Boron fiber-reinforced composites

In the aircraft industry advanced composites are penetrating into large markets. The first application of advanced composites was in the F14 Fighter in 1971. At that time, boron-fiber reinforced composite was used. Today, the materialsused to make the "Harrier AV-8B" consists of 25 wt advanced composites reinforced by carbon or aramid fibers. For commercial aircraft, advanced composites were applied for the first time in 1977, for the secondary structural parts of the DC-10. Afterward, the proportion of advanced composites used has risen from 3 wt on the Boeing 767 in 1982 to 20 wt of the Air-Bus A320 in 1987. Moreover, the application of advanced composites has extended into the primary structural parts of aircraft. In the future, we expect, the next generation aircraft will consist of more than 30 vjt% of advanced composites. 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

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. 

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. 

Fiber-reinforced composite materials such as boron-epoxy and graphite-epoxy are usually treated as linear elastic materials because the essentially linear elastic fibers provide the majority of the strength and stiffness. Refinement of that approximation requires consideration of some form of plasticity, viscoelasticity, or both (viscoplasticity). Very little work has been done to implement those models or idealizations of composite material behavior in structural applications. 

BFRP boron fiber reinforced polymer composites... 

Samples were cut from 3-mm-thick sheets of the following 15-ply uniaxial fiber-reinforced composites 0.14-mm boron/6061 aluminum S-901 glass/NASA Resin 2 type AS graphite/NASA Resin 2 and 0.14-mm boron/5505 epoxy. The mechanical properties of these materials, which had been selected by the National Bureau of Standards, Boulder, Colorado, as candidates for low-temperature structural applications, have been discussed in detail by Schramm and Kasen [ ]. NBS supplied the samples for the thermal expansion measurement [" ] and calorimetric studies conducted at Battelle. 

Drilling and machining can damage composites. Several techniques exist for producing quality holes in composites. Carbon, aramid, and boron fiber reinforced materials each require different drilling methods and tools. When composites are cut, fibers are exposed. These fibers can absorb water, which weakens the material. Sealants can be used to prevent moisture absorption in the clearance hole. Sleeved fasteners can also provide fits that reduce water absorption as well as provide tightness. 

The term reinforced plastic (RP) refers to composite combinations of plastic, matrix, and reinforcing materials, which predominandy come in chopped and continuous fiber forms as in woven and nonwoven fabrics. Other terms used to identify an RP include glass fiber reinforced plastic (GFRP), aramid fiber reinforced plastic (AFRP), boron fiber reinforced plastic (BFRP), carbon fiber reinforced plastic (GFRP), graphite fiber reinforced plastic (GFRP), etc. 

Multi-component ceramics allow the optimization of various physical properties. These include ceramics which form multi-component oxides as well as fiher-rein-forced ceramic matrix composites. However, the oxidation behavior of these materials is complex compared with the pure materials. The leading fiber-reinforced composites are silicon-based and contain continuous SiC fibers with coatings of graphitic carbon or hexagonal boron nitride. The oxidation of the fiber coating at intermediate temperatures is a major issue and models of this process are discussed for both carbon and boron nitride coatings. 

Glass, carbon, and the aramids are the most common fiber reinforcements incorporated into polymer matrices. Other fiber materials that are used to much lesser degrees are boron, silicon carbide, and aluminum oxide tensile moduli, tensile strengths, specific strengths, and specific moduli of these materials in fiber form are given in Table 16.4. Boron fiber-reinforced polymer composites have been used in military aircraft components, helicopter rotor blades, and sporting goods. Silicon carbide and aluminum oxide fibers are used in tennis rackets, circuit boards, military armor, and rocket nose cones. 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

Boron Trichloride. Approximately 75—95% of the BCl consumed iu the United States is used to prepare boron filaments by CVD (7). These high performance fibers are used to reinforce composite materials (qv) made from epoxy resius and metals (Al, Ti). The principal markets for such composites are aerospace industries and sports equipment manufacturers. 

By this time the industry required a more inclusive term to describe RPs, so composite was added. Thus the name in the plastics industry became Reinforced Plastic Composites. More recently they became known only as Composites. However composites identify many other combinations of basic materials (Table 6-18). The fiber reinforcements included higher modulus glasses, carbon, graphite, boron, aramid (strongest fiber in the world, five times as strong as steel on an equal-weight basis), whiskers, and others (Table 6-20 and Figs. 6-13 and 6-14). In... 

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. 

Wawner, F.E., Jr. (1988). Boron and silicon carbide/carbon fibers. In Fiber Reinforcements for Composite Materials (A.R. Bunsell cd.), Elsevier, Amsterdam, pp. 371-425. 

The majority of commercially available polymeric composites are reinforced by glass fibers, carbon fibers, aramid fibers (e.g., Kevlar) and, to a lesser degree, boron fibers. In some cases hybrid composites are made that contain combinations of fibers. 

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