Boron fibers properties

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. 

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)... 

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). 

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. 

Boron fibers, like any CVD fiber, have inherent residual stresses which originate in the process of chemical vapor deposition. Growth stresses in the nodules of boron, stresses induced due to diffusion of boron into the W core, and stresses generated due to the difference in the coefficient of expansion of the deposited boron and the tungsten boride core, all contribute to the residual stresses, and thus can have a considerable influence on the fiber mechanical properties. 

Table 6.7 summarizes the properties of boron. The high temperature treatment, indicated in this table, improves the fiber properties by putting a permanent axial compressive strain in the sheath. Commercially produced 142 xm... 

Table 6.7 Properties of boron fibers (after Dicarlo, 1985)."...

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 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. 

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. 

Reinforced pol)nners are those to which fibers have been added that increase the physical properties—especially impact resistance and heat deflection temperatures. Glass fibers are the most common additions, but carbon, graphite, aramid, and boron fibers are also used. In a reinforced polymer, the resin matrix is the continuous phase, and the fiber reinforcement is the discontinuous phase. The function of the resin is to bond the fibers together to provide shape and form and to transfer stresses in the structure from the resin to the fiber. Only high-strength fibers with high modulus are used. Because of the increased stiffness resulting from the fiber... 

Table 2.5 Typical Properties of Boron fibers and of other Commercially ...

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... 

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