Boron fibre

A final step in manufacture is to coat the fibre with SiC or B4C. This provides protection against reactions with other elements at high operating temperatures and ensures that the fibre retains its tensile strength at elevated temperature. Typically, the W wire substrate has an 8 pm diameter, the diameter of the boron fibre sl50pm, and the SiC or B4C coating is 4pm thick. 

The importance of carbon-fibre composite materials in the development of the space shuttle cannot be overemphasized. Reinforced carbon-carbon composites are used in the nose cone and wing leading edges to provide the resistance to thermal shock and stress required for re-entry into the Earth s atmosphere. Carbon-carbon composites are a particular group of carbon-fibre reinforced materials in which both the bulk material and fibres are carbon. The 

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Boron itself has been used for over two decades in filament form in various composites BO3/H2 is reacted at 1300° on the surface of a continuously moving tungsten fibre 12/tm in diameter. US production capacity is about 20 tonnes pa and the price in about 80(. The primary use so far has been in military aircraft and space shuttles, but boron fibre composites are also being studied as reinforcement materials for commercial aircraft. At the domestic level they are finding increasing application in golf shafts, tennis rackets and bicycle frames. 

BORON FIBRE CARBON FIBRE--IRON WHISKER-... 

Bouix, J., Vincent, H., Boubehira, M., and Viala, J. C., Titanium Diboride-Coated Boron Fibre for Aluminum Matrix Composites, J. Less Common Metals, 117(l-2) 83-89 (Mar. 1986)... 

Boron fibres and boron/carbon fibre hybrids... 

Boron fibres were used for specific aeronautic composites, for example by Grumman in the wings of the F 14, because of their structural properties. Now, because of their high cost, they are often replaced by carbon fibres. 

Boron fibres are made by means of a CVD process, a Chemical Vapour Deposition process. Boron trichloride is heated in an oven and as a result it breaks down ... 

When a composite is subjected to external forces, the energy of the matrix is only transferred to the fibres when there is question of a proper attachment. For that reason fibres are some-times provided with a layer of another material. An example boron fibres in an aluminium matrix are provided with a silicon carbide coating and as a result the fibres are called borsic fibres. The thermal expansion coefficient of a fibre and its matrix must correspond. Figure 14.9 is a representation of what takes place when a crack in a fibre-reinforced matrix grows. 

Boron of low purity is obtained by reduction of the oxide by Mg, followed by washing the product with alkali, hydrochloric acid and then hydrofluoric acid. The product is a very hard, black solid of low electrical conductivity which is inert towards most acids, but is slowly attacked by concentrated HNO3 or fused alkali. Pure boron is made by the vapour-phase reduction of BBr3 with H2, or by pyrolysis of B2H6 or BI3. At least four allotropes can be obtained under different conditions but transitions between them are extremely slow. For a discussion of the production of boron fibres, see Section 27.7. 

Fig. 27.14 Schematic representation of the assembly used for the manufacture of boron fibres by CVD using a tungsten substrate.

Fibre production began in 1972 at 100000 Ib/annum with plans to increase as the market develops. Large price reductions for the fibre were timed by Du Pont to coincide with the 1972 Farnborough air show bringing epoxy pre-preg fabric for instance down to 20-30 per lb, and projected costs are expected to be lower than for carbon and boron fibres. It has been reported that the fibre will be availaWe as collimated tape, chopped fibre, yams, rovings and as woven fabric. ... 

Composite parts Solid composite parts may be repaired by making carefully tapered scarf joints. Hot-bonded repairs are preferred provided that the surface is first adequately dried." Parts made from structural fibres, that is, carbon fibre, Kevlar or boron fibres and some glass-fibre parts require that each repair layer be of the same type as the original, or a direct equivalent, and laid up in the correct orientation. 

Vega-Boggio, J., Vingsbo, O. and Carlsson, J.O. (1977) The initial stages of growth and the origin of proximate voids in boron fibres. J. Mater. Sci., 12 1750-1758. 

Boron fibres High tensile strength and compressive load-bearing expensive... 

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