductile matrix. The eomposites of metal-metal type are eonsidered, first of all. However, the combination of a ductile matrix with brittle fibers can be easily accounted for, as well. As an example of the latter ease serves the aluminium alloy reinforced with boron fibers. 

Boron fibers are produced by chemical vapor deposition (CVD) onto a substrate filament (e.g. tungsten or carbon) and thereby consist of two components. They exhibit both metallic and nonmetallic properties, which is to be expected for pure boron due to its position in the periodic table. 

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 are used to reinforce different epoxy or light-metal matrices. To hinder interactions with metals, they should be protected by boron carbide deposits obtained by chemical vapor deposition. Boron carbide fibers can be prepared directly by reaction of boron obtained by the reduction of BCI3 on carbon fibers. ... 

A reinforcing fiber with high strength and modulus with 2.7 density. Primary purpose for this development was for the reinforcement of metal matrix and ceramic matrix composite structures used in advanced aerospace applications by the military. SiC fibers were developed to replace boron fibers in these RPs, where boron had its drawbacks principally degradation of mechanical properties at temperatures greater than 540C (lOOOF) and very high cost. 

The use of metal-reinforced fiber composite systems appears to offer an alternative to the typical boron- or graphite-filled systems for cryogenic application. 

The moduli of elasticity of fiber-reinforced plastics can exceed those of metals. Thus, the modulus of an epoxide resin reinforced with 60% boron fiber is higher than that of steel, and that with 30% boron fiber is about the same as that of titanium, whereas that with 30% glass fiber is higher than that of aluminum (Figure 35-16). Since the densities of fiber-reinforced epoxides... 

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